[sdiy] Which transistors for synth circuits?

[Michael E Caloroso]

Designers have the freedom to use their favored transistors.

For substitution in existing devices, some circuits are sensitive to
transistor selection.

In vintage synths, the voltage to current converter for the iABC input of
the OTA based SVF is one of them.  Scott Rider was designing an OBX
voicecard clone and when it was installed in my OBX the filter sounded
different.  We thought it was the 13700 in place of the 3080 but study of
the databook spec showed the control law was the same (the 13700 was
configured the same as the 3080).  Scott substituted the 2N3906 in place of
the 2N3905 on the original, so that was my next suspicion.  When I compared
the '06 against the '05 specs in my Fairchild databook, I found that the
'05 had a lower HFE range than the '06.  Sure enough, when the 2N3905 was
put in the voicecard clone now sounded the same.  The 2N3906 caused the SVF
cutoff scaling to change because the higher HFE range resulted a
steeper current/volt response, thus it was putting out higher current to
iABC for the cutoff control voltage.   That's why the filter sounded
different.

Other crucial parameters are max iC, min vCE, etc.  You need to know the
limits of your circuit, no single transistor substitutes in every circuit.
There are some VCA designs out there that rely on high beta PNP
transistors, which are not easy to acquire.

MC

On Fri, Dec 31, 2021 at 12:56 PM David G Dixon via Synth-diy <
synth-diy at synth-diy.org> wrote:

> Same.
>
> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Tom
> Wiltshire
> Sent: Thursday, December 30, 2021 2:55 PM
> To: cheater cheater
> Cc: synth-diy
> Subject: Re: [sdiy] Which transistors for synth circuits?
>
> [CAUTION: Non-UBC Email]
>
> I don't.
>
> I'm a lame-arse about stuff like that. I use 2N3904 and 2N3906 for
> basically
> everything unless there's some absolutely fundamental reason why not. It's
> amazing what you can get away with. If you were *really* concerned you
> could
> match them, but to be honest if you grab a few from the same bag you
> probably won't notice.
>
> Previously I used to use BC547/557 the same way. I don't even remember why
> I
> swapped over. It might have been the pinout rather than anything more
> important.
>
> > On 30 Dec 2021, at 18:18, cheater cheater via Synth-diy
> <synth-diy at synth-diy.org> wrote:
> >
> > Hi all,
> > I was wondering how you pick out transistors (as well as diodes) for
> > synth circuits like filters. For example moog filter, 303 filter, etc.
> > Do you look for anything in specific? I'm talking about a situation
> > where you don't want to use the exact part in the schematic, but want
> > to play around with parts and see what comes out of it.
> >
> > Best regards
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