How much DC offset is too much? And how to place caps?

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gnd

Well-known member
Joined
Jan 24, 2006
Messages
285
Hi all.

I'm wondering, for console channel, how much DC offset is too much? Is few milivolts ok to go without cap?

I've been playing arround with one channel, and got idea to remove some caps between opamp stages. It seems I could just throw them all out, and still stay within few milivolts offset. Am I asking for trouble, or is it ok to kick those cheap suckers out?

Are caps at line inputs absolutely neccesary? Why?

Are they necessarily placed inbetween each opamp stage?

How to determine the need for DC decoupling caps? Is it proper to make prototype without them, then measure offsets, and put caps only if needed?

If I remove all caps between stages, although I shouldn't, how will I know that I did something wrong? :shock: Will it be obviously wrong immediately, or will it only show later in some special situation?

thnx for help
gnd
 
In general DC content is mostly a cosmetic or touchy feely issue in console design. Even modest amounts of DC will cause switches to click when audio is switched between different DC levels (like mute switches). A second issues with DC is it will cause wiper noise in pots when rotated due to granularity in pot resistance and even wiper bounce. This is typical cause of scratchy pots.

An interesting tradeoff in console design is that users perceive an audio path with scratchy pots and clicky switches as grossly inferior to that same path with coupling caps added, despite the reality of even decent caps having some performance consequence.

JR
 
removing all the caps will probably leave you with added and unwanted sound. not sure if its the best idea, but you will find out soon enough. :grin:
 
Heres what Bill Whitlock, president of Jensen Transformers, Inc. said about DC offset when asked if the capacitor could be eliminated before the output transformer in a circuit I built.The DC voltage is about 1.5mv offset in my pre amp.I chose to not include the capacitor.I think the pre sounds fine without it but I would not recomend going any higher than about 1 to 1.5mv of offset without adding a capacitor or dc servo.
Eliminating caps between opamp stages in a console could get a bit tricky but i guess experimentation is what DIY is all about.
"Our JT-11-FL or JT-11-FLPCH would be a better choice for the
transformer.
The coupling capacitor can be eliminated only if the DC offset voltage
on
the transformer primary can be held to less than about 1 mV. If the
servo
holds the voltage under this limit, I'd recommend eliminating the
coupling
capacitor. I'd be happy to look over your schematic and send you my
comments
and/or recommendations."

Sincerely,
Bill Whitlock, president
Jensen Transformers, Inc.
 
Whitlock's response is interesting commentary towards answering a question that's been ignored so far in another thread, namely how much d.c. is permissible in typical audio transformers.

It would seem that the answer is: not much. Wonder what the d.c.r. of the suggested part is.
 
http://www.uaudio.com/webzine/2006/december/index2.html

if you put a cap remember to put a R to ground too, input current on op amp inputs could charge the cap until it explote.
 

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