copyright 2001 by Scott Swartz, all rights reserved

This design is a very simple clean booster that is perfect for switching between a clean rhythm and lead volume, or adding sustain to a distorted rhythm tone for leads. The circuit is a little like the classic rangemaster circuit, but implemented with a MOSFET transistor, arranged for more uniform frequency response, vastly higher input impedance, less distortion, well maybe its not much like the rangemaster after all, but it’s a cool title.

Calling it a 'clean boost' is a bit misleading, since it measures about 1.5% THD at 1 volt output, but this amount is barely perceptible and actually adds a little character and sparkle to the tone. This is due to the fact that the distortion is almost all second harmonic, which I was able to determine by checking the waveform with the fundamental removed using a distortion analyzer; What was left looked like a sine wave with twice the frequency.

Maximum gain available is about 8, which doesn't sound like much, but is plenty for my uses. Of course, the gain could be increased with a source bypass capacitor (try 20 uF), but distortion will also increase correspondingly. Another way of adjusting gain would be to make the source resistor variable.

 Construction

I built the pedal using a Hammond 1590B box. Since the circuit is so simple, I used terminal strip construction.

Here is the SCHEMATIC.

Check out this PICTURE to see the layout. Even though the box is small, if you build it in the proper order, there is plenty of room. Study the picture to see which components should be installed first; Start with the terminal strip, pot, and LED, as these end up under the input and output caps, as you can see in the pictures.

There is only one adjustment required after construction, the bias point of the MOSFET. The following section details how I settled on the bias point I recommend, which is 4.5 VDC at the drain of the MOSFET.

I was trying different resistors to see how low I could go on LED current and still have enough brightness, and I noticed that the sound of the pedal was changing when I connected the LED, it seemed a little more distorted. The LED draw was only around 1 ma and of course constant, so this was surprising.

To satisfy my curiosity about whether what I thought I was hearing was actually there, I hooked up an HP 200CD signal generator set for 100mv 1 kHZ on the input, and a Tektronix 561A oscilloscope and HP 334A distortion analyzer on the output. I was not hearing things, as connecting the LED raised the THD to 1.9% from 1.7%.

The LED current is not capable of modulating the power supply in any way, of course, so where is the extra distortion coming from? A little more checking gave the answer- the LED current draw changed the bias point of the MOSFET, causing a change of about 0.2 VDC in the drain voltage. It turns out THD generated is very dependent on the drain voltage.

I then ran a more extensive test, using an input of 180 mv which gave 1.36 Vrms output, and varied the bias point with a pot in the Vref divider. Below are the THD versus bias point results using a 2N7000 MOSFET. I later tried a BS170 MOSFET, and distortion was about 0.2% less at each bias point, but otherwise it behaved the same at the bias point was varied.

I then increased the signal generator voltage to the point of clipping and adjusted the bias point for the drain voltage that gave the largest voltage swing on both sides on the waveform by watching the wave on the scope. This point turned out to be 5.1 VDC on the drain, which you can see from above is not the optimum point for low distortion, unfortunately. The lowest distortion reading I was able to obtain was about 1% THD, but that was at a drain voltage that is too low for real world use.

Setting the drain voltage to 4.5 VDC is probably a good compromise. It is often stated that this is the number to shoot for, but here is some actual data to back it up.

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