[sdiy] tempco puzzle

[Chris Crosskey]

No you don't....not for a mixer or a VCA....humans can't hear that sor of
variation in loudness.....it's not worth the hassle...if it was pitch it
would be worth doing....

chrisc

> -----Original Message-----
> From: Seb Francis [mailto:seb at is-uk.com]
> Sent: 01 October 2002 03:38
> To: synth diy
> Subject: [sdiy] tempco puzzle
> 
> 
> Hi,
> 
> I'm building a Mixer/VCA with an SSM2018 I got lying around.  
> It has a CV tempco of -3500ppm/C, so I need to build the CV 
> input stage with a 3500ppm/C tempco.
> 
> The required gain of the CV input stage is -0.25, and I have 
> 1K 3900ppm/C tempcos.
> 
> The question is .. how to do it with *minimum* extra components?
> 
> The only way I can work out uses 3 inverting apamps:
> CVs => Inverting unity-gain summer
> => Inverting unity-gain
> => Inverting 0.25 gain with the 1K tempco + 150R 100ppm/C 
> resistor in the feedback loop
> 
> I've been racking my brains for hours trying to work out a 
> more elegant way to do this, but have got nowehere (maybe I'm 
> trying too hard over 3 opamps, but it's nice to find the 
> neatest solution ;-)
> 
> Single opamp + potential divider with tempco doesn't give a 
> near linear coefficient because the gain is not small enough.
> 
> There must be a neat way to do this!
> 
> Seb
> 
> 

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