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Mosfet Booster
© 1995-2010 Jack Orman
Please don't post the link to this page or pass it around to friends as it is a bonus
intended only for those who have purchased pc boards from AMZ. Thanks.

This is the schematic for the basic Mosfet Booster without the buffered output. There have been just a few changes from the original version that is on the AMZ web site.

A gate-stopper resistor R8 has been added between the input capacitor and the gate of the mosfet for extra protection. A jumper may be put in its place if you want to duplicate the original version as shown online.

Power supply filtering has been provided by the addition of R9 and C7. This is also optional and if you want the original version, leave out C7 and use a jumper for R9.

Because we are not using the buffered output in the basic version, C6 is not required and the output pad for it is left empty.

The value of R2 is listed as 62K ohms but it may have to be adjusted for best operating conditions. The dc voltage from the drain of Q1 to ground should be 5.0 to 5.5 volts, when powered by a 9 volt battery. The left end of R4 connects to the drain and is a convenient test point.

The 8-sided pads on the pcb are for wires to connect to the external parts. The input pad at the bottom left is for the signal from the input jack or footswitch. The pad marked with an "GAIN" connects to the gain control R6. The ground connections (Gnd) should be connected to the ground lug on either the input or output jack.

R10 is a 10M pulldown, which is optional, and is used to help prevent pops when the bypass switch is toggled. You can try the pedal without it and then add the resistor if the pop is noticeable. You may have to decrease it in value for more pop prevention is needed; use 4.7M instead of 10M for R10.
Parts List A 5k linear taper works fine for the Mosfet Boost except that most of the gain happens in the last quarter-turn of the knob. A reverse taper pot will have the response spread out more evenly over the entire sweep of the knob. I have asked Small Bear Electronics to stock the special pot for this circuit and they have added it to their stock list. Click here to go to the Small Bear stock page.
R3, R1010M
R4, R52.7K
R65k linear taper - Gain control
R8, R9220 ohms
C6not used
D19.1v zener diode 1N5239 or 1N757A
Q1, Q2BS170, 2N7000, NTE490, VN10LP or similar
LEDany choice
All resistors are 1/4w and capacitors are rated at 25 volts or higher.

Circuit Mods

OUT2 is the buffered output and C6 is used to couple it to the output. The gain control R6 must be disconnected from ground when the buffered output is being used. This can be done by using a 5K pot for R6 that has a switch on the back of it. When the gain is turned all the way down, the switch will disconnect the end of it going to ground.

Another way (shown in the drawing to the left) is to wire a second footswitch that toggles between OUT1 and OUT2, and at the same time will disconnect the ground connection on R6.

The 100K trimpot is used in place of the 62K resistor R2 as shown on the basic circuit. This allows the bias of the mosfet to be adjusted easily to the proper operating conditions - 5.0v from drain to ground. The optional 10M resistor is used to stop popping and should only be placed on the board if needed.

Additional Parts for Mods:
R2100K trimpot
R1010M resistor (anti-pop)

Tips and Techniques

Here is a Bode plot of the response of the AMZ Mosfet Booster as constructed with the pcb and instructions that you were sent. The plot was run with the drive control set for maximum gain. It probably will not actually yield 33db -- more like 28 or 29. The response is -3db at 35Hz which is plenty good enough for guitar and bass. The low E on the bass guitar is 41Hz so the circuit is down only about 1.5db for even the lowest note. The high end is ruler flat out beyond 20kHz.

I received some 2N7000 mosfet transistors recently and discovered that the pin configuration is different from the BS170 transistors that I had been using. While the BS170 is drain-gate-source when looking at the flat side with the pins down, the 2N7000 is source-gate-drain. Check the pin configuration of the transistors that you are using. The alternate pin layout only requires turning the mosfet 180 degrees and it will be aligned properly. (Align the transistor's flat side opposite the silkscreened print on the pcb.)

Parts List: I have made a complete parts list for use with the Mouser BOM Import tool. All you do is copy the parts list and paste into the tool to get an order assembled in the Mouser cart that you can then review and edit before placing an order. Download the parts list, which includes complete instructions. You will need to get a footswitch elsewhere since Mouser does not stock the blue 3PDT. I recommend Diystompboxes or Small Bear.

The hole in the middle of the pc board can be used for a screw mount but it is also sized to fit the 3/16" high standoff from Eagle Plastics (Mouser p/n 561-LAD187) which is also sold by Small Bear.

The back of the plastic mount has an adhesive pad. Peel off the covering from the pad and stick the standoff inside your box where you want theboard to be mounted, even on the back of a pot!

The Multi-Purpose pcb is shown in this photo but the mounting hole is the same on all AMZ pc boards.


If you are having trouble getting the circuit to work, here are some dc voltage readings to help troubleshoot the problem. With a meter set to read dc volts, place the meter's black lead on a ground point in the circuit and use the red lead to probe the pcb to get circuit readings. The dc volt readings should be close to those in the diagram.


Output Volume Control

If the Mosfet Booster is a too clean sounding for your amp setup, you can use this mod to introduce a bit of grunge into the sound right at the point of pick attack. The 5k pot is eliminated and the "Gain" pad from the pcb is connected to ground. A 100k audio taper volume pot is added at the output.

