LM3886 Power Amplifier, Dual or Bridge

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A compact dual power (or bridge) amplifier is easy to build if you have some electronics experience. Only a few parts are required. Of course it is even easier to build a mono amp. The crucial issues are the power supply and the cooling.

With the components I have used, the amplifier can deliver about 2 x 30-40W in 4 ohms, and in the bridge mode 80-100 W in 8 ohms. The transformer current is the limiting factor.

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Step 1: Electronic Design

The story is this; In Sweden we have municipal garbage and reuse stations. This is where you leave all things you want to get rid of (not food waste). So in the container for electronics I found something that looked like a home built amplifier. I nicked it (because it is not allowed to take, only leave). When I got home I checked what it was and I found that the power amp IC was the really popular LM3875. I started to build my own guitar amplifier with it, but the legs of the IC were short and somewhat damaged, so in the end I had to give up. I tried to get a new one, but the only thing on sale was the successor, the LM3886. I bought two, and I started in earnest. The idea was to build a compact guitar power amp, using two LM3886:s, either for two channels or in a bridge circuit. In my own scrap heap I had a CPU heat sink and a PC-fan, so the idea was to use the heat sink and the fan to build an amplifier without any external heat sink.

Step 2: Electronic Design (power Amp)

The design of the power amp is really straight forward, and follows the datasheet example in the absolutely excellent application note AN-1192 from Texas Instruments, which should be your bible if you want to use the LM3886.

The upper circuit is the non-inverting amplifier with the gain of 1 + R2/R1. The lower amp is inverting with the gain of R2/R1 (where R2 is the feedback resistor). For a bridge design the trick is to get the resistor values so that both circuits have the same gain. Using mostly standard resistors (some metal film resistors) and measuring the exact resistance I was able to find combinations that worked. The non inverting circuit gain is 1+ 132,8/3,001 = 45,25 and the inverting gain is (132,8+3,046)/1,015 = 45,27. I introduced a gain switch (SW1) to be able to increase the gain. It reduces the R1 value to get four times higher gain.

Non-inverting circuit : 1,001 k in parallel with 3,001 k gives (1 * 3) / (1+3) = 0,751 ohm. Gain = 1+ 132,8/0,75=177,92 = 178

The inverting gain is 179,1 = 179 , acceptable!

The small (and free) application "Rescalc.exe" can help you with resistance calculations (serial and parallel)

I wanted to be able to use the two amplifiers separately so a switch (SW2) for switching between stereo and bridge was required.

In "bridge" position the switch SW2A disconnects Input A and instead connects the B input (after the volume pot) both to Amp A and B. On the output of Amp A is the non-inverting voltage, with the inverted voltage on Amp B, which is the bridge. The SW2B disconnects Output A and and replaces Output B ground with Output A.

The input impedance of an inverting amp consists of the serial resistance (3k or 1k), therefore the volume control potentiometer must have a low value (10k). The capacitors on the input (2,5 uF) will be replaced since 2,5uF gives a low frequecy cut off at 80Hz.

The input tele jacks are connected so that when no plug is in jack A the signal is sent to both Amp A and Amp B (dual mono) .

Step 3: Electronic Design (Power Supply)

The power supply is a straight forward design with two large electrolytic condensers and two foil condensers and a bridge rectifier. The rectifier is the MB252 (200V /25A). It is mounted on the same heat sink as the power amps. Both the rectifier and the LN3686 are electrically isolated so no extra isolation is needed. The transformer is the 120VA 2x25V Toroid transformer from the amp that I found in the scrap heap. It can supply 2,4A which actually is a little low, but I can live with that.

In section 4.6 of AN-1192 the output power is given for different loads, supply voltages and configurations (single, parallel and bridge). The reason that I decided to implement the bridge design was mainly because I had a transformer that was not usable in a parallel design due to the low voltage. (The 100W parallel circuit requires 2x37V but the bridge design works with 2x25V).

The small application "PSU Designer II" from Duncan Amps is highly recommended if you want to make a serious calculation of transformer values.

