[sdiy] Voltage-controllable parametric EQ and other goodies
stewpye at optusnet.com.au
Mon Jun 24 23:20:30 CEST 2013
Sorry, I had another look and that won't work. You may be able to
re-arrange the summing stage so it will work though, but I have to go to
On 6/25/2013 7:12 AM, Stewart Pye wrote:
> Hi Tom,
> Couldn't you use two 2164 VCA's instead of VCA3, feed the output of
> each directly into A4 and A5 respectively, and do some control port
> pokey and jiggery to make what is essential a VC panner...
> Actually you might have to invert the signal before the VCA's, or use
> an I to V converter like A6 for both VCA outputs...
> On 6/25/2013 2:51 AM, Tom Wiltshire wrote:
>> 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>
>>> 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.
>>>> Message Received: Jun 22 2013, 05:47 PM
>>>> From: "Tom Wiltshire"
>>>> To: "synthdiy diy"
>>>> Subject: [sdiy] Voltage-controllable parametric EQ and other goodies
>>>> This is an interesting read from THAT Corp., full of good ideas:
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>>>> Synth-diy at dropmix.xs4all.nl
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