The Homebrew Electronics Uno-Mos pedal (a clone of the AMZ Mosfet Booster) uses this mod as does the BYOC Tri-Boost mosfet section. Here is the Uno-Mos schematic for your reference; the BYOC is online in the instruction manual for that product.

To understand how that changes the response of the circuit, read this article which explains the differences between variable gain circuits and those with fixed gain into output volume controls.

Also, moving the volume pot to the output makes the pedal much more susceptible to loading caused by the next pedal and can reduce frequency response when driving long cables as explained in this article. The fixed low output impedance of the original design is the preferred method to get consistent full-range response.

Mosfet Static Protection

DO NOT use an LED instead of the zener diode for protection of the mosfet. I do not know how this mod got started at the DiY forum but it is not acceptable. The reverse breakdown voltage of a typical LED is rated at 5v minimum but is typically 15v to 25v, which is way too high for proper protection of the mosfet.

Pulldown Resistors

Pads for pulldown resistors are included on the pcb for use if the footswitch pops when you switch it on. The output resistor R7 should aways be used but the input pulldown R10 is optional. I suggest trying the pedal first without R10 and add it only if needed.

P-Mos Booster

You can build a p-channel version of the mosfet booster using the AMZ pc board. None of the part values change except for the mosfet. You must reverse the power supply connection so that the voltage input is -9v, and the positive battery connection goes to ground. The electrolytic capacitors C3, C5 and C5 have to be turned around as does the D1 protection diode. Here is the p-mos schematic that reflects those changes. The p-channel transistor in the example is a BS250 mosfet.

Hum Reduction

If you are using an external power supply and are getting some hum from it with the Mosfet Boost, increase the value of R9 to 1k and it should eliminate the problem. You could even go as high as 2.2K if needed.

More Bass Response

No mods are necessary. The Mosfet Boost as shown in the schematic for the pcb that I sell will have full range response for bass and guitar. The version that is available online has full range response for a guitar - no mod will make it have more.

Decrease Input Impedance

The value of R3 determines the input impedance (Z), which is set at 10M ohms on the stock unit. If the sound is too crisp or bright, you can lower the value of that resistor to decrease the input Z. Try 2.2M or even 1M and see if that makes the sound more pleasing.

Alternate Gain Control

This is a versatile mod for the Mosfet Booster. It uses a 10k gain pot instead of a 5k and the capacitors will have to be mounted off the pcb. In the center position the gain is one (1). As you turn to the right, gain increases as with the basic circuit. However, if you turn it to the left of center, the smaller capacitor is brought into play and you get a treble boost! You can try other values for the smaller capacitor but 0.1uF is a good starting point.

Radio Frequency Noise

If you have a problem with Radio Frequency (RF) noise, here is a simple mod to eliminate it from the Mosfet Boost; increase the value of the C2 capacitor to 100pF or more - even as high as 470pF. You will not be able to hear any difference in the sound. You can also double the value of R8 for more hum reduction.

Mica capacitors

I have sometimes seen it recommended that C2 be a small mica capacitor instead of the ceramic disc that I recommend. It will make absolutely no difference in the sound since C2 is essentially out of the circuit at audio frequencies and not conducting, and it is being shunted by R1 anyway. Use the ceramic; it will be physically smaller, cheaper and easier to find.

Reverse Polarity Protection

Another mod that can be made to the circuit is at the R9 position. The resistor is removed and a diode substituted in its place for reverse polarity protection. A 1N5819 Schottky diode is what I would recommend although the common 1N914, 1N4148 or 1N4001 will also serve the purpose but with a greater forward voltage loss.

More Clean Headroom

An easy way to get more clean undistorted headroom from the Mosfet Booster circuit is to power it with two 9v batteries in series for a total of 18v. An external power supply that provides 18v can also be used. If you have a 12v or 15v adapter, those would be acceptable too. It is not recommended to go over 18v with the voltage power supply. No changes to the circuit are required to use these higher voltages.

Multiple Outputs

Out1 is the boosted output of the circuit and is the most often used signal. The pad marked Out2 is the output that can wired for the optional buffer switch as shown in the online article. If the buffer output is not used, the C6 capacitor can be eliminated as well.

Wiring Diagrams

If you need a little help with the Mosfet Booster pcb, click here for a drawing of the pcb connections. Different colored wires do not connect to each other. The green wire on the switch is a jumper. The other green wires that all connect together are the ground connections. The input and output jacks are automatically grounded to the metal case (and each other) so you can just connect the green wires to either of the ground lugs on the jacks. Also, here is another diagram for basic stompbox wiring.

Below is a closeup drawing of the jacks that shows the terminals. The metal tabs can be traced around through the body of the jack to find which go to the tip. The output jack is a mono version and the input jack is stereo so that it can do the battery switching. The battery negative wire (black) connects to the shorter of the two metal contacts on the jack as shown in the drawing. The tip (or longer contact) carries the audio.

Please don't post the link to this page or pass it around to friends as it is a bonus intended only for those who have purchased pc boards or transistors from AMZ. Thanks.

Download the instruction sheet and parts list for the pcb.

©2005-2010 Jack Orman
All Rights Reserved
Not for Posting on Any Web Site

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