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A Complete Guide to Design and Build a Hi-Fi LM3886 Amplifier

by Admin

Word: Editable PCB recordsdata are obtainable for this venture right here.

The LM3886 is among the most extremely regarded audio chip amplifiers within the DIY group. The explanation for its recognition is because of its very low distortion, minimal exterior elements, and low price. With the proper structure and element choice, you possibly can construct a superb sounding Hello-Fi audio amplifier that may rival high-end amps retailing for a number of thousand {dollars} or extra.

On this tutorial, I’ll go step-by-step by the amplifier design course of as I construct a 40 Watt stereo amplifier utilizing the LM3886. I’ll clarify what every a part of the circuit does, and present you the way to calculate the proper element values with examples from the amplifier I’m constructing. I’ll additionally present you the way to structure the PCB and wire the amplifier in an enclosure for minimal noise and hum.

My amplifier relies off of the similar circuit offered within the datasheet, with all the elective stability elements included.

BONUS: Obtain my elements record to see the elements I used to get nice sound high quality from this amplifier. I’ve additionally included the schematic and Gerber recordsdata for the ability provide I used.

I extremely advocate studying the datasheet earlier than constructing your amplifier. It has all the efficiency specs, absolute most rankings, schematics, and design suggestions:

Circuit Basics PDF Icon LM3886 Datasheet

Utility notice AN-1192 has further data that fills in gaps omitted of the datasheet. It additionally has schematics for bridged and parallel amplifier circuits:

Circuit Basics PDF Icon Overture Utility Word AN-1192

It’s additionally good to have the Overture Design Information. That is an Excel spreadsheet that calculates output energy, warmth sink dimension, achieve, and different helpful parameters:

Circuit Basics Excel Icon Overture Design Information

Since this can be a moderately lengthy article, listed below are hyperlinks to the completely different sections:

You too can take a look at this video to see a fast overview of the design course of. On the finish I wire up the amp so you possibly can hear what it feels like:

Issues to Determine Earlier than Beginning

Earlier than beginning to design your amplifier, it’s best to have an concept about how a lot output energy you wish to get out of it. Output energy is what you’ll usually see acknowledged because the Wattage score of an amplifier. The utmost output energy of the LM3886 is 68 Watts, however the precise energy you get will rely in your energy provide voltage and speaker impedance.

You’ll additionally must know the impedance of your audio system. You must be capable to discover your speaker’s impedance on the again of the speaker or within the consumer handbook.

Lastly, you’ll must know your enter voltage. That is the output voltage of the audio supply you’ll be amplifying. It might be within the machine’s consumer handbook, but when not, you may get a tough estimate by enjoying a 60 Hz pure sine wave (there are apps that may do that) at full quantity and measuring the AC voltage between floor and the left or proper channel with a multi-meter.

WARNING: THIS PROJECT INVOLVES WORKING WITH MAINS SUPPLY VOLTAGES THAT CAN SERIOUSLY INJURE YOU OR KILL YOU. BE SURE TO TAKE ALL NECESSARY SAFETY PRECAUTIONS, AND NEVER WORK ON A LIVE CIRCUIT!!

Decide the Required Energy Provide Voltage and Energy

Let’s begin by determining how a lot voltage and energy your amplifier will want from the ability provide. These calculations will let you know the proper voltage and VA rankings for the transformer you’ll use to energy your amp. This step is essential as a result of if the transformer’s voltage is just too low, the output energy of the amplifier will probably be lower than what you anticipated. If the transformer’s VA score is just too small, the amplifier may clip or distort the audio at larger volumes.

Required Energy Provide Voltage

Earlier than you’ll find the required energy provide voltage, you might want to calculate the amplifier’s peak output voltage.

Discover the Peak Output Voltage

Peak output voltage (Vopeak) is the utmost voltage measured throughout the amplifier’s speaker terminals. Your amplifier’s peak output voltage will rely in your desired output energy (Po) and speaker impedance, in accordance with this method:

V_{opeak} = sqrt{2times R_{L}times P_{o} }  R_{L} = Speaker  impedance  P_{o} = Average  output  power

The amplifier I’m constructing will probably be 40 Watts with 6 Ω audio system, so my peak output voltage is:

V_{opeak} = sqrt{2times 6  Omegatimes 40  W}  = sqrt{480}  = 21.9  V

Discover the Most Provide Voltage Wanted by the Amplifier

Now that you simply’ve discovered the height output voltage of your amplifier, you possibly can calculate the most provide voltage (Vmax provide). That is the voltage the amplifier wants from the ability provide to get the desired output energy.

To seek out the utmost provide voltage, take the height output voltage and add the voltage drop (Vod) of the LM3886 (Four V). Then consider your transformer’s regulation and the variation in your mains voltage.

Regulation is the rise in output voltage of a transformer when the load isn’t drawing present (i.e. the amp stops enjoying music). Regulation values can often be discovered within the transformer’s datasheet, however if you happen to don’t know your transformer’s regulation, a protected worth to make use of is 15%. The regulation of the transformer I’ll be utilizing is 6%.

Mains voltages can range as much as 10% relying in your location. It often peaks late at evening when persons are asleep and drops within the daytime when extra persons are awake and drawing present from the ability grid.

Use this method to calculate the utmost provide voltage required by your amplifier:

V_{max  supply} = pm( V_{opeak}+ V_{od})(1+Regulation)(1.1)  V_{opeak}= Peak  output  voltage  V_{od}= Voltage  drop  across  LM3886  Rightarrow 4  V  Regulation= Increase  in  voltage  when  transformer  is  unloaded  1.1= Variation  in  mains  voltage  (10  %)

For my 40 Watt amplifier, the utmost provide voltage it wants is:

V_{max hspace{1mm} supply} = pm(21.9hspace{1mm}V+4hspace{1mm}V)(1+0.06)(1.1)  =pm(25.9times1.06times1.1)  =pm30.2hspace{1mm}V

So my energy provide might want to ship a peak voltage of ±30.2 V for my amplifier to output 40 Watts into 6 Ω audio system. The ± image signifies that the voltage is +30.2 V on the constructive rail and -30.2 V on the adverse rail.

The subsequent step is discovering a transformer voltage score that may ship this most provide voltage.

Discover the Most Provide Voltage Output by a Transformer

Needless to say a transformer’s voltage score solely tells you it’s AC voltage output. The DC voltage will probably be larger after the bridge rectifier diodes in your energy provide convert the AC voltage to DC.

