LM3886 without electrolytic feedback capacitor and (hardly) no DC offset voltage

[AudioLapDance]

Andrew and Danielle,

Are there any PCBs that you guys think have a good layout?

Cheers,
Jeff

[AndrewT]

Some on this Forum.
Some for sale on the web.

[danielwritesbac]

Mark, have you tried 10k vs 390R instead of 20k versus 1k? The firmer 10k input load may reduce DC offset a lot. In this case, it would be easier to get left and right closer to matching, better for stereo amplifiers.

[zakman35]

Daniel, i tried your suggestion and it worked. But let me explain .

Psu= +/- 37V
I have install Rf=22K, Ri=680R Gain set to 33.
Rin= 680R Rg=22K
Rm= 39K
Dc offset = 30mV.

I change Rin= 330R Rg=10K everything else remain the same.
Dc offset = 6mV.

Since I was happy with the result, i didn't try further values for Rin + Rg, but i install a Cm cap of 100uf, as suggested by the datasheet, since i want to use the mute function with a little delay.

With the Cm 100uf installed and no other changes Dc offset = 20mV.

I try some smaller values for Cm and found that 22uf gave the best results of Dc offset = 4mV. (Rm was 39K)
Further degrease of Cm to 10uf result in Dc offset = 30mV.

So i kept the Cm to 22uf and increase the value of Rm from 39K to 49K and Dc offset reduced to 1.7mV!!!!!

According to the Datasheet RM≤(|VEE| − 2.6V)/I8, Mute pin has to be feeded with minimum of 0.5mA.

In my first try I had originaly install Rm= 39K = Im = 0.88mA= Dcoffset = 4mV.
With Rm = 49K = Im = 0.70mA = Dcoffset = 1.7mV

All the above tests was done with and without Ci = 220uf. Dc offset was the same with and without Ci installed.

So the 10K - 330R is a good combination for the input. In addition to that it seems the degrease of Imute current can further degrease the Dc offset.

I will try to do the same tests with another chip in order to clarify the results.

[Bigun]

Originally Posted by Marc Vi.

I am very suprised that this method is not applied more often. Please give your comments.

Marc.

Full marks - I love this idea. The matching of the input sections on the i.c. are probably pretty good because it's a monolithic device and they likely track well with temperature.

I think this approach is going to become de rigueur for chip amps !

Makes me consider building one (Daniel has been trying to persuade for awhile).

[linuxguru]

I guess the reason this is not used often is because Voffset varies not only with each LM3886 (requiring individual trimming) but also with mute current, temperature, etc.

BTW, I get less than 4 mV output offset on the MiniRef 3886 with a JRC4580 in the outer loop and no other special trimming.

[danielwritesbac]

Originally Posted by zakman35

. . .Gain set to 33.. . .I change Rin= 330R Rg=10K everything else remain the same. Dc offset = 6mV.. . .I try some smaller values for Cm and found that 22uf gave the best results of Dc offset = 4mV. (Rm was 39K). . .Further degrease of Cm to 10uf result in Dc offset = 30mV.. . .

And the next thing is to find which two resistors will give your preferred gain setting AND 0mv offset. There's some combinations (for feedback current) that are either better or worse.

Just notice that there's a wide variety of resistor selections that could result in a gain of about 33. You're not locked in to the datasheet's suggested 22k feedback resistor.

[zakman35]

And the next thing is to find which two resistors will give your preferred gain setting AND 0mv offset

Do you think that 1.7 vs 0 mV will have any audible differences?

[danielwritesbac]

Originally Posted by zakman35

Do you think that 1.7 vs 0 mV will have any audible differences?

If you can get two chips to do the exact same thing then your stereo should be pretty good; but, otherwise, the imaging could be off a bit on one side.

Also, you're going to want a speaker protector kit for when the chip has an accident, because you don't want a worn out chip to take the speakers out with it. And, DC coupled amplifiers have a long history of both shorter lifespan and sudden breakage. So, be sure to protect the speakers in some way.

The remaining problem of the DC coupled amplifier is one of inferior dynamics, but hopefully you can make up for that in the preamp or buffer circuit, and just helping the source a bit with a buffer could really help level the tone too.

[AndrewT]

I have a little bit of evidence, but not proper and thorough testing, that aiming for a low output offset by balancing all the input pair conditions results in lower output offset drift.

If I can build up an input pair that has an offset of 1mV, then I find it hardly changes over a big range of temperature.

But if that same LTP is deliberately mismatched then the output offset drifts a lot more.

That mismatching could be any combination of different: Ic, Ib, Vce, Vbe & Tj.

But do keep in mind my lack of proper testing !!!!