Decibel Dungeon

Eventually, I just had to try a Gainclone using the LM3886 chip instead of the LM3875. Apart from being able to provide more power than the LM3875, the LM3886 has a mute function. Perhaps most importantly, as pointed out to me by Xavier Cortez, the LM3886 can better drive low impedance (4 ohm) speakers (as I am finding out with my Ciare 250 drivers). Mute function - the ability to disconnect the speakers from the output of the amplifier.
The other main difference from the LM3875 is the pin connection arrangement. The LM3886 has an extra voltage supply pin (pin 5) for the positive supply. And pin 8 is used for the mute function. You can see full details of the LM3886 in the LM3886 data sheet.
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My circuit.

LM3886 circuit layout.

Once again, I have gone with an inverting circuit because that's what I prefer. I've shown the actual layout and connection of the components above and will add a circuit diagram when I have time. Beginners may find the above diagram easier to follow anyway.
If you have been familiar with building LM3875 chip amps, the main points to keep in mind are:
  • Pin 5 is connected to the positive rail. You can do this with a simple wire link from pin 5 to pin 1.
  • Pin 7 has to be connected to the signal ground star.
  • Pin 8, the mute pin is connected to the negative supply pin (pin 4) via a resistor. The resistor value depends on the rail voltage and for 35-37 volt rails a resistor value of 18-22K is recommended. Also connected to the mute pin is a 100uF capacitor, the other side of which is connected to the signal ground star. Note the polarity of this resistor! (negative to pin 8 and positive to signal ground star)
  • The signal input pins are different to the LM3875. Pin 9 is the inverting input and pin 10 the non-inverting input (ie the opposite way round to the LM3875 so be careful).
Other notes:
  • The 0R22 output resistor should be rated at 3 watts. This may limit you to using a wire-wound resistor which some consider to sound poor in this part of the circuit. A better solution for this part is to use two 2 watt 0R47 carbon film resistors in parallel.
  • The Zobel network on the output consists of a 100nF film capacitor in series with a 2R7 resistor (1/4 watt is fine here).
  • The DC blocking cap should be a quality film (the best that you can get - I use polypropylene types) with a value between 2.2uF and 4.7uF.
  • Keep all wiring as short as possible. Put the signal ground stars (shown as orange square) close to the chip, the power star between the capacitors on the voltage pins and the signal star right next to the chip as shown in the diagram above.
You can use whatever PSU you like. If using a single PSU, then connect the two channels together as shown in the diagram below. Use a thick piece of copper wire to join the power ground stars of each chip together and then connect the zero volt rail (coming from the PSU) to the centre point of the thick wire.

LM3886 circuit layout (both channels.

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What does it sound like?

In short, much like the LM3875! I would guess that in two identical configurations, it would be almost impossible to tell them apart. In other words, the LM3886 sounds very good!
So it's really a matter of how much power that you want from an amplifier, and if you require the mute function or not, as to whether you use the LM3886 or the LM3875.

The finished LM3886 (power) amplifier.
This is the finished LM3386 power amplifier which had a built-in PSU.

For another easy-to-follow LM3886 GC building guide, please click here for Mick Feurbacher's 'Three Resistor amp'.
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