Pro Audio Design Forum

"Single Stage Gives Logarithmic Gain Steps," Allen Windhorn, EDN, March 3, 1994

This circuit uses positive feedback to modify a linear taper pot to provide logarithmic gain control.
It's similar to the circuit trick Douglas Self used in the Soundcraft panpot.


"Single Stage Gives Logarithmic Gain Steps," Allen WIndhorn, EDN, March 3, 1994.

https://proaudiodesignforum.com/images/ ... 3_1994.pdf

Actually it is the gain relationship for most decent mic preamps made the last few decades.

Just imagine two op amps with the linear gain pot between the two - inputs... presto log gain function.

Ideally you want a nonlinear pot (slower at the low R end) because the log/exponential gain increase is too fast at the end hop off.

JR

Just imagine two op amps with the linear gain pot between the two - inputs... presto log gain function.

It looks like half an instrumentation amp at first glance.
But the pot is between the "+" inputs controlling the positive feedback not the negative feedback we are used to seeing.

mediatechnology wrote:

Just imagine two op amps with the linear gain pot between the two - inputs... presto log gain function.

It looks like half an instrumentation amp at first glance.
But the pot is between the "+" inputs controlling the positive feedback not the negative feedback we are used to seeing.

My bad... I was a little too fast on the trigger...

JR

I "double-taked" that one a couple of times too...

Sort of like when you see an op amp cascaded to an inverting buffer and overall NFB feedback taken into non-inverting input.
The first time you look at it you go "wait a minute. Was that drawn right?"

I need to find Self's Soundcraft pan pot with both positive and negative feedback.

So the question is, can a dual section pot varying two shunts be used in an instrumentation amp having both positive and negative feedback to obtain linear (or at least more linear) gain control vs. rotation?
The time is 09:16 on October 13, 2015.

Turns out you can enter a "Citation of prior art and written statements" into a patent file.
Cheaper than a re-examination....

mediatechnology wrote: The time is 09:16 on October 13, 2015.

Turns out you can enter a "Citation of prior art and written statements" into a patent file.
Cheaper than a re-examination....

I am confused again....

I did some research into trying to squash a dubious patent and you need to be careful if/when you trigger re-examination since a weak contest will just make the bad patent stronger.... Adding prior art to a file does not automatically spank it. In fact my understanding of the law is that the inventor is supposed to advise the PTO if (s)he even learns of prior art... I have sent prior art to inventors before...tends to shut them up.

JR

I think this would be useful as an input to an O'scope used in XY as an audio monitor. It could maintain a constant display size regardless of input level. NTP did that with the little XY display meter they had in a Danner cassette format.

I have the documentation for that somewhere. I'll try to dig it out and post the discrete version log amp they made.

Great idea Paul.

That would be useful. I'm always using the 'scope in X-Y and am constantly reaching to change two input attenuators.

I need to find Self's Soundcraft pan pot with both positive and negative feedback.

Soundcraft active panpot using positive feedback.


Soundcraft active panpot using positive feedback.