Protec V1572 Headphone-/Control-Amp Problem with Volume Regulation

[Trucki]

Hello !
The V1572 is a Euro-Format-Plug-In-Card for use as a Headphone Amp or small 10W StereoSpeaker Amp. Maybe some German Users know it.
It has the possibility to Remote Control the Volume via DC  0-5V. There is a 2150 VCA for this Task. On Board are two 5V Regulators to provide the Control Voltage.
I dont know how to Connect a Poti to realise the VolumeControl.
I hope someone have a Idea !
I attach a bad low resolution scan of the schematic. The red labeled are the Control Inputs for the DC-Voltage and the green are the VCAs.
Thanks for any suggestions,
Greetings Thomas

 

[Bo Deadly]

Just like the "Gain" pot. At least it looks like there's a trimmer there called "Gain". Just replace that with a pot with a knob. Although you probably want some flanking resistors to limit the gain / atten to something like +20dB / -80dB or whatever.

Then you might consider just disconnecting the circuitry on the other control port and ground that pin. That might reduce feed-through noise.

 

[Trucki]

Thanks for your Suggestion squarewave, but i dont want to change anything on the Card. I need the Gain Trimmer, because of its Original Function as a step-up amp to feed through to the next Headamp or something . And further i need a real Remote Control of the Level. A external poti with the Audiosignal fed through 40 meters of cabel isnt the real thing i think.
At the End it should work as it was meant to be, with a DC Control Voltage...
Any other Ideas ?

 

[Bo Deadly]

40 meters! Yikes. Not sure about that but I can give it a try.

In general, you just feed a voltage into one of control inputs in red. It doesn't matter which one.

However if you want a potentiometer on the end of a cable there are some potential issues. One is that VCAs are sensitive to feed-through noise. Meaning any noise in the control voltage will feed-through to the output of the VCA. So you will probably need shielded cable. A capacitor on the control input might also help depending on how quickly you want the VCA to react. You could even go so far as to make the control active.

Possible solution: Get a small die-cast pedal box with a 10K potentiometer wired to an XLR male output and connect pins 3 and 1 across the pot and wiper to pin 2. Then add an XLR female plug with a cable running into the unit with pins 3 and 1 connected to previously described "flanking" resistors. Pin 3's flanking resistor connects to V+ supply. Pin 1's flanking resistor connects to 0V. Then the wiper to the control port. Just don't leave out the flanking resistors. The one on pin 3 connected to supply limits current in the event of a short. Depending on which control port you connect to might dictate reversing pot wires 1 and 3 to get CW rotation to increase level (as opposed to decreasing).

If this is for a perminent installation, some foil shielded control cable could be fine. But 40 meters is pretty far.

Although it's been a while since I played with a VCA. This could all be nonsense. Try it on a breadboard first.