[sdiy] SSM2164 VCAs

Tue Jun 9 18:21:13 CEST 2009

George,

Thanks for the tip about the resistor!

I also had thought about the temperature compensation, and was wondering
whether there wasn't some way to use the 2164 for an expo converter.  Then
perhaps the other half could be used to drive an integrator for a triangle
core VCO.  This may be total stupidity, as I haven't thought it through at
all, but I am going to have a look at this idea later, when I get some free
time.

> -----Original Message-----
> From: George Hearn [mailto:georgehearn at btinternet.com]
> Sent: Tuesday, June 09, 2009 3:05 AM
> To: 'David G. Dixon'
> Cc: synth-diy at dropmix.xs4all.nl
> Subject: RE: [sdiy] SSM2164 VCAs
> 
> If it is of any use I've had a good play with the SSM2164 and can share my
> experiences!
> 
> The transfer function of an SSM2164 deduced by experiment is
> approximately:
> 
> Iout=Iin*exp(-3.489*VC).
> 
> This is approximately 33mV/db so limiting the control range to 0-3.3V
> would
> yield 0 to -100db attenuation.
> 
> By using the linearized VCA combination circuit (two Lin VCAs from one
> SSM2164) you get some very nice side effects.
> 
> 1.	Temperature independence.  The VCA gain has a tiny (in theory zero)
> temperature co-efficient.  This gives it very good DC performance making
> it
> good for modulation amount VCAs etc.
> 2.	The linearized circuit has a non inverting summing control port.
> You have a VCA with the transfer function.  Iout=Iin*(Ictrl/Iref).  Where
> Iref can be fixed to give Iout=k*Iin*Ictrl.  The current inputs and
> outputs
> make circuit integration nice and easy.  Both in/out and control port are
> all ground referenced nodes with the inputs allowing summing.
> 3. 'Off' really is 'off'.  The VCA has a very high maximum attenuation.
> 
> There is also a not so nice side effect that the gain response becomes
> non-linear below a certain level.  In most applications this is not
> significant as it is at very low gain.
> 
> One other thing.  In the EDN schematic there is no resistor from IC1 opamp
> output to the control ports of the ssm2164 (pin 3,6).  This means that
> when
> the VCA is 'off' 15V is produced across the two 5k input resistances in
> parallel ( eq. 2.5k) dissipating almost 100mW of power in the device.
> This
> produces noticeable heating and can be fixed by simply adding a 9k1
> resistor
> in series with the output of IC1.  For 15V supplies a larger resistor here
> would reduce the maximum attenuation.  George
> 
> http://www.edn.com/contents/images/233728f1.pdf
> 
> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of David G. Dixon
> Sent: 09 June 2009 07:05
> To: 'Scott Juskiw'; 'sdiy DIY'
> Subject: RE: [sdiy] SSM2164 VCAs
> 
> Scott,
> 
> That's awesome!  I will have to study your circuit more closely.  I'm
> pretty
> much sold at this point on the simple switched log/linear Bareille version
> of the Gallo/Irwin circuit, and I'm thinking of just a dual VCA on a 1U
> panel, and then just cranking a few of these out as needed.
> Alternatively,
> I might do a 2U panel and put some mixing capability in there (again, a la
> Bareille, albeit a bit less complicated).  This would achieve what your
> TLN-132 can do; namely, the blending of both log and linear VCA responses
> for the same input signal.  I've got lots to think about, and it's making
> me
> very sleepy...
> 
> Cheers,
> Dave
> 
> 
> > -----Original Message-----
> > From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> > bounces at dropmix.xs4all.nl] On Behalf Of Scott Juskiw
> > Sent: Monday, June 08, 2009 10:07 PM
> > To: sdiy DIY
> > Subject: Re: [sdiy] SSM2164 VCAs
> >
> > With regards to the lin/log argument for VCAs, I don't think it's a
> > good idea to assume that straight line envelopes should always feed
> > log VCAs. I believe you should do what "sounds" best. Particularly
> > with log VCAs, there are instances where 10db/volt works well, and
> > other instances where 20 db/volt works well, or something in between.
> > I built some VCAs that support both linear and log inputs at the same
> > time which I find quite handy when I want to get really picky about
> > envelope shape. Scroll down to the bottom of this page and listen to
> > the two panning examples. One is 100% linear input, the other is 100%
> > log input (with 16db/volt response IIRC).
> >
> > http://www.tellun.com/motm/diy/tln132/TLN-132.html
> >
> > In both cases a triangle wave (very linear 0-5 volts) is used to
> > modulate the VCA amplitude. The results are quite different for the
> > linear and log cases. If I was using this in a recording I'd probably
> > use a mixture of 80% linear with 20% log.
> >
> > On 8-Jun-09, at 10:25 PM, David G. Dixon wrote:
> >
> > > Thanks Dave, Aaron and Tim for their excellent advice and links.  It
> > > would
> > > appear that the Irwin-Gallo-Lamm-Bareille axis is all I need to go
> > > forth and
> > > amplify!  It's interesting that the Irwin method of linearizing the
> > > 2164
> > > appears to be very similar to the Sims method for linearizing the
> > > 13700....
> > >
> > > To answer Tim's concern, I was going to put in switches to have the
> > > option
> > > of linear or log response, for the very reasons you alluded to (analog
> > > envelopes should feed linear VCAs, and triangles (such as my lovely
> > > triple
> > > LFO) should feed log VCAs).
> > >
> > > Now I've got to go and study (and possibly simulate) both Sims' and
> > > Irwin's
> > > schemes to convince myself that I really understand what the heck is
> > > going
> > > on there, and to determine whether it's strictly necessary for VCA
> > > applications and therefore justifies doubling the IC requirements.
> > >
> > > Cheers,
> > > Dave
> >
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