[sdiy] Which transistors for synth circuits?

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Sat Jan 1 11:04:30 CET 2022


It may be an indication of design convention. After all, reducing the 
number of different parts to stock is quite often a good idea.

But if a mere extension of spec'ed hFE range changes properties of the 
design, I'd consider this to be poorly designed. For real BJTs, the spcs 
for hFE are already pretty wide, and you never know which end you'll 
get. Sure enough, if there's a whole device family with minor 
differences in specs (for hFE it's often several overlapping ranges, or 
a few limit values for CE breakdown), it's likely they'll be sorted 
after production according to measured values, so it's not too likely 
you'll end up with parts right on the edge of specs. But still, any 
design that relies on hFE - which varies quite a lot in production, and 
is heavily dependent on bias and environmental conditions - i'd consider 
very poor design. In the end, even replacing one transistor with another 
one of identical part number, but with hFE sitting on the other end of 
spec range, would alter circuit properties as significantly as 
substituting a 2N3906 would do, or even more. The same goes for a 10°C 
change in ambient temperature. Emitter degeneration has been invented 
for a reason, and is pretty cheap to implement.

Bests,
Florian

On 01.01.22 04:09, Michael E Caloroso via Synth-diy wrote:
> 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 
> <mailto: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
>     <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 <mailto:synth-diy at synth-diy.org>> wrote:____
> 
>         Same.
> 
>         -----Original Message-----
>         From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org
>         <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 <mailto: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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