October 14, 2008

Early To Rise

A while back, in the Buchla Yahoo Group, I got involved in a discussion about why some 200e modules will not advance a 245/246 Sequencer and subsequently my 243 Programmable Pulser module. I suggested that it was the slow rise time of the 249e, and likely every other 200e module, that caused the problem. Ezra wrote that he thinks it's the voltage level of the pulses, since they only peak at 10 volts, not 15 volts like the old black knobbed modules from the 246 era. Well, I am not one to let sleeping dogs lie, so Reed and I did a little research on as many modules as we had available and my scope. This is what we found.

  • 249e - 8.5 volt peak - 300µS rise time - does not step the 246
  • 281 - 15 volt peak - 10µS rise time - steps 246
  • 230 - 15 volt peak - 5µS rise time - steps 246
  • 257 pulse from 281 sent through - 10 volt peak - 80µS rise time - steps 246
  • 248 all pulses out - 14.75 volt peak (no spike, just gate) - 500nS rise - steps 246
  • 248 CV out alternating between high and low - 200µS rise - steps 246
  • 225e modded for bigger pulses - 12.75 volt peak - 150µS rise time - steps 246
  • 259 square wave out of mod oscillator - 10 volt peak - 5µS rise time - steps 246
  • 259 saw wave (falling even though the panel shows rising) - 10 volt peak - 5µS rise time - steps 246

The 246 schematic specifies that it's "input sens." is 3V, 100µS rise. The 284 schematic specifies it's "nominal input sensitivity: 5V (5 pulse, 3V sustain)" and "required rise time (15v):3ms.). Interestingly, the 284 would trigger from the 249e. Where does that leave us? Why does the 225e modded to have a higher gain on the output opamps work if it's not about gain? Why is 10 volt enough coming from the 257, but not the 249e? Why is 10 volts not enough if the "sens" of a 246 is 3v? These are all very good questions. I have a guess.

The issue is not wither the rise time or the voltage level, it's both. Specifically, The rise time to the threshold. In the 246's case, the rise time from 0 to 15v needs to be 100µS. On the scope, the rising voltage slope is not linear. It appears to be an RC curve, rising quickly at first and slowing as it approaches the asymptote. So the "rise time" is not actually the issue, but the speed of the rise. If the pulse's total voltage is 15v (WAY above the threshold), it will be moving much faster as it passes 3v than if it is only rising to 8.5v.

My assumption is that the output of the microprocessor in the 249e is PWM that is filtered to make continuous voltages and there's no way to raise the cutoff frequency of that filter to speed up that rise time.

The gain on the pulse outputs could be pumped up, like I did to that 225e, or an opamp comparator could be added between the pulse input jack and the circuit of the sequencer/pulser. A 281 could be triggered with very short time constants and it's big daddy pulse output used to step the sequencer. A banana to 1/8" lead could be used to send the sluggish pulse into a 230 to sharpen it up. I'm considering changing the input section of the Pulser to be more understanding.

I'll close with a quote from the 242 clock schematic. I have not tested any 100 series modules for rise time, but they seem to be way faster than anything 200e...

"pulse width: ~50µS
risetime: ~3µS
[<10µS required to operate models 123, 146 sequencers. when these are phased out, we may increase to ~20µS]"

1 comment:

Quad said...

what about 281e and 250e are they any different ?