[sdiy] Voltage-controllable parametric EQ and other goodies

Tom Wiltshire tom at electricdruid.net
Mon Jun 24 18:51:04 CEST 2013

Hi Steve,

"The cut/boost pots were a bit awkward" - yes, I rather agree.
I've been modelling some of the circuits in LTspice, and those "boost/cut" pots are the major sticking point.
There's the "symmetric" design, which needs a switch to move between boost and cut, as you mention. Or there's the "asymmetric", which certainly is. Neither is quite ideal, but I've not come up with anything better so far, though I've learned quite a lot, both about EQ and about LTspice!
My own take on it is to try and get a single VC parametric EQ stage into a single SSM2164. There'd be a 2-pole SVF (2 VCAs), a Q control (another VCA) and the last VCA for the "Cut/Boost". Ideally, I'd like to be able to optionally use the LPF or HPF outputs to make the channel into a shelving EQ. Then you'd be able to stick 3 or 4 of these blocks together and you'd have a full voltage-controllable EQ. Now, if I can just find a decent solution for the boost/cut. Of course, another way to go is to take a hint from another current thread, and have a simple "Amount" control for each band, like the Moog 907/914, thereby avoiding the problem.


On 24 Jun 2013, at 15:19, Steve Ridley <spr at spridley.freeserve.co.uk> wrote:

> Tom
> Good ideas - old ideas. I've seen a mid/late 1980s Calrec mixer which used these ideas - when they were still dBx VCAs - to do all the eq.
> The cut/boost pots were a bit awkward - a pot plus a comparator to operate a polarity switch as the pot crossed zero. I wish I had one of those eq cards, but I think I 
> have/had a schematic somewhere - A2 - never got around to scanning it.
> Steve
>> Message Received: Jun 22 2013, 05:47 PM
>> From: "Tom Wiltshire" 
>> To: "synthdiy diy" 
>> Cc: 
>> Subject: [sdiy] Voltage-controllable parametric EQ and other goodies
>> This is an interesting read from THAT Corp., full of good ideas:
>> http://www.thatcorp.com/datashts/AES13-031_Digitally_Controllable_Audio_Filters.pdf
>> Enjoy!
>> Tom
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