[sdiy] TR-909 undersized DC blocking caps

[Stewart Pye]

Hi Ritchie,

Actually most channels (including the kick drum) have 1k in series with 
12k which makes it 13k. This should result in a -3dB point of 12.2Hz.

Regards,
Stewart.

Richie Burnett wrote:
> Just thought I'd put this info out there as i've not seen it mentioned before...
>
> A couple of years ago I did some repair work on a customer's TR-909 and
> discovered that the DC-blocking capacitors in the final output mixer are grossly
> undersized.  The impedance of the mixing busses (used to mix the individual
> sounds into left and right channels) are around 1k1.  With the existing 1uF
> coupling capacitors (C153, C154) this makes a high-pass filter with cutoff
> frequency around 140Hz!  Considering that the fundamental frequency for the kick
> and most of the toms are well below this figure, it does a good job of stripping
> most of the bottom-end punch out of the stereo mix outputs.
>
> Replacing C153 & 154 with 10uF components restored the bottom-end to the stereo
> mix outputs so they sounded the same as the individual instrument outputs.
>
> Maybe the bass-light stereo mix behaviour is well known to seasoned TR-909
> users?  ...and maybe Roland did it for some reason?  Either way, i've not heard
> it mentioned before.
>
> -Richie,
>
> (NB. The same arguments apply to the Introspectiv-9090 as it's C4 and C5 are
> also 1uF)
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>
>