To seek out the utmost DC provide voltage output by a transformer and energy provide, take the transformer’s AC voltage score and consider a 1.41 improve in voltage from the rectifier diodes, the 10% mains provide variation, and the transformer’s regulation:

V_{max hspace{1mm} supply}=(V_{transformer})(1.41)(1.1)(1+Regulation)  V_{transformer}=Transformer hspace{1mm} voltage hspace{1mm} rating  1.41=Voltage hspace{1mm} increase hspace{1mm} from hspace{1mm} rectifier hspace{1mm} diodes  1.1= Variationhspace{1mm}inhspace{1mm}mainshspace{1mm}voltagehspace{1mm}(10hspace{1mm}%)  Regulation= Increasehspace{1mm} inhspace{1mm} voltage hspace{1mm}when hspace{1mm}transformer hspace{1mm}is hspace{1mm}unloaded

I attempted the above calculation with a transformer rated at 18 V AC to see if it may provide the 30.2 V most provide voltage wanted by my amplifier. With an 18 V transformer, I might get a most provide voltage of:

V_{max hspace{1mm} supply}=(18 hspace{1mm} V)(1.41)(1.1)(1+0.06)  =29.6 hspace{1mm} V

29.6 V is fairly near the 30.2 V most provide voltage wanted by my amplifier, however let’s calculate precisely how a lot output energy I’d get with this transformer.

Discover the Output Energy from a Transformer’s Voltage Ranking

To calculate the output energy you’ll get from a selected transformer’s voltage score, use this method:

P_{o}=frac{(frac{V_{max hspace{1mm} supply}}{(1+Regulation)(1.1)}-V_{od})^{2}}{2times R_{L}}  V_{max hspace{1mm} supply}=Maximum hspace{1mm} supply hspace{1mm} voltage  of  power  supply  V_{od}= Voltagehspace{1mm}drophspace{1mm}acrosshspace{1mm}LM3886hspace{1mm}Rightarrow 4hspace{1mm}V  Regulation= Increasehspace{1mm} inhspace{1mm} voltage hspace{1mm}when hspace{1mm}transformer hspace{1mm}is hspace{1mm}unloaded  1.1= Variationhspace{1mm}inhspace{1mm}mainshspace{1mm}voltagehspace{1mm}(10hspace{1mm}%)  R_{L} = Speakerhspace{1mm}impedance

Utilizing the utmost provide voltage I calculated for an 18 V transformer (29.6 V), the output energy I’ll get is:

P_{o}=frac{(frac{V_{max hspace{1mm} supply}}{(1+Regulation)(1.1)}-V_{od})^{2}}{2times R_{L}}  P_{o}=frac{(frac{29.6 hspace{1mm} V}{(1+0.06)(1.1)}-4 hspace{1mm} V)^{2}}{2times 6 hspace{1mm} Omega}  =frac{(frac{29.6 hspace{1mm} V}{1.166}-4 hspace{1mm} V)^{2}}{12}  =frac{(21.4)^{2}}{12}  =frac{458}{12}  =38.2 hspace{1mm} W

38.2 Watts of output energy is fairly near my purpose of 40 Watts, so an 18 V transformer will work high-quality.

Required Transformer Energy

Now let’s discover a minimal VA score for the transformer that may energy your amplifier.

First you’ll must calculate the entire energy (Psupply) required by the amplifier. Whole energy depends upon the utmost provide voltage output by the ability provide, the amplifier’s peak output voltage, and the speaker impedance. The method to make use of is:

P_{supply} = 2 times V_{cc}(frac{V_{opeak}}{pitimes R_{L}}+QPSC_{total})  V_{cc}= Maximum  supply  voltage  of  power  supply  V_{opeak}= Peak  output  voltage  QPSC_{total}= Total  quiescent  power  supply  current  (from  datasheet)  R_{L}= Speaker  impedance

I’ve already calculated the utmost provide voltage of an 18 V transformer (29.6 V), and the height output voltage of my amplifier (21.9 V). The full quiescent energy provide present (QPSC) is given within the LM3886’s datasheet as 85 mA.

So my 18 V transformer wants to produce the amplifier with at the very least:

P_{supply} = 2times 29.6 hspace{1mm} V(frac{21.9hspace{1mm}V}{pitimes 6hspace{1mm}Omega}+0.085hspace{1mm}A)  = 59.2times(1.16+0.085)  =59.2times1.25  =74hspace{1mm}W

The full energy can now be used to discover a minimal VA score in your transformer.

Convert the Whole Energy to a Transformer VA Ranking

To transform the entire energy to a transformer VA score, a normal rule of thumb is to multiply it by an element of 1.5:

74hspace{1mm}Wtimes1.5 = 111hspace{1mm}VA

That is the VA required for every channel, so for a stereo amplifier powered by a single transformer, simply double it:

111hspace{1mm}VAtimes2 = 222hspace{1mm}VA

Discovering a transformer with a VA of 222 will probably be onerous, however you can round-up to the following closest worth and use a 250 VA transformer or bigger.

Decide the Correct Warmth Sink Measurement

The LM3886 wants a warmth sink giant sufficient to dissipate the warmth it generates or it can rapidly turn out to be broken. The minimal dimension of the warmth sink may be discovered by calculating its most thermal resistance (in °C/W).

First although, you’ll must know your LM3886’s most energy dissipation (Pdmax), and the thermal resistances within the path warmth takes from the chip die to the ambient air.

Discover the Most Energy Dissipation

The utmost energy dissipation is the restrict at which the LM3886’s inner SPiKe circuitry is enabled. Sound high quality is severely compromised when the SPiKe circuitry is enabled, so to forestall this we want a warmth sink with a thermal resistance low sufficient to dissipate the utmost energy dissipated by the LM3886. Pdmax depends upon the utmost provide voltage of your energy provide and your speaker impedance:

P_{dmax} =frac{(2times V_{cc})^{2}}{2timespi^{2}times R_{L}}  V_{cc}= Maximum  supply  voltage  of  power  supply  R_{L}= Speaker  impedance

The most provide voltage output from my energy provide is ±29.6 V, and I’ll be driving 6 Ω audio system, so my Pdmax is:

P_{dmax} =frac{(2times 29.6 hspace{1mm} V)^{2}}{2timespi^{2}times 6hspace{1mm}Omega}  = frac{3505}{118.4}  =29.6hspace{1mm}W

So my warmth sink wants to have the ability to dissipate 29.6 Watts of energy to forestall activation of the SPiKe safety circuit.

Discover the Most Thermal Resistance of the Warmth Sink

There are three resistances to warmth circulation away from the LM3886:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Thermal Resistance Diagram

θjc: The thermal resistance from the chip’s junction (the die) to the case.

θcs: The thermal resistance of the hole between the chip case and the warmth sink.

