AMZ Mosfet booster - does the trimpot really do much?

[dap9]

Hey guys.  I put together Jack's MOSFET booster and everything works fine, but I really hear no difference when I tweak the 100k trimmer.  I really dig the sound of the circuit, but I just want to make sure I'm gettin' everything I can get out of it.  I used MarkM's layout w/ the C5K pot.  Good stuff, but if anyone can give me some insight on my trimmer issue, I'd really appreciate it.

Thanks!

[petemoore]

  100k trimmer...can't say's I remember...I guess it'd be shown on the schematic you're working from.
  See GEO, 'Updates to Mosfet Boosters', if it's boosting and the mosfet is nicely biased, there's nothing much more to it.

[amz-fx]

Hey guys.  I put together Jack's MOSFET booster and everything works fine, but I really hear no difference when I tweak the 100k trimmer.  I really dig the sound of the circuit, but I just want to make sure I'm gettin' everything I can get out of it.  I used MarkM's layout w/ the C5K pot.  Good stuff, but if anyone can give me some insight on my trimmer issue, I'd really appreciate it.

The trimmer is used to adjust the bias voltage of the mosfet. It is usually set to the optimum level for the most clean headroom...  between 4.0 and 5.0v on the drain of the mosfet is good.

If it is tweaked way out of adjustment, you would expect to hear distortion but if the input signal is small enough and the gain is turned down, then the bias can be significantly off the usual setting and the pedal will still sound clean.

Adjust for 4.5v +/- and leave it alone after that 

regards, Jack

[alanlan]

  between 4.0 and 5.0v on the drain of the mosfet is good.

You can't get less than just over half rail on the drain  i.e. the same drain current flows through R4 and R5, in which case if you have the drain at say 4V that means 5V across R4 and by definition 5V across R5.  5V + 5V = 10V doesn't work.  If you can get 4V it means that your supply voltage is somewhat less than the nominal 9V.

If you want best headroom in this circuit the drain should probably be at around 6V leaving 3V across the MOSFET.

[amz-fx]

The same DC current flows through R4 and R5 but at the AC frequencies of the guitar, the big fat bypass capacitor across the source resistor is shunting the signal to ground (through the gain control). At max gain, as the mosfet varies from a few ohms to many megaohms, the output signal can swing (almost) 0v to 9v... 

I usually tweak the trimmer for 4.5v to 5.0v for my own use, but 6v could be good to experiment with.

regards, Jack

[dap9]

Thanks guys.  I realized that I didn't have my trimmer wired up properly.  MarkMs diagram has the wiper connected to that one lug (I'm guessing it's #1) and nothing to the other lug.  When I turn the trimmer CCW, I get no signal until about 1/2 way and from there on (going CW), I don't really hear much difference in terms of gain/headroom.  Should that other "lug" be connected to ground?

Thanks.  I'm really digging the sound of this boost, Jack!

[amz-fx]

The free lug should be connected to the wiper (preferably) or can be left open.

regards, Jack

[dap9]

The free lug should be connected to the wiper (preferably) or can be left open.

regards, Jack

Thanks Jack.  I have it opened now, and have the aforementioned problem - no sound when all the way CCW.  I'll hook up the wiper to the free lug.  Hopefully that'll allow me to experiment w/ the full range.

[alanlan]

Like petemore said - why are you using a 100K pot - it should be 5K shouldn't it?

The same DC current flows through R4 and R5 but at the AC frequencies of the guitar, the big fat bypass capacitor across the source resistor is shunting the signal to ground (through the gain control). At max gain, as the mosfet varies from a few ohms to many megaohms, the output signal can swing (almost) 0v to 9v... 

At full gain, the source will be sat at the steady state DC level i.e. if the drain is at 5V then the source will be at 4V assuming 9V rail (as explained before). 

The drain cannot possibly go below below the source.  So it can swing to 9V but it can't swing below the source.  That's why I'm saying the drain needs to be biased higher than 5V.  The signal will be able to swing 4V up but only a volt or so down from the 5V quiescent level.  If asymmetry is your thing then this is fine but if you want an equal up down headroom then the drain should be biased higher up.

An interesting thing about this circuit is that if you don't use the gain pot and just use it as a nearly unity gain buffer or phase splitter, the biasing should be altered from the high gain scenario to get best headroom.  At the end of the day I suppose you can just tweak and listen.

