Pro Audio Design Forum

It seems for extreme boost or cut, the VCA may be more linear with differential CV,, however at extreme cut how important is THD when swamped by noise floor? And I don't plan to use extreme boost...

JR

My console buddies have shared their VCA drive approach. They use differential control voltage and NOT by just using a simple inverter between the two control ports. They were advised by THAT to use separate pads and buffers at + and - control ports. One inverting buffer and one non inverting buffer...

Thanks.. that agrees with my gut feeling..

In the German article the improvement even at low levels was more than the expected 3dB from doubling up devices. So it looks like it was canceling out something in the older technology VCA.

JR

Thanks for the article... I don't spreiken zie Deutsche, but the plots sure look pretty. At lower levels on the plot they show an expected improvement similar to throwing two VCAs in parallel (but a little better than expected). The best I can guess is curves ein and zwie are 1 VCA trimmed for lowes...

Yup while this is bad for our opamp count, I'm leaning toward differential and buffered EC port drive so that's +2 opamps for pretty much just the buffer. I would ASSume there isn't a major problem from static assymetry at the control ports since so many applications drive just one, so perhaps we ca...

+1

Merry Christmas... looks like t-shirt weather for my 5 mile run today.

I'll be glad when 2008 is over... brutal year for markets, and stuff...

I predict 2009 will be more fun...

JR

My friends at the console company have used a lot of THAT VCAs gave me some pointers based on their experience and review of their design by the lads at THAT. I will digest those and distill it down for our project. FWIW they use pads and opamp buffers after the pads... so low gain opamp noise is no...

Merry Christmas! Wayne, couldn't we input our CV to an inverting amp with "attenuation" in the feedback loop and then drive the opposite polarity Ec input with the output from that? That would eliminate the R divider. Probably a reason not too but I was curious. Roger Even an inverting op...

Yes... I used to scale control voltages up to maybe 10dB/V and use TL074s with their 10s of mV offset and 3 uV of noise routinely in side chains. edit- we might want to terminate other control port with similar resistance to pad.. IIRC this made a difference on some old VCAs. /edit JR PS.. Yes Merry...

driving both ports is more opportunity to add noise via CV. Yes it is though if correlated they might cancel. I don't know if they spec a CMRR for the control ports but with the scale factor of only a few mV per dB I wouldn't have high expectations. Using both ports is more likely to involve uncorr...