Beginner's Distortion Design

[mattthegamer463]

I've built a few, so I decided I'd take a crack at designing.

A MXR Dist+ with an adjustable negative feedback resistor, BMP-esque tonestack and a 12 position rotary switch for switching diode configurations.



Is this not too shabby or does it need a lot of work?  I just thought some input could spice this up into something of value to someone.

If the rotary switch portion isn't clear than heres my hand-drawing of the setup, with planned diodes.  Recommendations of diode configs/types is majorly recommended.

http://img.photobucket.com/albums/v321/mattthegamer463/Guitar%20Pedals/DSC01299-Copy.jpg

Just wondering what you guys think.  Thanks.


Matt

[svstee]

I like the rotary switch idea and the tonestack. I'm wondering how useful haveing the negative feedback resistor will be though. Maybe put it in series with a fixed value?

[PRR]

It'll work. Sometimes best to slap solder and find out what happens.

On paper (does anybody do paper these days?).......

C4 seems large.

Why R7? I could see 10K here, but 1Meg is killing precious signal, adding hiss, and begging for radio reception.

Why R5 AND R6? They do very nearly the same thing.

Not gonna ask why 741.

When R10 is more than about 1K (20% rotation), the diode effect against R9 may be very subtle.

Have you simmed or tested this specific tone control? It seems to have a deep mid-dip and not a ton of variation. I have seen similar except with a resistor to ground at the C6 R11 joint (node 12).

[mattthegamer463]

Why R7? I could see 10K here, but 1Meg is killing precious signal, adding hiss, and begging for radio reception.

Why R5 AND R6? They do very nearly the same thing.

Not gonna ask why 741.

When R10 is more than about 1K (20% rotation), the diode effect against R9 may be very subtle.

Have you simmed or tested this specific tone control? It seems to have a deep mid-dip and not a ton of variation. I have seen similar except with a resistor to ground at the C6 R11 joint (node 12).

R7 is a mistake, a lazy copy-paste mess up.  Fixed.

R6 does seem useless, now that I've read more into negative feedback.  I'll switch with a normal 1M resistor.

I could go with a NE5532 or something, still thinking about that. 

What would you suggest to remedy that R10 issue?  Log pot?

I haven't tested the tone control, but it was in the Pentadriver Submini tube pedal I built based on Frequencycentral's schematic and it sounds pretty nifty to me.

[PRR]

> R10 issue?

Try R9 5K or 10K.

> I could go with a NE5532

Oh, I have no objection to 741. It's not a great audio chip, but people today diss it much more than it deserves.

5532 is not a good choice with 1Meg resistors. R1 R2 R3 aim to set the input at 4.5V, or at least near half-way up the total battery. This is true if there is NO current into the "+" input. It will be very nearly true for a FET chip like TL071. It will be close enough for 741. But the 5532 has fat input transistors for low-low hiss with low to medium impedance. These fatties draw significant DC current: 1uA. A worst-spec 5532's input current through 1Meg (actually 1.5Meg) will force 1.5 Volts in R3, throwing the in and out pins off of half-way up the battery voltage. About +3V bias. Which might be OK, but 5532 was aimed at higher-voltage systems, won't pull-down as far as some other chips. You may only have 1V of negative swing left, which would make a diode-selector somewhat moot.

TL071 is a great choice. 741 if it is on your bench begging to work; not "wrong", just unfashionable.

[Electron Tornado]

Kind of looks like a variation on a theme:

http://www.generalguitargadgets.com/pdf/ggg_isd9_sc.pdf

Does it still have a good volume level without a booster stage after the tone stack?

[mattthegamer463]

I haven't tested it, its just an idea. I need some pots and a rotary switch before I can prototype it.  I'll nab a few TL071s and see what happens.

[GibsonGM]

I'd like to see a switchable tone stack, too, or maybe just a defeat switch.  That would make an already sick idea VERY sick )  Not a lot of missing diode configs with 12 options! 

