180° VCA Circuit Idea

[Bo Deadly]

I just scibbled this circuit up:



The idea is that you can continuously adjust between the non-inverted and inverted signal but when CV=0V, the signal should be fully attenuated. So it could be used as a conventional VCA but it might have some properties that are nice for some other reason like faster response because the LDR that is turning "on" will be fast. Or maybe it has nice distortion characteristics because of the balanced nature of the circuit.

Anyways, I have no idea what it might be good for. Clearly it's a solution in search of a problem.

 

[JohnRoberts]

I did a bunch of bench work back in the 70s looking for low cost alternates to VCAs. I didn't have any luck back then, while VCAs have significantly improved in the decades since.

One of my more promising pursuits with LDRs was using a center-tapped LDR with one half in an opamp NF path to linearize the control law.

Build yours and tell us how it sounds, measures, etc.

JR

 

[Bo Deadly]

One day maybe. Right now I'm supposed to be writing code.

As a VCA this isn't special. In fact I think the control law might be wonky because of the logarithmic behavior of the LTP. What I thought was interesting, or at least different, is that it can sweep continuously from positive phase, attenuate down to some null where the two paths cancel (albeit limited by the imperfections of the LDRs) and then to negative phase. So it sweeps over 180 degrees of phase (title of post corrected). That might be useful for a filter of some sort like a voltage controlled boost / cut.

 

[JohnRoberts]

In fact the current sharing in a LTP responds logarithmically so if the current output is driving something linear you can cobble together a dB law control function. Not saying you should pursue this but I don't think the control law is you biggest hurdle.

JR

 

[L´Andratté]

Basically a crossfader.
I have build verions of that circuit and found they can work nice in a "musical" way.
Though the control law graph probably resembles something like JR´s raised eyebrows (;
link

 

[Potato Cakes]

I may have a problem for such a solution. I am working on a way to have a 1 kHz (or some other sine wave frequency) tone to control the level of an analog signal. Yes, this can be done via a side chain in a compressor, but what I'm after is just a VCA type circuit to be inserted in the signal path or to attenuate a parallel inverted signal that when combined with the original source it causes phase cancellation and the attenuation thereof allows the original signal to pass, creating the perception of a volume control. The purpose is to be able to use a digital console to control the output level of an analog processor without the need to integrate a MIDI or GPIO circuit in said analog processor by adjusting the level of the tone from a post fader send. I think this makes sense but I may need to make a sketch of what I'm trying to explain.

Thanks!

Paul

 

[neil.johnson]

I just scibbled this circuit up:

View attachment 80500

The idea is that you can continuously adjust between the non-inverted and inverted signal but when CV=0V, the signal should be fully attenuated. So it could be used as a conventional VCA but it might have some properties that are nice for some other reason like faster response because the LDR that is turning "on" will be fast. Or maybe it has nice distortion characteristics because of the balanced nature of the circuit.

Anyways, I have no idea what it might be good for. Clearly it's a solution in search of a problem.

You've "invented" a four-quadrant multiplier. Using LDRs. These days you're more likely to use a multiplier chip (AD633, MC1496, etc) or a linear VCA like an LM13700, or a couple of SSI2164 expo VCAs in an Irwin lineariser. Much faster response compared to the tediously slow LDRs, and also almost guaranteed to have the correct gain as you sweep the CV.

Neil