[sdiy] Unused SSM2164 cell

[David G Dixon]

Yes, Neil, I know.  I figured it out.

What someone needs to do is to derive the transfer function for this thing
and determine whether the network has any effect at unity gain with no
current input.  Until that is done, it's all idle speculation whether the
network is needed or not with an unconnected amp.

I'd give it a try, but I'm too busy catching up on school work after an
extended absence.


> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl 
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Neil Johnson
> Sent: Monday, July 20, 2015 2:43 PM
> To: synth-diy diy
> Subject: Re: [sdiy] Unused SSM2164 cell
> 
> David wrote:
> > The RC circuit at the input of 2164 is a "snubber" circuit. 
>  AFAIK, it 
> > is not for eliminating HF oscillation.  It is for protecting the 
> > transistors inside the 2164 from current switching transients.  If 
> > nothing is connected to the input, then the RC does 
> nothing.  Leave it off.
> 
> Umm....no.
> 
> To quote the SSM2164 datasheet:
> 
> "Additionally, a 500 Ohm resistor in series with a 560 pF 
> capacitor must be added from each input to ground to ensure 
> stable operation."
> 
> Or, put another way, it's a lead-lag compensator:
> http://www.ti.com/lit/an/sloa020a/sloa020a.pdf
> 
> It's obvious why it is needed once you understand the 
> internal structure and history of the SSM2164 design.
> 
> So even with no signal passing through the 2164 there is 
> still the potential for undesirable instability.  Sometimes 
> you get lucky, sometimes not.
> 
> Cheers,
> Neil
> --
> http://www.njohnson.co.uk
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