[sdiy] CMOS Synth

[Osamu (Sam) HOSHUYAMA]

Hello Magnus,

I'm so sorry that I missed your name in the acknowledgment.
I appreciate your inspiring discussion on CMOS modules.

Magnus Danielson wrote:
> Ah, finally someone done what I've proposed... a CMOS based synth!
> 
> Actually, there been precursors like the Pearl Syncussion. It's a mix of CMOS,
> bipolar transistors and op-amps.

Interesting.  Is there any schematic available on the net?
I love the designs of those days by Pearl.  They are really original.

> Hmm... do you have the GX1 service manual? I'd love to read it!

I don't have one.  I had some inquiries about GX1, but the schematic on
my site is
all the information I have.  I'm sorry.

> You should look for my conceptual idea of a 4069 based CV-ADSR. There's many
> conceptual flaws in it, such as voltage range, but it is a cross-over of many
> different use of 4069. I have this toy idea of how to build a synth with only
> the 4069 as the active element (well, there's plenty of passive uses one could
> dream up as well). The restriction to 4069s is a bit more obsessive, but it is
> quite a bit of extra mind-bending which one needs at times...

Yes.  Your schematic was also very inspiring.

Another very inspiring schematic was Pearl PH-03 phaser, which gave me
an idea of smooth (exponential) and wide-range sweep for VCF.

> >   * Power consumption is large because all the amplifiers (inverters)
> >     are operated in class A.  (Heatsinks are needed for regulators.
> >     I used 2 sets of 3 terminal regulators to distribute the heat, and
> >     to reduce noise with VCF and soft sync with VCOs.)
> 
> Maybe you should run them at a lower voltage or use current limiting resistors
> on the power-feeds, possibly with caps over the terminals.

The first module, VCF (2 pole), requires 15v to obtain wide sweep
range.  
That's why I decided +-15v operation.  I should've consider unbalance
operation 
e.g. +10v and -5v.

Actually, without current limiting resistors, signal coupling through
Vdd 
caused serious troubles.  For example, two modules couldn't share one
package.  
The noise generator easily oscillated.  The reason I abandoned 4-pole
VCF was 
also coupling through Vdd.  I tried 3 structures, however, couldn't make
them 
stable.

> >   * VCF and VCA are a little noisy and have some thump with fast attack.
> 
> And the balance-trimming didn't help you? Toss a sine on the CV input and
> trim balance to minimize the signal on the output.

In the VCF, I didn't put the balance trim pot.  I'm regretting that a
little.
With the VCA, drift is quite large.  I think I should try DC feedback
technique
used in old Korg VCAs.  That's the reason I didn't complete the other
VCA, 
which I'm thinking to use for CV delay.

Thank you very much for your detailed comments.

Sam Hoshuyama
-- - ---  - -- ------ - -------- -- - ---- -
Osamu (Sam) HOSHUYAMA      houshu at rogers.com
                              Ottawa, CANADA
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