AMZ Mosfet Booster

The booster is a single class A gain stage based on a mosfet transistor. The BS170 mosfet is specified in the schematic though a 2N7000 or similar device could be used. Other alternates include the NTE490, VN10LP or Zetex ZN3306A, and the IRF511 sold at Radio Shack even works though the maximum available gain will be slightly less.

Since the mosfet is an enhancement mode device, the biasing required is not quite as simple as that used for jfet circuits. Resistors R1 and R2 form a voltage divider that establishes the reference voltage (Vr) that is connected to the gate of the mosfet through R3. The value of R3 essentially establishes the input impedance of the circuit, which is 10M as shown here.

A zener diode is connected from the transistor's gate to source in order to protect it from static charges or over-voltage inputs. If the transistor chosen has an internal zener protection, D1 could be deleted. Capacitor C2 is optional, but may be included as a prevention against radio frequency interference.

Depending on the device used, maximum gain should be about 35dB and minimum ~3 dB. The source resistor is bypassed by a large capacitor in series with a variable resistor used as a gain control. The minimum resistance setting yields maximum gain.

The dc voltage measured from point A on the schematic to ground should be 4.5v to 5.5v with the circuit idling and no input. The value of R2 should be tweaked until the point A voltage is correct. If two or more of these circuits are mounted in the same enclosure, only one R1/R2 network is necessary and the R3 from each stage can be connected to the Vr point.

The Mosfet Booster has a 10M input impedance that will not load down any guitar that is plugged into it, and the moderately low output Z is capable of driving almost any circuit that follows. In addition, with the gain cranked up, any amp input can be over-driven for a smooth distortion sound.

Current requirements are low so the battery will last a long time. A fresh alkaline 9v is recommended.

Build this simple low noise booster and be surprised at the transparent clean boost you can get with it.

NOTE: The frequency response of the AMZ Mosfet Booster is flat and extends down low enough for both bass and guitar use. There is no need to mod the design to add more bass; it will not make any audible change.

Revised 9/14/2001

A simple modification can increase the versatility of the Mosfet Booster. The addition of a couple of components allows for an extra buffered output. For R6, select a 5k pot that has an SPST switch mounted on its back side that is opened when the pot is turned to its minimum setting. This allows the bypass capacitor C5 to be completely taken out of the circuit when the switch is open and Output 2 can be used as a buffered output. Output 1 may be used as well if needed.

The signal gain is essentially unity from both outputs with the switch open. Output 1 is phase inverted and output 2 is non-inverting. Some clever use could probably be made of the differential drive of these outputs for an octaver or other effect. When R6 is turned up, the switch closes and the circuit functions as normal and provides a boosted signal via Out 1. Out 2 should not be used when the switch is closed since it will not provide a full range signal.

An alternate way to control the outputs uses a DPDT switch connected to switch between outputs while also switching R6 to ground. This would allow a tap of the switch to alternate between the boosted output (Out 1) and the buffered unity gain out (Out 2). It would probably be a good idea to put a 1M resistor from each output to ground to prevent pops when the switch is activated.

PC Boards for this project are available on the catalog page. Make your own mosfet booster & modify it with a tone control or other variations! Each board comes with complete instructions and a link to a page of special mods and tips.	Pre-assembeled Mosfet Booster modules are now available too!
Click here for some photos of a module that I installed in a 1590A box.

The question sometimes comes up: How long have mosfets been used for audio applications?

I have a book entitled "Design of VMOS Ciruits", copyright 1980, that clearly shows the use of small signal mosfets for audio purposes and even has a datasheet for the BS170 in it. I bought it right after the first edition was published and began experimenting with mosfets (the BS170 specifically) for guitar boosters. Radio Shack even carried the BS170 back in the early 1980s so it was readily available to anyone that wished to try it out.

But we can go back further than that. Look at these scans of publications going as far back as 1967.   [Application1]   [Application2]

The use of mosfets in audio applications has been around for almost 40 years...