Little Jim - a Marshall inspired MOSFET distortion pedal design

[rankot]

How important is V_th in this case?

[Phend]

I can't answer the threshold question, maybe one of the others can...

[Phend]

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 ?

[11-90-an]

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...

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

[Phend]

Thanks 11 90 an , your help is much appreciated.

[rankot]

Just finished: Little Jim MOSFET overdrive with P-MOS BS250 transistors, V_th 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?

[rankot]

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)?

[Gus]

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]

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]

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

[rankot]

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:

[Vivek]

Hello Ranko,

I was going to input Little Jim into LTSPICE

is it possible that you share your file with BS170s

Thanks.

[rankot]

NMOS version:

Code Select Expand

Version 4
SHEET 1 3204 680
WIRE -160 -208 -160 -240
WIRE -112 -208 -160 -208
WIRE 0 -208 -32 -208
WIRE 96 -208 0 -208
WIRE 128 -208 96 -208
WIRE 0 -160 0 -208
WIRE 96 -144 96 -208
WIRE 560 -144 560 -176
WIRE 656 -144 560 -144
WIRE 960 -144 960 -176
WIRE 1056 -144 960 -144
WIRE 1312 -144 1312 -176
WIRE 1408 -144 1312 -144
WIRE -160 -128 -160 -208
WIRE 560 -96 560 -144
WIRE 656 -96 656 -144
WIRE 960 -96 960 -144
WIRE 1056 -96 1056 -144
WIRE 1312 -96 1312 -144
WIRE 1408 -96 1408 -144
WIRE -160 -16 -160 -48
WIRE 0 -16 0 -80
WIRE 0 -16 -160 -16
WIRE 96 -16 96 -80
WIRE 96 -16 0 -16
WIRE -160 0 -160 -16
WIRE 560 48 560 -16
WIRE 656 48 656 -32
WIRE 656 48 560 48
WIRE 720 48 656 48
WIRE 848 48 784 48
WIRE 960 48 960 -16
WIRE 1056 48 1056 -32
WIRE 1056 48 960 48
WIRE 1088 48 1056 48
WIRE 1200 48 1152 48
WIRE 1312 48 1312 -16
WIRE 1408 48 1408 -32
WIRE 1408 48 1312 48
WIRE 1456 48 1408 48
WIRE 1712 48 1712 32
WIRE 656 64 656 48
WIRE 1056 64 1056 48
WIRE 1408 64 1408 48
WIRE -480 96 -560 96
WIRE -288 96 -480 96
WIRE -160 96 -224 96
WIRE -112 96 -160 96
WIRE 16 96 -32 96
WIRE 64 96 16 96
WIRE -160 128 -160 96
WIRE 1712 128 1712 112
WIRE -480 144 -480 96
WIRE 16 144 16 96
WIRE 112 144 112 128
WIRE 176 144 112 144
WIRE 288 144 240 144
WIRE 400 144 368 144
WIRE 608 144 400 144
WIRE 848 144 848 48
WIRE 1008 144 848 144
WIRE 1200 144 1200 48
WIRE 1360 144 1200 144
WIRE 112 176 112 144
WIRE 400 176 400 144
WIRE 1408 224 1408 160
WIRE 1408 224 1328 224
WIRE 576 240 576 224
WIRE 656 240 656 160
WIRE 656 240 576 240
WIRE 976 240 976 224
WIRE 1056 240 1056 160
WIRE 1056 240 976 240
WIRE 1456 256 1456 48
WIRE 1552 256 1520 256
WIRE 1712 256 1712 208
WIRE 1712 256 1632 256
WIRE 1776 256 1712 256
WIRE -480 272 -480 224
WIRE -160 272 -160 208
WIRE -160 272 -480 272
WIRE 16 272 16 208
WIRE 16 272 -160 272
WIRE 112 272 112 256
WIRE 112 272 16 272
WIRE 576 272 576 240
WIRE 656 272 656 240
WIRE 848 272 848 144
WIRE 1056 272 1056 240
WIRE 1200 272 1200 144
WIRE 1408 272 1408 224
WIRE -480 288 -480 272
WIRE 400 288 400 256
WIRE 1328 288 1328 224
WIRE 1712 288 1712 256
WIRE 976 304 976 240
WIRE 576 384 576 336
WIRE 656 384 656 352
WIRE 656 384 576 384
WIRE 848 384 848 352
WIRE 976 384 976 368
WIRE 1056 384 1056 352
WIRE 1056 384 976 384
WIRE 1200 384 1200 352
WIRE 1328 384 1328 352
WIRE 1408 384 1408 352
WIRE 1408 384 1328 384
WIRE 656 400 656 384
WIRE 1056 400 1056 384
WIRE 1408 400 1408 384
WIRE 1712 400 1712 368
FLAG -480 288 0
FLAG -160 0 0
FLAG 656 400 0
FLAG 1776 256 Out
IOPIN 1776 256 Out
FLAG -160 -240 V+
FLAG 112 32 V+
FLAG 128 -208 Vr
FLAG 400 288 Vr
FLAG 560 -176 V+
FLAG 1056 400 0
FLAG 960 -176 V+
FLAG 848 384 Vr
FLAG 1408 400 0
FLAG 1312 -176 V+
FLAG 1200 384 Vr
FLAG 1712 400 0
FLAG 1712 32 0
FLAG -560 96 IN
IOPIN -560 96 In
SYMBOL voltage -160 -144 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 9
