[sdiy] what opamp for "warmth"? (I know, I know...)

René Schmitz synth at schmitzbits.de
Sun Apr 9 15:36:45 CEST 2023

On 09.04.2023 01:52, Richie Burnett wrote:
>> I couldn't find the 2SD1469 ... what's the part designation on the schematic?
> Q47 and Q48 underneath where it says "Snare Drum" on the Voicing Board schematic.
> I noticed that people online building hardware comes of the 909 snare drum voicing circuitry recommend fitting  ZTX1051A devices in these locations. That is a 4A rated switching transistor! Apparently a general purpose small-signal NPN transistor here doesn't give the right sound, but they don't say why!

I wouldn't interpret too much into this. Has someone really made a 
double blind study with statistical significance?

Our ears are so easily fooled by our expectations.

The ZTX seems to come from the 909 clone of Trevor Page.

He writes:

"A number of transistors were tried in place of the 2SD1469; ZTX1051A 
worked the best.
These transistors are available from Rapid Electronics in the UK."

I read that as: "I use it because I liked it and I know where to get them".

The rest is copy pasta. RE-909 mentions the same part. The origin story 
eventually gets lost and myths are created.

BC635 would be a closer match, and would have the same pinout.

The reason to use a switching transistor could be it's lower Vce(sat). 
Which would maybe reduce the DC offset / feed through.

Then again these tend to have lower beta, which makes more DC feed through.

As we said before, most of the artefacts would be masked anyway, and 
hence I can't see how it would make a significant difference to use this 
extra part.

That it potentially makes a difference now, when you try to recreate the 
909 sound (or as said above maybe it's actually "season to taste") is a 
different question,

compared to back in the day where the designers had the freedom to 
define what that sound is to begin with.

Why they used different parts in different places?

Totally hypothetically:

The designer originally intended to use 2SD1469 for all these VCAs.

Then at a time it was realized they could use the same jellybean NPN.

And overlooked to change the ones in the snare. (Maybe this was done 
with pen and paper still...)


There were several people working on different parts, and the guy doing 
the snare had different preferences then the one doing the bass or clap.


The designer changed his mind mid way, and didn't want to re-test parts 
that had already been finalized.

What I'm trying to say is, that there are possibly more mundane 
explanations for certain part choices.



synth at schmitzbits.de
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