Here's one that I've been thinking about for a while but never got around to working on....let's say you find a nice schematic for a tone stack, FMV or whatever.

What you're looking at could be (should be) "idealized" for the source driving it...maybe a gain stage of some sort.

Ok - so I don't have the right value pots...what if I scaled the stack component values to what I have at hand...and drove it with a buffer to avoid loading my source?   I mean, they do it in amps...

Understandably, this could also require more make-up gain, of course.

Seems that we use buffers for just this situation, no?  Should offer a low enough source Z to be able to drive "almost anything"...right?

Just a thought, I have no specific circuit I'm working with.   I've not bothered with a lot of designs because of 'values'...seems this may help you avoid that problem?

Like, reading about a particular tone stack, then you see "rather low input impedance..."...this seems like the way to solve that.

> drove it with a buffer to avoid loading my source?

The original values may have been picked with source impedance in mind. Driving with a near-zero impedance may change things.

The base reason though is cost. "If" you can do a job without adding buffers, why add buffers? In commercial products, every penny spent hastens the nearly inevitable bankruptcy (or dime-on-dollar buy-out of a coveted brand name).

Yes, if you are dodging a $20 shipping fee to get the "right" pot, a 19 cent opamp is a valid choice.

I've recently made a bass and high cut circuit in my guitar. The original had passive pickups and rather high pot values (500k and 1M). I did mine with lower pots (around 100k) as my guitar had active pickup (which is basically a passive pickup with a buffer in it). The result was the same as the original. So if you have a lower source impedance than the original you can scale down the component values.

As PRR said the tone stack are usually designed with a specific source. After a valve stage the tone stack has to be high impedance not to load down the driver. This also means that the source impedance is "part" of the tone stack and having a lower source impedance can change the frequency response of the tone stack.

QuoteUnderstandably, this could also require more make-up gain, of course.

Actually if you have an ideal source driving you need less make-up gain as there is less damping.

QuoteOk - so I don't have the right value pots...what if I scaled the stack component values to what I have at hand...and drove it with a buffer to avoid loading my source?

In DIY having a buffer made of spare parts is better than ordering new pots. But in an industrial situation ordering the new components is cheaper than adding a couple more to a design as you would order pots anyway.

Thanks guys, I thought so.  No desire to make things I'm messing with more complex, but at times I may have the pots for one model of stack, then want to try something else and they're not there, that kind of thing.    Nothing commercial here!   

Just thought, if buffer has low output impedance, then you can raise the source to the tone stack simply by inserting a series resistor. So if it needs 10k source and op-amp is driving, just add 10k in.

Quote from: anotherjim on July 02, 2017, 11:22:12 AM
Just thought, if buffer has low output impedance, then you can raise the source to the tone stack simply by inserting a series resistor. So if it needs 10k source and op-amp is driving, just add 10k in.

Nice, good point Jim.    I see now that if it was designed for something like 10k, and you have a source of 600R, you'll get a different response from the stack than what was intended...so it seems that maybe some figgering is needed, by looking at what was driving it - what you changed with respect to the intended source...cool, another thing right there in front of me that I wasn't seeing, LOL!