Maestro FSH1 a.k.a. Madbean Sharkfin on the bench
A look at the LFO and sample&hold section
and a fix for the noise generator
last update: Oct 11, 2014

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Index

Abstract
Fixing The Noise Generator
Reference

Abstract

This is an closer look at the FSH-1īs[1],[2]
CV (control voltage) generator in lieu of driving the Colorsound Dipthonizer™ clone with the  random values this produces. The envelope generator and VCF section are disregarded.

The FSH-1
īs CV part consists of a  of a LFO section that triggers a sample and hold circuit, which samples a noise generator plus some more filtering and level shifting circuitry.

The sonic result of using this in the Dipthonizer 13x00 was inferior, so it was abandoned. However a sure fix for the noise generator has been found and tested.

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Fixing The Noise Generator

According to plenty of internetīs stomp box builderīs experiences the noise generator part is seemingly notoriously vulnerable.  It is not known, what their problem was, presumably too little noise from the noise generation transistor Q4,
which leads to a lot of mojo regarding transistor choice.

The original circuit uses a reverse biased transistor with the base open, which is very strange. It is not easily explainable, how this is supposed to work, no reference could be found that supports that wiring, however it is easily comprehensible that this does not work in many cases.

In contrast, a better known circuit is to use the base-emitter junction of a transistor in reverse, which exhibits an excellent zener diode breakthrough at about 6 Volts. Zener junctions are known to be noisy. Naturally, a 6V (or thereabouts) zener diode will work too.

The subsequent OPA  (operational amplifier) is there to boost the noise signal to a useful level.
The first transistor that was tried worked like a charm - why should it not.

The unused collector pin can be tied to the base or being left open - this makes no difference. Unstable existing designs can thus be fixed by just soldering those two pins together. Any old NPN transistor should work.

Note that some component values have been adapted to suit the Dipthonizer 13x00īs CV range.
For j-fets BF245A type work well, type B or C may not work due to the increased voltages needed to turn them off.

Since it was originally planned to install this subcircuit into the same housing as the
Dipthonizer 13x00, the power supply section of the FSH-1 had to be adapted for the single-ended circuit the latter uses. What has been termed "GND" on the FSH-1 now is termed "VRef" on the "Random CV" subcircuit.

A ultra-bright LED was used with a comparably large current limiting resistor. Not that increased brightness was being asked for, but low current demand. The increased brightness however helps the LED to be appreciated since the pulse with of the sample pulse is very small.

Although the random CV subcircuit works great per se, it did not sound too great together with the vowel sounds and thus did not make its way into the Dipthonizer 13x00 housing.

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Schematic for the FSH1īs Random CV subcircuit, adapted for the Dipthonizer 13x00

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Reference

[1] Madbean Pedals, Sharkfin, http://www.madbeanpedals.com/projects/Sharkfin/Sharkfin.pdf
[2] Tonepad, Maestro Filter Sample Hold, http://www.tonepad.com/getFile.asp?id=138

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Update History
  • Oct 11, 2014  first release
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