AMZ Mosfet Booster

[soupbone]

I built a AMX Mosfet Booster a while back,and I didn't like the result at the time.My main beefs are the lows are boosted,and the 5k rev.log taper's volume doesn't go to "0" when you turn the volume down.I had an idea of using a 500k rev.log taper micro amp style.Has anybody tried this,and would this even work?

[Kipper4]

Hey Soupbone.
Did you use the Sw1 switch?
Jack say in the article this should give unity.

Here

http://www.muzique.com/schem/mosfet.htm

[soupbone]

Hey Soupbone.
Did you use the Sw1 switch?
Jack say in the article this should give unity.

Here

http://www.muzique.com/schem/mosfet.htm

No,I didn't do the switch mod.Does this mod allow you to have the pot at 0,when you turn it down?Thanks!

[antonis]

The 5k pot (rev log..) is for controlling the gain (which is 35dB - 3dB) so you can't use it as a volume control..
(or you refer to another schematic...)

You may add a 100k A pot (it's actual value depents on what you want to drive next) after the output capacitor..

[induction]

I built a AMX Mosfet Booster a while back,and I didn't like the result at the time.My main beefs are the lows are boosted,and the 5k rev.log taper's volume doesn't go to "0" when you turn the volume down.I had an idea of using a 500k rev.log taper micro amp style.Has anybody tried this,and would this even work?

The Mosfet Booster and the Micro Amp are very different topologies (mosfet gain stage vs. op-amp gain stage), and the behavior gain pots are both dependent on other components in the circuit, so you can't just swap the pot values and expect them to behave like each other. Even so, in both cases, the Level pot controls the gain, not the output volume. And neither one will go to zero volume. The easiest way to get that is to do what antonis said and add an output volume control. If you only want one knob, dial the gain pot to your favorite setting, disconnect it, measure the resistance between lugs 2 and 3, and replace it with a fixed resistor.

If you want to cut lows, reduce the input cap, output cap, or the cap between the gain pot and the mosfet source leg.

[Kipper4]

As Induction says change the input and output caps if you want less bass (wool) in the signal.
I guess it's intended to push the preamp tubes in the amp. Although it might sound better if it was pushing the power amp tubes if you have an effects loop on the amp?
There's little point in having a booster that goes to zero.
I wasn't too impressed with the micro amp I built.
I like the mad professor sweet honey and Hermida zen drive. I really like the way the focus feature (tone) works on the sweet honey.
Where all differant but this is enough break up for me to retain dynamically controlled playing and not completely dessimate the signal into fuzz territory.
It's a bit difficult to be sure what exactly you want to get out of it Soup.
Try pushing the edge of break up tube amp with the mosfett booster and if you still don't like it maybe try another.
Have you built a rat yet? Absolute classic sound. Might not be your taste or what you wanted but worth a build.

[midwayfair]

Why do you need your boost to go to 0? 

Anyway, the gain pot is 5K because that's the next value up after the 2.7K source resistor. The control increases gain when the resistance and capacitance through the drive pot and 100uF exceed the fixed 2.7K source resistance for frequencies that are passed by the capacitor (which is everything on the guitar). Hopefully that sentence wasn't too convoluted. Anyway, it's a C taper because very little gain increase happens before you get the gain pot's resistance below that 2.7K, and the last few hundred ohms increase gain at an increasing rate. The value and the taper give the smoothest control over the boost increase. A 500K pot will do nothing until the very last smidge of the pot's rotation.

If yours is boosting bass, you most likely did something wrong or added something to it (perhaps a treble cut?). The stock design does not increase or decrease bass or treble at any meaningful frequencies. If you want a little bass cut, I've found that a 1uF cap on the emitter sounds really good -- that's the setting I'm using on the one on my board (I have a 10uF on a switch for full-range).

[Mark Hammer]

Just like tube stages, the gain on that MosFet transistor is partly set by the resistance of the path from the source to ground.   The 2k7 resistor from the source to ground sets the basic gain.  The 5k pot and 100uf cap provide an alternate path.  As the pot resistance gets reduced, there is a lower-impedance path provided for AC through the 100uf cap (and you don't want to provide a path for DC to be amplified), such that gain is increased for audio-frequencies, beginning at a point set by the value of the cap.  As long as the pot resistance is greater than the fi8xed resistor (2k7), the alternate path provides for no additional gain beyond the default.

The smaller the cap value, the higher the point in the spectrum where that additional gain begins.  The same circuit, but with a 22uf cap instead of 100uf, would provide the identical boost, but commencing 2 octaves up from wherever the original started.  Drop the cap down to 4u7 and you move it up a little over another 2 octaves.

One of the things a person can do is play with the cap values, and even use multiple caps.  Imagine you have a 10k linear pot.  At its midpoint of rotation, there is 5k on one side of the wiper, and 5k on the other.  Run a 100uf cap from one outside lug to ground, and a 2u2 cap from the other outside lug to ground.  As you rotate in one direction, from the midpoint, you increase the resistance in series with the one cap, and decrease the resistance in series with the other.  Adding more to 5k does nothing, but subtracting from 5k begins to add gain for that portion of the spectrum dictated by the cap value.  As described, the circuit becomes a full-range booster in one direction, and a treble booster in the other.

[soupbone]

Thanks guys!I wanted the volume to go to "0" like a normal booster should.Thanks to all for the ideas!

[Hatredman]

In short: the gain pot IS NOT A VOLUME CONTROL. If you want a volume pot, you need another one.

[antonis]

I wanted the volume to go to "0" like a normal booster should.

So you need a Boooster which gradually turns to Buffer...
(I like the way you think..)