θsa: The thermal resistance from the warmth sink to the ambient air.

Extra energy will probably be dissipated when any of the thermal resistances within the path to ambient air are lowered. θjc is a property of the plastic case enclosing the die, so we will’t do something to decrease that.

θcs may be lowered by utilizing a thermal paste between the chip and the warmth sink. Thermal paste has a thermal resistance of round 0.2 °C/W, however the actual worth of the kind you utilize ought to be obtainable from the producer.

The best solution to cut back the entire thermal resistance is by decreasing θsa with a extra environment friendly warmth sink. Warmth sinks with a decrease θsa are higher at dissipating warmth.

The warmth sink will dissipate the height energy produced by the amplifier (Pdmax) if its thermal resistance (θsa) is lower than or equal to the worth calculated with this method:

theta_{sa} =frac{[(T_{jmax}-T_{amb})-P_{dmax}(theta_{jc}+theta_{cs})]}{P_{dmax}}  T_{jmax}= Maximumhspace{1mm}junctionhspace{1mm}temperaturehspace{1mm}Rightarrowhspace{1mm}150hspace{1mm}^{circ}Chspace{1mm}(fromhspace{1mm}datasheet)  T_{amb}= Ambienthspace{1mm}temperature  theta_{jc}= Thermalhspace{1mm}resistancehspace{1mm}fromhspace{1mm}junctionhspace{1mm}tohspace{1mm}case  theta_{cs}= Thermalhspace{1mm}resistancehspace{1mm}fromhspace{1mm}casehspace{1mm}tohspace{1mm}heathspace{1mm}sink  P_{dmax}= Maximumhspace{1mm}powerhspace{1mm}dissipation

The LM3886 is manufactured in two completely different packages, the LM3886T and the LM3886TF. The LM3886T has a metallic flange on the again of the case, and the LM3886TF is all plastic. The plastic case of the LM3886TF offers it the next θcs:

  • LM3886T: θcs = 1 °C/W
  • LM3886TF: θcs = 2 °C/W

Tjmax is the utmost junction temperature, or the temperature on the chip’s die above which the thermal shutdown circuitry is enabled. The datasheet offers a price for Tjmax of 150 °C.

Tamb is the ambient temperature in °C that the amplifier will probably be operated at. A typical worth for Tamb is room temperature (25 °C).

So the utmost thermal resistance (θsa) of the warmth sink for my amplifier with a Pdmax of 29.6 W is:

theta_{sa} =frac{[(150hspace{1mm}^{circ}C-25hspace{1mm}^{circ}C)-29.6hspace{1mm}W(2hspace{1mm}^{circ}C/W+0.12hspace{1mm}^{circ}C/W)]}{29.6hspace{1mm}W}  =frac{62.2}{29.6}  =2.1hspace{1mm}^{circ}C/W

So I’ll want a warmth sink rated at lower than or equal to 2.1 °C/W to make sure it could actually dissipate the utmost energy produced by the LM3886.

Right here’s one channel of my amplifier hooked up to a correctly sized warmth sink:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Assembled PCB With Heat Sink

Calculating the Part Values

Now that you simply’ve calculated the ability provide and warmth sink necessities, the following step is to search out values for the elements within the amplifier circuit. I’ll be utilizing the schematic beneath. It’s mainly the identical because the one within the datasheet, however with the elective stability elements included:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Circuit Schematic

Word: The elements are labeled as they seem within the datasheet.

Right here’s a diagram of the LM3886’s pin structure in your reference:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - LM3886 Pinout Diagram

Discover the Minimal Achieve Required

The achieve may be set to any worth above the LM3886’s minimal of 10 Vo/Vi, however so as to get your required output energy, it must be above a sure minimal worth. The minimal achieve setting of your amplifier will rely in your enter voltage, speaker impedance, and output energy in accordance with the method:

A_{V} geqfrac{sqrt{P_{o}times R_{L}}}{V_{in}}  A_{V}= Gain  (frac{V_{o}}{V_{i}})  V_{in}= Inputhspace{1mm}voltage  R_{L}= Speakerhspace{1mm}impedance  P_{o}= Outputhspace{1mm}power

I plan on utilizing an iPhone because the audio supply for my amplifier, which has an output voltage of 1 V. The output energy I’ll get with my transformer and energy provide is 38.2 W, and the impedance of my audio system is 6 Ω. So my minimal achieve is:

A_{V} geqfrac{sqrt{38.2 hspace{1mm}Wtimes 6hspace{1mm}Omega}}{1 hspace{1mm}V}   geqfrac{sqrt{229}}{1}   geq 15.1

So I’ll must set the achieve to at the very least 15.1 Vo/Vi if I would like 38.2 Watts output energy into 6 Ω audio system with a 1 V enter voltage.

Setting the Achieve

The achieve of the LM3886 may be set by altering the values of resistors Ri and Rf1. These resistors kind a voltage divider that determines the voltage on the inverting enter (pin 9) of the LM3886:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Rf1 and Ri Determine the Amplifiers Gain

Setting the achieve too excessive may trigger distortion. Setting it too low may make your amplifier too quiet. A good achieve setting that’s not too excessive to trigger distortion, however not too low to provide you a superb vary of quantity is round 27 to 30 db.

The achieve is calculated with this method:

A_{V} = 1+frac{R_{f1}}{R_{i}}  A_{V}= Gain  (frac{V_{o}}{V_{i}})  R_{f1}= Resistancehspace{1mm}ofhspace{1mm}R_{f1}hspace{1mm}inhspace{1mm}Ohms  R_{i}=Resistancehspace{1mm}ofhspace{1mm}R_{i}hspace{1mm}inhspace{1mm}Ohms

This offers you the voltage achieve (Vo/Vi), or the issue of amplification. To transform voltage achieve to decibels (db) achieve, use this method:

Gain_{db}=20 times log_{10} (frac{V_{o}}{V_{i}})

Greater worth resistors create extra Johnson-Nyquist noise, so it’s finest to search out an Rf1 / Ri ratio that gives your goal achieve with low resistor values.

I made a decision on a achieve of round 27 db (22.Four Vo/Vi) for my amplifier. To maintain the resistances low, I began out by setting Ri to 1 kΩ. Then I rearranged the achieve method to unravel for Rf1 with a achieve of 22.Four Vo/Vi:

R_{f1} = R_{i}(A_{V} - 1)  R_{f1} = 1000  Omega(22.4 - 1)  =21,400  Omega

I’m going to make use of Vishay-Dale PTF collection metallic movie resistors all through my amplifier, however the closest worth I may discover was 20 kΩ. However utilizing a 20 kΩ resistor for Rf1 would make the achieve:

A_{V} = 1+frac{20000hspace{1mm}Omega}{1000hspace{1mm}Omega}  =1+20  =21hspace{1mm}frac{V_{o}}{V_{i}}  = 26.4 hspace{1mm}db

Which is shut sufficient to 27 db, and above the 15.1 Vo/Vi minimal achieve required for my desired output energy, enter voltage, and speaker impedance.

