[sdiy] Spice model for CA3080 OTA

[Carsten Toensmann]

It sometimes happens in the OTA based VCF of my Formant modular system. The 
filter "crashes" and has to be "rebooted". It happens when the cutoff pot 
has reached a certain position or above. Sometimes the system has to be 
powered off, sometimes the filter starts working again when the cutoff is 
turned back (meaning anti clockwise). I would say this filter is quite 
similar to the ARP filter beside the difference that all FET transistors are 
FET opamps in my filter.

Perhaps the OTA shuts down if the controll current reaches a certain level, 
without being destroyed. So a test with a higher resistance in the control 
current might help.

Hope this makes sense...

-----Ursprüngliche Nachricht----- 
From: rburnett at richieburnett.co.uk
Sent: Wednesday, October 16, 2013 2:34 PM
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Spice model for CA3080 OTA

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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