Console VCA bypass ALMOST working. Please help!

[jdurango]

Hey fellas. Working on a popularly suggested VCA bypass for my Sound Workshop Series 40 console. I've got some Alps Japan 10ka faders  which measure around 9.3k with a DMM, so I'm assuming 10ka means 10k impedance audio taper.

Basically, at bottom of fader travel, I'm still getting about 100mVPP with roughly 7vPP @ 1k coming into the fader using mic input.

Talked to a tech who originally built these consoles. He said fader audio boosters and buffers are on a separate card and no extra circuitry should be required to bypass VCA's, but he wasn't sure of correct impedance.

Here's a vid with details

https://youtu.be/k7vxAUihiJ4

I've attached the best schem I having showing vca/fader amp that is still being utilized with the bypass active, although schems for the VCA which was removed won't be in til some time this week. Any idea what's going on? Adding a 4.7k resistor to input cuts signal to fader down to about 4vPP but significantly lowers output ripple at bottom of travel. Doubt this is a tenable solution though. Watcha think? Thanks fellas!

 

[synthiaks]

Schematic?

Are all channels the same? Without knowing how the circuit looks for the Fader Gain my first guess would be to look there.

Maybe there is some Differential action that doesn't happen the way it should there.

 

[jdurango]

Schematic?

Are all channels the same? Without knowing how the circuit looks for the Fader Gain my first guess would be to look there.

Maybe there is some Differential action that doesn't happen the way it should there.

Just attached a schem of the most relevant circuit I can find to the original post. I don't currently have schems for the VCA that was removed, but this shows the fader gain amp. And yes, all chans are exactly the same.

I see when I added that 4.7k resistor to the fader input, I effectively increased the resistive load on the fader, possibly that helped match impedance better? I could decrease value of feedback loop resistors on the booster amp to make up for loss at the input, but I'd also be amplifying the same post fader leakage I'm trying to get rid of.

Thanks for the help!

 

[synthiaks]

Well if all of them are the same I guess it must be something that the circuit now  lacks when the VCA is gone.

 

[abbey road d enfer]

This symptom really suggests a weak ground connection. I see that the fader ground seems to circulate via several connectors, maybe they are oxydized? For verification, you may try to reinforce the ground connection with a wire and alligator clips from the fader ground to the negative of C13.

 

[jdurango]

This symptom really suggests a weak ground connection. I see that the fader ground seems to circulate via several connectors, maybe they are oxydized? For verification, you may try to reinforce the ground connection with a wire and alligator clips from the fader ground to the negative of C13.

Ground was one of the first things I checked. It's definitely solid, but it's not going directly to the inverting in ground on the fader driver amp. I'll try wiring it directly there as you suggest. That's a good idea!

Also checked mechanical travel. Removed screws entirely and operated fader outside channel strip frame/chassis. Same result.

 

[jdurango]

I understand some faders actually have a little switch at the bottom hooked to seperate wires coming out, that shorts the wiper to ground specifically to prevent signal from not completely cutting off.  Often 6 wire faders. Mine are basic 3 wire pots.

 

[radardoug]

David Kulka gave you the answer over on Gearslutz, so why are you double posting? Your faders aren't good enough. End of story.

 

[jdurango]

David Kulka gave you the answer over on Gearslutz, so why are you double posting? Your faders aren't good enough. End of story.

A) I just found that post.

B) How is it that even cheap $0.50 pots don't suffer from this same problem.

If you're not trying to actually be helpful, please don't reply.

 

[pucho812]

David Kulka gave you the answer over on Gearslutz, so why are you double posting? Your faders aren't good enough. End of story.

can you link I would love to see what kulka said...

 

[radardoug]

If cheap pots dont suffer from it, then that points it straight back at the faders you have. Have you tried a Penny and Giles there?

 

[synthiaks]

B) How is it that even cheap $0.50 pots don't suffer from this same problem.

This is kind of vital information when asking for help...

With part A everything works as it should, when I change part A to part B it does not work anymore.

 

[jdurango]

This is kind of vital information when asking for help...

With part A everything works as it should, when I change part A to part B it does not work anymore.

I didn't say I tried substituting a cheap pot, just that they generally don't exhibit this behavior, meaning it's unlikely to be a problem with the faders themselves. They're not top of the line P&G, but there's no reason they should perform any poorer than a cheap rotary pot. If that logic held, every console that didn't have P&G faders and Bourns pots on every channel would be leaking signal all over the place.

 

[jdurango]

can you link I would love to see what kulka said...

Yup, here you go. Figured it was easier to paraphrase than copy and paste the whole explanation:

https://www.gearslutz.com/board/geekslutz-forum/1069219-console-master-fader-wiring.html

 

[synthiaks]

Yup, here you go. Figured it was easier to paraphrase than copy and paste the whole explanation:

https://www.gearslutz.com/board/geekslutz-forum/1069219-console-master-fader-wiring.html

Well, there you go then.

 

[jdurango]

So being unwilling to accept that all but the very best pots leak a significant amount of audio I tried cleaning several of them and retesting out of circuit with a function gen and scope. Everything seemed fine so I popped one back in circuit after testing Vin and Vout pre, post fader and post booster IC. Works perfectly now and has less noise than the VCA's! Only about 20mv of noise with gator clips and no discernible signal leak even after the LF351 10x makeup gain.

After cleaning and faderlube, these Alps faders feel great! The P&G on the VCA's are great quality but are like sliding around on ice.....almost feels like they'll move if you sneeze. The Alps remind me of the nice smooth steady movement of a Manfrotto fluid filled tripod head, if anyone is familiar with that. Provides a little smooth gliding resistance which makes jerky/abrupt movement less likely.

I do love the covers over the fader mechanism on the P&G that protect crap from falling down inside them, but it also makes them a PITA to quickly clean vs the Alps. The Alps also have solder lugs for wires whereas the P&G do not. Overall, I think I'll be very happy with this mod.....and got all 48 faders for about $10 in a salvage Soundtracs console....so definitely fits my budget

 

[abbey road d enfer]

So, all's well that ends well...