[sdiy] Which transistors for synth circuits?

Michael E Caloroso mec.forumreader at gmail.com
Sat Jan 1 04:09:47 CET 2022


No it's an indication of design convention.  If you study the circuits of
legacy Oberheims, almost all of the PNPs are '05 - that was their
convention.  I don't remember any '06 used.

MC

On Fri, Dec 31, 2021 at 4:09 PM Mike Bryant <mbryant at futurehorizons.com>
wrote:

> Ø  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.
>
>
>
> Isn’t that more an indication of a poor design ?  If the current/volt
> response is critical then it should have been reduced with an emitter
> resistor.
>
>
>
>
>
>
>
> *From:* Synth-diy [mailto:synth-diy-bounces at synth-diy.org] *On Behalf Of *Michael
> E Caloroso via Synth-diy
> *Sent:* 31 December 2021 18:47
> *To:* David G Dixon
> *Cc:* synth-diy
> *Subject:* Re: [sdiy] Which transistors for synth circuits?
>
>
>
> 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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