Designing Blend and Feedback Controls: What are the Impedance Rules?

[thehallofshields]

Ive had good success using Jfet Buffers for Mix controls, but I'm often jealous of designs that are able to accomplish Clean/Dry Blends without using buffers. Flanger and Delay units often do the opposite and send the Output back to the Input under the names Feedback, Regeneration or Resonance; is this ever done without buffers?

I have a basic understanding of Impedance, Phase and DC Coupling, but the results of trying to jump signal around circuits has been very hit-or-miss for me.

I was hoping for some tips or general guidelines on getting Signal to flow in the direction you want.

Right now I'm working on a circuit with a Dual Opamp as Input and Output Buffers, maybe that would be the easiest place to start.

[FiveseveN]

I was hoping for some tips or general guidelines on getting Signal to flow in the direction you want.

Make one point in the electromagnetic field a different potential from another and current shall flow between them, proportional to the impedance in its path. 
Chips lilke the MN3005 are internally buffered, that's one thing to take into account.

[merlinb]

Ive had good success using Jfet Buffers for Mix controls, but I'm often jealous of designs that are able to accomplish Clean/Dry Blends without using buffers. Flanger and Delay units often do the opposite and send the Output back to the Input under the names Feedback, Regeneration or Resonance; is this ever done without buffers?

The circuits you mentioned are usually built with opamps. Opamps tend to have very low output impedance, so they can often be thought of as providing a buffering action even when they are also doing another job like providing gain or acting as an all-pass filter. You get two for one!

In other circuits the blend may not be intedned to go from 100% dry to 100% wet. Perhaps 75% dry to 75% wet is good enough, in which case they may be able to forego any explicit buffers, managing simply with the "moderately low" source impedance of two sources, and maybe a high-value pot.

[anotherjim]

I think you're asking for "passive mixing"? Just feeding each input via resistors all connected at one common "output".
It works, but has limitations.
Back in the stone age, many small PA and "Keyboard" amps happened to be a standard guitar amp chassis with a 3 or 4 input passive mixer (and another 100 or so units of currency tacked onto the price) . They work fine until you put something with strong low frequency (like a drum machine) in and every kick drum hit gates everything else out.
But in a delay and such, you can get away with it, but it's not ideal. It isn't too hard to build a proper virtual earth mix function into an opamp you were using anyway.

[R.G.]

I was hoping for some tips or general guidelines on getting Signal to flow in the direction you want.

Ohm's law

Down at the bottom of this, you have to know what the driving impedance to a network is, the network itself, and the load impedance. Then you have to do the math to figure out where the current flows and the resultant voltages from that current flow.

To simplify things, you can sometimes use the rule of ten: if something is ten times bigger than something else, you can ignore the something else in most cases, at least for a first pass.  Search for my ID ("R.G.") and "10:1".

[PRR]

> jealous of designs that are able to accomplish Clean/Dry Blends without using buffers.

Why?

Buffering is cheap as chips nowadays.

[thehallofshields]

I was hoping for some tips or general guidelines on getting Signal to flow in the direction you want.

Ohm's law

Down at the bottom of this, you have to know what the driving impedance to a network is, the network itself, and the load impedance. Then you have to do the math to figure out where the current flows and the resultant voltages from that current flow.

Okay. I can do Ohms law! Except with a single Voltage and simple resistances, not several signals and complex impedances.

So... what am I missing?

[thehallofshields]

> jealous of designs that are able to accomplish Clean/Dry Blends without using buffers.

Why?

Buffering is cheap as chips nowadays.

I'm sorry I meant designs that use passive mixing to reduce parts count.

[merlinb]

I'm sorry I meant designs that use passive mixing to reduce parts count.

I think you need to show a picture of the sort of circuit you're talking about. You'll get a more informed answer