MOTM

On Wed, 17 Aug 2005, Larry David wrote:
> I have no opinion about SMT in general so this is not a critical post - just 
> a question.  I have wondered for some time if the physics of smaller parts - 
> like SMT parts - and I guess transistors especially, causes them to respond 
> differently in VCOs, VCFs, etc, than the older, larger parts.  Perhaps this 
> is just psychological, but my OB-8 sounds more organic (more variation in 
> timbre as you play different notes - like an acoustic instrument) than say 
> the Andromeda I played a few times or the Polyevolver I played with at the 
> store.  Is there a correlation between the larger, single resistors and 
> transistors used in the earliest analog synths and their "superior" sound (I 
> realize this is subjective, but many seem to notice it) as compared to modern 
> analog synths that use SMT components?  (I know the OB uses Curtis chips so

I think I can answer this. (However, more experienced engineers should 
feel free to correct me where I'm missing something, I haven't been doing 
this long and I'm mostly a hack :)

Would an equivalent SMT circuit sound different than a through-hole 
version of the same circuit? At some level: always. The components are 
different. If any general statement can be made about SMT parts versus 
through hole parts it's that one can generally create circuit designs with 
a *much* higher signal bandwidth than with through-hole components.

A through hole resistor is more than just a perfect ideal resistor. It is 
also a bunch of little capacitors and inductors, it's just that it's 
"mostly" a resistor ;) Chip resistors have less of those effects. You 
could say that a SMT part is more resistor than the equivalent 
through hole component. This difference gets to be a problem as your 
signals get faster. Heck, at higher speeds the shape of your copper trace 
starts to be a problem :)

Would you be able to tell in any other capacity than a scope? Well, that 
would depend on how much the circuit design relied on the "side effects" 
of through hole componentry. Here's an anecdote:

It's interesting, when compared to the doepfer in my studio, my MOTM 
sounds at times like a "digital" synth. This is largely due to the 
abnormally clean signal path in Paul's designs. I like the doepfer because 
it is so damn filthy, the circuits bleed all over themselves and interfere 
in complex unpredictable ways. They both use through-hole 
components, but one is considerably less susceptible to external 
interference and as such sounds very different. The MOTM sounds very very 
clean in comparison. It all has to do with the circuit design.

To answer your question I would say that in the hands of a skilled 
designer the difference wouldn't amount to jack squat. (It might also be 
interesting to point out at this moment that *my* MOTM-300 sounds 
different than *your* MOTM-300 ;)

> it falls somewhere between say Moog modular and Andromeda).  MOTM is by *far* 
> the best sounding current production synth (analog or digital) I've heard 
> (well, my Voyager is a close second, but nothing else comes close, imho).  Is 
> this just because of the excellent ckt designs or do the components affect

In the overall gradated scale of things that ultimately determine the 
relative sound "quality" between two synths I would say that 
psychoacoustics and the circuit design play a *much-much* higher role than 
component choice :) I say this owning a MOTM, doepfer, Evolver KB, and 
various other assorted VA/A synths.

> the sound?  I.e. if Alesis or someone were to take Paul's designs and mass 
> produce them - putting the whole PCB on a single SMT chip (as much as that 
> would be possible) - would they sound any different?

Yes. That would be a very different circuit :) If Alesis where to design a 
board, taking into account component placement with respect to the original 
design, well, that would be a different story :)

> Larry David (not wanting to stir up anti-SMT trouble :)

Nah, I think it's great! It's all new, and asking questions is a good 
thing (tm) Maybe in a few years DIY'rs and other synth enthusiasts will be 
trading tips on the best toaster oven for their latest creations (black 
& decker IR oven ;)

--mikes, who enjoys hand baking SMT boards.