Coupling caps, upgrade, similar values

Hi,
If you know the input impedance of the next piece in the gear lineup, you can calculate the needed value to prevent roll off and possible distortion. Google for this formula. It seems most gear is designed for very low input impedance, one size fits all. I find that most values can be reduced(I have all tube equip). I would think that going from 5 to 3 should only be a problem(sound wise) in the one in a thousand. That said, remember if you change that next piece of gear for something else, you need to qualify you choice of capacitance value for the new gear!

2x3.3 in parallel would be closer than a 3.3 as a replacement. 2x10uf in SERIES will be 5uf?
It is possible that TOO much capacitance would be worse than not enough....even if the 2x3.3 in parallel is closer.

Caps have 'tolerance', too. The measure of how far any caps individual measurement will be from the marked value.
some, the most common are 20%. Better is 10% while cost no-object can be 5%. The last get you into hi-priced goods.

In general, do not try to re-engineer a product. In some circuits, caps and resistors are chosen together. If you substantially change the cap, the resistor will be the wrong value. I don't know this is true, but you can probably buy tested / select caps of matching values. This is nutty, since nothing else in your amp/pre would than match, anyway.

Let me guess, you are trying to buy a better sounding cap?
If you are stuck on a name and they don't have the right values, you probably should pick another name.

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.

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

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 :)