If you’re constructing a stereo amplifier, you need Ri and Rf1 to have shut resistance tolerances. If these resistors range a lot between the 2 channels, the positive aspects will probably be completely different and one channel will probably be louder than the opposite. Metallic movie resistors with a tolerance of 0.1% or much less are excellent.

Balancing the Enter Bias Present

After setting the achieve, the following step is to steadiness the enter bias currents by selecting values for Rin and Rb:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Balancing the Input Bias Current With Rin, Rb, and Rf1

If the currents on the non-inverting enter (pin 9) and the inverting enter (pin 10) are completely different, a voltage will develop between them. This distinction in voltage will probably be amplified as noise.

The inverting enter sees the resistance of Rf1 and the non-inverting enter sees the resistance of Rin and Rb in collection. You already discovered a price for Rf1 whenever you set the achieve of the amplifier. The values of Rin and Rb are chosen so collectively they equal the worth of Rf1. This may make the present on the non-inverting enter equal to the present on the inverting enter. To seek out values of Rin and Rb for a selected Rf1, use this method:

R_{in}+R_{b}approx R_{f1}

I used the worth given within the datasheet for Rb (1 kΩ). So with Rf1 at 20 kΩ, the worth of Rin that balances the enter bias present for my amplifier is:

R_{in}+R_{b}approx R_{f1}  R_{in}approx R_{f1} -R_{b}    approx 20000 hspace{1mm} Omega - 1000 hspace{1mm} Omega   approx 19000 hspace{1mm} Omega

You’ll in all probability be capable to discover a 19 kΩ resistor obtainable with the kind of resistors you’re utilizing, however 20 kΩ is the closest worth I may discover with Vishay-Dale PTF resistors so I’ll must settle with that.

Set the Low Frequency Cutoff on the Amplifier’s Enter

Cin is in collection with the non-inverting enter. It’s major operate is to dam any DC current within the audio supply, whereas permitting AC (the audio sign) to move. DC within the audio supply must be blocked or it will probably be amplified together with the audio sign and create a excessive DC offset on the audio system. This distorts the audio, which we don’t need for apparent causes.

Along with the DC blocking operate, Cin and the enter resistor (Rin) kind a excessive move RC filter that units the low finish of the amplifier’s bandwidth on the non-inverting enter:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Cin and Rin High Pass Input Filter

This filter’s cutoff frequency (also called the -3db level or nook frequency) is the frequency at which the filter begins to work. In a excessive move filter, frequencies beneath the cutoff frequency will probably be attenuated (muted). In a low move filter, all frequencies above the cutoff frequency will probably be muted. We’ll use mixtures of low move and excessive move filters to set the amplifier’s bandwidth and enhance stability.

The cutoff frequency (Fc) of this filter may be discovered with the equation:

F_{c}=frac{1}{2pi R_{in}C_{in}}  R= Resistancehspace{1mm}ofhspace{1mm}R_{in}  in  Ohms  C= Capacitancehspace{1mm}ofhspace{1mm}C_{in}  in  Farads

The equation may be rearranged to discover a worth of Cin for a selected Fc:

C_{in}=frac{1}{2pi R_{in}F_{c}}

You discovered the worth for Rin whenever you balanced the enter bias currents, so all you want now’s to decide on a cutoff frequency. The decrease restrict of human listening to is 20 Hz, so the Fc ought to be effectively beneath that to forestall attenuation of bass frequencies. Decrease than 2 to Four Hz is right.

I are likely to take heed to music with plenty of bass, so I made a decision on a reasonably low Fc for my amplifier. I began with 1.5 Hz, however you need to use larger or decrease values if you would like. Simply be sure you keep effectively beneath 20 Hz or the bass will probably be weak.

With an Fc of 1.5 Hz, the worth of my Cin must be:

C_{in}=frac{1}{2times pi times 20000hspace{1mm} Omegatimes 1.5hspace{1mm} Hz}  =frac{1}{188,496}  =0.0000053hspace{1mm} F  =5.3hspace{1mm}mu F

A 5.Three µF capacitor will probably be onerous to search out, however an in depth worth that’s pretty frequent is 4.7 µF. The Fc with a 4.7 µF capacitor could be:

F_{c}=frac{1}{2 times pi times 20000hspace{1mm} Omega times 0.0000047 hspace{1mm} F}  =frac{1}{0.591}  =1.69hspace{1mm} Hz

An Fc of 1.69 Hz is fairly near my desired 1.5 Hz, so a 4.7 µF capacitor ought to be good.

Since Cin is instantly within the path of the audio enter sign, the sort of capacitor used could have an affect on sound high quality. Electrolytics, ceramics, and tantalum capacitors ought to be averted. A superb high quality polypropylene metallic movie, and even higher a polypropylene metallic movie in oil capacitor will sound finest right here.

Set the Low Frequency Cutoff on the Suggestions Loop

A second excessive move filter exists within the suggestions loop with Ri and Ci:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Set the Fc of the Feedback Loop High Pass Filter

The cutoff frequency of this filter ought to be Three to five instances decrease than the Fc of the Cin Rin excessive move filter on the enter. If the Fc of this filter is larger than the enter filter, the amplifier will move low frequencies to the suggestions loop that it can’t deal with. This will create a voltage throughout Ci and trigger DC voltage to seem on the inverting enter, which will probably be amplified and trigger distortion. Due to this fact, the enter filter (Cin and Rin) ought to decide the decrease bandwidth frequency of the amplifier, not the suggestions loop filter (Ci and Ri).

The enter filter defines the low finish of the bandwidth, however Ci nonetheless has an impact on the bass response. With smaller values of Ci, the bass will probably be softer and have much less punch, however with bigger values of Ci, the bass will probably be tighter and have extra influence.

