[sdiy] amateur question about modifying the ms-20 ota filter

[René Schmitz]

Hi Emily,

On 12.12.2022 11:14, Emily Straight via Synth-diy wrote:
> i'm tweaking the rene schmitz <https://www.schmitzbits.de/ms20.html> 
> implementation of the ms-20 filter for a project, and i was hoping to 
> use the emitter followers on the 13700 rather than separate op amps if 
> possible, it seems, like, vaguely wasteful not to or something. in 
> simulation this is resulting in killing the resonance on the negative 
> side of the waveform, however. is this because of a mistake i made 
> setting it up, or is the 13700 buffer fundamentally unsuited for the 
> purpose? i found one schematic 
> <https://aisynthesis.com/wp-content/uploads/2020/05/MS20v2.5.pdf> that 
> appears to do the same thing so i'm guessing the former, though idk 
> maybe theirs sucks too lol. i also put a vca in the feedback path 
> rather than a potentiometer, maybe that's causing problems? i have 
> essentially zero experience in analog electronics if you couldn't tell
>
The original circuit from Korg used LM13600 and its buffers. 
http://machines.hyperreal.org/manufacturers/Korg/MS-synths/schematics/KLM-307.GIF

They used 4.7k and 10k for the emitter resistors (4.7k in the highpass, 
10k in the lowpass section), maybe try 4.7k too.

So there is in principle nothing wrong with using them.


I used the opamp buffers primarily because I was using CA3080s which 
needs an extra buffer.

There are various mixed versions around because people built it using 
13700, some with the LEDs in the resonance like on mine.


With the darlington buffers there is a DC shift, this results in more 
"thumping", i.e. the DC shifts with the frequency control.

Opamp buffers are the cleaner solution in this regard.


I would try lowering the amplitude in your simulation, maybe you are 
exceeding the dynamic range of the right circuit.


Cheers,

  René

--
synth at schmitzbits.de
http://schmitzbits.de
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