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Discrete ladder filter calibration issue
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author Discrete ladder filter calibration issue
Pav
Hi ,
P2 and p3 calibrations show my NPN transistor matching was ok but P1 only Trimmed to 1.7mv. The user guide suggests the matched pair of PNP might be poorly matched.

Ok I can accept that..the PNP pair in question was my first attempt at matching...though I matched to 0.5mv throughout...I may have picked up the wrong pair.

My question is what if anything is the impact and recommendation in this scenario?
Implicit I guess is to replace.

..filter sounds excellent. Id rather not remove and replace the offending pair as chance I do some damage.

Also I had to use a TL 072CP for U3 temporarily as I only had 2 AC variants. Could that have had any adverse effect?

Thanks in advance
Synthbuilder
Pav wrote:
... P1 only Trimmed to 1.7mv.


That's fine. You only have to worry about it if it gets into the tens of millivolts.

Quote:
though I matched to 0.5mv throughout...


Yes, but you are only matching at one current. Vbe between the transistors in each of the pairs varies with emitter current and temperature. So you may well have perfect matching at one value of emitter current but not at another. However, this is nothing to be concerned about in this application. All we need to be as close is as we can be.

Quote:
My question is what if anything is the impact and recommendation in this scenario?


Keep them as they are. Excessive mismatching will cause an unwanted DC offset on the Q2 output relative to Q5. This can make the filter sound different when driven hard.

Quote:
I had to use a TL 072CP for U3 temporarily as I only had 2 AC variants. Could that have had any adverse effect?


It'll be fine. The ACP, with its slightly lower offset, is useful for U1 but in the other two locations you can use any unity gain stable dual op-amp.

Tony
Pav
Thanks for the insght Tony .
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