The method beneath provides you with a place to begin for the worth of Ci:

C_{i}geqfrac{sqrt{2}times(R_{in}+R_{b})times C_{in}}{R_{i}}

I already discovered values for Rin, Cin, Rb, and Ri, so the worth of my Ci ought to be higher than:

C_{i}geqfrac{sqrt{2}times(20000 hspace{1mm} Omega+1000 hspace{1mm} Omega)times 0.0000047F}{1000 hspace{1mm} Omega}   geqfrac{0.14}{1000 hspace{1mm} Omega}   geq 0.00014 hspace{1mm} F   geq 140 hspace{1mm} mu F

Rounding as much as the following frequent capacitance worth offers 220 µF. Lets see what the cutoff frequency could be with that. We are able to use the Fc equation with Ri and Ci:

F_{c}=frac{1}{2 times pi times 1000hspace{1mm} Omega times 0.00022 hspace{1mm} F}  =frac{1}{1.38}  =0.72hspace{1mm} Hz

Now I’ll test to see if 0.72 Hz is Three to five instances decrease than the 1.69 Hz Fc of my enter filter:

frac{1.69 hspace{1mm} Hz}{0.72 hspace{1mm} Hz}  =2.3

It’s 2.Three instances decrease. Lets attempt some bigger values for Ci to see if we will’t do higher than that. Repeating the Fc calculation for a 330 µF capacitor offers 0.48 Hz.

frac{1.69 hspace{1mm} Hz}{0.48 hspace{1mm} Hz}  =3.5

3.5 instances decrease is okay, however I may be capable to do even higher with a 470 µF capacitor. Repeating the calculations once more with a 470 µF capacitor offers an Fc of 0.34 Hz.

frac{1.69 hspace{1mm} Hz}{0.34 hspace{1mm} Hz}  =4.9

A 470 µF capacitor will set the Fc of my suggestions loop filter to 4.9 instances decrease than the Fc of my enter filter. That is nice, so I’ll use a 470 µF capacitor for Ci.

Ci can be within the audio sign path, so a superb high quality capacitor ought to be used. The capacitance will in all probability be too excessive to make use of polypropylene, so that you’ll possible have to make use of an electrolytic. Nonetheless, there are good high quality audio grade electrolytics just like the Elna Silmic II or Nichicon KZ collection which shouldn’t adversely have an effect on the sound high quality.

Set the Excessive Frequency Cutoff on the Amplifier’s Enter

Rb and Cc kind a low move RC filter that units the higher restrict of the amplifier’s bandwidth on the non-inverting enter:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Rb and Cc Low Pass Input Filter

Within the datasheet, Cc is proven linked between the non-inverting enter and the inverting enter. In that configuration, Cc filters radio frequency and electromagnetic interference picked up by the enter wires. Sadly, it additionally will increase the prospect for oscillation. A greater means is to attach Cc from the non-inverting enter to floor as proven within the picture above. That means Cc nonetheless filters radio frequencies, but it surely additionally acts as a low move filter that may set the higher restrict of the amplifier’s bandwidth.

The Fc of this filter ought to be set effectively beneath the bottom radio broadcast frequency in your space, and effectively above the 20 kHz higher restrict of human listening to. Broadcast radio frequencies within the USA are:

  • FM: 87.5 to 108 MHz
  • AM: 535 to 1605 kHz

I selected to begin with an Fc of about 250 kHz. It’s effectively beneath the bottom AM broadcast frequency (535 kHz), so radio frequencies and most electromagnetic interference ought to be filtered out. It’s additionally effectively above the higher 20 kHz frequency of human listening to, so larger audio frequencies received’t be attenuated.

To discover a worth for Cc that provides an Fc of 250 kHz, I’ll simply rearrange the cutoff frequency method:

F_{c}=frac{1}{2pi R_{b}C_{c}}Rightarrow C_{c}=frac{1}{2pi R_{b}F_{c}}  C_{c}=frac{1}{2times pi times 1000 hspace{1mm} Omega times 250000 hspace{1mm} Hz}  =frac{1}{1.57 times 10^{9}}  =6.36 times 10^{-10}hspace{1mm} F  =636 hspace{1mm} pF

Since 636 pF will not be a typical worth, I’ll spherical as much as 680 pF. With a 680 pF capacitor, the Fc turns into:

F_{c}=frac{1}{2 times pi times 1000hspace{1mm} Omega times (6.8 times 10^{-10} hspace{1mm} F)}  =frac{1}{4.27 times 10^{-6}}  =234 hspace{1mm} kHz

So a 680 pF capacitor will set the higher cutoff frequency to 234 kHz, which is shut sufficient to my desired Fc of 250 kHz. Cc can be within the sign path, so a superb high quality capacitor ought to be used. The most effective dielectric varieties for audio capacitors within the picofarad vary are silver mica or polystyrene.

Stability Parts Rf2 and Cf

Rf2 and Cf dampen resonance within the suggestions loop and improve stability:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Rf1, Rf2 and Cf

Rf1, Rf2, and Cf kind a low move filter within the suggestions loop, however as you possibly can see from the method within the datasheet, calculating the Fc of this filter is sort of difficult:

F_{c}=frac{R_{f1}times R_{f2}(s+frac{1}{R_{f2}times C_{f}})}{(R_{f1}+R_{f2})(s+(frac{1}{C_{f}(R_{f1}+R_{f2})}))}

One of the best ways to find out values for Rf2 and Cf is with circuit simulation software program comparable to LTSpice. That’s past the scope of this text although, so I’m simply going to make use of the values given within the datasheet.

However if you wish to experiment, reducing the worth of Cf will increase the higher Fc of the bandwidth, and rising the worth will decrease it.

The Zobel Community

Csn and Rsn kind a Zobel community at the amplifier’s output:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Rsn and Csn Form the Zobel Network

The Zobel community is used to forestall oscillations brought on by inductive hundreds. It additionally prevents radio frequencies picked up by the speaker wires from getting again into the amplifier’s inverting enter by way of the suggestions loop.

At excessive frequencies, the impedance of Csn may be very low, so excessive frequency present is shorted to floor. Rsn limits the excessive frequency present so there isn’t a direct brief to floor, which may exceed the present restrict of the LM3886. Due to this fact, smaller values of Rsn make the Zobel community extra environment friendly at filtering radio frequencies, but it surely additionally will increase the cutoff frequency, which in flip reduces it’s effectiveness.

The datasheet offers a price of two.7 Ω for Rsn, and a price of 100 nF for Csn. This makes the Fc:

F_{c}=frac{1}{2  pi  R_{sn}  C_{sn}}  =frac{1}{2  pi  (2.7  Omega)  (1 times 10^-7  F)}  =frac{1}{0.0000017}  =589  kHz

589 kHz is pretty excessive, particularly because the lowest frequency of AM radio broadcast is 535 kHz. As a way to deliver this all the way down to a extra affordable stage, I made a decision on utilizing 4.7 Ω for Rsn and 220 nF for Csn, which lowers the Fc all the way down to 154 kHz:

F_{c}=frac{1}{2  pi  (4.7  Omega)  (2.2 times 10^-7  F)}  =frac{1}{0.0000065}  =154  kHz

154 kHz is effectively above the 20 kHz restrict of human listening to, and effectively beneath any radio frequencies the speaker wires may decide up.

