[sdiy] Transistor questions

[David G Dixon]

To clarify, it was just a simulation, using NI Multisim, that showed the 2SC945/2SA733 circuit being much faster than the 2N3904/2N3906 circuit.  I don't actually know whether that is the case or not.  In fact, I haven't even tried to calibrate the VCOs yet (I haven't had time).


 

-----Original Message-----
From: cheater cheater [mailto:cheater00social at gmail.com] 
Sent: Sunday, March 02, 2025 10:02 PM
To: David G Dixon
Cc: synth-diy at synth-diy.org
Subject: Re: [sdiy] Transistor questions

[CAUTION: Non-UBC Email]

maybe it's a highly inductive current source, i.e. the breadboard you
put this in. solder it up and see if it changes. you don't even need a
pcb, just solder up a spider.


On Mon, Mar 3, 2025 at 6:21 AM David G Dixon via Synth-diy
<synth-diy at synth-diy.org> wrote:
>
> Hey Synth DIY Team,
>
> I have built sawcore VCOs with two-transistor cores based on the Korg PS-3100.  I initially used 2N3904 and 2N3906, and they worked great, but simulations suggested that the saw reset time would be around 30ns, which was more than I wanted, because I want perfect tracking up to about 6kHz without high-frequency compensation.  Hence, I tried many other transistor models, and settled on 2SC945 and 2SA733.  My simulations suggested that the reset time would only be 2 or 3ns, and this is fast enough.
>
> My question is this: given that the 2N390X transistors have higher transition frequencies and lower collector capacitances than the old Japanese transistors (according to All Transistors), why should the latter give significantly faster saw reset times?  What is the actual figure of merit for this?
>
> Cheers,
> Dave Dixon
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