[sdiy] TR-909 undersized DC blocking caps

[Stewart Pye]

Richie Burnett wrote:
>> 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.  

Again, I disagree. If you want to talk about the impedance of the "mix 
bus" then it's basically zero ohms. The series impedance from the bass 
drum op amp output to the 1uF coupling cap is 13k.

Are you saying that if you do a sim of a summing op amp with Rf of 20k 
(I cant read the value on the schematic), a common 1uF coupling cap, a 
13k input resistor going to your signal source and 9 other 13k resistors 
going to ground that you get a HPF cutoff of 120Hz?

Regards,
Stewart.