Parallel Signal Path for VCA Compressors (eg S S L)

[matthias]

no prob..

for full dry signal I'll have a bypass switch...

wet/dry pan-like control would be nice, but if it would be possible to do it without an extra buffer I'd go for the dry-mix-control.

unfortunately it seems that I'll need 2 extra buffers in any case...

I think now it's time to build a prototype.

is that ok ?

 

[Boswell]

The circuit is now fine for the 0 - 100% dry addition version.

It you want the wet/dry wipe (janitorial configuration), take the grounded end of the pot R10 and put it instead on the output of OP1. The OP4 buffer stays, but drives the output coupling capacitor (C2), which no longer connects to OP1. You lose OP3 and its associated components R20, R18, C4, and R21.

For consistency, the gain of the input INA137 is 1/2 or -6dB, and that of the output DRV134 is 2 or +6dB.

 

[matthias]

thanks,

but for the wet/dry configuration I would still need an non-invertnig buffer between the summing point and the drv134 input?

I would do it like that:

mat

 

[Boswell]

For the pan pot configuration, you still need the buffer, with gain, as you have drawn it. However, you also need a pair or resistors between the outputs of OP1 and OP2 and the respective ends of panpot R4, otherwise you short out the op amps at the ends of the resistor travel.

For myself, I can't see that the extra complexity and gain needed for the panpot configuration is worth the effort over the simple wipe. You won't get the standard panpot feel in this circuit anyway because of using a non-inverting amplifier at the output (which you need to preserve the phase) as opposed to the virtual-earth configuration of the standard panpot.

An alternative cunning plan is to change OP3 to virtual earth (adding the extra resistors between the pot ends and the driving amplifiers) and interchange the DRV134 outputs (pins 2 and 3 on the output XLR) to correct the phase inversion. The component values given in
http://www.groupdiy.com/index.php?topic=8360&highlight=crush+blend are relevant once more.

 

[DrFrankencopter]

I'm a little late to this thread, but why can't you just put in a 10k pot wired as an attenuator just after the buffer/inverter in the dry path, then feed the attenuated signal to the op amp just after the VCA as a virtual earth summer (make the dry path summing resistor 10k)?

It won't go full dry...but I don't really see the point of being able to do that anyways.

OR, how about putting a VCA in the dry path too, summing its output with OP1 (virtual earth). Set the dry path VCA up so that is has a bias on its CV to keep it at -120dB gain when your blend pot is at the 'wet' extreme. The 'wet/dry' pot then makes a CV that gets summed with this bias. Invert this CV and sum it with the CV going to the wet path VCA, and you should be able to blend between wet and dry. You might be able to get away without inverting the CV by putting it on the CV- port of the VCA (not sure if VCAs can be driven from both CV+ and CV- simultaneously).

Cheers,

Kris

 

[matthias]

Hi Kris,

you mean something like this ?

 

[DrFrankencopter]

Yeah....something like that.

There's probably some reason why it won't work....I'm curious to see what the comments are.

Cheers

Kris

 

[Crusty2]

I went out to the shop and tried it with a single inverter after the pot, summing with a 10k. It worked great! In fact too much effect range for me. A little goes along way. I think I'll lower the range a bit. Probably shoud add a cap too.

Neat effect, really seems to change the sound of the compressor somehow...

 

[Boswell]

Yes, that will work, as you have found out. The only quibble I would have is that the pot is not linear in this configuration, but if that doesn't matter, it's a minimal parts solution.

If you want to decrease the available dry signal in the mix, you could pad the top of the pot R22 with another resistor, or you could increase R20/decrease R18.

By the way, what are you using to drive the VCA control voltage?

 

[Crusty2]

I have these cards that my uncle gave me. He uses them as a control card in a compressor he makes. It works on its own too, with a few other parts. I've been helping him test it and stuff boards and such. It has the 4301 on it, and I've been experimenting on my own with it. I've blown three up so far (!), but he has tubes of them...