Q about combining circuit blocks

[jdub]

Howdy-

Just in general, are there any basic guidelines for combining various circuits together?  For example, say one wanted to link a compressor circuit (eg the So Simple Compressor) to an overdrive (a D+ for instance).  Or even, say, linking two of Tim Escobedo's circuit snippets together.  Is it as simple as running the output from circuit A into the input of circuit B, just like one would run one pedal into another?  I've been playing around with this as a learning experience and can't say I've been very successful.  I'm familiar with coupling via capacitor (or even with a transformer), but am I correct in deducing that there are other issues at play here, such as impedance? Or just plain incompatibility? Sorry if I'm missing something here- I'm just trying to wrap my brain around this stuff.

I'm not even sure if there are "general" approaches to this or if it must be determined on a case-by-case basis, but if anyone has the time (or the inclination), some tips or suggestions would be very helpful to me.  If need be, I can present specific examples.  Thanks!  

[R.G.]

Just in general, are there any basic guidelines for combining various circuits together?  ...
I'm not even sure if there are "general" approaches to this or if it must be determined on a case-by-case basis, but if anyone has the time (or the inclination), some tips or suggestions would be very helpful to me.  If need be, I can present specific examples.

It's very much like jigsaw puzzles or those little shuffle-the-squares puzzles. The pieces all potentially fit together any way you shuffle them - but the trick is to get the edges where they meet to match. There are general guidelines, which are below:
1. The impedance levels where the blocks meet have to be compatible. Sorry - you were going to get to impedance levels eventually, better to hit you with it first. You're going to have to know what impedance is and what compatible is as regards impedances. Fortunately, this is fairly easy. See "introduction to impedance" below.
2. The signal levels have to be compatible in terms of (a) DC and (b) AC voltages and currents. For most cases, the DC levels will not be compatible, except by lucky accident, so you will have to do something to get the blocks not to mess up each other. For audio, this "something" means blocking the DC levels between the two pieces so they don't interfere, and that means a capacitor which lets AC signal through and blocks DC. This is so important that most audio circuits already put in a DC-blocking capacitor on their input and output anyway. If this is true, the original designer of your "block" has taken care of this for you already, and the lucky accident has already happened.

For (b), most often you want the AC signal to be a voltage, and you want it fully transferred from one block to another. That means the capacitors on the driving output and the receiving input have to be big enough to do the transfer and not lose bass signal. Again, the original designer has probably done this for you.

But you have to then get the signal levels to match the way you want them. if the driving signal is too big for the receiving input, you can get distortion ranging from a little to massive and possibly ugly. You may want distortion, and this may be OK, but if you don't want distortion, like if you're trying to get a phaser input to work, you have to make the input signal smaller until the distortion goes away. If the driving signal is too small, the receiving circuit may not produce enough signal for you, or the signal may be noisy because you had to amplify it up so much that you also amplified up the inherent noise too much, too.

Introduction to impedance
For this simple introduction, you can think of impedance as resistance. Something that puts out a signal that can supply a relative lot of current without sagging has a low internal resistance, so it qualifies to be thought of as a low impedance source. Something that forces out a specific current, whatever voltage that causes to happen is a high impedance source. Something that eats only a tiny current no matter what voltage you put on it is a high impedance load. Something that eats a lot of current for any voltage put into it has a low impedance.

Lets go back to the garden hose analogy. Voltage is like water pressure. A hose can have a lot of pressure in it. If the nozzle is only opened a miniscule amount, water sprays out very forcefully, but not much total water flow, few gallons, of it comes out. The nozzle, under these conditions, is a high voltage, high impedance water (current!) source. Lots of pressure, not much flow. If you open the nozzle up a lot, then much more water comes out, still spraying. The nozzle has changed from a high impedance (high resistance to flow) source to a lower resistance (higher flow) source. More water, still sprays.

If the hose has a low pressure (voltage) inside it, then the nozzle won't spray water very hard at any opening/setting. It doesn't have enough pressure/voltage. And if you open it, water flows out, but limply. Maybe a lot of water flow if you open it up a lot, but a low-pressure flow still.

Now imagine a fire hose. This is a low impedance HOSE. Even at low pressures, lots of water flows. If you hook up a garden hose to a fire hose, less water flows because the garden hose is a higher impedance than the fire hose. Impedance is relative.

[jdub]

Wow, thanks for the in-depth reply, R.G.  My thinking/understanding has been leading me in this direction, but the real-world application has been a stumbling block. My suspicion is that impedance issues have been my main problem.  Your explanation is a great help.

One question concerning level matching: can this be as basic as a level (volume) trim between the output of one block and the input of another, adjusting until appropriate & then replacing with a fixed-value resistor?  Any other tips for making output/input levels play nicely?

Thanks again!

[R.G.]

One question concerning level matching: can this be as basic as a level (volume) trim between the output of one block and the input of another, adjusting until appropriate & then replacing with a fixed-value resistor?  Any other tips for making output/input levels play nicely?

Yes, it can be as simple as a volume control/voltage divider from one stage into the next. This only works when you have too much voltage on the driving block for the receiving block to take, of course. If you don't have enough to drive the receiving block correctly, you'll have to stick an amplifier block between them to get the signal level up to what the receiving block needs.

Do not fall prey to thinking you need matched impedances. Matched impedances are sometimes important in power applications. They are almost never important in low level audio signal work. Instead you want deliberately mis-matched impedances. In particular, you want the driving impedance to be less than one-tenth of the receiving input  impedance. This ensures that 90+ percent of the audio signal voltage actually gets into the receiving input, instead of being lost in the two impedances acting like a voltage divider.

[jdub]

Gotcha.  Eventually all this stuff will sink in.  Thanks again for the help, R.G.

[robmdall]

Thanks again for the help, R.G.

I agree. Thanks R.G. for the education once again!!

Rob