Since Rsn might want to shunt excessive currents to floor if the amplifier oscillates, the ability score ought to be at least 1 Watt. Csn ought to have low ESR and low ESL, with a voltage score higher than the rail to rail swing of output voltage. To attenuate inductance, find the Zobel community shut to the output pin (pin 4) and hold the traces brief.

The Thiele Community

How to Design a Hi-Fi Audio Amplifier With an LM3886 - The Thiele Network

Whereas the Zobel community reduces oscillations brought on by inductive hundreds, the Thiele community reduces oscillations brought on by capacitive hundreds, often as a consequence of lengthy speaker cables. It additionally prevents radio frequencies picked up by the speaker wires from getting again into the amplifier’s inverting enter by the suggestions loop.

Inductors have a low impedance to low frequency present and a excessive impedance to excessive frequency present. Audio indicators are comparatively low frequency, so they are going to circulation by the inductor uninhibited. Excessive frequency oscillation present will probably be impeded by the inductor and be pressured to circulation by the resistor, which is able to dampen it.

The datasheet recommends a 10 Ω, 5 Watt resistor in parallel with a 0.7 µH inductor. In a stereo amplifier, there will probably be one Thiele community per channel. They need to be situated away from the amplifier’s enter circuitry to forestall interference from the magnetic fields generated by the inductor. A superb location is close to the speaker output terminals, separated a bit or at 90° angles to one another to forestall magnetic area coupling between them.

Making the Inductors

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Assembled Thiele Network

The inductors for the Thiele community are wire wound air core varieties, made by wrapping enamel coated wire (magnet wire) round a cylindrical object. For the reason that inductor will probably be carrying the total output present of the amplifier, the wire ought to be heavy gauge. 12 to 18 AWG could be good. Use this Single-Layer Air Coil Calculator to learn how many turns you want for a selected wire diameter and coil diameter.

Or you possibly can calculate the inductance your self with this method:

L=frac{d^2 times n^2}{(18 times d)+(40 times l)}  L=Inductance  in  mu H  d=Coil  diameter  in  inches  l=Coil  length  in  inches  n=Number  of  turns

I used 14 AWG magnet wire in my construct because it’s thick and simple to search out. The diameter of 14 AWG is 1.62814 mm. I deliberate on utilizing a screwdriver shaft with a diameter of 11 mm to kind the coil. Getting into this data into the inductance calculator, I discovered I’d want about 12 wraps to get a 0.7 µH inductor.

Energy Provide Decoupling Capacitors

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Power Supply Decoupling Capacitors

The LM3886 has one adverse energy provide pin (pin 4) and two constructive energy provide pins (pins 1 and 5). The adverse provide pin wants it’s personal set of decoupling capacitors and the constructive provide pins share a separate set of decoupling capacitors.

The massive decoupling capacitors present an enduring supply of reserve present when the low frequency output of the amplifier is excessive. Bigger values will enhance the bass response. Typical values are between 470 µF and 2200 µF.

The medium worth decoupling capacitors provide further present for mid-frequency output. These ought to be someplace between 10 µF and 220 µF.

The small worth decoupling capacitors present present in a short time to assist the amplifier output larger audio frequencies. Additionally they filter noise and radio frequency interference within the energy provide.

The decoupling capacitors additionally compensate for the parasitic inductance and resistance of the ability provide wires and traces resulting in the the chip’s energy pins. Inductance and resistance inhibit the circulation of present, which will increase with longer wires and traces. For the reason that energy provide is comparatively distant from the chip, inductance and resistance are an issue. To maximise present circulation to the chip, the decoupling capacitors ought to be positioned as shut as potential to the chip’s energy pins.

Capacitors with decrease equal collection resistance (ESR) and decrease equal collection inductance (ESL) are the perfect varieties to make use of right here.

Analysis by Tom Christiansen exhibits {that a} 4.7 µF ceramic X7R capacitor in parallel with a 22 µF electrolytic and 1000 µF electrolytic has considerably higher efficiency than the paralleled 100 nF, 10 µF, and 470 µF capacitors really useful within the datasheet. That’s what I’ll be utilizing in my amplifier.

The Mute Circuit

Rm, Cm, and D1 kind the mute circuit:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - The Mute Circuit

When the present flowing out of the mute pin (pin 8) is lower than 0.5 mA, the amplifier’s output is muted, and when the present is bigger than 0.5 mA, the output is un-muted.

To un-mute the amplifier, we have to discover a worth for Rm in order that the present allowed to circulation out of pin Eight is bigger than 0.5 mA. That may be discovered with this method:

R_{m}leqfrac{|V_{ee}|-2.6 hspace{1mm} V}{0.0005 hspace{1mm} A}   |V_{ee}|= Absolute  value  of  the  negative  supply  voltage

For my amplifier operating on a ±29.6 V provide voltage,

R_{m}leqfrac{29.6 hspace{1mm} V-2.6 hspace{1mm} V}{0.0005 hspace{1mm} A}   leqfrac{27hspace{1mm} V}{0.0005 hspace{1mm} A}   leq 54,000 hspace{1mm} Omega

So, my Rm will must be lower than 54 kΩ for the present at pin Eight to be higher than 0.5 mA.

Rm and Cm create a time fixed that slowly decreases the present on the mute pin when energy to the amplifier is shut off, and slowly will increase the present when the amp is turned on. The 16 V Zener diode (D1) blocks present flowing out of pin Eight till the diode’s breakdown voltage (16 V) has been reached. This produces a tender begin/cease impact that progressively will increase or decreases the amount as an alternative of abruptly slicing it.

The time it takes the present to ramp up and down may be adjusted by altering the values of Rm or Cm in accordance with the method for the RC time fixed:

T=R_{m} times C_{m}  T= Time hspace{1mm} in hspace{1mm} seconds  R=Resistance hspace{1mm} in hspace{1mm} Ohms  C=Capacitance hspace{1mm}in hspace{1mm}Farads

For instance, if I need a one second lengthy tender begin, I may arbitrarily set Rm to 10 kΩ, then discover a worth for Cm:

T=R_{m} times C_{m}Rightarrow C_{m}=frac{T}{R_{m}}   C_{m}=frac{1 hspace{1mm} Second}{10000 hspace{1mm} Omega}  =0.0001 hspace{1mm} F   =100 hspace{1mm} mu F

So setting Rm to 10 kΩ, and Cm to 100 µF will give me a one second lengthy tender begin.

