Pot impedance..

[jeth]

PRR..Don't want to be pushy as you (and others) have already given me more time than I deserve..BUT..can you help me on this? I really want to get to the bottom of it. Yor previous explanations worked for me....
I've been searching the metas for clues, and if I wasn't stuck in my current location I'd just go and buy a decent textbook and start filling the (giant) gaps in my learning, but I can't..PLEASE! :?

 

[PRR]

I'm not sure what the question is.

Find mixer schematics and study them. It is much easier to learn by example than to invent stuff out of thin air.

 

[jeth]

Sorry if the questions not clear...

I've searched around for as many mixer schematics as possible. The problem is that no mixer I have seen is configured like what I am aiming for.... The usual arrangement (that I am sure you're familiar with) is for each input module to have it's own buffered fader, this source then feeds a number of busses, and the level of each bus is controlled by means of a buffered fader after the bus mix stage.

Say I have four sources, and four busses. All the sources simultaneously feed all four busses...so each bus sees the same four input sources. What I need to be able to do is have a seperate level control for the signal between ONE of the sources and each bus mix stage, so that the level of this source into each bus can be controlled without effecting the other sources levels in the bus, or the level of that source into the other 3 busses.

My question was really if it makes sense (or works at all) to buffer after these pots to raise the parallel R, and improve the load seen by the source that has to feed the four pots in parallel. I would have to use quite large value pots to keep the load reasonable, and I wondered if lower value pots with following buffers would be a better option in terms of noise and loss of signal.

 

[bcarso]

So---if you are unsure of your sources' drive capability, buffer each of them---with a buffer that will easily drive four pots. Buffer the wipers out of each of the four pots.

This is the most complex but the most certain to have no loading problems if you do the math right.

So for a given source: source output to buffer input; buffer output to four pots; each pot wiper to another buffer input; each of those buffers' outputs to the input mix resistor of each bus.

That's five buffers per source. Twenty buffers for four sources and four buses. Overkill in many cases, but it allows low Z pots, no concern with loading source outputs of more-or-less arbitrarily high impedance, no interaction among buses, and no concern about crosstalk.

 

[jeth]

Thanks for the reply Bcarso, Your description is exactly what I was proposing, just wasn't sure it would have the desired effect, or be worthwhile.

I think if I had parts available that would enable my input stages to drive the loads I could do without the buffers on the pots, but I don't so it's probably safest. One added advantage is that sweeping the pot won't influence the input resistance, and therefore the gain of the mix stage.

 

[jeth]

> general rule of loading the pot with mix R of equal value...what happens when this rule is broken..Is it just a deviation from the taper, or worse?

PRR answered this one for me before, but this applies to log pots, right? What if i want to use lin pots and fake the law by making the mix R close to 15% of the pot value? There's really no way to get around using larger value pots in this circumstance, as the mix R needs to stay at a reasonable value. 10k pot would need 1.5k mix R....Is there a way to avoid high value pots in this situation?

 

[PRR]

> use lin pots and fake the law

That puts "center" at -15dB instead of -6dB or -8dB, true.

But 10% is just -24dB. (Not that different from an unloaded linear which gives -20dB here.)

Loading a pot will not give you a useful -30dB or -40dB range.

Tapered pots are useful when you really need a wide range.

 

[jeth]

It's just that I had read that "law faking" by use of either a resistor to ground from the wiper, or simply loading the pot with around 15% of it's value, would give a result "better" than the taper of most "true" log pots.

The reasoning was that "true" log is usually achieved by use of seperate sections joined, giving an approximation of the log curve. Several sources suggested that a fake law on a linear pot is actually closer to the ideal log taper.

:?: :?: :?:

 

[bcarso]

Naah. I daresay you were told wrong. The pot element is a continuous resistive strip with a selectively tapered amount of deposited material.

There are other circuits that approximate nonlinear laws with segments, like diode-resistor networks that appear in sine wave shapers for function generators, but I've not seen this done with pots.

The one thing that linears, being uniform per arc segment, or unit length for linear geometries, will do for you is enhance tracking on ganged multiple section pots, just because it's easier to be uniform than nonuniform.