[sdiy] TR-909 undersized DC blocking caps

[Richie Burnett]

> No, they're not in parallel. The other end of the 1k resistors go to 
> individual op amp outputs. I stand by my previous statement.

 Each individual instrument's buffered output is tied to each mix busses
 through around 13k of resistance.  Since there are ten of these buffered
 individual outputs the impedance at the mix bus is roughly 13k / 10 = 1.3k.
 Each mix bus is then coupled through a 1uF capacitor to the virtual earth 
of
 the summing amplifier.  This gives a zero at DC, and a pole at 122 Hz in 
the
 transfer function.  i.e. A first-order highpass with fc=122Hz in the left
 mix amplifier, and likewise in the right mix amplifier.  (A quick PSpice 
sim
 also proves the break frequency is in the region of 120Hz.)

> You may hear some improvement on the bass drum by increasing the 
> capacitor, but if it was a high pass at 122Hz as you state the bass drum 
> would be pretty much unusable from the mix output.

 I wouldn't go as far as to say it is unusable, since the effect of the
 capacitors is only a first-order highpass filter.  However it definitely
 makes the BD sound less weighty and more clicky.

 The TB-303 has similarly under-sized coupling capacitors between the VCF
 output and VCA input.  I've also thought this was a strange design decision
 for something that is after all a bass synthesiser.

 -Richie,