Author Topic: Little Jim - a Marshall inspired MOSFET distortion pedal design  (Read 6970 times)

rankot

Re: Little Jim - a Marshall inspired MOSFET distortion pedal design
« Reply #100 on: February 20, 2021, 02:04:25 PM »
How important is Vth in this case?
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Phend

Re: Little Jim - a Marshall inspired MOSFET distortion pedal design
« Reply #101 on: February 20, 2021, 02:51:15 PM »
I can't answer the threshold question, maybe one of the others can...
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Phend

Re: Little Jim - a Marshall inspired MOSFET distortion pedal design
« Reply #102 on: February 23, 2021, 04:30:34 PM »
Next to the LED....
Looking at the circuit on page one, what is the purpose of C13 (10u) ?
Is it in essence in parallel with C12 ?

If C13 was not there what might the effect be...
Of course I can try it, and will, but technically what does it do ?
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11-90-an

Re: Little Jim - a Marshall inspired MOSFET distortion pedal design
« Reply #103 on: February 23, 2021, 09:21:59 PM »
C13 (i think) is an anti-LED-pop capacitor, which, as its name says, stops the LED, when triggered, from potentially making a pop.

R15, R7 and R8 are configured in a voltage divider to provide the mosfets with VDD (around 8.56v, as per schematic. R15 and C12 form a lowpass filter to filter PSU noise from 9v to VDD... or something along the lines of that... :icon_lol:

So no, C12 and C13 are not parallel with each other...
flip flop flip flop flip

Phend

Re: Little Jim - a Marshall inspired MOSFET distortion pedal design
« Reply #104 on: February 24, 2021, 05:29:41 PM »
Thanks 11 90 an , your help is much appreciated.
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rankot

Re: Little Jim - a Marshall inspired MOSFET distortion pedal design
« Reply #105 on: March 07, 2021, 08:56:12 AM »
Just finished: Little Jim MOSFET overdrive with P-MOS BS250 transistors, Vth is 3.1V. I made this PCB so it can be made with both N-MOS or P-MOS devices. Now I have to make N-MOS one so I can compare noise levels :)

Thanks Jonny, this one will be labeled as Little Jonny once I make a box for it!



It's the same one as in my schematic on previous page, but I have omitted D1 and D2. In case I decide to put some static electricity protection, which diodes do you guys suggest and where should I put 'em?
« Last Edit: March 07, 2021, 03:29:06 PM by rankot »
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rankot

Re: Little Jim - a Marshall inspired MOSFET distortion pedal design
« Reply #106 on: March 07, 2021, 03:24:43 PM »
Jonny, how to choose the right bias? I have tried with trim pot there, and it sound reasonably good from approx 2.5 to 4.3V, but it also seems that bias affects low end - higher bias, more lows and lower bias, more highs. Is it normal (or I'm just trippin)?
« Last Edit: March 07, 2021, 03:29:28 PM by rankot »
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Gus

Re: Little Jim - a Marshall inspired MOSFET distortion pedal design
« Reply #107 on: March 07, 2021, 03:53:47 PM »
Why not use three low noise BJTs  after a higher input resistance follower. It can be  SF or is can be another follower design.

Some thing from the past
https://www.diystompboxes.com/analogalchemy/sch/vulcan.html

Also look at the Bee Baa and BMP, with BMP biasing think about how the gain is set you don't need the C to B to be 100k

Also an emitter degeneration BJT circuit without the emitter resistor bypass cap can be triode like
 





jonny.reckless

Re: Little Jim - a Marshall inspired MOSFET distortion pedal design
« Reply #108 on: March 09, 2021, 02:09:05 AM »
Jonny, how to choose the right bias? I have tried with trim pot there, and it sound reasonably good from approx 2.5 to 4.3V, but it also seems that bias affects low end - higher bias, more lows and lower bias, more highs. Is it normal (or I'm just trippin)?

Yes I've noticed that too, the DC bias point seems to affect the perceived amount of bass in the signal. Generally with N channel devices I try to get the drain voltage between 2/3 and 3/4 of VDD. This gives the best looking signal on a scope with the top half of the waveform fat and rounded, and the bottom half cycle clipped pretty hard. From there, it's just a matter of personal taste. The audible difference is quite subtle until you get towards the extremes, both of which sound bad to my ears.

jonny.reckless

Re: Little Jim - a Marshall inspired MOSFET distortion pedal design
« Reply #109 on: March 09, 2021, 02:11:19 AM »
Why not use three low noise BJTs  after a higher input resistance follower. It can be  SF or is can be another follower design.
Some thing from the past
https://www.diystompboxes.com/analogalchemy/sch/vulcan.html
Also look at the Bee Baa and BMP, with BMP biasing think about how the gain is set you don't need the C to B to be 100k
Also an emitter degeneration BJT circuit without the emitter resistor bypass cap can be triode like
Yes, I've done guitar preamp designs with BJTs and more often JFETs. The Little Jim was specifically an attempt to see if I could make something simple work with MOSFETs, since I wasn't aware anybody had done that at the time (I subsequently found out about the box of rock which is a similar idea but voiced differently). You get a more aggressive, crunchy sound with MOSFETs compared with JFETs which works well but it's not subtle.

For using BJTs the Peavey Transtube style circuit works really nicely in my opinion. The Vulcan is a nice circuit but again, it's a bit noisy, and the bias is tricky to get just right.

My "go to" approach will always be NJFETs. I love the sound of them.
https://www.diystompboxes.com/smfforum/index.php?topic=123572
https://www.diystompboxes.com/smfforum/index.php?topic=118627
« Last Edit: March 09, 2021, 02:20:01 AM by jonny.reckless »

rankot

Re: Little Jim - a Marshall inspired MOSFET distortion pedal design
« Reply #110 on: March 09, 2021, 04:30:08 AM »
Yes I've noticed that too, the DC bias point seems to affect the perceived amount of bass in the signal. Generally with N channel devices I try to get the drain voltage between 2/3 and 3/4 of VDD. This gives the best looking signal on a scope with the top half of the waveform fat and rounded, and the bottom half cycle clipped pretty hard. From there, it's just a matter of personal taste. The audible difference is quite subtle until you get towards the extremes, both of which sound bad to my ears.

OK, I will try to check this on scope, this description is quite useful! What is position of Gain pot for that? I tried to match simulated waveform of NMOS circuit, but it is more like symmetrical clipped signal with gain at max. Like this:


« Last Edit: March 09, 2021, 04:34:19 AM by rankot »
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