[sdiy] Spice model for CA3080 OTA

[rburnett at richieburnett.co.uk]

If the output of the filter sometimes mutes and sticks to one of the 
rails, I'd hazzard a guess that there might be an op-amp in there 
somewhere who's output undergoes phase-reversal when one of the inputs 
goes outside the normal operating range.  A normal cascaded OTA resonant 
filter has negative feedback around it to achieve resonance, but if an 
op-amp anywhere in this loop undergoes phase-reversal then this will 
become positive feedback.  Positive feedback at DC will drive the output 
to one of the rails and keep it there.

Just a thought, but without schematics it could be almost anything!

-Richie,

On 2013-10-16 13:21, Pete Hartman wrote:
> Part of the issue I'm having is while I think I theoretically
> understand what's going on (OTA -> capacitor -> buffer x 2) I don't
> have a lot of practical experience with FETs and feedback paths yet to
> be certain what I *should* be seeing.  So I want to simulate it and at
> least get a better idea.  The caution about SPICE is a good one
> though, I'll keep that in mind.
> 
> As for schematic the main differences between this and the original
> ARP 4023 schematic are a trimmer between the rails with a 475K
> resistor on the wiper tied to the control voltage input, and using
> OPA134's instead of LM301's.  The original doesn't show a value for
> the balance trim, but this is using a 50K.  The output has a DC
> blocking capacitor and a 1K resistor, and there is additional
> circuitry for attenuating/summing audio and control inputs, but none
> of that is surprising and all of it appears to work fine.  I'm
> hesitant to share the actual schematic, but I'll check with the PCB
> manufacturer.
> 
> I appreciate everyone's input and the models, I have what I wanted,
> now I just have to find time to apply it (hopefully before the
> weekend).
> 
> Thanks
> 
> Pete
> 
> 
> 
> 
> On Wed, Oct 16, 2013 at 7:02 AM, Neil Johnson 
> <neil.johnson71 at gmail.com> wrote:
>> Hi,
>> 
>> Pete Hartman wrote:
>>> I'm looking for a CA3080/LM3080 Spice model
>> 
>> Don Sauer has probably more detail on CA3080/LM13600/LM13700 SPICE
>> models than you need:
>> 
>> http://www.idea2ic.com/LM13600/SpiceSubcircuit/LM13700_SpiceModel.html
>> 
>> But to be honest I wouldn't use SPICE to debug this circuit.  Sit down
>> with a scope, schematic, and some quiet time to think through what it
>> actually happening, and why that is different from what should be
>> happening (you won't know if SPICE is lying to you).
>> 
>> Can you share the schematic?
>> 
>> Neil
>> --
>> http://www.njohnson.co.uk
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