The Prologue-4

KA4HJH terrymbowman at
Wed Mar 4 20:45:28 CET 1998

Tony's orignal message:

>Now a question, have any of you had any problems with a UK produced MIDI-cv
>converter called the Prologue. Mine has two faults of note, one, it
>occasionally latches up, ie the gate outputs go high and the CV rises on all
>four of its output channels. Actually, it sounds rather groovy when it does
>this but it is not supposed to. And secondly, there is a audible slew in the
>output CV, so it sounds like a very fast glide. The Prologue uses 10n
>sample/hold caps buffered by a LM324 op-amp for its CV outputs. Replacing
>the surface mount caps with smaller ones leads to audible ripple as the MPX
>clocks. Methinks I will have to replace the MPX chip, caps and op-amps.
>Quality design it is not!
>Anyone else use one of these things and found an easier way out??

As you suggest, you could tweak the performance some by using
lower-impedance analog switches, slightly smaller caps and higher-impedance
op-amps, but none of this will fix the fundamental problem--the sampling
rate is too damn low! Have you checked it with a frequency counter or a
'scope? Thomas Henry's 4 channel device in the July '89 issue of Electronic
Musician used a 1kHz sampling rate (with 100nF caps and TL074's), and
included a bypassable pot in series with the S/H cap for adjustable
poratmento, thus turning a bug into a feature (the bypass was

Check out the circuit designs and software for the PAiA and Midwest Analog
Products MIDI/CV converters. I haven't heard this sort of complaint about
them. Now what were their respective URL's...?

Oh yeah, is the 324 one of those chips susceptible to latch-up?

"It's not a bug, it's your solder flux--I mean, it's a feature."

Terry Bowman, KA4HJH

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