[sdiy] OTA performance (was SSM chip reissue)

[Mattias Rickardsson]

Possibly - if the variation is stretched across the whole range.
Is it? I always thought that the phase-lag problem was accelerating at
the top end, not spread out linearly-ish over the whole audio range
causing something that could be perceived as a loss at the low end.
But perhaps not? :-)

/mr


On 27 April 2017 at 18:05, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
> 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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>
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