active filter with voltage controlled amplitude - Page 1
Forum section: electronics / projects

author: Pferd
author_rank: Contributor
timestamp: August 29, 2021, 07:13:42 pm
content: Hello everyone,I am in need of some help with my circuit. My project is a guitar preamplifier and I want to be able to replace the actual potentiometers that normally set the equalizer values (Bass Mid Treble Presence) with a Control Voltage input each, so it can be controlled via a control voltage that sets the according amplitude. I hope it is somehow understandable what I want to achieve. I attach the circuit that is working an is using regular pots. It consists of two opamps for the main route and of other active band pass filters that I´ve drawn in blocks to simplify my circuit here. In this design it is working excellent because the middle position of the pot is keeping the audio signal unmodified, as it should be, and then it can be adjusted the same +dB as -dB.For the gain and volume control I´ve already successfully modified a great VCA circuit that uses standard BJTs from the page sound-au.com, project 213, I´ll add the link down below. Now I´m wondering if it can be somehow accomplished to modify my filter circuit so that it is controllable through a VCA-stage each. Would it work if I just send the audio signal through every active filter and each one gets its own VCA afterwards. then the result of them all gets mixed in an opamp and that´s it? Or is there a sleeker solution that I´m missing here? When I´m doing that way the +dB won´t be the same as the -dB, am I right?Oh, and I don´t want to use digital pots or specialized ICs for the job. I know that this would work too. And even if it´s for educational purposes only, now that I started using discrete transistors and op amps in my design I want to keep it that way:)https://sound-au.com/project213.htm---> Figure 3, the enhanced VCAThanks in advance,Pferd

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 29, 2021, 07:57:41 pm
content: You really ought to look at LM13700. It's basically a replacement for 12 discrete transistors, with better matching and thermal that you can never, ever hope to achieve otherwise. By all means, do it once or twice discrete; learn why we don't do things the tedious way, well, since... the over half a century that OTAs have been around, it's still only catching up to 60s-70s tech, it's still embarrassingly old tech, there's no shame in using them, it's still plenty anachronistic, alright?The next best thing might be a full DSP solution, and yes I see you're eschewing digital pots and such for this, but it might be a good learning project for later. Especially for effects that are essentially impossible to realize in pure analog electronics, like reverb. It doesn't need anything fancy, you can do a poor job with a mere ATMEGA; I made a passable one with XMEGA (32MHz CPU + 12-bit ADC/DAC):https://github.com/T3sl4co1l/Reverb/Possibly stock Arduino libraries on a ARM (Due, Teensy, etc.) would be enough power to do this without special code (i.e., ~half my project is AVR assembler to get enough performance to be practical; ARM is many times more powerful, plus arm-gcc actually manages to optimize its output). It's a good way to learn about what DSP is, how it works, how to write and calculate filters, etc. And how to write hard-real-time systems in software, use interrupts, etc. Definitely not a "first timer" project, but something to look at when you're ready.Tim

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2021, 09:51:12 pm
content: An alternative to an operational transconductance amplifier like the LM13700 is an analog transconductance multiplier like the AD633, but the LM13700 is intended for audio and getting better performance would be challenging. The LM13700 is the way to go at least initially.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 30, 2021, 01:17:33 am
content: Alsoconsiderablycheaper!(AD633 is good for those very rare cases when you really do need an accurate, and stable, full range (4 quadrant) multiplication. I don't have a clue what that would amount to these days -- but back before DSP was so trivial, it was hard to beat.)Tim

author: Wolfram
author_rank: Frequent Contributor
timestamp: August 30, 2021, 09:23:18 am
content: With the increasing popularity of analog synthesizers, good and reasonably cheap VCAs have become available again. Check out parts like the SSI2164/V2164/AS2164, CEM3330, AS3360, AS3364, among others. Many more can be found on the websites of Sound Semiconductor Inc, Alfa RPAR, Coolaudio and Curtis Electromusic.