[sdiy] CMOS Synth

[Magnus Danielson]

From: "Osamu (Sam) HOSHUYAMA" <houshu at rogers.com>
Subject: Re: [sdiy] CMOS Synth
Date: Mon, 03 Feb 2003 11:35:08 +0900

> Hello Magnus,

Dear Osamu-san,

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

Oh, THANKS!

:-) <- Just a happy smile! ;O)

> 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?

Not yeat! ;O)

(Prohibited to say, but there is a reason for a Syncussion being on my table
disassembled... )

> I love the designs of those days by Pearl.  They are really original.

Indeed.

I love their OTA/integrator setup from which they make both triangle/saw
oscillators and filter cores. There is also a very interesting feature about
the Syncussion filter I will love to come back to...

> > 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.

Well, don't blame me for asking! ;O)

> > 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.

Ah! So you've seen it! ;O)

I've gotta clean that thing up after a try-out.

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

Hmm... got to have a look at that schematic then. Don't have it.

> > >   * 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.

Hmm... which makes me think... isn't there a way to counter-act on that or use
it? Sure there is... I've just got to figure out the design for it ;O)

> > >   * 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.

Yes, DC feedback could be a way to deal with it.

If you got DC drift problems, then you certainly needs to handle those,
trimming isn't going to solve it I realize. Trimming is just good for static
errors which is fairly unstable with temperature, supply voltage and signal.

> Thank you very much for your detailed comments.

My pleasure, my pleasure!

Cheers,
Magnus