Quad VCA, AC or DC coupled with normalized input

From circuitbending to homebrew stompboxes & synths, keep the DIY spirit alive!

Moderators: Kent, luketeaford, Joe., lisa

Post Reply
User avatar
Haralds:Werk
Wiggling with Experience
Posts: 342
Joined: Fri Sep 09, 2016 3:33 am
Location: Germany
Contact:

Quad VCA, AC or DC coupled with normalized input

Post by Haralds:Werk » Mon Aug 03, 2020 8:28 am

This is one more module for my quad system. It contains four VCA with normalized input, so you can distribute one input signal to four output channels. With each channel under voltage control you can move the input signal through the room dependent on the control voltages. I have two versions build one AC and the other DC coupled. Beside using this VCA's for signal distribution you can use every VCA independently. The gain to voltage ratio is linear. This is done because it is easier to predict the performance. The distribution of the signal must be steered through the control voltages. The implementation follows the recommendations from an article at Open Music Labs from October 3, 2015 (Minimizing distortion in OTA's)

Specs and features
• Four VCA with normalized input
• Usable as four independent VCA as well
• Linear gain to voltage response
• Individual Volume, CV and Bias controls
• Low distortion implementation
• Runs on +/-12V and +/-15V (with minor resistor value changes for best performance)
• Power consumption around 35mA each rail

The documentation and the Gerber files for download can be found in my website

Quad_VCA_front.JPG
Quad_VCA_main_090_schematic_01.png
Quad_VCA_DC_control_201_schematic.png
Quad_VCA_AC_PCB.JPG
Quad_VCA_main_PCB.JPG
Quad_VCA_back_view.JPG
You do not have the required permissions to view the files attached to this post.
www.haraldswerk.de
Home of Next Generation Formant

Studioalethea
Wiggling with Experience
Posts: 336
Joined: Mon Dec 23, 2019 2:20 am
Location: Seattle

Re: Quad VCA, AC or DC coupled with normalized input

Post by Studioalethea » Mon Aug 03, 2020 7:50 pm

What does the bias knob do?

I have been recently looking for a quad VCA whose behavior when normalled is actually 90 degrees phase-shifted down the line. So CV 1 (original), CV2 normalled is 90 degrees from CV 1, CV3 is 180 from CV1, etc.

User avatar
Haralds:Werk
Wiggling with Experience
Posts: 342
Joined: Fri Sep 09, 2016 3:33 am
Location: Germany
Contact:

Re: Quad VCA, AC or DC coupled with normalized input

Post by Haralds:Werk » Wed Aug 05, 2020 8:53 am

Studioalethea wrote:
Mon Aug 03, 2020 7:50 pm
What does the bias knob do?
With the bias you can open the VCA by hand. And it comes in handy when you are doing AM with a CV source which has only AC output. Say an LFO with -5V to +5V. Then you can attenuate the CV 1/2 with the CV pot and open the VCA 1/2 with the bias pot. This gives you the full LFO range without any gaps on the negative site of the LFO:
Studioalethea wrote:
Mon Aug 03, 2020 7:50 pm
I have been recently looking for a quad VCA whose behavior when normalled is actually 90 degrees phase-shifted down the line. So CV 1 (original), CV2 normalled is 90 degrees from CV 1, CV3 is 180 from CV1, etc.
Good luck. This is usually done with a quadrature LFO running the phase shifted outputs in the four CV inputs from 4 VCA. That 's what I build my quad VCA for. A suitable LFO is here.
www.haraldswerk.de
Home of Next Generation Formant

Quinie
Common Wiggler
Posts: 73
Joined: Thu Jul 09, 2015 3:14 pm
Location: Netherlands
Contact:

Re: Quad VCA, AC or DC coupled with normalized input

Post by Quinie » Sun Nov 29, 2020 6:04 am

And gerbers for panels. I have not tested these yet but the lfo fits nicely quite sure these will do also.
You do not have the required permissions to view the files attached to this post.

Quinie
Common Wiggler
Posts: 73
Joined: Thu Jul 09, 2015 3:14 pm
Location: Netherlands
Contact:

Re: Quad VCA, AC or DC coupled with normalized input

Post by Quinie » Wed Jan 13, 2021 11:08 am

Thx again Harald.
20210113_170704[1].jpg
You do not have the required permissions to view the files attached to this post.

User avatar
Haralds:Werk
Wiggling with Experience
Posts: 342
Joined: Fri Sep 09, 2016 3:33 am
Location: Germany
Contact:

Re: Quad VCA, AC or DC coupled with normalized input

Post by Haralds:Werk » Wed Jan 13, 2021 11:51 am

Very nice!
www.haraldswerk.de
Home of Next Generation Formant

Post Reply

Return to “Music Tech DIY”