pucho812
Well-known member
for those that do not have and would like to DIY some.
http://www.waltzingbear.com/Schematics/API/API_2520_schematic.pdf
http://www.waltzingbear.com/Schematics/API/API_2520_schematic.pdf
A PEE EYE vienty Vient y cinco Schematic located in here
- Thread starter pucho812
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featherpillow
Well-known member
Interesting.
I've never looked at a schematic for one of these before. That input pair looks relatively slow. It seems there's a 10% imbalance in current between the collectors of Q1 and Q2, which would give rise to a large amount of THD if I understand it correctly.
That THD would be all 2nd and 3rd order, though, and it would rise with frequency if I'm not mistaken.
I hear that this opamp has a characteristic midrange "growl." I think that would explain at least some of it, the rest being attributed to the imbalances between the collector currents of Q3 and Q4, and whatever nonlinearity comes about in the VAS.
I've never looked at a schematic for one of these before. That input pair looks relatively slow. It seems there's a 10% imbalance in current between the collectors of Q1 and Q2, which would give rise to a large amount of THD if I understand it correctly.
That THD would be all 2nd and 3rd order, though, and it would rise with frequency if I'm not mistaken.
I hear that this opamp has a characteristic midrange "growl." I think that would explain at least some of it, the rest being attributed to the imbalances between the collector currents of Q3 and Q4, and whatever nonlinearity comes about in the VAS.
SSLtech
Well-known member
...the "trim" pin is to allow a 'fariable' resistance (one that the pixies tweak for you!) to be placed in parallel with R4, presumably balancing the current in the input pair, trimming the output offset and reducing the distortion if that's indeed the mechanism...
Or so I think...
Keith
Or so I think...
Keith
featherpillow
Well-known member
That would allow you to adjust or eliminate distortion in that input stage LTP . But it's interesting that the value of R7 is shown in the schem as 3K-5K, and the emitter R's are given values of 3.9K for R6 and 3.3K for R7. This means that stage is specifically set up to give us some harmonic distortion, and doesn't allow for any adjustment of that distortion in the way that the first stage does.
I'd guess that C5 is there for stability???
