[sdiy] Box-O-Trix

[Seb Francis]

[CC'ed to list in case this is useful info for other people building the 
Box-O-Trix]

Romeo Fahl wrote:
> what's the deal with those 5.1K tempcos? How important is it to use 
> them as opposed to non-tempcos?
>

Well you could probably get away with not using them.  The gain-control 
input of the VCA varies +0.33%/C, and equally the gain-control output of 
the RMS level detector varies the same amount.

So it sort of cancels out, except the make up gain & threshold control 
settings then will vary by 0.33%/C.  This means that on a hot day your 
compressor could have slightly different settings as on a cold day.  
Also as it warms up the settings will change.

e.g. For a 10deg temperature change that's a 3.3% change in gain & 
threshold voltage.  What this means in terms of dB depends on the actual 
settings since for unity gain the control voltage is 0mV and thus is 
unaffected by the temperature.  If you do the maths it works out as for 
every 10dB away from unity gain there is a 1/3dB error per 10deg.  For 
example, a make up gain setting of +10dB would change 1/3dB for 10deg 
change in temperature, and a threshold setting of -20dB would change 
2/3dB for 10deg change in temperature.

So as you can see the error from using normal resistors is not very 
significant unless the temperature in your studio varies a lot.

Seb