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Old 29th June 2015, 07:08 PM   #1
grantos is offline grantos  United States
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Default Bypass electrolytic caps in audio chain?

While replacing the electrolytic caps (10/43uF and 25V) in my in/out audio signal path of my audio interface, is it worthwhile to also bypass with ceramics? Like a 0.1uF 50V X7R? on each? Or is this primarily used for PSU caps?
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Old 30th June 2015, 06:43 AM   #2
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It may cause a 'tanking' resonance which depending on the circuit may upset things and cause some oscillation.

Stefan
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Old 30th June 2015, 07:13 AM   #3
AndrewT is offline AndrewT  Scotland
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Don't think bypass. This implies placing a low esr capacitor across an electrolytic to allow high frequency to pass more easily.

Think supply rail decoupling.
This is placing medium esr capacitors with ultra low inductance at the point of current change. This is AT the POWER Pins of the current changing device.

Decoupling is just as important in the digital side as in the analogue side. The digital chips change current very quickly. It's this CHANGE in current that must be decoupled at the power pins, to help prevent the supply rail to other devices being starved of clean power.
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Last edited by AndrewT; 30th June 2015 at 07:16 AM.
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Old 30th June 2015, 07:16 AM   #4
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Ah never noticed it was coupling caps. Have the amps got coupling caps? If so you could just do away with them altogether. Normally gives a nice boost
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Old 30th June 2015, 10:17 AM   #5
DF96 is offline DF96  England
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Coupling caps are there for a reason.

'Bypassing' coupling caps is unnecessary. If it sounds different with a 'bypass' then there is something wrong with the design or the bypass arrangement.
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Old 30th June 2015, 02:32 PM   #6
grantos is offline grantos  United States
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Quote:
Originally Posted by AndrewT View Post
Don't think bypass. This implies placing a low esr capacitor across an electrolytic to allow high frequency to pass more easily.

Think supply rail decoupling.
This is placing medium esr capacitors with ultra low inductance at the point of current change. This is AT the POWER Pins of the current changing device.

Decoupling is just as important in the digital side as in the analogue side. The digital chips change current very quickly. It's this CHANGE in current that must be decoupled at the power pins, to help prevent the supply rail to other devices being starved of clean power.
OK, I'm confused, and bear with me because I just learned about all this stuff last weekend Here's the PSU of my audio interface:

Click the image to open in full size.

Based on stuff I read I had already planned to bypass C12 and C18 with 0.1uF ceramic caps. C12 and C18 are my coupling caps for the 15V rails, right? So, to bypass those or not?

But there are also smaller 10uF electrolytic caps in my audio in/out signal path (not shown in the schematic), not really sure what they do but I think they maybe filter the signal before the op amps, and I was wondering about bypassing these with ceramics too. I had read in a few places that electrolytics suck for audio chains and that bypassing with small ceramics helps the sound, by allowing higher frequencies, as you mention.

Am I way off track?
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Old 30th June 2015, 02:39 PM   #7
DF96 is offline DF96  England
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If you don't know what the caps do why do you think they might need bypassing?

Don't believe everything you read, especially if you read it about audio on the internet.
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Old 30th June 2015, 02:53 PM   #8
grantos is offline grantos  United States
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Quote:
Originally Posted by DF96 View Post
If you don't know what the caps do why do you think they might need bypassing?
Care to make this a learning experience, or no?


Quote:
Originally Posted by DF96 View Post
Don't believe everything you read, especially if you read it about audio on the internet.
I've read in several places from several "audio guru" types on the internet that you should bypass electrolytic caps with small ceramics, and I've also read the opposite. There seem to be two camps but I don't have the background to discern between the two. Can you or someone else please elaborate and help me clarify so I can take the best course of action?
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Old 30th June 2015, 03:50 PM   #9
DF96 is offline DF96  England
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All capacitors have some series inductance and resistance. This means that as frequency rises their impedance falls but not to zero. After the minimum the impedance rises again with frequency, but now it is inductive not capacitive. The frequency of minimum impedance can range from some kHz (large electrolytic) to hundreds of MHz (small ceramic cap with very short leads). In any case, the capacitor is still a small impedance for some frequency range either side of the minimum. Nothing awful happens if you go some way beyond the minimum!

I suspect the 'audio gurus' have read that impedance can rise with frequency, have been alarmed by this, and wrongly concluded that the solution is to add a parallel small cap as a so-called bypass. So for a start they are solving something which is not usually a problem. Secondly, their 'solution' can actually raise the net impedance above the minimum as the small extra cap is now in parallel with the inductance of the big cap. Fortunately, the big cap may be a bit lossy so the parallel resonance is well damped so little harm is done. If adding a 'bypass' results in a real change in sound (i.e. a change you can hear with your ears - no peeking!) then it is likely that this is caused by the increase in impedance above the minimum - so you are achieving exactly the opposite of what you wanted.

Coupling capacitors are often small electrolytics which are OK over the whole audio range and some way above it. No need for bypassing.

Decoupling capacitors may be larger electrolytics and here there could be a problem of impedance rise at higher audio frequencies (although not always). In such cases it is important that the bypass is placed right at the circuit and not just across the big cap. Then the resistance of the wiring helps damp any parallel resonance. In some cases the real problem is not the big cap but the length of the wires to it.

To understand the problem and decide which camp to believe you need to learn about the following issues:
- first order passive CR filters
- coupling and decoupling caps - what they do (they are first order CR filters, coupling caps form part of high pass filters while decoupling caps form part of low pass filters)
- parallel and series LC resonance, and Q
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Old 30th June 2015, 04:43 PM   #10
grantos is offline grantos  United States
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Thank you! I see that I have some learning issues, but for now, am I understanding the basics correctly?

Decoupling caps along with the RC network in the PSU help filter and shape the DC current, which drives the amps. The digital audio output from those amps is AC, which is HP filtered by coupling caps to remove any DC signal.

Quote:
Originally Posted by DF96 View Post
Decoupling capacitors may be larger electrolytics and here there could be a problem of impedance rise at higher audio frequencies (although not always). In such cases it is important that the bypass is placed right at the circuit and not just across the big cap. Then the resistance of the wiring helps damp any parallel resonance. In some cases the real problem is not the big cap but the length of the wires to it.
So if I placed 0.1uF ceramic cap flush with the underside of the PCB between the leads of my decoupling caps (C18, C12, C60, and C22, right?), would this be beneficial?

And no need to bypass the coupling caps.

Really appreciate the help!
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