[sdiy] Best AVAILABLE OTA for building new designs?

[David G Dixon]

I have actually implemented something similar on my "Delta Quadrant" dual multiplier module -- a "Limiter" circuit that keeps the output to maximum 10Vpp regardless of the position of the "Quadrant" control.  For 10Vpp input signals, 2-quadrant multiplication results in an output amplitude of about 16Vpp, whereas 4-quadrant multiplication gives about 10Vpp.  With linearized 2164s it is easy to do this sort of thing, but I'm not quite sure how to do it with 48 independent VCAs taking control voltages from 48 independent ADSRs.

I think what I might do instead is implement some sort of accounting technique that counts up how many keys are depressed (active gates), and adjusts all of the VCAs accordingly based on some sort of logic.  I can simply sum a negative "adjustment" voltage into all of the VCAs in parallel.  The amount of adjustment can even be put under panel control.  I'll have to think about this a little bit.  It would have to be pretty subtle so as not to have sudden changes in loudness when keys are released.


-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Gordonjcp
Sent: Monday, August 12, 2024 11:48 PM
To: synth-diy at synth-diy.org
Subject: Re: [sdiy] Best AVAILABLE OTA for building new designs?

[CAUTION: Non-UBC Email]

On Tue, Aug 13, 2024 at 07:54:24AM +1000, Adam (synthDIY) wrote:
> 
> 
> > On 12 Aug 2024, at 11:38 PM, Mike Bryant <mbryant at futurehorizons.com> wrote:
> > 
> > I think you need to watch some videos of the late Keith Emerson.  He'd happily have twenty or more keys pressed on a Hammond, some of course held down with knives :-)
> 
> David, you might be able to use this as a "feature"...  have a front panel control labelled "Knife Factor" that switches in a compressor with auto-make-up gain... 

Given that compressors work by controlling their gain based on output level - a negative feedback loop - I have this feeling that you could probably sum all the VCA CVs to give a "total output demand" voltage, which could be fed back to the DAC Vref (taking us back around to the multiplying DAC question) to depress the output voltage and keep the total level approximately constant.

"Approximately" because you'd want it to have a bit of dynamics, you wouldn't want one note to be as loud as ten, but maybe five should be as loud as ten.

-- 
Gordonjcp

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