Ok, I'm a synth geek. To prove it I decided to grok the Grinder circuit
and built one on my prototyping gig, it's so simple. The only part
value that differed from the schematic was the 4n7 instead of the 5n6
cap. I fed it a variety of input waveforms, all 10V peak to peak, from
a MOTM-300 VCO, varied the pots and tried different capacitors, while
monitoring the output by scope and on audio.
I first ran the circuit without *any* lag capacitor. In this
configuration it is nothing more than an inverting amplifier. The LAG
pot does almost nothing, while the DRIVE pot varies the gain roughly
between unity and 0.01 (very small). I discovered that at the smallest
gain setting with the LAG pot near the lowest resistance, a parasitic
oscillation occured. This may have been what Larry Hendry experienced
that led him to limit the lower resistance of the LAG pot.. The
parasitic also happens with the lag cap in the circuit. I was able to
eliminate the parasite with a 100pf cap across pins 1-2 of the LM358.
(A 47pf wasn't enough.) This dulls the sound a tad, but I traded that
for the stability.
With the parasitic eliminated and the 4n7 lag cap reinserted, I
continued to experiment. (The 4n7 value seems fine.) My next
observation would be expected from an analysis of the circuit: the gain
is highly dependent on frequency. As frequency of the input goes up, so
does the gain. Way up! The lag circuit is a simple low pass filter,
and it is in the feedback loop of the inverter. As the frequency rises,
more energy is conducted to ground through the lag cap, allowing less
signal back into the input as negative feedback. This causes the op amp
to increase its output in an effort to establish equilibrium between its
+ and - inputs. The consequence is that the Grinder output level
increases rapidly as the input frequency rises, and it quickly reaches
saturation, the output becoming nearly a 30V peak to peak square wave!
This isn't necessarily a bad thing. But the output can go into clipping
easily.
So I decided to make another modification: I added a 1K resistor to
ground at the output, just like the voltage divider on the Pulse Out of
the Wave Multiplier. (That extra 1K resistor on the pulse output is on
the board and parts list, but it is not on the schematic). This cuts
the gain by half, and limits the output to a 15V peak to peak signal.
Since so much gain is available, it's not a problem to cut it in half.
In summary, my two mods are:
1. Add a 100pf cap betwen pins 1 and 2 of the LM358 to eliminate parasitics.
2. Add a 1K resistor to ground at the Grinder Output to limit the output
level.
I must say that there are some pretty cool sounds obtainable, especially
with a sawtooth wave on the input.
I haven't tried other chips yet. I will before I'm done.
-Richard Brewster
Larry Hendry wrote:
Show quoted textHide quoted text
>--- Richard Brewster <pugix@...> wrote:
>
>
>>As for the capacitor, I don't have a 5n6 cap on hand. So I think
>>I'll try a 4n7, which is pretty close.
>>
>>
>
>1. You know you are officially a synth geek when you use the phrase
>" what I have on hand."
>
>2. I had an issue with the control range on mine. I put a 150K
>resistor in series with the 1M pot so that the minimum value cannot
>go gelow 150K.
>
>Larry H.
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