Infinite Melody modification and a strange discovery
2004-02-14 by Richard Brewster
The internal white noise source, which I added to my Infinite Melody
(rev 0), needed to have its level adjusted. The Sense pot was only
useful in a small range near the middle. Much above or below center and
the LED was always off or always on, indicating not much change in the
bit presented to the 4015 (IC2). So I poked around today with my
scope. I found two interesting things, one entirely unexpected.
First, IC1, the LM358 on the bipolar supply, did not produce clean
pulses for clocking the 4015. I saw a lot of the noise passing from the
input to the output at pin 1. Junk in the middle range. So I changed
it for a TL072. The IC socket made this easy. Now the pulses are very
sharp and clean. The other LM358 (IC2) runs at LFO rates. It is on a
single supply and does not have a problem producing a good clock.
Next I proceeded to adjust the level of my white noise generator (which
feeds the normal lug of the Noise In jack) until I got a nice range for
the Sense pot. It looked just right on the bench. I set the white
noise output to be about 400 millivolts AC RMS, which is about the same
level as the white noise output of my MOTM 101 (which is set lower than
Paul Schreiber's suggestion of 850 mv). Ok, now I install the IM into
the MOTM cabinet and whoa! The RMS measurement from the IM white noise
is now *1400* mv! And the sense pot range is no good now, which is why
I got into all this today. What gives? I decided to adjust the module
while powered by the MOTM cabinet instead of my bench supply. Pulled
out the module on its power cable and redid the adjustment. I was
really surprised by what came next. Just installing the module back
into the cabinet raised the noise output. In fact, simply moving it
inward and outward would change the level by up to 1V RMS. I installed
the IM near to the Power One 3A power supply and over the power
distribution board. Could this be it? Not sure, but it's likely. My
noise circuit is built on a simple perf board with no ground plane.
There is a lot of amplification in the first gain stage, so it must be
picking up something when inside the cabinet. For a quick fix, I
adjusted the noise level trimpot so that I get the desired level when
the IM is installed. I will try moving the IM to a different location
in the cabinet at some point. I am planning a "digital" section with a
separate power supply and the IM will go into that section.
Now that the Sense range is more useable, I am getting a better
distribution of notes and it is much more controllable. The IM is one
of my favorite modules.
-Richard Brewster
(rev 0), needed to have its level adjusted. The Sense pot was only
useful in a small range near the middle. Much above or below center and
the LED was always off or always on, indicating not much change in the
bit presented to the 4015 (IC2). So I poked around today with my
scope. I found two interesting things, one entirely unexpected.
First, IC1, the LM358 on the bipolar supply, did not produce clean
pulses for clocking the 4015. I saw a lot of the noise passing from the
input to the output at pin 1. Junk in the middle range. So I changed
it for a TL072. The IC socket made this easy. Now the pulses are very
sharp and clean. The other LM358 (IC2) runs at LFO rates. It is on a
single supply and does not have a problem producing a good clock.
Next I proceeded to adjust the level of my white noise generator (which
feeds the normal lug of the Noise In jack) until I got a nice range for
the Sense pot. It looked just right on the bench. I set the white
noise output to be about 400 millivolts AC RMS, which is about the same
level as the white noise output of my MOTM 101 (which is set lower than
Paul Schreiber's suggestion of 850 mv). Ok, now I install the IM into
the MOTM cabinet and whoa! The RMS measurement from the IM white noise
is now *1400* mv! And the sense pot range is no good now, which is why
I got into all this today. What gives? I decided to adjust the module
while powered by the MOTM cabinet instead of my bench supply. Pulled
out the module on its power cable and redid the adjustment. I was
really surprised by what came next. Just installing the module back
into the cabinet raised the noise output. In fact, simply moving it
inward and outward would change the level by up to 1V RMS. I installed
the IM near to the Power One 3A power supply and over the power
distribution board. Could this be it? Not sure, but it's likely. My
noise circuit is built on a simple perf board with no ground plane.
There is a lot of amplification in the first gain stage, so it must be
picking up something when inside the cabinet. For a quick fix, I
adjusted the noise level trimpot so that I get the desired level when
the IM is installed. I will try moving the IM to a different location
in the cabinet at some point. I am planning a "digital" section with a
separate power supply and the IM will go into that section.
Now that the Sense range is more useable, I am getting a better
distribution of notes and it is much more controllable. The IM is one
of my favorite modules.
-Richard Brewster