Impedance and coupling cap values

[Taylor]

I'm wondering why it seems to be that coupling caps in front of an active component tend to be smaller than those after the active component.

In all of the buffers on AMZ:

http://www.muzique.com/lab/buffers.htm

In the TS:

http://www.geofex.com/article_folders/tstech/tsxtech.gif

etc....  Is there a reason why the rolloff frequency would be different with a high impedance source than a low impedance source or something?

[merlinb]

A buffer (should) present a high input impedance, and is usually used to drive a lower load impedance. Hence the input cap can be smaller  than the output cap, for the same roll-off frequency.

[Derringer]

the output doesn't have to have that huge cap on it

I think he just drew them that way because you have no idea what the input Z of the following circuit will be ... could be reaaaaaallllll low

[earthtonesaudio]

Smaller cap values = higher impedance.  So at the input you can get away with a smaller cap because the bias resistor is usually large, and the corresponding voltage divider formed by the series cap + shunt resistance is small.
Conversely, if the input resistance is low you need a larger cap to pass the necessary frequencies, like in a Fuzz Face.

Same situation on the output; most of the time you want a low driving impedance, and the shunt resistance of the next input is unknown, so you use a large value cap.

[PRR]

> all of the buffers on AMZ

In addition to what Merlin, Derringer, and Alex said (hi-Z in, lo-Z out).....

AMZ's cap values are not "minimized".



The lowest impedance around the input LOOP (source, C1, R1+JFET) is 1Meg. For -3dB at 20Hz we could use 0.01uFd.

The lowest impedance around the output LOOP (JFET Source + R2, C2, load) is ~~~1K (load shorted, only JFET impedance) but for good system performance the load won't be shorted and probably should be higher than R2. Say 10K load. Total loop ~~~1K+10K= 10K. For -3dB at 20Hz we could use 1uFd.

However at 9V ratings, in DIY quantities, film-caps as large as 0.1uFd are the same price as 0.01u or smaller. 1uFd film is costly, but 10uFd electrolytic is cheap.

So while 0.1u and 10u are not minimized values, they are (for 9V DIY) minimized cost.

If you are going to make a million, or squeeze it into a plug, you might want a sharper budget-pencil or physically smaller parts. 0.01u may be a penny cheaper in 1000s. 2uFd may be available a millimeter smaller.

But in general, we design inputs for HIGH impedance and outputs for LOW impedance. That's not essential, just a custom you can't fight. As an extreme example, a bass amp may have 0.1uFd into 470K input, and 4,700uFd out to 8 ohm load.

[merlinb]

Say 10K load. Total loop ~~~1K+10K= 10K. For -3dB at 20Hz we could use 1uFd.

However at 9V ratings, in DIY quantities, film-caps as large as 0.1uFd are the same price as 0.01u or smaller. 1uFd film is costly, but 10uFd electrolytic is cheap.

So while 0.1u and 10u are not minimized values, they are (for 9V DIY) minimized cost.

In addition, electrolytic coupling caps are usually 'oversized' in pro-audio equipment in order to reduce the distortion they introduce (i.e., to push the distortion way down into the sub-20Hz inaudible region). Admittedly, I doubt the AMZ design was too concerned with that!

[EATyourGuitar]

can I mod my phantom octave to play nice with line level sources? I mostly use it with my cell phone or keyboard. doesn't sound like the same pedal using with guitar vs synth. its a lot less fuzzy with a line level source.

[ayayay!]

If you are going to make a million, or squeeze it into a plug, you might want a sharper budget-pencil or physically smaller parts. 0.01u may be a penny cheaper in 1000s. 2uFd may be available a millimeter smaller.

That's so important to remember for newer DIY'ers.  Even as little as 3-4 years ago, you just didn't have the kind of easy access to these parts as you do now.  The schem may be right, but that doesn't mean you have to do it exactly that way.  It's more important to know what you're doing.  You might save on parts, pesos, or both, which may help reduce forehead slapping after that next build.