[sdiy] OTA performance (was SSM chip reissue)

[Richie Burnett]

Lack of resonance at LF end = Too much resonance at HF end ?

-----Original Message----- 
From: Mattias Rickardsson
Sent: Thursday, April 27, 2017 4:44 PM
To: Richie Burnett
Cc: SDIY List
Subject: Re: [sdiy] OTA performance (was SSM chip reissue)

Yes, I also use phase-lead to reduce possible increase at the HF end
(even if I've had a hard time actually selecting the perfect value
there since they didn't seem to make much of a difference). But this
wouldn't explain why some units lack resonance at the LF end, and some
don't as much, would it? So I'm still clinging to the possibility of
another cause.

/mr


On 27 April 2017 at 17:26, Richie Burnett <rburnett at richieburnett.co.uk> 
wrote:
> The few analogue SVFs that I've built from OTAs & op-amps always seemed to
> prefer oscillating at the HF end of the scale.  I'd always put this down 
> to
> additional phase-lag around the loop caused by the OTAs and their op-amp
> buffers not having infinite bandwidth (instant transient response).
>
> If you take a look at this schematic of a state-variable filter from
> Roland's Jupiter 6, you can see small capacitors C107/108 that form a
> phase-lead network in the feedback path, presumably in an attempt to
> compensate for excess phase-lag at HF and even out the resonance over the
> audio spectrum:
>
> http://www.florian-anwander.de/roland_filters/JP6.jpg
>
> -Richie,
>
>
> -----Original Message----- From: Mattias Rickardsson
> Sent: Thursday, April 27, 2017 4:03 PM
> To: Richie Burnett
> Cc: SDIY List
> Subject: Re: [sdiy] OTA performance (was SSM chip reissue)
>
> On 27 April 2017 at 16:57, Richie Burnett <rburnett at richieburnett.co.uk>
> wrote:
>>>
>>> But if the gains match well at 0dB, pretty well at -20dB, and not so 
>>> well
>>> by the time you're down at -80dB, then you'd expect resonance to drop 
>>> off
>>> at
>>> lower frequencies.
>>
>>
>> Or it could do the opposite, depending which of the two integrator gains
>> is
>> lowest/highest!  Like in that paper you referenced about varying SVF gain
>> by
>> manipulating the integrator gains.
>
>
> True!
> This is intriguing. Does this indicate that the loss of resonance
> towards the bottom end is actually caused by another effect than the
> gain mismatch? Possibly capacitor leakage (e.g., through zeners) or
> something else?
>
> /mr
>
> ---
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