[sdiy] OTA performance (was SSM chip reissue)

[Mattias Rickardsson]

Considering using the "gain side" of the 2164 CV in the top end of filters:

I probably shouldn't have suggested this. I looked back in my
measurement experiment notes and wouldn't really recommend it, since
the noise grows faster than the gain grows on the "gain side". The
filter would be more noisy when open. In the 2164 the best SNR is
reached with maximum signal into a roughly 0 dB gain setting.

/mr


On 27 April 2017 at 19:20, Tom Wiltshire <tom at electricdruid.net> wrote:
>
> On 27 Apr 2017, at 16:23, Mattias Rickardsson <mr at analogue.org> wrote:
>
>> On 26 April 2017 at 11:51, Tom Wiltshire <tom at electricdruid.net> wrote:
>>> On 26 Apr 2017, at 09:06, Roman Sowa <modular at go2.pl> wrote:
>>>
>>>> At low cut off frequency, when integrator currents are really small, Zeners are damping the oscillation, because most of the current is eaten up by the zener, not the cap - at that small current their voltage is way much smaller than, say, at 1mA, the top of VCF frequency.
>>>
>>> I've got some evidence this is true. I built an 2164 SVF and swapped the 30/33K input resistors for 15K, thereby doubling the currents. This improved the situation somewhat, but it's only a octave's worth of improvement. But running the filter "hot" helps.
>>
>> I just swapped the 30k resistors to 15k for another reason: to double
>> the signal level and get the noise down almost by half. If it also
>> reduces any effect of zener leakage it's a nice bonus! :-)
>>
>> Also, I guess raising the gain (with lowered resistors or increased
>> VCA gain) would imply that the integrator capacitor values be
>> increased accordingly - which would also reduce the leakage problem,
>> right?
>>
>> But note that 15k is too low input resistance for signals above 7 V.
>> Clipping might occur, and in the 2164 clipping isn't something you'd
>> want. The SSM2164 datasheet uses 30k for maxing out the signal with
>> +/- 15 V supplies, but with full signal swing using 15k would only be
>> safe with +/- 8 V supplies.
>>
>> Speaking of "running the filter hot"... I usually just use the
>> "attenuation side" of the 2164 CV in SVFs, letting a CV of 0 V (which
>> gives 0 dB gain) set the maximum frequency and then the audio range is
>> covered by positive CVs of roughly 0-2 V (giving lower gains of 0 dB
>> to -60 dB). Would it be worthwhile to use some of the "gain side" of
>> the 2164 as well, like perhaps 15 dB to -45 dB, bringing the
>> well-speced optimum 0 dB point into the audio band and avoiding the
>> most imprecise high-attenuation regions? I tend to avoid it in filters
>> due to the rather uncontrolled THD that slowly grows on the "gain
>> side"... I get a bad feeling allowing this to happen in the highest
>> frequency region, but how far do you guys usually go?
>
> The same thought had occurred to me as a result of this discussion and a note in a THAT Corp datasheet I read recently - perhaps try some gain as well as some attenuation. I haven't tried it, and usually organised things for 0dB on down. If you're building a filter, you don't need much more than 66dB gain range anyway (11 octaves roughly covers the audio spectrum), so we're not pushing the chip to its lowest point.
>
> Tom
>
>
>