electro-music.com   Dedicated to experimental electro-acoustic
and electronic music
 
    Front Page  |  Articles  |  Radio
 |  Media  |  Forum  |  Wiki  |  Links  |  Store
Forum with support of Syndicator RSS
 FAQFAQ   CalendarCalendar   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   LinksLinks
 RegisterRegister   ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in  Chat RoomChat Room 
 Forum index » DIY Hardware and Software
Trimless VCA
Post new topic   Reply to topic Moderators: jksuperstar, Scott Stites, Uncle Krunkus
Page 1 of 1 [4 Posts]
View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
Author Message
uniqview



Joined: Jun 15, 2020
Posts: 7
Location: Silicon Valley, California

PostPosted: Mon May 31, 2021 11:52 am    Post subject: Trimless VCA
Subject description: A good performing VCA made without any trim pots.
Reply with quote  Mark this post and the followings unread

I have updated my electronic music blog with a post about a new VCA I recently designed.

I wanted to see if it was possible to achieve the performance goals, including very small control voltage feedthrough, yet require zero trimmers. The topology I developed does this, and has excellent performance in most respects.

The circuit design would admit alternative component choices for the active devices, which are a precision matched transistor array, and a precision op amp.

To acheive the specific goals, I made certain engineering tradeoffs with passive components that other SDIY'ers might or might not find favorable to their needs or aesthetics. Much of the design relies on components matching tightly, so for that reason there are several ±0.1% resistors.

The VCA works really well in my superstrip breadboard test. It sounds great, and it's simple. It only uses 24 parts, with only 2 ICs. I didn't post scope fotos in my blog, because I wanted to test on a PCB implementation, which I've not yet done.

There's more details in my blog post, but to summarize some basic performance the VCA is designed for ±2.5V audio with DC signal path, and 0-5V linear-responding control voltage. It has excellent waveform fidelity at 20 KHz, and full power bandwidth to at least 50 KHz. I don't have an Audio Precision tester, but with FFT on my Tek scope, the main harmonic contribution is a 3rd-order spur at -45 dBc, everything else is down in the mud. I'm reasonably sure this VCA is not a high-fidelity type, and it likely doesn't have 100 dB dynamic range. But I also think the requirements for a synthesizer VCA are also more relaxable.

When controlled to be off, the VCA output measures "quiet" on a scope when viewed at 20 mV/div, with very apparent little noise. It also sounds quiet when I hook up a sound amplifier. The main reason I wanted a trimless design was control voltage feedthrough that worked -- many VCAs I've studied or built over the years have interacting trims, and control voltage feedthrough is a tradeoff. So I made a VCA topology that appears to remove audible "clicks" that come from fast control signal changes, or even the control voltage as an audio signal, when the input audio is at 0V.

_________________
Applying Bipolar Junction Transistors for New Sound
Back to top
View user's profile Send private message Visit poster's website
Blue Hell
Site Admin


Joined: Apr 03, 2004
Posts: 23665
Location: The Netherlands, Enschede
Audio files: 267
G2 patch files: 320

PostPosted: Mon May 31, 2021 1:53 pm    Post subject: Reply with quote  Mark this post and the followings unread

Looks like a nice design .. was wondering .. you state +/- 2.5V for the signals .. that seems a bit low for synth levels?
_________________
Jan
also .. could someone please turn down the thermostat a bit.
Posted Image, might have been reduced in size. Click Image to view fullscreen.
Back to top
View user's profile Send private message Visit poster's website
uniqview



Joined: Jun 15, 2020
Posts: 7
Location: Silicon Valley, California

PostPosted: Mon May 31, 2021 2:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
Looks like a nice design .. was wondering .. you state +/- 2.5V for the signals .. that seems a bit low for synth levels?


Well, that's an implementation choice I'm making for my synthesizer.

The levels could be changed to ±5V or ±10V, with simple component changes. I don't feel those signal levels are that useful, and lower signal levels are more conservative in terms of the slew-rate performance, signal fidelity, current swing, and other aspects of the output drivers.

The rationale is that ±2.5V signal levels align typical module outputs toward a RMS volume level that is much closer to +4dBU for direct use in mixing and recording. Esp. if 600Ω module outputs are terminated with 600Ω inputs into a CODEC, mixer, or whatever.

_________________
Applying Bipolar Junction Transistors for New Sound
Back to top
View user's profile Send private message Visit poster's website
Blue Hell
Site Admin


Joined: Apr 03, 2004
Posts: 23665
Location: The Netherlands, Enschede
Audio files: 267
G2 patch files: 320

PostPosted: Mon May 31, 2021 3:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ah ok, I understand, thanks!
_________________
Jan
also .. could someone please turn down the thermostat a bit.
Posted Image, might have been reduced in size. Click Image to view fullscreen.
Back to top
View user's profile Send private message Visit poster's website
Display posts from previous:   
Post new topic   Reply to topic Moderators: jksuperstar, Scott Stites, Uncle Krunkus
Page 1 of 1 [4 Posts]
View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
 Forum index » DIY Hardware and Software
Jump to:  

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum


Forum with support of Syndicator RSS
Powered by phpBB © 2001, 2005 phpBB Group
Copyright © 2003 through 2009 by electro-music.com - Conditions Of Use