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Robert Casey
 
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Default Is there a standard or convention for AC or DC coupling for amp inputs?

Is there a widely followed standard or convention in the audio industry
on whether amps should have coupling caps on their input jacks (to
keep stray DC bias from tuners or CD players or tape machines from
goofing up bias on tube grids)?. Or is it expected that all audio sources
are not to have any stray DC bias riding on the audio? And that there
should never be any DC bias leaking from the amp to the source?

If there is a convention, then one could avoid having a coupling cap
inside the source feeding the output jacks, only to feed into a 2nd
coupling cap in the amp.

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Phil Allison
 
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"Robert Casey"

Is there a widely followed standard or convention in the audio industry
on whether amps should have coupling caps on their input jacks (to
keep stray DC bias from tuners or CD players or tape machines from
goofing up bias on tube grids)?. Or is it expected that all audio sources
are not to have any stray DC bias riding on the audio?



** There is no convention and the majority of tube amp inputs connect
directly to the first stage grid.


And that there should never be any DC bias leaking from the amp to the

source?


** It is rare to see any significant DC bias voltage on an amp's input.



If there is a convention, then one could avoid having a coupling cap
inside the source feeding the output jacks, only to feed into a 2nd
coupling cap in the amp.



** That is why input caps are omitted where possible.

With SS amps input coupling caps are standard practice - to remove sub
sonics and any residual DC offset from transistor base bias current.




............... Phil



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TubeGarden
 
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Hi RATs!

Just open the casework and look to see if there is a coupling cap on the input
or the output. The hope that everyone follows some fine rule is noble, but,
doomed.

Yes, two caps in a row are not the best solution, but, it is hit or miss.

If you do not look in every case, you don't know.

And two good caps in a row are better than two cheap caps. Just for reference


One is all that is needed, barring some strange topology

Happy Ears!
Al


Alan J. Marcy
Phoenix, AZ

PWC/mystic/Earhead
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Mikkel C. Simonsen
 
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Robert Casey wrote:

Is there a widely followed standard or convention in the audio industry
on whether amps should have coupling caps on their input jacks (to
keep stray DC bias from tuners or CD players or tape machines from
goofing up bias on tube grids)?. Or is it expected that all audio sources
are not to have any stray DC bias riding on the audio? And that there
should never be any DC bias leaking from the amp to the source?


The convention is to use caps on both inputs and outputs. Some poweramps
have inputs (often labelled "LAB" for some reason) that are DC coupled.

I never use coupling caps on inputs, as most equipment should have
either coupling caps on the outputs or DC-servos. But if you're a
manufacturer and have to make "safe" products, you install caps on both
inputs and outputs - whether they are needed or not...

Best regards,

Mikkel C. Simonsen
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Phil Allison
 
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"Mikkel C. Simonsen"

The convention is to use caps on both inputs and outputs.

Some poweramps
have inputs (often labelled "LAB" for some reason) that are DC coupled.



** That would only be on SS amps - and darn few of them are actually DC
coupled with full gain.

The Crown DC300A and a few relatives.




............... Phil




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Tom Schlangen
 
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Hi Robert,

Is there a widely followed standard or convention
in the audio industry on whether amps should have
coupling caps on their input jacks


AFAIK no. Just for that reason I simply use a DPDT
micro switch in my projects, which adds/deletes a
pair of suitable coupling caps after the RCA jack
(or selector switch) just to be armed for worst case :-)

Actually, I _had_ a problem with DC offset from an old
SS tuner (Marantz ST510) I plugged in as the very
first audio source to my very first tube amp project ...

Tom

--
MS-DOS is the worst text adventure game I have ever played:
Poor vocabulary, weak parser and boring storyline.
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