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Coupling caps, upgrade, similar values

Hi all,

One question I have with regards to changing or upgrading signal coupling caps in a preamp (or any amp in that matter). Replacing the cap with a capacitance value that is not identical to the original, but that of a similar value ... wouldn't this change the output impedance of the preamp, as well as bass roll off frequecy, sound etc, etc. Cos then you're actually changing two variables, brand and value ... so how can you make an accurate judgement of the difference in sound? eg. If the orignal coupling cap is a 5.0uf, and you change it to a 3.3uf or a 10.0uf ... what happens?

Sorry, but I'm all new to this. Thanks for your time.

Regards,
David
linnmaster
magnumpi205
504 posts
 
Keep it close, also can double up in series as Magfan states. Or you can add in series like 4.7uf t .1uf to get closer to 5.0 uf. Don't try to engineer a new circuit.
pauly
456 posts
 
David, it depends in what part of the circuit the coupling cap is located.

If the cap is between two amplifying stages within the same amplifier/pre-amplifier, it is safest to keep the capacitance as close as to standard as possible. Going bigger can cause some amps to oscillate, going smaller may cause the amp to roll off some of the bass signals early.

If you are referring to an output coupling cap, Optimationman is correct in that the value depends on the input impedance of the next device. I have reduced a 8uF output coupling cap to 1uF on a phono preamp with no negative impact, mainly because the input impedance of my pre-amp was sufficiently high to allow this.

Most pre-amp designers go big on output coupling caps to ensure compatibility with other equipment.

Regards
Paul
linnmaster OP
45 posts
 
Thanks for all the support.

I believe the cap is an output coupling cap as one end of it connected to the volume control.

I don't plan on re-engineering the circuit, and would like to keep it to the same value ... but some "really good" caps aren't available in specific values ... hence my question.

But I think I'm clear with what to do now ... will post another thread if something blows up :)
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