[sdiy] Voltage-controllable parametric EQ and other goodies

Stewart Pye 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 
work now...

Stew.

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...
>
> Regards,
> Stewart.
>
> 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.
>>
>> Thanks,
>> Tom
>>
>> 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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