[sdiy] TR-909 Rimshot - Pulse Input

[mark verbos]

Jim,

The cap, resistor and diode that the trigger passes through are setting 
the pulse's width, level and making sure it's all positive. However, the 
three sections are not oscillators exactly. They are bandpass filter 
stages set up to ring when a pulse comes into them. It's what they call 
a twin-T.  So they resonate at three different frequencies, with three 
different decay lengths, then they are mixed together and limited by the 
2 diodes D91 and D92. The mix of the 3' volume is then shaped by the 
master envelope via transistor Q65 set up as a simple VCA.

If you are looking to mod the circuit, you could consider adding 
controls to the bandpass filters to tune them differently. Changing the 
resistors going to ground from between the caps will do that. Maybe add 
a pot in series with a smaller resistor. Decay control of the master 
envelope (R401) or gain into the diode limiting (maybe volume controls 
in front of of R408, R415 and R417) would do something interesting too.

good luck,

Mark



carlile at ece.utexas.edu wrote:

>I'm building a copy of the TR-909 rimshot circuit.  I understand that a trigger 
>pulse is fed to 3 oscillators to start them ringing and these sounds are then 
>mixed.  The trigger passes through a capacitor, diode, and resistor before 
>making it to the oscillators.  Can anyone tell me what the purpose of these 
>components are for?  I'd guess some conditioning or shaping of the trigger is 
>going on, but I'm not sure exactly what is happening.  I'm looking at the 
>rimshot circuit here:
>http://hardware.freepage.de/cgi-
>bin/feets/freepage_ext/41030x030A/rewrite/raf909/rimshot.htm
>
>I'd appreciate any help on this. THanks,
>-Jim
>
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