commercial synth circuits

[Magnus Danielson]

>>>>> "DaGS" == Debby and Gene Stopp <squarewave at jps.net> writes:

Hi Gene (and all you others of course)!

 DaGS> How the heck did I miss the original posts? Ah well, new baby, new job, no
 DaGS> sleep, no time, the once beloved email list has become the occaisional
 DaGS> diversion for lurking...

Those RR (Real Reality) distractions and quite disturbing, don't you agree?

 DaGS> Anyway, while I do appreciate the concept that perfect operational
 DaGS> parameters should override any parts-count considerations, the fact is that
 DaGS> the 3130-FET-LM311 VCO core is both accurate and simple, a tribute to the
 DaGS> designer. Interestingly, the Roland 700 modular has pretty much the same
 DaGS> design.

This is a pretty amazing design. The more that I have gotten into it
the more impressed have I gotten. I have done the homework on it a few
times, each time with more details emerging.

BTW. What resettime where the ASM-1 VCO being designed for?

 >> I do not prefer the resistance in series with the timing cap method of
 >> high-end compensation.  The steady state voltage drop across the resistor
 >> at high frequencies alters the end points of the sawtooth waveform. This in
 >> turn affects the purity of the other waveforms usually derived from the
 >> sawtooth.  However, it has the benefit of accomplishing high-end
 >> compensation very simply, with only one component. This comes down to
 >> personal design philosophy and goals.  Mine is generally to achieve the
 >> best performing, most precise circuits, with only a secondary consideration
 >> to the number of components needed.  I recognize the desire of others to
 >> create simple, inexpensive designs, and there is clearly a need out there
 >> for that design approach.  The great value of the DIY list is providing a
 >> forum for all types of designs
 >> to be shared, so everyone can find what they want.

For the ASM-1 will this rise not be very noticable until you get up to
about 10kHz or so... I am not very happy about wave distorsion myself,
but I see it as an acceptable compromise. The neat thing is that when
this resistor is correctly selected one can completely eliminate the
frequency distorsion of the reset time. The reset waveform will never
be perfect, my ASM-1 VCO has a falltime of about 736 ns and many other
paths in the audio cursuit will not have similar speed. One can extend
the compensation to eliminate the DC offset error as well as the
amplitude error. Keeping the time from touchdown to start of the
rising ramp is also necessary. In my ASM-1 VCO I was forced to add a
cap in the reset cursuit for it to properly reset, this was due to
lack of propper understanding of the cursuit at the time, so my full
resettime was about 1.5 us which does not near the resettime that the
resistor and intergrator cap will compensate for.

Balancing these things will be a key to minimize waveform
distorsion. The resetperiod in itself is a wavedistorsion, so...

There also comes a time where improvements gets out of hand and does
not really contribute to the function of the cursuit and only
contributes to the designers foolish wish of making a "perfect"
design. Getting things as right as possible within certain limits is a
good thing. I daily encounter the question wiether a specific part of
a design effort is worth taking or not, the right answer only emerges
after asking the right questions and prior to that it is just
gutfeelings all the way.

Cheers,
Magnus