Re(2): Lin detune,MS20 Core,Polysix/Trident

[jh.]

> With a ref current of 6,8uA  and a charge
> cap of 4,7n it have a frequency range of 62Khz to 0,0074Hz!!!

Yes, you can actually tune the offset voltage of the opamps by ear:
With no input CV applied, turn the pot until the tone disappears and
you get no click for a minute ...
Or adjust linear detuning: with no CV applied, adjust offset until
you hear it clicking at the desired beat rate.

> I cant even measure it with a frequency counter, nor a scope,
> i had to sit with a stop watch and count the seconds dragging
> away!! I used just standard stuff like BC557,BC547 and a LF353
> as JFET buffer.

I guess I'll try some of my standard transistors as well then. So
far I've used the japanes transistors from the Korg schemos.
I can imagine leakage currents are an issue here, and the discharge
time will depend of the transistor type as well ...

> It seams to be quite linear to, i haven't measured yet but
> it changes its amplitude very little at the very far end of
> the frequency spectrum, Has anyone any experience of the
> Korg oscillators linearity? Jurgen?

No measurements from my side. I'm only using it over
a couple of octaves anyway; the rest is done with dividers
(as in Minikorg). But in the MS-20 it's full range, and I've
never heard anybody complain about the MS-20's linearity.


> Another thing that's puzzles me are that in the Trident and
> Polysix they just bypass each OSC with one 100n cap!!!
> The osc must be very Soft sync friendly, and that in a
> 16 VCO machine!!!

I'm reluctant to believe that it's less critical than other designs.
Maybe the single cap is combined with a clever layout that
works like a small inductance ? (I've never seen the Trident
layout - just guessing.)

> And just look at the temp compensating stuff for the Polysix,
> a vactrol adjusts the resistor division ration of the CV voltage!!
> ,also since the vactrol compensation voltage are one of the mux
> channels the cirquit compensates each CV channels Ron value and
> any temp variation that might occur in the 4051 mux. Not entirely
> perfect, but Brilliant, just damn brilliant!

Truely brilliant. And rather perfect in a practical sense as well: The
expo converter's temperature drift is fully compensated, but the
(partly desirable) linear drift of the buffers after the expo conv
is not.

> It seams that there was a old and new version of the Polysix
> the old had a discrete DAC and no mid tune cirquit but the same
> vactrol compensation, the Trident on the other hand used
> the same expo converter as the Old Polysix but no vactrol
> but instead a heated uA726 and a resistor called TH1 in
> series with the CV voltage and parallel with a series resistor.

There is one drawback of that method, of course: It's hard to
perform expo modulation on VCOs with shared expo converters.
The Trident is quite lacking here.

> Back to the Trident, what we have missed are that in the
> design of the Trident they have created a controllable
> offset voltage to the charge transistor, by pushing the
> thresh hold level more and more positive we charge the
> cap sooner and sooner, linear detune folks!
>
> In the case of the Korg engineers, they where probably only
> looking for a simple way (low component count) to have a
> detune function.If they had chosen to implement it before
> the exponential convertor they would have needed quite a lot
> of CV's and multiplexers.

You're right - I never noticed that.
I thought the linear detuning effect that gives the Trident its gorgeous
bass sound were due to remaining tuning errors (offset voltage drift),
but looking at the schemos again: yes, right, they do their intentional
controlled detuning as you described. (And with a lot of bypass caps
here, too.)
Does this mean the Trident has *only* linear detuning ? Then I'd expect
it to sound quite boring in the upper registers. I always thought the linear
detuning feature an *addition*, not a replacement, for expo detuning.
Just like the Moog Taurus: linear detuning due to offset voltage drift,
plus a Detune pot that works the ordinary (expo) way.
It's been a long time since I heard a Trident, so please correct me if
I'm wrong here. I think it's also time for me to get the full service
manual (I only have a copy of the VCO page).

> But on the other hand, they did implement the string
> section in the Trident!!!

And that wonderful trigger selector for the Brass section.
(This comes from the PS-series, btw.)

> 11:Has anyone listen real close to how close the Trident
>    osc's can beat?

Can't answer any of your questions, but #11 is the one I'd like
to get an answer, too !

JH.