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A mosfet boost circuit for electric guitar from Jack Orman.
Note: added pseudo log pot
Note: This should probably be redesigned using a JFET, or a BJT
The gain is controlled via the pot below the large bleed cap.
Note: With an input of 150 mV peak (100 mV RMS), the signal output can reach 410 mV peak (290 mV RMS).
Note: this is representative of a single coil Stratocaster style pickup with passive electronics, played at moderate or quiet levels. However, this type of pickup is capable of outputting much higher voltages. Some single coil pickups can produce up to 5 volts!
Orman typically creates this style of gain control in his circuitry. He also includes the zener protection diode on the front end, as has been included here. This provides some protection against accidental input voltages that might kill the mosfet transistor. It does not protect against accidental high voltage DC coming from defective valve (tube) amplifiers.
EC does not yet have JFET transistors, so this circuit is ideal. It is not an attempt at creating an artificial EC version of a JFET. Rather, it is a reminder that MOSFET devices such as the BS170 have strong merits of their own.
NOTE: the biasing of the gate is achieved using the T resistor network. The 10M resistor hangs off the gate line and meets the potential divider pair. I keep 1M and 10M values, but like many other humans, I do not keep an extensive range of high values. So this configuration makes it easy to get the voltage you want, which in this case should be just above 5 Volts on the gate.
The actual input impedance here is determined by the 10M resistor, as the input impedance of the MOSFET itself is practically infinite.
The 10uF above the 10M resistor effectively grounds any unwanted signal appearing from the supply.
The input signal can swing to about a volt peak to peak before the circuit becomes problematic.
Just like JFETS, MOSFET transistors are notoriously differing in their specifications; even in the same batch there may be considerable variation in the VTO voltage. In EC, we can set this parameter. Here it is set at 450mV. This is what one might expect from a BS170
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