Step 4: Electronic Design (Step Down Regulator and Fan Control)

The requirement of the fan at full speed is 12V 0,6A. The power supply provides 35V. I quickly found out that the standard voltage regulator 7812 will not work. The input voltage is too high and the power dissipation of (roughly) 20V 0,3A =6W requires a large heat sink. Therefore I designed a simple step down regulator with a 741 as the controller and PNP transistor BDT30C working as a switch, charging a 220uF capacitor to the voltage of 18V, which is a reasonable input for the 7812 regulator that provides power to the fan. I did not want to have the fan working on full speed when not needed, so I designed a variable duty cycle circuit (pulse width modulation) with a 555 timer IC. I used a 10k NTC resistor from a laptop battery pack to control the duty cycle of the 555 timer. It is mounted on the power IC heat sink.The 20k pot is used to adjust the low speed. The output of the 555 is inverted by the NPN transistor BC237 and becomes the control signal (PWM) to the fan. The duty cycle changes from 4,5% to 9% from cold to warm.

The BDT30 and the 7812 are mounted on a separate heatsink.

Note that in the drawing it says PTC instead of NTC (negative temperature coefficient) , in this case from 10k to 9,5k when I put my finger on it.

Step 5: The Heat Sink

The Power amps, the rectifier and the PTC-resistor are mounted on the copper plate of the heat sink. I drilled holes and made threads for the mounting screws using a thread tool. The small veroboard with the components for the power amp is mounted on top of the power amps to ensure as short cabling as possible. The connecting cables are the pink, brown, lilac and yellow cables. Power cables are of a higher gauge.

Note little metal stand by the red cable in the lower left corner. That is the single central ground point for the amplifier.

Step 6: Mechanical Construction 1

All major parts are mounted on the 8 mm plexiglass glass base. The reason is simply that I had it and I thought it would be nice to see the parts. It is also easy to make threads in the plastic for the mounting of the different components. The air intake is under the fan. The air is forced thru the CPU heat sink and out thru the slits under the heatsink. The slits in the middle was a mistake and are filled with plastic from a glue gun.

Step 7: Amplifier Without the Case

Step 8: Mechanical Construction 2

The front panel is made of two layers; a thin steel plate from a PC and a piece of mint green plastic that remained when I made a new pickguard for my Telecaster.

Step 9: Front Panel From the Inside

Step 10: Wooden Casing

The casing is made from alder wood from a tree that fell in a storm. I made some planks using a carpenter's plane, and glued them together to get the required width.

The cutouts in the casing is made with an electric wood router.

The sides, the top and the front are glued together, but I also secured the construction with screws thru the small pieces in the corners.

To be able to remove the wooden casing, the back side is separately held in place by two screws.

The grey plastic pieces have threads for the 4 millimeter screws for the bottom and the back.

The small grey piece in the corner is a little "wing" that locks the front panel so that it does not bend inwards when you plug in the tele jacks.

Step 11: The Backside of the Amplifier

On the back there is the mains inlet, the power switch and a (not used) connector for preamp power

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    21 Discussions

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    Jimbo724

    26 days ago

    I've never built anything like this but I like the idea of having a super simple guitar amp when I don't "need" the mindboggling capabilities of my Marshall CODE50. For a mono amp using only one LM3886 chip, how would the design be simplified? Obviously, one can draw a line between the 2 channels and omit everything on one side of the line, but which side should be kept? and which components would become unnecessary and could be omitted? Would only the amplifier schematic change? or would the step-down regulator and power supply, too, need fewer components?

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    haawaJimbo724

    Reply 25 days ago

    Hi, I would use the non-inverting side (the upper amp) because of the higher input impedance. You an also get higher gain, so that you can plug in you guitar directly. In that case increase the values of the volume pot and the 47k resistor so that you reduce the load of the guitar mics. A stomp box after the guitar will also take care of the load issue. Increase the value of the feedback resistor (132,8k) for higher gain.
    The power supply must be the same, but you should decide how much output power you want, that decides the voltage and the effect (VA= VoltAmpere) of the transformer. To make it even simpler I would remove the fan and use a big heatsink. Or have a 12 V fan with a separate 12V adapter. I designed my step down and fan control just for the fun of it.
    Good Luck!

