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

[zakman35]

Hi Marc, this is more about learning to control - adjust offset (as with your case).

This is my original schematic.
I have remove Cin, Ci & Cc
Rin =10K
Rb=330 R

 

[Marc Vi]

Hallo Zakman35,

Don't remove Ci because the input resistance will be dependent on the output resistance of the pre-amplifier and that may change the output offset voltage of the LM3886 quite a lot.

And, if there is an output offset voltage on the pre-amplifier, that output offset voltage will be multiplied by 33 !!

Don't remove Cc because it prevents noise and - if necessarely - oscillation.

Marc.

 

[danielwritesbac]

I measure dc voltage directly on the pins of the chip, both rails are exactly the same (+36.2V, -36.2V)

Good. Now, what is the AC (noise content) voltage of both DC rails?

 

[zakman35]

Ac noise = 0V on both dc rails.

 

[Marc Vi]

Can we return to the original subject: LM3886 without electrolytic feedback capacitor and (hardly) no DC offset voltage.

Zakman35 can you reply on that and on my last post?

Marc.

 

[zakman35]

Don't remove Ci because the input resistance will be dependent on the output resistance of the pre-amplifier and that may change the output offset voltage of the LM3886 quite a lot.

And, if there is an output offset voltage on the pre-amplifier, that output offset voltage will be multiplied by 33 !!

Don't remove Cc because it prevents noise and - if necessarely - oscillation.

I install Ci back again (no changes at offset).
My pre amp (minidsp) is AC coupled at the output, so no dc from there.

I will try tomorrow to install back the Cc cap and report back.

 

[danielwritesbac]

Having eliminated so many other possibilities, I think there's 6mv worth of ground loop.

 

[zakman35]

Anyway, back to the topic.

What about adjusting - reducing positive's rails voltage to trim output offset to approximate 0mv. Is that an acceptable practice in power amps?

 

[danielwritesbac]

Sure. You can run some led's on that rail, and you can add two of 4k7 1/4w bleeder resistors. It will make a very mild difference.

 

[Marc Vi]

No, No, No, this isn't a solution at all!! (Or I don't understand it)

To trim out a offset voltage of let's say 6 mV, you have adjust the positive rail for a multiple of that. And that reduces your output power and induces distortion much earlier on one flank.

Why not use the way I proposed?

Marc.

 

[danielwritesbac]

Why not use the way I proposed? Marc.

With the LM3886, your solution is probably better.

 

[Marc Vi]

Adjusting the supply voltage on one side in order to trim out the offset voltage is a bad solution for every amplifier.

Marc.

 

[AndrewT]

The output offset should be substantially independent of the supply rail voltages in any opamp style amplifier.

However, a VAS stage without CCS supply/source will change it's Iq when supply voltage changes (both rails changing together), and this changed Iq with push/pull more or less current through the Collector load of the input transistor. This changed collector current will change the offset. This is one of the big disadvantages of a bootstrap feed to the VAS in a P3A (ESP) amplifier.

 

[0x4452]

Marc, I successfully applied your tip in my DIY studio monitor (with LM4766 - another BJT chipamp).

Previously, due to the low feedback resistance (200R and 3k3) in my design, I needed a large feedback capacitor. There was a slight but audible distortion, which is now gone or at least below my threshold 🙂.

Schematic here: http://rdimitrov.info/blog/show.php?entry=Minimalistic%20HIFI%20Bi-Amplified%20Speaker%20System

Thanks!

 

[mlloyd1]

0x4452:

thanks for the link. i found a new band i like - Pink Tomato.
🙂
any recommendations?

sorry for being off-topic ...
mlloyd1