Upgrading OPAMPS in old equipment and Coupling Caps

[abechap024]

Hello,
I recently fixed and modded an old quantum qm168 board. I upgraded the op amps to more modern precision op amps with very low offset.

I replaced all the coupling caps with new polar electrolytic caps. I have long since put the board back together and have been using it for a while now.

Now I'm thinking about if the opamps don't have any dc offset to polarize the polar coupling capacitors won't the distortion be higher than if I replaced them with bi-polar or eliminated some of them all together?

I'm planning on opening up the board when I head back to the studio this weekend and taking some voltage measurements to verify this but just wondering if anyone else has had any experience and thoughts with this.

Thanks

PS the IMD could be lower....it hangs around -75 to -80......

 

[adamboon]

If a coupling cap has 0v DC across it, direct couple it! (at least try it)

 

[abbey road d enfer]

This subject has been discussed a number of times.
There is a series of articles by Cyril Bateman
Capacitor Sounds Original version Pub. Electronics World July 2002 - C. Bateman.
There's one thing that is not made clear in Bateman's article.
Electrolytic caps measure the worst of all types.
But distortion in caps appear only when there's significant AC voltage across them. Coupling caps are generally made very large so as to present a quite low impedance even at the lowest frequency of interest. So the voltage across them, and hence distortion, are yet quite low.
But sometimes designers use electrolytic caps in filter stages, in particular in HPF, where large values are often needed. So they choose electrolytics because they are cheaper and less bulky than film caps. It's a design flaw.

 

[Samuel Groner]

IMD could be lower...

This has likely absolutely nothing to do with AC coupling.

If a coupling cap has 0 V DC across it, direct couple it!

There's nothing like 0 V DC in real world. It might be 10 mV, 1 mV or 100 uV but even the later requires extensive attention to DC precision if it is to held up in a reasonably wide temperature range.

How much offset is tolerable? It all depends on the design. If the offset causes DC current to flow through the wiper of a pot almost nothing can be tolerated. If it flows just through the resistance element of a pot we can be a bit more generous.

Samuel

 

[abechap024]

Ok. I was under the impression that if a electrolytic capacitor wasn't biased with DC on one side then they produce more distortion then if they are. Isnt that why SSL fed a polarizing voltage to the middle of 2 back to back caps?

 

[tmuikku]

Ok. I was under the impression that if a electrolytic capacitor wasn't biased with DC on one side then they produce more distortion then if they are. Isnt that why SSL fed a polarizing voltage to the middle of 2 back to back caps?

Is the polarization voltage there just to protect the caps from reverse polarization which could destroy them?