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    Jimbo724haawa

    Reply 25 days ago

    I need to breadboard a mono version without a fan. Am I correct to assume that I need to supply +35V/-35V power to the breadboard? The instructables for breadboard and bench power supplies seem to max out at +12V/-12V. Is there an EASY way to supply the necessary power?

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    haawaJimbo724

    Reply 24 days ago

    No, you certainly do not need +-35 V. The 3886 works from +-9 volts. In the data sheet page 17 fig. 37 (http://www.ti.com/lit/ds/symlink/lm3886.pdf) you can see that you get about 15 W in 4 ohms with a supply voltage of +- 12 V. The easiest way to get power maybe is to buy two laptop power supplies (18-19 V 3-4 A) e.g. https://www.amazon.com/Replacement-Satellite-L875D-S7332-PA3097U-1ACA-PA3714U-1ACA/dp/B07RL5K812/ref=sr_1_3?keywords=laptop+power+supply&qid=1573897066&sr=8-3
    (or look in the scrap heap)
    The you will get nearly 40W in 4ohm output. Good luck!

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    Jimbo724haawa

    Reply 24 days ago

    Good to know. Thank you!
    BTW, I salvaged the power supply depicted in the attached image from a dead DVD/BlueRay player for a TV. The power supply plugs directly into the wall (120V) and apparently supplies "unswitched12V." The transformer with the yellow tape is labeled "68-AT07A" and "TDK KA 382805"; I can't find specs for those part numbers. The 2 resistors are each labeled "470uF25V." I can't read the 2 polarized capacitors. I assume this is a single-rail supply and not suitable for the LM3886? The Sony web site described the srv2257uc as a Switching Regulator, but I cannot find any specs.
    I have an old Apple MacBook and iMac which I can cannibalize for their power supplies, but are they likely to be more suitable for the LM3886?

    P.S. I found the following site http://www.decdun.me.uk/gainclone_psu.html which has an elementary tutorial for complete nubes like me on building gainclone power supplies. The dead link to Rod Elliot's article on grounding/earthing appears to belong to https://sound-au.com/earthing.htm

    power supply top.jpg
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    haawaJimbo724

    Reply 24 days ago

    Sorry, I do not think that the power supply is usable. The Apple Power supplies are maybe be usable. What do the labels say? Outputvoltage? Power? Amps?

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    Jimbo724haawa

    Reply 23 days ago

    Thanks.

    Apple does not make a lot of detailed specs available because they seem to expect the use of complete drop-in replacement parts. For the MacBook, this is all I could find:
    55-watt-hour lithium-polymer battery
    60W MagSafe Power Adapter with cable management system
    Line voltage: 100V to 240V AC
    Frequency: 50Hz to 60Hz
    The technical/repair manual(s) seems to show very little in the way of conversion or transformation between the battery and the wall wart and the motherboard.
    Between the battery and motherboard, I see absolutely nothing in the schematics.
    Between the wall wart and the motherboard, I see a very small "DC-In Board," shown in the attached photos, and for which I could find no specs whatsoever.
    My guess is that I would need to use the wall wart + the DC-in board, and connect the 5 pins on the DC-In board to the amp. It's extremely unlikely I would ever find a schematic of these pins, and although I own a multimeter I don't really know how to use it for exploratory purposes.
    Perhaps the power supply in the old iMac would be a better candidate. There's no box between the wall outlet (mains) and the power-chord receptacle on the iMac, so the entire power supply presumably is inside the iMac. That'll take more poking around.

    DC-in board 2.pngDC-in board 3.pngDC-in board 4.pngDC-in board 5.pngMacBook MagSafe DC-In Board.png
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    Jimbo724Jimbo724

