[sdiy] MIDI-CV convertor idea

[Justin Herrmann]

Hi Tom,
 
I'd put the SPICE models of the op amps you intend to use into your LTSpice simulation. Some op amps are really weak and need a light load to perform as expected. As long as U1 and the op amps that create your NoteCV1 and NoteCV2 have enough current sourcing/sinking capability, you should be okay. U1 being an ideal voltage source is what gives you isolation between NoteCV1 and NoteCV2.
 
Justin
 
 

Sent: Sunday, October 25, 2020 at 8:50 AM
From: "Richie Burnett" <rburnett at richieburnett.co.uk>
To: "SYNTH DIY" <Synth-diy at synth-diy.org>, "Tom Wiltshire" <tom at electricdruid.net>
Subject: Re: [sdiy] MIDI-CV convertor idea
There *shouldn't* be any crosstalk between the channels because the negative feedback around the op-amp on the left makes its output impedance very low indeed. It's almost a perfect voltage source. At least it is at low frequencies.

I'd be more concerned with whether the DACs that provide the other voltage sources have a constant and low output impedance over their output voltage range.

If it was me, I would use virtual-earth summing amps with separate offset and gain trimmers for each. But if I'm only going to make one of something for my own use, I'm inclined to go overkill to avoid potential issues and worry less about the pennies.

-Richie,

---- Tom Wiltshire wrote ----
 
Hi All,
 
I’d like a sanity check on something I’m working on, please.
 
I’m designing a polyphonic MIDI-CV convertor. There are various ways this could be done, but I chose to generate the Note CVs and the Pitchbend CV separately, and then add them together. This means that the Note CV only needs to be updated when a new note arrives for that voice, and the Pitchbend CV can be updated much more frequently and will affect all voices. Otherwise every time the pitchbend alters, *all* the voices need to be updated.
 
Ok, so that’s the idea. Since I don’t want the Note CVs to get inverted, I either need to use an inverting mixer followed by another inverting op-amp to flip the voltage the right way up again, or I have to use a passive mix followed by some make-up gain. I decided to try the second way to save the extra op-amp.
 
HOWEVER…a passive mix doesn’t use a virtual ground node, so there’s the potential for bleed through from one channel to another. So I did a quick simulation of the situation to see if this was a problem, shown attached. U1 takes a 0-4V signal from the Bend DAC and turns it into a +/-83mV signal. This is then fed to both mixers. The other mix input comes from the appropriate Note DAC, also 0-4V. Each mixer is then followed by x4 make-up gain, which compensates fro the /2 loss in the mixer and adds an extra x2 to turn the Note CV range into 0-8V and the pitch bend range into the standard +/-2 semitones.
 
In the simulation this works perfectly. But I don’t quite believe it. I was expecting to see some limited bleed through on Output1 from the 0-4V pulse wave at NoteCV2 (not a realistic note CV, but a good stress test). There’s nothing, it’s clean as a whistle. Can anyone see anything I’ve forgotten or done wrong? Or should I believe the sim?
 
Thanks,
Tom
 
 
 

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       Electric Druid
Synth & Stompbox DIY
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