[dap9]

Let's see if this works...  I'm using this layout below.  it calls for a 100k trimmer.  I have a C5k pot.  I have the trimmer wired up as in the pic, but I don't have log 3 hooked up to anything.  I just now tried to wire it to the wiper, but then I get no sound at all despite where I put the trimmer.  I'm troubleshooting now to see if I have any bridges or anything like that

[dap9]

Well I tried adding wiring lug 3 to the wiper and it cut out all sound.  I'm pretty sure I have all the other stuff wired correctly.  What I hear is a very decent clean boost w/ the Vol. pot to about 3/4 of the way up.  After that I get some very nice OD-type qualities.  I have a DMM, I have a rough idea of how to check the voltages; if I'm assuming correctly, I put the black lead on ground and then put the red lead at the points on the MOSFET that were mentioned.  Do I need to have it hooked up to the adapter?

Thanks for helping out a noob, guys!

[dap9]

bumparoo

[alanlan]

Please measure the DC voltage at the following points:
drain of MOSFET (junction with R3)
source of MOSFET (junction with R4)
gate of MOSFET

Please repeat with the trimmer set to each end i.e. fully CW and CCW.

[ItZaLLgOOd]

That layout shows the wiper already attached to the lug with a trace so you should be good to go.  Turn and burn

[slacker]

Sounds like you've wired the trimmer up wrong. Like ItZaLLgOOd said on Mark's layout the bottom leg of the trimmer (VR1) is connected to the trace that goes to C2 and R5. The wiper (middle leg) and the top leg are both connected to the trace that goes to +9 volts so you shouldn't have a leg unconnected.
If you connect the trimmer up like this then it will behave like you've said yours does. You'll get no sound with it turned all the way in one direction then as you turn it the other way the sound will come in.
With the battery or adaptor plugged in put the black probe of your DMM on ground and the red probe where R3 connects to the Mosfet then adjust the trimmer until you get 4.5 -5.0 volts DC.

[dap9]

Please measure the DC voltage at the following points:
drain of MOSFET (junction with R3)
source of MOSFET (junction with R4)
gate of MOSFET

Please repeat with the trimmer set to each end i.e. fully CW and CCW.

Sorry it took so long to get back to you guys - busy wknd.

Ok, here's what I came up w/ my DMM:

Drain of MOSFET:  CCW = 4.87   CW = 7.33
Source   "    "     :  CCW = 4.86   CW = 2.4
Gate                  :  CCW = 4.83   CW = 2.3

To recap:  I get NO sound when fully CCW.  When I turn the trimmer about half way I get signal, but at the drain, the reading has to be at least 5.8 (and the signal is fine and the effect is doing what I believe it should be doing - sounds really good to me fwiw)

I noticed w/ the DMM just now, when I was checking the gate w/ the trimmer fully CCW, I had sound when the positive lead was connected.

Lemme know if this sounds good/not good.  Thanks!

[dap9]

bump

[alanlan]

It's like I said, it needs biasing up around 6V and perhaps more depending on use.

There seems to be a common belief that drains and collectors should always be biased at 4.5V.  This is all very well for op-amp circuits but not always the case (in fact quite often not) for discrete transistor circuits.  You have to take into account the swing available across the transistor.

Anyway, it sounds like you've got it sussed.  The reason your voltmeter probe caused audio to appear is probably due to loading of the voltmeter itself  i.e. the bias circuit is high impedance at the gate so if you load this point it will reduce the gate voltage and change the bias.

[dap9]

It's like I said, it needs biasing up around 6V and perhaps more depending on use.

There seems to be a common belief that drains and collectors should always be biased at 4.5V.  This is all very well for op-amp circuits but not always the case (in fact quite often not) for discrete transistor circuits.  You have to take into account the swing available across the transistor.

Anyway, it sounds like you've got it sussed.  The reason your voltmeter probe caused audio to appear is probably due to loading of the voltmeter itself  i.e. the bias circuit is high impedance at the gate so if you load this point it will reduce the gate voltage and change the bias.

I wish I could tell you that what you've written isn't Greek to me, but I'm still learning.  However, I appreciate it, and I'm gonna do my research!  But if I get you correctly, I'm on the up and up and I'm not gonna blow anything up.  I appreciate the help and I'm gonna read, re-read, and re-read again what you've written.  Then I'm gonna do my research to fully understand it.  So thanks again for taking the time to respond!  I'm still blown away by how good this circuit sounds for as "simple" as it is!!!