Does the signal current loop around an amplifying device include any local decoupling - diyAudio
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Old 20th May 2017, 01:57 AM   #1
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Default Does the signal current loop around an amplifying device include any local decoupling

i know it is, but let us hear what others have to say?
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Old 20th May 2017, 03:10 AM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by TonyTecson View Post
i know it is, but let us hear what others have to say?
Draw the AC equivalent circuit. Now put back in the power supply capacitors.
If you have to break any lines to do that, then yes they are in the path.
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Old 20th May 2017, 03:57 AM   #3
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so are you saying it is not a simple yes or no?
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Old 20th May 2017, 04:26 AM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by TonyTecson View Post
so are you saying it is not a simple yes or no?
Usually it's a yes, unless there's a regulator circuit, or a current source after the cap.
Some circuits, like a diff amp, draw equal and opposite signal currents at the same time,
so little signal current gets in the supply if they are well balanced. A cathode follower tube
following a voltage amplifier tube is a combination that can have little signal in the supply,
since they also have equal and opposite signal currents.

Last edited by rayma; 20th May 2017 at 04:31 AM.
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Old 20th May 2017, 04:29 AM   #5
JMFahey is offline JMFahey  Argentina
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Ultimately they are in the signal path but "at the other side of a wall" which is the preamp/power amp itself, which may have better or worse supply rejection.

Completely different from being, say, used as coupling caps between signal source and load.
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Old 20th May 2017, 04:38 AM   #6
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thanks guys, keep em coming....
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Old 20th May 2017, 06:50 AM   #7
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by JMFahey View Post
Ultimately they are in the signal path but "at the other side of a wall" which is the preamp/power amp itself, which may have better or worse supply rejection.

Completely different from being, say, used as coupling caps between signal source and load.
Best answer I think.

I always argue it like this...

If the PSU cap was directly in the signal path then a DC coupled amplifier would not be able to maintain a DC voltage across the load, or to be able to deliver a low frequency squarewave (say 1Hz) with no apparent droop.

How can that be so if the cap/s are in the signal path ?

Answers on a postcard
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Old 20th May 2017, 07:26 AM   #8
udok is offline udok  Austria
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They are definitly not in the signal path (at least in my definition of signal path).

They have a secondary task: To provide and filter power for the active parts which
are in the signal path.

Best wishes,
Udo
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Old 20th May 2017, 07:40 AM   #9
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Since in your question you ask whether they are in the "signal path" rather than the signal path, then I would say "yes".
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Old 20th May 2017, 07:43 AM   #10
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Quote:
Completely different from being, say, used as coupling caps between signal source and load.
yes, discounting coupling caps which is clearly in signal path....

i posted this in another forum....see if you agree...

Quote:
take the OPT in a push pull amp, it is biased at 300 volts say and the OPT center tap is connected to a psu filter cap while the outer ends to plates of output tubes, this is dc biasing condition...but what about ac? the center tap at ac acts like the fulcrum in a see-saw. it is at ground zero in terms of ac.....guess what? it is in the signal path....if you can only see that as dc alone, then you are looking at half of the picture, you also need to see in terms ac...just as dc biasing has a + an -, so does ac. it has a loop that can not be broken...
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