[sdiy] Best AVAILABLE OTA for building new designs?

[cheater cheater]

What you want to do is to sum one note from all octaves in one op amp,
this way distortion will be fully harmonic for the most part except
for oscillating filters. Then, sum all 12 notes together.

You probably also want to add some sort of unison option that divides
the voice cards in two so that eg C5 now plays C3 in addition to the
original C3 card. You can set the unison to 2 or 4 and transpose. The
above arrangement with op amps will work well for that.

IMO you shouldn't be doing any compression in a keyboard. Being able
to vary the volume and therefore dynamics of a song by varying the
amount of notes being played is part of musical training for any
keyboard musician. Subverting that is a bad idea. Outboard compressors
exist and can be used. At least make it optional.

If you want good air circulation around your chips with as few fans as
possible, arrange your cards in a very long duct, like an Xbox Series
X. Leave at least 8mm between cards to prevent pressure build up and
use a slow static pressure fan from noctua. Note that the duct has to
be sealed, so that air doesn't seep out, or you will not get good
ventillation by the final cards.

You want the first cards to be the highest notes (get the freshest
air) and the last ones to be the lowest notes (if those drift it
sounds good). High notes drifting sounds bad, low notes drifting
sounds good.

You probably want a duct where you can unscrew the top and inspect the
cards. Add a gasket between the parts that screw together, like in a
water sealed electrical box. You can 3D print the duct, and leave
space for some rubber piping or one of those stick-on window gaskets
you can buy in a home improvement store.

On Wed, Aug 14, 2024 at 7:50 PM el macaco via Synth-diy
<synth-diy at synth-diy.org> wrote:
>
> The Alesis Andromeda has a mod source of a voltage that increases with the number of voices.
>
> I think in the tips and tricks document they mention this to reduce clipping in popy patches with loud monosynth like voices.
>
> Es
>
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> ________________________________
> From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Mike Bryant <mbryant at futurehorizons.com>
> Sent: Tuesday, August 13, 2024 4:38:15 AM
> To: Mattias Rickardsson <mr at analogue.org>; Gordonjcp <gordonjcp at gjcp.net>
> Cc: synth-diy at synth-diy.org <synth-diy at synth-diy.org>
> Subject: Re: [sdiy] Best AVAILABLE OTA for building new designs?
>
> The overload problem is one reason to go software.  Lots of algorithms available for setting the starting phases of each tone so that you are unlikely to overload the output.  Though avoid Schroeder-phase harmonic tone complexes which have an absolutely level envelope as the sound suddenly changes to a rising chirp.
> ________________________________
> From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Mattias Rickardsson <mr at analogue.org>
> Sent: 13 August 2024 10:06
> To: Gordonjcp <gordonjcp at gjcp.net>
> Cc: synth-diy at synth-diy.org <synth-diy at synth-diy.org>
> Subject: Re: [sdiy] Best AVAILABLE OTA for building new designs?
>
> Sounds like we are starting to overdesign things a bit here... :-D
> Btw the "counting notes and keeping total volume constant" idea reminds me of one of the Unison modes in Roland Jupiter-4, where you depending on the number of depressed keys get 4-voice unison, 2x 2-voice unison, 3-voice poly or 4-voice poly. The level loss in this mode is remarkable and not very nice, especially when playing a 3-note chord and getting only 3 voices playing compared to playing 1 note and getting 4 voces playing.
>
> Anyway, I don't think the all-polyphonic voice summing issue should be too over-thought, but just try to combine
> - a summing where everything fits without clipping
> - a level control that can gain even more so that clipping could occur but a melody player would have enough output level.
>
> One of the trickiest bits is that resonant filters make peak levels vary a LOT more than you first might think, so you'll have to count with a peak voice level that is much higher than many typical sounds.
>
> Hopefully this is less of a problem in a true PS3100 voice where resonances are very moderate. In the gainstaging of polysynth designs in general, there is a huge advantage if you manage to keep filter resonances limited - everything level related gets easier, but on the other hand it's hard to make a firm resonance limiter without adding unwanted distortion or side effects to the filter sound.
>
> /mr
>
> On Tue, 13 Aug 2024 at 08:49, Gordonjcp <gordonjcp at gjcp.net> wrote:
>
> 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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