Looking for a quick mod idea for a Mosfet boost

[Bill Mountain]

Howdy!

I'm building a boost for a friend and I'm looking for some quick ideas for a mod because the enclosure I'm using has an extra hole for a switch or maybe a knob.

I'm probably going to build a single knob mosfet boost so I was wondering what guitarists might also like the boost to do (I'm a bass player so my needs are usually different).

Hi/Low switch?

Treble Bleed?

Treble Boost?

Bass Boost?

Bass cut?

Tone Knob?

Extra LED?

Diode Switch?

Gain boost?

Extra Boost Stage?

Are there are two-knob mosfet boost circuits worth looking at?

[Brisance]

maybe add switchable clipping diodes?

[GibsonGM]

Rather than diodes, which would make it a distortion and no longer a boost (ok, on a switch, it's still an OPTION, I get it   how about a bass CUT? 

As a lead player, I know I prefer to be a little less muddy when I go for a solo.   So I build MY MOSFET boosts with this in mind.   I tend to use smaller in/out caps to do this, but you COULD very easily add a variable HPF to the output....you'd then have the option of keeping a fatter tone if you want it, or dialing that low end back some. 

[acehobojoe]

switch a 47uF to ground connected to the source... or possibly a 100uF. This will increase the gain a lot.. which will cause the mosfet to clip. Be careful with mosfets.... they aren't as robust as other transistors.

[Bill Mountain]

What frequencies to you cut?

I had also considered a switchable notch filter but I'm not sure what notch frequency would be advantageous for guitarists.

I do think switchable cabs or a cap blend at the beginning would be nice and simple.

I'm trying to avoid a lot of R&D because I wanted to have it done before a jam on Sunday.

[midwayfair]

The one I use is just Jack Orman's MOSFET booster with a switch inside to go between a 1uF cap in series with the gain pot and a 10uF. I usually leave it on the 1uF.

If you want to do the notch filter with a really good tone control, my favorite spot is 250Hz. This is the "mud" frequency.

Here's what I would do: Hardwire one MOSFET boost to ~2x gain (6dB) by using a drain resistor of 39K and a source resistor of 20K (you can use 22k, it's fine), then follow it with a Big Muff Control with the following values:

Jack's BMP tone control picture:


Pot value: 250K
In place of the 39K: 270K
In place of the 4nF: 1nF
In place of the 22K: Omit
In place of the 10nF: 5.6nF

This makes a ~-8db cut at 250Hz and gives you a low pass shelf filter that includes a treble boost as well as a treble cut, and a low-pass filter all the way CCW.

Then follow that with a standard MOSFET booster, except maybe put the source bypass cap on a switch to go between the big cap and a 1uF, which will let you run it as a mids/treble booster. Also, you'll have to make the 10M going to Vb a 1M and the input cap a 22nF to get the proper cutoffs.

You should be able to find layouts easily for the MOSFET booster and for an add-on BMP tone stack, so there shouldn't be any R&D involved.

[Bill Mountain]

Thanks man!  This is perfect.  I also have a three knob enclosure I could use for gain/tone/tone-bypass.

[petemoore]

  This guy:

  http://www.muzique.com/schem/mosfet.htm
  Has boost, read the article, note as above mentioned that R6 value 'feeds' the amp device, this and the biasing design of the circuit is what makes it boast 35db boost, it is ''loud''.
  The output capacitor is where the any DC blocking tone control could be. A passive TC doesn't add, it takes away signal content.
  There are many ways that the source components [R6 in particular] could be made switchable.
  If making one this time, I'd ponder and experiment with adding resistance of some sort parallel with R6, make it switchable so two or three levels of boost would be available.
  1 Bypass = gain level I
  2 Pedal engaged = adjustable gain boost level II
  3 Pedal engaged and R6 value dropping pot engaged = adjustable gain boost III 
  After some use the bypass switch might rarely be used.
  I would also put adjustable gain boost III [R6 parallel pot] on a 3PDT of 'the distortion bypass switch, this way when the distortion is kicked on, the gain of the boost is kicked up to drive the distortion harder. Then he'd have it all except Boost II feeding distortion, [accomodations of course could be made for that too]:
  A Bypass
  B Gain Boost II
  C Gain Boost III
  Hard boosted overdrive when the booster is engaged.
I used to check my spellling but refuse to look up words like spelling or accomodations or because it turns out the auto spell checkers are better at highlighting correct spelling than errors, I'm pretty sure the above has no spelling errors.

