[sdiy] Ultrarough draft of my hot-rodding of the Music Easel ring mod

[Grant Richter]

Actually, I looked at it again and you have a simple typo for the non  
inverting buffers.

Take IC6a, pin 1 and 2 are connect together only, and the junction of  
the 1K and 100K goes to pin 3. Pin 3 can not be grounded as it is the  
input.

The same with IC4B. Simple mis-draw.

I believe pins 5 & 6 are switched on IC6B, it is currently wired as a  
comparator with hysteresis.

The preamp should be normalized to MODIN not SIGIN, which is the  
carrier input.

For an onboard sine source, your best bet would be a single op-amp  
Hewlett design with an incandescent lamp for soft limiting.
Both the ICL8038 and XR2206 have sine waves with a THDs of 5%, this  
is not good.
The Hewlett design should have a THD of less than 1%, much better.
This sine wave can be normalized to the SIGIN jack, then if you need  
voltage control of the carrier frequency, you can patch it externally.

The on-board sine source can then be a simple potentiometer control.  
A suggested range is 10 Hz to 1kHz.
Adding a wider range is not musically useful, at a 1kHz carrier  
frequency everything is shifted so high it's almost inaudible.
Th simple sine source should have a panel output also with a 1K  
resistor in series.

And again if you need a higher carrier frequency you can patch it  
externally.

It would also be a real good idea to change R112 and all the others  
to 1K resistors. An LM837 will source 40ma into a 600 ohm load at 25  
volt p-p. That is quite a bit of power. Buchla control voltages are  
all unipolar from 0 to 10 or 15 volts. Don put the 220 ohm resistors  
INSIDE the feedback loop of the op-amp and was partially relying on  
the current limiting internal to the 741s to keep things under  
control. The LM837s can drive a mile of 600 ohm twisted pair wire and  
will happily blow things up if allowed to. Definitely raise the  
current limiting resistors to the now standard 1K.

Here is an example feedback patch that should be quite metallic and  
noisy.

Patch the output of IC1a to the SIGIN jack, you are now multiplying  
the modulation signal with a distorted version of itself.
Patch MIXOUT back to preamp in, leave it normalized to MODIN.
Patch the simple sine output to the wet/dry control mix input.
Crank up the preamp gain until the output starts to howl.

In the dry mode, the feedback should produce a simple pitch with  
pretty much distortion.
In the wet mode you are feeding back a product multiplied by a  
distorted version of itself, which should produce multiple attractors.
And you will be fading between the two modes at a rate set by the  
simple sine source.
If the chaotic function is jumping attractors at an audible rate,  
then the audio rate crossfade will interfere and cause a rhythmic  
pattern which is a beat frequency between the sine frequency and  
attractor jumping rate.

I just reread the above and it sounds like Martian even to me, but I  
think it would work that way. If not, it will be doing something  
crazy anyway.

Another variant would be to use one of the half wave outputs to the  
wet/dry mix.
The half wave rectified "outputs" have a low impedance two diode  
drops away, but I bet if you rammed the simple sine INTO the node,  
the impedance would be high enough  for the simple sine frequency to  
"mess up" the current flow in that leg, which would also produce some  
weird modulation or distortion, but not hurt the electronics since we  
have the 1K current limit.

On Nov 27, 2008, at 11:02 AM, Aaron Lanterman wrote:

>
> On Nov 27, 2008, at 6:52 AM, Grant Richter wrote:
>
>>> Questions:
>>>
>>> * Any other cool mods I should add?
>
> Grant, you rock.
>
> - Aaron