VCA noise

[Mick Bailey]

I had a lateral though with this and tried re-scaling the resistors to reduce noise. I now have the 270R resistors in both legs, but have reduced the input resistors R to 1k5 and K3R to 1k8. At these values the 270R resistors really make a difference to noise and are pretty much the ideal value. I also installed a bypass switch to A-B the circuit. To get around the signal loading I've used the spare half of the opamp as an input buffer, with 1M input resistance. The noise level has dropped significantly and the circuit attenuates nicely with no discernible bleedthrough. I installed a bypass switch and experimented with resistors to get near unity gain (thought there is a very slight overall boost). I'm now pleased with the results and intend building it into an old wah shell.

The noise level is such that the residual noise under full attenuation is lower than the pickup noise from the guitar when at full output. I think when it's built up on a PCB and with short leads inside a metal shell it will be even better.

The circuit is sensitive to FET and resistor matching, otherwise there can be bleedthough if random parts are selected.

[danmc]

yay!

I'm curious, did you put the 270R just in series with the JFETs or the somewhat different arrangement that Dave H posted a schematic of?

[nickb]

I'd invite you to consider an op-amp circuit with the + input grounded and a resistor from input source to - Input = R1 and from - input to output of R2. Would you agree that the gain is -R2/R1? Now add a third resistor, R3, from - input to ground. The gain from input to output is still -R2/R1 up until you make R3 so small relative to R2 that you make Av_op_amp * (R1||R3)/(R2 + R1||R3) become not so huge. With 30 Ohms for R3 and 30k for R2, you have about 1000x attenuation. The NE5532 has about 10 Mhz unity gain frequency and so at 1 kHz you still have a gain of about 10,000 from the op-amp and the loop gain including the feedback network is about 10 and so while not infinite, the error is small.

Now consider a second circuit. Just a standard non-inverting amplifier. R1 from ground to - input of the op-amp and R2 from - input to output and the input source drives the + input. Gain is 1+R2/R1. Now add R3 from - input to ground and vary it. In this case you vary the gain substantially (as long as R3 is small enough to have a strong influence on R1||R3). The gain is now 1 + R2/(R1 || R3). This is why the gain through the plus half of the VGA varies.

-Dan

Yes, of course. Thx. I don't know why I didn't see it before as it's so obvious. I think I'll go stand in the corner for a bit...

[Mick Bailey]

yay!

I'm curious, did you put the 270R just in series with the JFETs or the somewhat different arrangement that Dave H posted a schematic of?

Originally I'd read Dave's suggestion in post #6 as being in series with Q1 drain - this is why I wasn't getting enough 'off' attenuation - it just adds to the Rgs and what's needed here is a low value. When I used the circuit in #15 that's when I got much better results.

I did try increasing the value from 270R in an attempt to improve the noise. This has the effect of moving the FET from behaving like a variable resistor into a more abrupt on/off. The result was that whilst the noise level reduced, it narrowed the useable pot sweep to about 1/5 of the pot rotation.

I need to look through some opamp specs to see how low the gain-setting resistors could drop and the device still source enough current. Maybe there's a better choice than the NE5532.

[Mick Bailey]

Yes, of course. Thx. I don't know why I didn't see it before as it's so obvious. I think I'll go stand in the corner for a bit...

I'll stand there with you, because that's how I saw it working originally.