The 741 will give you "chip distortion" in a starved-current kind of mode, like the original, due to the 1M bias resistors.   A '71 will not distort as much, esp. if you lower the meggers to between 10k and 100k.   It's the main difference between Dist+ and Marshall Guvn'or, really. Neither way is "better".  Be sure to pop both chips in the socket to see what it sounds like!!!! 
Good concept, you're going to have to post 12 clips, ha ha.

[mattthegamer463]

I like the idea of a switchable tone-stack.  Maybe I should just go all out 3-band EQ?

[arawn]

you could do a dual bandpass with a low and high. Just when youi do the calculations make sure there is a little crossover in the middle.

[mattthegamer463]

Fiddling with the idea of a 3-band EQ.



EQ is shamelessly stolen from http://freecircuitdiagram.com/2008/11/25/3-band-graphic-equalizer-circuit/. It should make a decent starting point to play with.



Ordered the pots, rotary switch and diodes from smallbear, so when those arrive I'll proto this and see how it goes.

[mattthegamer463]

I've been thinking, would there be any benefit to a second, switchable amplification stage?  Before or after the diodes, for extra gain if desired?  I'm just wondering if that would be overkill.  Maybe a switch to relocate the 2nd stage from before the diodes to after the diodes?  That could cause an interesting clipping situation.

[Joe Hart]

Not gonna ask why 741.

So it'll sound awesome, silly! Noise is just an added benefit.
-Joe Hart

[mattthegamer463]

Fiddled with a layout in Sketchup:



And built the diode switcher sub-board:





1N270, NOS Ge (from smallbear) 1N1418, 1N914, 1N4001, and red LEDs in various configurations.  The 1N1418, 1N914, 1N4001, and red LEDs feature a symmetrical setup as well as an asymmetrical setup.

[petemoore]

  741 is the only thing makes a DIST+ sound IME.
  The noise, slight increase seems small because I always seem to have better sources of noise...this is a distorter, a little noise is your friend.
  Not to say the slight noise can't be cured with a different opamp and work, gives a different tone though.

[slotbot]

hey Matt

2 things i would suggest

1) put an output capacitor between the the output of the 2nd opamp and the volume knob. there will be a dc voltage (~1/2 Vcc) at the output if you run the opamp of a single supply as your doing. You probably want to get rid of it so this wont pop every time you switch it in and out of the signal chain probably around .1uf - 1uf is ok

2) double check the pin out of the second opamp, compared to the diagram you linked, you have the + and - terminals reversed.

looks neat otherwise

[meffcio]

Fiddled with a layout in Sketchup:

Whoa, dou you have some templates for the pots, jack etc? Or did you make everything from scratch?
Or are the templates already in Sketchup? Never used that ;p

[Fender3D]

1N270, NOS Ge (from smallbear) 1N1418, 1N914, 1N4001, and red LEDs in various configurations.  The 1N1418, 1N914, 1N4001, and red LEDs feature a symmetrical setup as well as an asymmetrical setup.

Sorry for the noobish question:
do 1N1418, 1N914, 1N4001 really sound different?
Also, for what about the noize...
at high gain you'll have plenty no matter the op-amp used...

[mattthegamer463]

1N270, NOS Ge (from smallbear) 1N1418, 1N914, 1N4001, and red LEDs in various configurations.  The 1N1418, 1N914, 1N4001, and red LEDs feature a symmetrical setup as well as an asymmetrical setup.

Sorry for the noobish question:
do 1N1418, 1N914, 1N4001 really sound different?
Also, for what about the noize...
at high gain you'll have plenty no matter the op-amp used...

Theres quite a difference between Ge and Si diodes, and LEDs have a huge difference, I find.

My schematic has changed quite a bit, I will update it momentarily.  Thanks for inputs, all.

[PRR]

> shamelessly stolen from

Steal with pride, but also with care. You have the "+" and "-" inputs swapped.