SYMBOL voltage -480 128 M0
WINDOW 123 24 124 Left 2
WINDOW 39 24 152 Left 2
SYMATTR Value2 AC ac 1
SYMATTR SpiceLine Rser=11k
SYMATTR InstName V2
SYMATTR Value SINE(0 .07 440)
SYMBOL cap 0 144 R0
SYMATTR InstName C1
SYMATTR Value 150p
SYMBOL res -144 224 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R2
SYMATTR Value 1Meg
SYMBOL res 672 368 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R4
SYMATTR Value 4k7
SYMBOL res 576 0 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R9
SYMATTR Value 4k7
SYMBOL res -16 80 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R11
SYMATTR Value 10k
SYMBOL cap -288 112 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName C3
SYMATTR Value 100n
SYMBOL njf 64 32 R0
SYMATTR InstName J1
SYMATTR Value J112
SYMBOL res 96 272 M180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R1
SYMATTR Value 4k7
SYMBOL cap 240 128 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C2
SYMATTR Value 220n
SYMBOL res -128 -192 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R5
SYMATTR Value 10k
SYMBOL res -16 -176 R0
SYMATTR InstName R7
SYMATTR Value 8k2
SYMBOL cap 80 -80 M180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C7
SYMATTR Value 10µ
SYMBOL res 384 160 R0
SYMATTR InstName Gain
SYMATTR Value 10k
SYMBOL cap 784 32 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C4
SYMATTR Value 22n
SYMBOL cap 592 336 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C5
SYMATTR Value 220n
SYMBOL res 1072 368 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R3
SYMATTR Value 4k7
SYMBOL res 976 0 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R6
SYMATTR Value 4k7
SYMBOL cap 1152 32 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C6
SYMATTR Value 22n
SYMBOL cap 992 368 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C8
SYMATTR Value 10µ
SYMBOL res 864 368 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R13
SYMATTR Value 100k
SYMBOL res 1424 368 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R8
SYMATTR Value 4k7
SYMBOL res 1328 0 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R10
SYMATTR Value 4k7
SYMBOL cap 1520 240 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C9
SYMATTR Value 220n
SYMBOL cap 1344 352 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C10
SYMATTR Value 10µ
SYMBOL res 1216 368 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R15
SYMATTR Value 100k
SYMBOL res 1728 384 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R12
SYMATTR Value 100k
SYMBOL res 1648 240 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R14
SYMATTR Value 22k
SYMBOL res 384 128 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName Gain1
SYMATTR Value 1
SYMBOL res 1728 224 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName Tone
SYMATTR Value 10k
SYMBOL cap 1728 112 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C12
SYMATTR Value 100n
SYMBOL nmos 608 64 R0
SYMATTR InstName M1
SYMATTR Value BS170
SYMBOL nmos 1008 64 R0
SYMATTR InstName M2
SYMATTR Value BS170
SYMBOL nmos 1360 64 R0
SYMATTR InstName M3
SYMATTR Value BS170
SYMBOL zener 592 224 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 0
SYMATTR InstName D1
SYMATTR Value BZX84C15L
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL zener 992 224 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 0
SYMATTR InstName D2
SYMATTR Value BZX84C15L
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL zener 1344 224 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 0
SYMATTR InstName D3
SYMATTR Value BZX84C15L
SYMATTR Description Diode
SYMATTR Type diode
TEXT -472 -56 Left 2 !.tran 0 .1 .08
TEXT -472 -24 Left 2 !;.ac oct 16 10 50k
TEXT -472 -88 Left 2 !.param dr=500
TEXT -488 352 Left 2 ;ALOG
TEXT -408 352 Left 2 !;.step param Rx list 1 180k 360k 540k 720k 900k 920k 940k 965k 999k
TEXT 1544 -96 Left 2 !;.noise Out In oct 32 10 20k