The Remaining Schematic

Now that we’ve seen the way to calculate the element values, we will begin designing the PCB structure and wiring scheme. If you happen to don’t need do all the calculations we did above, you need to use the values I used. Right here’s the ultimate schematic:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Circuit Schematic

Word: The element labels match the labels on the PCB structure offered beneath. Click on on the picture to edit the schematic or change element values.

Designing the Floor Format

The bottom structure of your amplifier has a serious impact on sound high quality. With a correctly designed grounding structure, the amplifier’s output will probably be utterly silent when the supply is linked and no music is enjoying. With a poorly designed floor structure, the amplifier can produce a really noticeable hum or buzzing sound.

The important thing to a superb grounding structure is to maintain low present grounds separate from excessive present grounds. Low present grounds are the bottom feeds to the enter circuitry and suggestions loop. Excessive present grounds are the bottom feeds to the ability provide decoupling capacitors, the Zobel community, and the audio system. Excessive currents flowing by the low present floor conductors will create a DC voltage that may present up on the amplifier’s enter and get amplified as noise.

To separate the low present grounds from the excessive present grounds, we are going to create a number of floor networks:

  • Audio enter floor: Floor for the audio enter cable
  • Sign floor: Floor for the enter circuitry – Rin, Cc, and Ri/Ci
  • Speaker floor: Floor for the audio system
  • Energy floor: Floor for the ability provide decoupling capacitors, Zobel community, mute capacitor, and the bottom pin of the LM3886

These grounds ought to join solely as soon as at a set of terminals referred to as the primary system floor. The principle system floor is situated as shut as potential the reservoir capacitors on the ability provide. The major system floor will connect with the mains earth wire by way of a floor loop safety circuit (defined later), and the amplifier chassis.

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Power Supply Main System Ground

The person floor networks are linked to the primary system floor in order that larger present grounds are nearer to the reservoir capacitors. The diagram beneath exhibits the way to order the bottom connections:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Grounding Diagram

The speaker grounds and audio enter grounds are routed instantly from their terminals on the chassis to the primary system floor.

Designing the PCB Format

PCB design additionally has a serious affect on the efficiency of your amplifier. Beneath I’ll talk about the rules I used to design this PCB structure. The PCB is for a single channel, so for a stereo amplifier, you’ll must construct two boards:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - PCB Layout

Word: The elements on the PCB structure match the schematic above. You possibly can click on on the picture above to edit the PCB structure, change the element footprints, and order PCB’s.

The PCB was designed with EasyEDA’s on-line design software program. EasyEDA is a full suite schematic and PCB design software program/manufacturing service that’s free to make use of and provides nice costs on customized PCB manufacturing.

Ordering PCBs

If you happen to click on on the “Fabrication Output” button in EasyEDA’s PCB editor, you’ll be taken to a web page the place you possibly can order the PCB. You’ll be capable to select the copper thickness, PCB thickness, colour, and order amount:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Ordering the PCB

I ordered 5 boards for $17.10 USD and so they have been delivered in about 10 days. The completed boards look nice. All the traces and printing got here out very clear and exact, and there have been no defects on any of the boards. Right here’s one of many PCBs:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - PCB TopHow to Design a Hi-Fi Audio Amplifier With an LM3886 - PCB Bottom

PCB Design Tips

Excessive currents flowing by the ability provide and output traces will create magnetic fields that may generate currents within the suggestions loop and enter traces in the event that they’re routed parallel to one another. This will distort the enter sign, so it’s finest to maintain them far aside or route them at 90° angles. Putting their PCB terminals on reverse sides of the board will make it simpler to maintain them separate whenever you route the traces.

Any house between traces of the identical circuit will create a loop that may transmit or obtain electromagnetic fields. The traces for the ability provide feeds and energy floor ought to be routed shut collectively to cut back the loop space. Likewise, the audio enter and sign traces ought to be routed shut to one another. A straightforward solution to reduce the loop space is to make use of floor planes on the underside layer of the PCB, which I’ve completed on this structure.

The facility floor and sign floor are the one floor networks on the PCB. Each has its personal electrically remoted floor aircraft on the underside layer. For the reason that energy floor carries excessive currents and the sign floor carries low currents, they’re saved separate till they join on the major system floor. On the highest layer of the PCB, the ability provide, output, and Zobel community traces are routed over the ability floor aircraft. The enter and suggestions loop traces are routed over the sign floor aircraft. The traces for the ability provide feeds have been made very large to reduce the resistance and inductance.

The suggestions loop ought to be saved as brief as potential to cut back the loop space. I trimmed the leads of the suggestions resistor (Rf1) and soldered it on to pins 9 and three to maintain the loop space as small as potential:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Feedback Loop Resistor

Inductance inhibits the circulation of present and creates resonance with a capacitor that’s in collection. Since inductance will increase with hint size, it’s finest to maintain all traces as brief as potential. That is particularly essential for the ability provide decoupling capacitors, suggestions loop, enter circuitry, and Zobel community. Preserve the elements for these circuits proper up towards the chip’s pins so the traces will probably be brief.

Now we have extra suggestions and tips on designing PCBs in our  Make a Customized PCB article, so test that out if you happen to’re .

Wiring it All Collectively

The LM3886 is a Hello-Fi chip amp, so I used top quality audio grade elements for my amplifier:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Amplifier Components

The full price got here to about $118 for each channels, not together with the chassis, energy provide, and wiring elements. You possibly can construct it for lots much less with cheaper elements if you happen to’re on a funds, simply be sure you change the element footprints within the PCB structure.

Solder and Soldering

Earlier than soldering the elements to the PCB, use a chunk of high-quality grit sandpaper to take away any oxidation from the element leads. This provides you with a stronger solder joint and higher electrical conductivity.

To carry particular person elements in place whereas soldering, use a putty like Sticky-Tac on the highest facet of the PCB. Begin soldering the smallest elements first, and work your means as much as the bigger elements.

Attempt to keep away from the usual 60/40 tin lead solder and use a 63/37 eutectic solder as an alternative. 60/40 solder has a large melting vary, and when it’s on the decrease finish of the vary it turns into pasty. If the element strikes within the pasty part, it could actually create a chilly solder joint. The smaller melting vary of eutectic solder makes the solder set quicker and provides a greater electrical connection.

