New version and (Re: Haible_Juergen's comments on VCO)

Rick Jansen sscprick at horus.sara.nl
Mon Jun 12 13:20:28 CEST 1995


Juergen Haible wrote:

> (2) The electronic switch that discharges the cap is controlled
> by 0 ...+5V, but it switches signals in the +5 ... +10V range.
> Are You sure the switch You use is capable of that?

I thought so, but the data sheet is a bit confusing. The HC(T)4066 
can switch voltages up to 12V, but it may be that this means voltages
between -6V and +6V. In the latest version the cap voltage is 
less than 5 V, with control signals of 5V.

There is also a 4316 quad switch, which contains a level translator
so you can switch analog voltages outside the range of the digital
control signals. But I have no data on Ron for this ic, so I stick
to the HC4066.
 
> (3) Personally, I would avoid coupling capacitors where ever
> possible. You have so many components already, why not
> adding a little level-shifting instead of just AC-coupling?
> Every VCO can be a very useful LFO ...

I've thought about this too. Using a capacitor seemed so easy.
I could of course shift the sawtooth down. Is this what is done
usually?  The new circuit uses level-shifting.

> (4) I would replace the current source by a simple resistor.
> I'd invest in a good on-board voltage regulator instead.

I now use an LM329 (6.9V) reference and fix all voltages
with it, i.e. the reference current, and both comparator levels.
(I'll go for the two-comparator solution, one comparator detecting
the upper, tho other the lower level.)

A bit controversial may be that the power supply for the digital
(sorry!) chips is derived with an opamp from the reference voltage.
 
> (5) You could implement linear FM by simply adding one
> resistor ... but You surely know that!

Uh, not really in this schematic? You'd have to introduce control
after the exp generator, where would you do that? 

> (6) The heater, the heater!!! No I know what someone recently
> meant with "flametorch"! You have such a good heat regulated
> circuit in Your VCF - based on this omnipresent application note -
> Why not use it for the VCO? 

Well, the sole reason is that the 3046 transistors do not have
great specs for a *very* good exp generator. The bulk resistance is
rather big, and the current gain is only about 100. So, I decided
to go for a really good matched pair like the LM394 or MAT-02.
I think flametorch is a bit overstated :) 's just that that 
piece of aluminium acts as a stabilizing factor in the whole process,
like a flywheel. So quick variations in the environment have no effect.
I'd have gone for a temperature of, say, 45 degrees. 

Now I know there is a MAT-04 I have gone back to the transistor array
version with substrate heating. The current circuit is a LM3046 one,
but is easily modified for MAT-04.

> (7) Unfortunately, You also have used the same tuning scheme
> as in FORMANT. Offset tuning and scale tuning are very, very dependant
> on each other, and this almost drove me crazy.

True, I have experienced this too. Although the real value
for the offset in the Formant VCO's isn't that important, as 
you'll tune them to any pitch you like. I haven't bothered getting
the same frequency for exactly the same position of the coarse 
frontpanel potmeter for the different VCO's. 
 
> The triangle converter looks interesting. Where do You have the circuit 
> from?
> How does it work, exactly? (must be a resistor to gnd at the input, btw)

It's a circuit that is in both S. Franco's Design with Opamps and
W.G. Jung's IC-Opamp Cookbook. It's a circuit evolved from others,
I added a short explanation at the bottom of the wave shape converter.
This is a unity-gain saw-to-triangle circuit.

There are two new gifs on the web page depicting the current state,
again, comments of course very welcome!

  http://www.sara.nl/Rick.Jansen/Emusic/VCO/VCO.html
  
    gif of LM3046 based sawtooth generator (31258 bytes) 
    gif of sawtooth conversion (29132 bytes) 

Rick Jansen
__
rick at sara.nl   http://www.sara.nl/Rick.Jansen
_____________________________________________
S&H's a module and s&h's looking good



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