PMOS version:

Code Select Expand

Version 4
SHEET 1 3204 680
WIRE -160 -208 -160 -240
WIRE -112 -208 -160 -208
WIRE 0 -208 -32 -208
WIRE 96 -208 0 -208
WIRE 128 -208 96 -208
WIRE 0 -160 0 -208
WIRE 96 -144 96 -208
WIRE 560 -144 560 -176
WIRE 656 -144 560 -144
WIRE 960 -144 960 -176
WIRE 1056 -144 960 -144
WIRE 1312 -144 1312 -176
WIRE 1408 -144 1312 -144
WIRE -160 -128 -160 -208
WIRE 560 -112 560 -144
WIRE 960 -112 960 -144
WIRE 1312 -112 1312 -144
WIRE 656 -96 656 -144
WIRE 1056 -96 1056 -144
WIRE 1408 -96 1408 -144
WIRE -160 -16 -160 -48
WIRE 0 -16 0 -80
WIRE 0 -16 -160 -16
WIRE 96 -16 96 -80
WIRE 96 -16 0 -16
WIRE -160 0 -160 -16
WIRE 560 48 560 -32
WIRE 656 48 656 -32
WIRE 656 48 560 48
WIRE 960 48 960 -32
WIRE 1056 48 1056 -32
WIRE 1056 48 960 48
WIRE 1312 48 1312 -32
WIRE 1408 48 1408 -32
WIRE 1408 48 1312 48
WIRE 1712 48 1712 32
WIRE 1312 64 1312 48
WIRE -480 96 -560 96
WIRE -288 96 -480 96
WIRE -160 96 -224 96
WIRE -112 96 -160 96
WIRE 16 96 -32 96
WIRE 64 96 16 96
WIRE -160 128 -160 96
WIRE 656 128 656 48
WIRE 1056 128 1056 48
WIRE 1408 128 1408 48
WIRE 1712 128 1712 112
WIRE -480 144 -480 96
WIRE 16 144 16 96
WIRE 112 144 112 128
WIRE 176 144 112 144
WIRE 288 144 240 144
WIRE 416 144 368 144
WIRE 560 144 560 128
WIRE 560 144 416 144
WIRE 608 144 560 144
WIRE 960 144 960 128
WIRE 960 144 848 144
WIRE 1008 144 960 144
WIRE 1312 144 1312 128
WIRE 1312 144 1200 144
WIRE 1360 144 1312 144
WIRE 112 176 112 144
WIRE 416 176 416 144
WIRE 656 256 656 224
WIRE 704 256 656 256
WIRE 848 256 848 144
WIRE 848 256 768 256
WIRE 1056 256 1056 224
WIRE 1104 256 1056 256
WIRE 1200 256 1200 144
WIRE 1200 256 1168 256
WIRE 1408 256 1408 224
WIRE 1408 256 1328 256
WIRE 1456 256 1408 256
WIRE 1552 256 1520 256
WIRE 1712 256 1712 208
WIRE 1712 256 1632 256
WIRE 1776 256 1712 256
WIRE -480 272 -480 224
WIRE -160 272 -160 208
WIRE -160 272 -480 272
WIRE 16 272 16 208
WIRE 16 272 -160 272
WIRE 112 272 112 256
WIRE 112 272 16 272
WIRE -480 288 -480 272
WIRE 416 288 416 256
WIRE 848 288 848 256
WIRE 1200 288 1200 256
WIRE 1712 288 1712 256
WIRE 656 304 656 256
WIRE 1056 304 1056 256
WIRE 1328 304 1328 256
WIRE 1408 304 1408 256
WIRE 848 384 848 368
WIRE 1200 384 1200 368
WIRE 656 400 656 384
WIRE 1056 400 1056 384
WIRE 1328 400 1328 368
WIRE 1408 400 1408 384
WIRE 1712 400 1712 368
FLAG -480 288 0
FLAG -160 0 0
FLAG 656 400 0
FLAG 1776 256 Out
IOPIN 1776 256 Out
FLAG -160 -240 V+
FLAG 112 32 V+
FLAG 128 -208 Vr
FLAG 416 288 Vr
FLAG 560 -176 V+
FLAG 1056 400 0
FLAG 960 -176 V+
FLAG 848 384 Vr
FLAG 1408 400 0
FLAG 1312 -176 V+
FLAG 1200 384 Vr
FLAG 1712 400 0
FLAG 1712 32 0
FLAG -560 96 IN
IOPIN -560 96 In
FLAG 1328 400 0
SYMBOL voltage -160 -144 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 9
SYMBOL voltage -480 128 M0
WINDOW 123 24 124 Left 2
WINDOW 39 24 152 Left 2
SYMATTR Value2 AC ac 1
SYMATTR SpiceLine Rser=11k
SYMATTR InstName V2
SYMATTR Value SINE(0 .07 440)
SYMBOL cap 0 144 R0
SYMATTR InstName C1
SYMATTR Value 150p
SYMBOL res -144 224 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R2
SYMATTR Value 1Meg
SYMBOL res 672 400 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R4
SYMATTR Value 4k7
SYMBOL diode 576 128 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL res 576 -16 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R9
SYMATTR Value 4k7
SYMBOL pmos 608 224 M180
SYMATTR InstName M1
SYMATTR Value BS250P
SYMBOL res -16 80 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R11
SYMATTR Value 10k
SYMBOL cap -288 112 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName C3
SYMATTR Value 100n
SYMBOL njf 64 32 R0
WINDOW 3 60 3 Left 2
SYMATTR Value J112
SYMATTR InstName J1
SYMBOL res 96 272 M180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R1
SYMATTR Value 4k7
SYMBOL cap 240 128 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C2
SYMATTR Value 220n
SYMBOL res -128 -192 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R5
SYMATTR Value 9k1
SYMBOL res -16 -176 R0
SYMATTR InstName R7
SYMATTR Value 10k
SYMBOL cap 80 -80 M180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C7
SYMATTR Value 10µ
SYMBOL res 400 160 R0
SYMATTR InstName Gain
SYMATTR Value 10k
SYMBOL cap 768 240 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C4
SYMATTR Value 22n
SYMBOL cap 672 -32 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C5
SYMATTR Value 220n
SYMBOL res 1072 400 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R3
SYMATTR Value 4k7
SYMBOL diode 976 128 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D2
SYMATTR Value 1N4148
SYMBOL res 976 -16 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R6
SYMATTR Value 4k7
SYMBOL pmos 1008 224 M180
SYMATTR InstName M2
SYMATTR Value BS250P
SYMBOL cap 1168 240 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C6
SYMATTR Value 22n
SYMBOL cap 1072 -32 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C8
SYMATTR Value 10µ
SYMBOL res 864 384 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R13
SYMATTR Value 100k
SYMBOL res 1424 400 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R8
SYMATTR Value 4k7
SYMBOL diode 1328 128 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D3
SYMATTR Value 1N4148
SYMBOL res 1328 -16 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R10
SYMATTR Value 4k7
SYMBOL cap 1520 240 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C9
SYMATTR Value 220n
SYMBOL cap 1424 -32 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C10
SYMATTR Value 10µ
SYMBOL res 1216 384 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R15
SYMATTR Value 100k
SYMBOL pmos 1360 224 M180
SYMATTR InstName M3
SYMATTR Value BS250P
SYMBOL res 1728 384 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R12
SYMATTR Value 100k
SYMBOL res 1648 240 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R14
SYMATTR Value 22k
SYMBOL res 384 128 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName Gain1
SYMATTR Value 1
SYMBOL res 1728 224 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName Tone
SYMATTR Value 10k
SYMBOL cap 1728 112 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C12
SYMATTR Value 100n
SYMBOL cap 1344 368 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C11
SYMATTR Value 10n
TEXT -472 -56 Left 2 !.tran 0 .1 .08
TEXT -472 -24 Left 2 !;.ac oct 16 10 50k
TEXT -472 -88 Left 2 !.param dr=500

Sorry for the late response, I was on a summer holiday! 

[Vivek]

Thanks for the LTSPICE files

I can now see the same schematic that you posted




A) When I ran it, it said BS170 model not found, so I added this directive:

.model BS170 VDMOS VTO=1.824 RG=270 RS=1.572 RD=1.436 RB=.768 KP=.1233 Cgdmax=20p Cgdmin=3p CGS=28p Cjo=35p Rds=1.2E8 IS=5p Bv=60 Ibv=10u Tt=161.6n

Does this match the model you used ?


B) I see that there is place near R9, R6 and R10. Is that for you to try out capacitors ? What works best ?


C) I see that the Zeners are not connected. Is that correct ?

D) Please teach me about the noise analysis you did. Thanks

E) I get the "Mid Hump" at around 20Khz !! Do my results match yours ?

[GibsonGM]

Just out of curiosity - how does one take the list that Rankot posted and import it to LT Spice?  I've never had the need to do this, but now I am interested Thx

[Vivek]

Copy it from here

make a new text file

call it "NMOS Little Jim.asc"

Paste the text into it

Open with LTSPICE

find that some models are missing

Find that some characters are not in the correct character set and cause peoblems

fix the problems

Thank the person who posted the Net List

[rankot]

Sorry, I thougth those MOSFET models are part of the standard library. I have them there like this:

Code Select Expand

.model BS170 VDMOS (VTO=1.824 RG=270 RS=1.572 RD=1.436 RB=.768 KP=.1233 Cgdmax=20p Cgdmin=3p CGS=28p Cjo=35p Rds=1.2E8 IS=5p Bv=60 Ibv=10u Tt=161.6n mfg=Fairchild Vds=60 Ron=1.5)

.model BS250P VDMOS (pchan Rg=160 VTO=-3.193 RS=2.041 RD=0.697 IS=2E-13 KP=0.277 Cjo=105p LAMBDA=1.2E-2 RB=0.309 Rds=1.2E8 Cgdmax=57p Cgdmin=5p CGS=47p TT=86.56n BV=45 IBV=10u mfg=Fairchild Vds=-45 Ron=14 Qg=1.8n)

Zenners are there to protect MOSFET in real life, I just wanted to see if that kind of protection affects the tone.

Empty space around those resistors in NMOS version are simply there because I was too lazy to erase lines. If you compare N/PMOS versions, you will see those capacitors are on their places.

I copied that noise analysis line from somebody other's file, but didn't run it yet. 

I don't get a hump, and at 20kHz it certainly can't be mids

[Vivek]

I will try again, using your models

[rutgerv]

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

I'vr recently made a DIY version of an AMT soldano clone with JFETs. AMT has An interesting way to simulate tubes with JFETs and I've been wondering if the same technology could be used to create a circuit with the sound of little Jim but less noise. One things in which the AMT circuit excels is low noise at high gain, but I love the tone of the little Jim. What do you think?

[Vivek]

Please post link to the AMT Soldano schematic.

Thanks