DSE mini-synth rants

[Tom May]

">>>marjan<<<" <urekar.m at EUnet.yu> writes:

> Hi all,
> 
> I suppose you're all in holiday mood, but when you get back,
> take a look at this very interesting project:
> 
> articele:   vinyl.plattsburgh.edu/minisynth.html
> samples :   http://members.optusnet.com.au/~shudder/minisynth.htm

I have one of these that I put together in 1986.  About all I ever did
with it was blast guitar chords into it and let the PLLs go nuts
trying to lock on which is not very useful, IMO.  I didn't have any
clean sound sources back then.  The samples above using cleaner sound
sources are a lot more reasonable.  Just a few months ago I pulled it
out and traced the circuit since I had lost it.  I remembered there
were some interesting ideas there.  But after I traced it out I
decided the circuit realization wasn't that great and put it away
again.

> Right now, I have too many projects started but maybe I'll 
> consider this one sooner or later.
> 
> Anyway, the schematic has some errors. Here's what I've found:
> 
> R41 mic input level pot shouold have lower pin to gnd but
> all other components on the same rail should be at V/2 (R2/3
> divider) so dot on R41 gnd connection is wrong - others
> (I think) are OK at V/2.
> 
> C6 at V/2 divider should be C2
> 
> IC3 pin 14 should be connected to IC1 pin 8 output (schmitt)
> according to text but theres dot connection missing (otherwise
> there's no sense to it), but cathode of D4 is connected to IC4
> pin 12 which is wrong, I guess it's also dot on the wrong place
> (pin 12 should go to IC2,3 input pins 14 and scmitt out.)
> 
> R39 is missing from PCB overlay.
> 
> Also, if you install separate pll input for IC2, shared R11 pull
> down resistor will stay only with IC4 pin 13, so you'll need
> another pulldown at pin 12.
> 
> I don't like LFO sharing IC with amps and schmitt. I'd consider
> making two LFOs in separate 324. LFO wave is square RC filtered
> which's strange, I'd use triangle from cap, maybe use remaining
> opamps from another 324 for bufferig. Tri/square switch.
> Level pot for each LFO (before 4M7 resistors). Range sw for LFOs.
> Mix both sections into one output. Maybe simple RM (XOR)?
> 
> Now, anybody have idea how to add detune pot to one of the oscs,
> just add CV to pin 12 via res or variable res at pin 11?

Their stunt with pin 12 only works (at least I think it works)
because they're applying an AC waveform there.

> If you find some more errors, or have some nice mod, please
> share...
> 
> 
> Cheers,
> 
> marjan

fTom.