More on Fav Designs

[J.D. McEachin]

Paul wrote:

> First, no VCO has a "sound", sterile or otherwise. They may have THD,
>but a pulse is a square is a saw. But what they DO have is drift and have
>linearity errors. 

But THD and drift are such an important part of a VCO's "sound" - how can 
you just disregard them to justify a VCO's lack of "sound"?  Sometimes 
you want a clean, clinical VCO like a 3340, sometimes you want something 
whacky like a 303 VCO.  It's easier to build a VCO you like the 
sound of than to take a 3340 and selectively mess it up to sound the way 
you want in different circumstances.

Have you ever listened to the sawtooth duty cycle modulation in a 
Chroma? 

How many oscillators can do thru-zero PWM?  Thru-zero FM?  Not every VCO 
design lends itself to these capabilities, does it?

Would you consider sync to be a part of the VCO design?  Roland decided 
the sync in the 3340 wasn't "Roland" enough, so they bypassed the sync 
on the chip & put in their own sync circuit - sure enough, it sounds very 
"Rolandish" compared to other 3340 designs.

Then there's some of the VC waveshapin from Buchla & Serge...


> This is a QUICK overview. > > a) #1 overlooked component in a VCO is the
integration capacitor. You just can't chunk any old cap in there. Why? The
quick answer is that real-world caps have ESR (effective Series
Resistance), leakage, temperature drift (just like a transistor!) and
(what we worry about) something called dielectric absorbtion. All of these
contribute errors. 

> 
> The best cap to use is a metallized polypropolyne. > > b)#2 is the
integrator used in the op-amp. A perfect opamp has zero input bias
current. But in the real world, the input bias current "steals" current
into the integration cap. So what? Well, the problem is this input bias
current DOUBLES every 10C. And, for low frequencies, the input bias
current on cheesy opamps can approach the integration current. 

> 
> c)#3 discharge transistor. Best advice is use a DMOS FET, like an
2n7000. They have a factor of 10 (to 1000!) lower ON resistance. This
eliminates errors at higher frequencies. 

Woo - great stuff.


> The 3080/LM13600 > ============== > > Hey, these are OK for poking
around. Heck, I bought them in tubes of 100 for years. But they suffer
from: 
> a) control voltage feedthrough. This translates to audible 'thumps' when
the CV rapidly changes (ie fast attack)

Major pain in the ass.

> b) they like current SOURCES to control them, which are harder to manage
(but doable)

> c) their THD/noise is between AM and FM radios which is OK if you can
live with it

But they distort in a really cool way...besides, why do you need better 
than AM quality when amplifying a CV?


> I like quiet VCAs with NO feedthrough.

I like having a choice between clean & nasty.  Again, it's easier to make 
a 3080 soound like a 3080 than it is to make a CEM VCA sound like a 3080.

> 
> The 3310 EG > ========= > > All agree, this is the chip. So concern over
the availability. Gene, I GOT 1200!! Now, this is certainly enough for
now. So don't be shy using it. I will design an equivalent from discrete
later this year.

Woo! must save for a few dozen - VC EGs are cool!


 > > Power Supplies > =========== > > It's hard to beat a
723/D40xxxx pass transistor. This is a case where 'vintage' is better. I
will ask Walt Jung for permission to publish his "super high tech" power
regulators. They have 1/1000th of the residual noise of a LM7812.
Amazing!! Can be built for about $8/side. 

Did Walt ever publish this?


> > Keep the dialog rolling. > > Paul Schreiber

Amen and hallelujah, Brother!

JDM