Stereo LM3886 amp who wants to be mono!

[Gabster]

Hi,

I've built a simple stereo amp using LM3886's. There's two LM3886's bridged on each channel. The power supply is built from a Plitron 2x22VAC 400VA transformer and 60,000 µF of capacitance for each channel.

When I input an audio source in one channel, everything is fine and the sound is pretty good. However, when both channels are hooked to the audio source, the sound looses all its bass and depth. What the hell is going on? I don't think that's the kind of problem caused by a ground loop and I strongly believe there's enough power in this amp to supply the two channels.

Below is a picture of the problem. Note that the board in the lower left corner was a regulation board...I converted it to a 'do nothing' board. It already had the bridge and connectors for the transformer so I thought it could still be useful.

Any help appreciated,
Gabriel

[HDTVman]

Are you sure that you don't have the phase reversed in one channel? That would make the bass go away. That would be easy to do with a bridged amp. Just try reversing the speaker connection on one channel. If that fixes it that's the problem.

Later
Bruce

PS: Nice looking amp. Good job.

[Gabster]

That was the problem! Very instructive problem. Thanks a lot.

[Gabster]

Another thing that bugs me is a little buzzzzzz when no music is played (not loud enough to be heard when music is playing).

When the volume pot is at one end or the other, the buzz is as loud as it can be. On the other hand, when the pot is centered the buzz is almost absent.

I'm sure this is not a big deal, but I'm lacking experience to solve it by myself.

Help appreciated.

regards,
Gabriel

[SkinnyBoy]

Why so many capacitors?? :P lol..

[ppl]

when i have had this problem it has been from a Ground loop or Instability in the Amp. My first recomendation is put a series resistor of about 2k between the Amp's input and the Pot's wiper, if no effect then a pair of 10 ohm resistors between each RCA jack's ground and the ground wire going to the Amp Board. make sure that the RCA jacks are insulated from the Chassis with washers. (real Importent). BTY what circuit did you use for the Bridge LM-3886 Amp?

[SkinnyBoy]

Originally posted by ppl
BTY what circuit did you use for the Bridge LM-3886 Amp?

I don't think you need a circuit.. I think the LM3886 has the input issolated from ground.. you can ofcorse connect it (as you would with a single amp) but when bridging I think you can just reverse the input to one of teh amps... hope this is right, or I'm posting incorrect info again.. :P lol

[Gabster]

ppl, I'll try what you say...thanks.

[ppl]

hopfully it will work good for you, also it looks like your wire routing has all the Power,Input and output wires running paralell to each other try puting the input at Right angle to the others and separating them. Good Luck

also see the assembly manual to the PM21 at http://www.marchandelec.com/articles.htm this is a pdf file on there Kit containing two LM-3886

[MWP]

Originally posted by SkinnyBoy

I don't think you need a circuit.. I think the LM3886 has the input issolated from ground.. you can ofcorse connect it (as you would with a single amp) but when bridging I think you can just reverse the input to one of teh amps... hope this is right, or I'm posting incorrect info again.. :P lol

I think you need to go study the LM3886 some more SkinnyBoy.

The LM3886 is just a powerful opamp.
Simply design one LM3886 as a non-inverting amp with a certain gain, and then design another LM3886 as inverting with the same gain.
The output of the non-inverting is the +ve to your speaker, the inverting is the -ve.
Easy.

Beware, you cannot power 4ohm loads with 2 bridged LM3886s as the overcurrent protection on the IC will cut in and clip the signal.
You can however, use 4 LM3886s in bridge/parallel to power 4 and 2 ohm loads.

And Gabster... yes, why so many smoothing caps?
The LM3886 has quite a high PSRR, all those caps are not needed.

If you experiment enough, bridged LM3886s can give a lot of power.
I got 220WRMS into 8ohms @ 1kHz with under 0.08% THD+N out of mine!
It had 8 channels (that 16 LM3886s) powered by 2x500VA 30-0-30 torids that was all mounted in a 3U 19" rack mount case.

This is it: