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Old 9th December 2012, 04:06 PM   #1
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Default Questions about AC coupling and opamp biasing

I'm trying to pick some better capacitor values for the part of a circuit shown below (dont ask me what it's for, you'll just laugh). Before this final amplifying stage there's a bunch of transistors summing signals from some DACs and a low pass filter (22nF to ground). The signals are 0-5v with 2.5v offset.

When I calculate the cutoff frequencies ot the bias filter and the output AC coupling (assuming I'll connect the the outputs to the line level input of a Hi-Fi amplifier) I get: Output = ~0.03Hz, Bias = ~7Hz.

I've read somewhere that the bias cutoff should be 1/10 of the amp cutoff to minimize the noise from the bias circuit. Is that right?

The design on the picture is doing the opposite, and I'd like to correct that if it's possible (or even necessary?). I'd like to use a pair of 10uF MKP4s for the output AC coupling. 100uF seems a bit overkill. But can I use a smaller value? How "high" can you make the cutoff frequency before it starts getting audible?

If I go with 10uF caps on the output, I'll have to use 22000uF caps on the bias instead of the 100uF to get 1/10 of the output cutoff (unless I'm doing all of these calculations wrong... there's a good chance of that). I guess I can do that by paralleling 10x 2200uF. But will it have some bad side effects (22000uF sounds huge to me)? Is it even worth the trouble?
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Old 9th December 2012, 04:16 PM   #2
DF96 is offline DF96  England
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Quote:
How "high" can you make the cutoff frequency before it starts getting audible?
It depends on your loudspeakers. If they only go down to 40Hz then there seems no point in sending 0.03Hz to the power amp; it may even be harmful to sound, as any subsonics from your sources will keep the power amp busy.
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Old 9th December 2012, 05:03 PM   #3
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Good point

Mine only goes down to 44Hz. So it would be safe to cut the output somewhere around that?
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Old 9th December 2012, 09:10 PM   #4
DF96 is offline DF96  England
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No, go a bit lower - maybe 10-15Hz. This reduces phase shift. You can always push the bias decoupling down a bit more, but no need to take it too far.
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Old 9th December 2012, 09:37 PM   #5
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Sounds good. I had a feeling I was getting into the territory of ridiculously overdoing things
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Old 9th December 2012, 09:43 PM   #6
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Your bias circuit looks ok.
The 470r seems a bit low, I use 10k's in my mixer circuit with 10 op amps on the bias line.

If you want to lose the 2v5 bias put a capacitor on the input to the op-amps, this will kill any DC.
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Old 10th December 2012, 11:01 AM   #7
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Quote:
Originally Posted by nigelwright7557 View Post
Your bias circuit looks ok.
It's not my design, but thanks anyway

Quote:
Originally Posted by nigelwright7557 View Post
The 470r seems a bit low, I use 10k's in my mixer circuit with 10 op amps on the bias line.
I've been told that they need to be as small as possible (within reason) to keep the feedback resistors out of the equation. And the whole circuit only has a single 5v rail to work with, so the current draw is quite low.

Quote:
Originally Posted by nigelwright7557 View Post
If you want to lose the 2v5 bias put a capacitor on the input to the op-amps, this will kill any DC.
That would be the optimal solution. But at the moment the I'm limited by the 5v supply.
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Old 10th December 2012, 04:16 PM   #8
DF96 is offline DF96  England
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I would increase the 470, too. This will reduce the DC and subsonic gain, so the circuit will cope better if the input bias is not exactly 2.5V.
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Old 10th December 2012, 06:26 PM   #9
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Ahh yes, I think I had my info on AC and DC coupling mixed.

The resistors only needs to be low if the amp is DC coupled, right?

I'll replace them with 10K
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