[sdiy] Question about my high freq compensation

[Ray Wilson]

Hi Oren

Thanks for looking at the circuit. You are absolutely correct. The source
for this circuit originated in Musical Applications of Microprocessors pg.
194 top schematic and you are 100% correct. I am going to have to correct
all of my oscillator schematics and PCBs. All I can say is "Oy Yoy Yoy" and
thank you. Grant Richter also suggests that the diode also be in thermal
contact with the transistor pair package.

Thanks to both of you for checking it out.

Ray


----- Original Message ----- 
From: "Oren Leavitt" <oleavitt at ix.netcom.com>
To: "Ray Wilson" <raywilson at comcast.net>; "Synth-Diy"
<synth-diy at dropmix.xs4all.nl>
Sent: Tuesday, December 09, 2003 1:08 PM
Subject: Re: [sdiy] Question about my high freq compensation


> Hi Ray,
>
> It looks like R13(2) will have to be connected to the output of the
exponential servo (IC1-A) instead of that of the CV summer (IC1-B) for this
to work correctly.
>
> In looking at the VCO core, with an MPF102 JFET with a few hundred ohms of
'on' resistance and a 0.02uF timing cap, the discharge time may be quite
large, requiring alot of HF compensation.
> The discharge time, the pulse appearing at (syo), should ideally be less
than a microsecond.
>
> HTH,
> Oren
>
> -----Original Message-----
> From: Ray Wilson <raywilson at comcast.net>
> Sent: Dec 8, 2003 11:09 PM
> To: Synth-Diy <synth-diy at dropmix.xs4all.nl>
> Subject: [sdiy] Question about my high freq compensation
>
> Hi List
>
> I would greatly appreciate it if some of you really smart people would
look
> at my oscillator here:
>
>
http://www.musicfromouterspace.com/analogsynth/OctVCO_LM394_FixedSquare.html
>
> and let me know if I have goofed on the high frequency compensation.
>
> My explanation (which may be incorrect is this) is this:
>
> Here is what I think is happening. I know the circuit looks wierd but  you
> must remember that as the voltage gets higher on the inputs of the
inverting
> summer made up of IC1-B and associated resistors that the voltage on the
> output of IC1-B gets lower and lower. I think the high freq compensation
> works by adding a secondary path for current to pull the base of the top
> transistor a little lower as the input voltage gets higher and
subsequently
> the frequency gets higher and higher so that the discharge time of the
> integrating capacitor (which becomes more significant at higher
frequencies)
> is compensated for by causing the oscillator to oscillate a bit faster
than
> it would without the secodary current path.
>
> Please let me know if I have goofed so I can fix it or if I got it right
in
> which case I can have a beer and celebrate.
>
> Thanks and Cheers
>
> Ray
>
>
>
>
>
>