[Bill Mountain]

That's two votes for a modded AMZ mosfet boost.  I like the idea of gain setting resistor but like you said once the preferences is decided then it'll never get used again.  It's worth a shot.

I also remembered that I had an empty big muff pcb that I can use.  I can retrofit a mosfet into a npn circuit so that's an option.  I would just need to squeeze some protection diodes in there  It would easily allow for more than one tranny and a tone pot.  I could also adjust several components with a switch.

Damn.   This last minute gift for my guitarist is turning into quite the experiment!

[Bill Mountain]

Here's what I came up with using my BMP PCB:

A JFet buffer (basically a JFet boost with no gain) into a level pot into a wide open Mosfet with a switch to select between two different source resistors (for a gain boost) and a switch to select between two different bright caps.  I already had an enclosure with three holes.

I though long and hard about the EQ.  I know that's something I would want but I don't think he'd use it.

I'll post a schematic once I can confirm it's working.

[antonis]

switch a 47uF to ground connected to the source... or possibly a 100uF. This will increase the gain a lot..

Depenting on Source's resistor and frequency...

(you can name it as frequency variable gain... )

[Gus]

Bill
Something I posted before http://www.diystompboxes.com/smfforum/index.php?topic=105324.0 note the gain control

Depending on the MOSFET circuit when you adjust gain with switched source resistors you can also change the biasing.  Maybe you want that maybe you don't.

If using a power MOSFET you also have gate capacitance you need to charge and discharge by the buffer and volume control
This can roll off the high end at max volume setting and roll off more at lower volume setting because of the series resistance of the level/volume control
This can be a good thing
Adjustable by output resistance of the buffer and volume control

I have built opamp buffer or gain stage to power MOSFET circuits they can be fun to adjust.  Drain current vs transconductance, biasing, EQ between stages etc.

If you are using a JFET input buffer to a passive guitar or bass it is easy to change the input resistance.  In the following link the input resistance changes from a bootstrapped input resistance to a LPB like input resistance you can do this by adding a parallel input resistor with a JFET buffer to go from high to lower 
http://www.diystompboxes.com/smfforum/index.php?topic=99139.0

[Bill Mountain]

So...this is what I ended up with.  Using the BMP PCB made it easier (even though I was up until 2:30!!!) but I had to make some concessions.  Because I didn't use the AMZ or RG's circuit, the Mosfet stage didn't have as much boost as I wanted so I increased the gain of the input stage so the circuit would have plenty of boost overall.

Q1 is a J201 and Q2 is a BS170.

The bright switch is great.  The gain pot is 1M and it uses an on/off/on switch so it goes from Low mid boost (1n)/to no boost (off)/to hi boost (220p) except at max gain of course.

The gain switch is noisy so if I was going to do it again I would leave the 1k in the circuit and just switch the 100R in and out.  The gain difference is quite noticeable but the bias only changes from 4.5V to 5.5V so I'm really happy with that.

Thank you all so much for your help.  I really didn't know what I was going to build when I started this thread.

Gus, thank you again for your excellent circuits.  They are always well thought out.  I had unfortunately finished building this before I saw your post.

I tried this out on my bass as well and I was quite happy with it so I may build it again and incorporate some of your guys' other suggestions.

[Gus]

Bill

If you don't want the bias to change you can cap couple the 100 ohm, However you might like the bias shift.

[antonis]

wrong post...