Right here’s one channel of my amplifier after I soldered the elements:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Assembled Amp

Discovering a Chassis

You’ll want an enclosure to maintain the PCBs and wires contained and to mount the enter, output, and energy connectors. Metallic enclosures are the perfect sort as a result of they protect the amplifier from interference brought on by fluorescent lights, radios, and cell telephones. Sadly it may be onerous to discover a chassis that matches every little thing and appears good too. After plenty of looking, I discovered an organization referred to as Hello-Fi 2000 that manufactures some actually good metallic enclosures. Their web site is in Italian, however it may be translated to English. I ordered their 330×280 mm Galaxy mannequin with a 10 mm black anodized aluminum entrance panel and it seems nice:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Amplifier Chassis

Additionally they do customized drilling and printing, so I had them customise the again panel:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Amplifier Back Panel

Earlier than you order a chassis, do a check structure of the transformer, energy provide, amplifier PCBs, and warmth sinks. Then measure the total dimensions to ensure the enclosure will match every little thing.

Wiring Format Contained in the Chassis

After the PCBs have been assembled and you’ve got a chassis, it’s time to wire every little thing collectively. The wiring structure is simply as essential because the PCB structure and grounding structure. Use the diagram beneath as a information for wiring the varied elements collectively:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Master Wiring Diagram

Click on on the picture to view a bigger model.

The purpose with wiring is to cut back or get rid of electromagnetic interference between excessive present and low present wires. The audio enter wires and sign floor wires are probably the most delicate to interference from surrounding magnetic fields.

The facility provide wires, speaker output wires, transformer, rectifier diodes, and AC mains wires are a serious supply of magnetic fields. To cut back interference, hold the audio enter and sign floor wires away from these elements or run them at 90° angles if separating them is unavoidable. If you happen to orient the enter facet of the amplifier PCBs close to the enter terminals on the chassis, the wires can be saved brief and away from sources of interference.

Any house between the wires of the identical circuit will create a loop that may transmit or obtain electromagnetic fields. To attenuate the loop space, the next units of wires ought to be twisted collectively tightly:

  • Scorching and impartial AC mains wires from the enter terminal to the transformer
  • AC zero and secondary voltage wires from the transformer to the ability provide
  • V+, V-, and energy floor wires from the ability provide to every amplifier PCB
  • Speaker output and speaker floor wires from the amplifier PCB/major system floor to the chassis terminals
  • Audio enter and enter floor wires from the enter terminals to the amplifier PCBs

Three energy provide wires (V+, V-, and energy floor) join the ability provide’s DC output to every amplifier PCB. These wires ought to be thick, as brief as potential, and twisted collectively tightly. I used 14 AWG, however something bigger than 18 AWG ought to be high-quality.

Solely low currents circulation by the enter wires and sign floor wires, so that they don’t must be heavy gauge. I used strong core 22 AWG, which works effectively as a result of it may be twisted into a decent coil.

Audio enter cables operating from the supply to the amplifier chassis can decide up interference. If this turns into an issue, you possibly can set up a 1 nF capacitor between every enter terminal floor and the chassis to filter it.

The mains earth wire ought to be secured on to the chassis with a bolt and ring terminal. I’d additionally use a lock nut or lock washer to forestall it from getting unfastened. All metallic elements of the amplifier (like the warmth sinks) ought to be electrically linked to the chassis to supply a path to earth for any mains voltages that would contact them within the occasion of a fault.

The principle system floor connects to the bottom safety circuit (mentioned beneath), which then connects to the chassis. The bottom safety circuit can connect with the chassis on the bolt the place the mains earth wire is linked to the chassis, or at a separate location.

The 2 Thiele networks are situated near the speaker output terminals. To forestall interference between the inductors, they need to be spaced aside or oriented at 90° angles to one another.

Right here’s how I put in every little thing inside my chassis. The appropriate channel PCB is mounted the wrong way up in order that the enter facet of the board is near the RCA and three.5 mm enter terminals. On this association, the warmth sinks present some shielding from the Thiele networks and the AC wires resulting in the transformer:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Wiring Layout in Chassis

Click on on the picture to view a bigger model.

The Floor Loop Safety Circuit

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Ground Loop Protection Circuit Assembled PCB

GROUND LOOP PROTECTION CIRCUITS MAY BE ILLEGAL IN SOME AREAS. PLEASE CHECK YOUR LOCAL ELECTRICAL CODE OR CONSULT AN ELECTRICIAN BEFORE INSTALLING THIS…

Whenever you join a powered audio supply to your amplifier, magnetic fields from the supply’s transformer and energy provide wires may be coupled into the bottom wires of the audio enter cables. That is know as a floor loop, and it could actually create hum in your amplifier’s output.

A floor loop safety circuit will break the bottom loop present:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Ground Protection Circuit Schematic

Underneath regular working situations, low voltage floor loop currents circulation by the resistor (R1) to earth (the chassis). The resistor reduces this present and breaks the bottom loop. If a excessive present fault happens, the fault present can circulation by the diode bridge to earth. Word that the chassis MUST be electrically linked to the mains earth wire to forestall mains voltages on the metallic chassis within the occasion of a fault. The capacitor is there to filter any radio frequencies picked up by the chassis.

If a floor loop safety circuit is used, all enter and output terminals have to be electrically remoted from the chassis. In any other case, the bottom loop safety circuit will probably be bypassed solely by the enter/output floor wires that connect with the primary system floor.

The bottom loop safety circuit may be onerous wired, but it surely’s slightly neater to mount the elements on a PCB. The “PSU 0V” terminal connects to the primary system floor. The “Chassis” terminal connects to the chassis:

How to Design a Hi-Fi Audio Amplifier With an LM3886 - Ground Protection Circuit PCB Layout

Click on on the picture to edit the structure, change element footprints, and order the PCB.

How Does it Sound?

The amplifier I constructed sounds extremely good. It’s the perfect amp I’ve ever owned by far. The bass may be very deep and clear. You possibly can actually really feel it. The highs are clear, however not harsh in any respect. I can hear particulars in songs that I by no means knew have been there. Belief me, if you happen to construct an amp with the LM3886 you’ll not be dissatisfied. It undoubtedly lives as much as it’s fame as a Hello-Fi amplifier. The video firstly of the publish provides you with an concept of what it feels like.

This could about cowl most of what you’ll must construct a superb sounding Hello-Fi amplifier with the LM3886. Because of the size of this publish, I made a decision to not cowl the ability provide intimately, however I could accomplish that sooner or later.

If you happen to’re enthusiastic about constructing different amplifiers, we even have a tutorial on making a 25 Watt amplifier with the TDA2050, and making 10 Watt stereo and bridged amplifiers with the TDA2003 as effectively.

Thanks for studying… In case you have any questions on this construct, be sure you depart it within the feedback beneath and we’ll attempt to reply it. And be sure you like, share, and subscribe if you happen to discovered this useful! Discuss to you subsequent time…

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