    Reply 23 days ago

    Thank you for tolerating all my newb questions.
    According to Wikipedia, "The maximum voltage supplied is ... 16.5 V DC for the 60 W units supplied with MacBook...."
    I have an extra 85W MagSafe for my MacBook Pro (not canniblizable at this time). According to Wikipedia, "The maximum voltage supplied is ... 18.5 V DC for the 85 W units....."
    The pins in the MagSafe (before the DC-In Board) are reversible (it doesn't matter which end is up or down, or on one side or the other), but I assume the DC-In Board changes that, given the different wire colors. If need be, I can deconstruct the MagSafe connector on the Macbook end and expose the pins, although the DC-In Board may simplify connecting to an amp.
    A pin schematic is attached.
    I see the following in the Wikipedia article: "The MagSafe connector pins allow for the adapter to be inserted in either of two orientations. The first and second pins on each side of the tiny central pin have continuity with their mirror pins. ... The inner large pins are V+ (...16.5 / 18.5... V DC). Measuring with no load will give 6.86 V DC for MagSafe and about 3 V DC for MagSafe 2; the full voltage is provided after a ~40 kOhm load is applied for one second." I wonder whether these details would require some components between the MagSafe connector and the amp to be built; maybe that's what the DC-In Board handles.Hmmm.
    My ultimate objective is a one-channel mono LM3886 amp. Would it be desirable, or even possible, to somehow (?) turn the 16.5V into +-8.25V or the 18.5V into +-9.25V?

    MagSafe connector pins..png
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    haawaJimbo724

    Reply 23 days ago

    You need two supplies that can deliver at least 12V each. Even if you get the MacBook supply to work, you still need another one, so my recommendation is still to buy two low cost supplies. The 3886 requires at least +-9 V , but at such a low voltage the amp will not be usable in any practical sense.

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    Jimbo724haawa

    Reply 23 days ago

    Thank you again for being so patient with all my questions. I will let you know what I build and how it turns out.

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    haawaJimbo724

    Reply 22 days ago

    Looking forward. Be careful when you solder. Good luck!

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    haawaJimbo724

    Reply 23 days ago

    I am sorry, but I can't help you with the MacBook thing. I suggest that you buy the 12$ power supply at Amazon (link above), strip the cables and use your voltmeter to find "Ground" and "18V" If that works out, buy another one and use those two as the +- 18 V power supply. Or build the amplifier and use whatever you have to provide +-10 - +-12V (four USB chargers will provide +- 10V). Not much power but you will know if the amp works.

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    Jimbo724haawa

    Reply 25 days ago

    Thank you for taking the time to respond!

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    MauricioO4

    24 days ago

    Good project, just thinking if the fan is really necessary? It appears oriented to an acrilic surface, It should be extracting the heat from the box or the power modules...

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    haawaMauricioO4

    Reply 24 days ago

    The fan sucks air into the housing and forces the cool air to pass between all the thin metal sheets of the heat sink and out thru the four slits under the heat sink. I would not say that it is unnecessary. Whitout fan I would have to have a heat sink outside the housing.

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    MauricioO4haawa

    Reply 23 days ago

    Good, I hadn't noticed the holes in the acrilyc, sorry

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    ve6cmm

    26 days ago

    This is a very nice looking project. I wish that I could do woodwork like that! One minor comment on the amplifier design is that the lower channel has the audio applied to the inverting input, always. This will cause a change in the stereo soundscape, not that my ears would hear the difference these days.

    I know why you did this, it's so that you can get the inversion for a bridged amplifier. Another way to do it would be to switch the input to the bottom amplifier from the input to the amp to the positive input of the IC when in stereo mode, and to the inverting input when in bridging mode. All it would take is to have the upper amp always connected to it's input and to use SW2A to switch the incoming audio between the non inverting input in STEREO to the inverting input in BRIDGED. You you may have to add a 4PDT switch to do this, depending on what you actually used for your stereo/bridged switch!

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    haawave6cmm

    Reply 25 days ago

    Thank you for your good comments. Good tools for woodwork and a little training is all that is needed (and somewhere to be). Try it. Yes you are of course completely right about the circuit improvement. I think a 4PDT switch is required, although if you just don't use input B when in "bridge" you could always have it connected to the positive input. I think I will change my amp, I don't like the low input impedance of the inverting input. I made a quick sketch. Note that SW2A must switch between ground and input A.

    IMG_7720.JPG
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    ve6cmmhaawa

    Reply 25 days ago

    That'll do it!

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    Alex in NZ

    4 weeks ago

    Nice project :-) Well scrounged and thank you for sharing your work :-)