More gain from Dist + with LED clipping diodes?

[Ell]

How do I get more gain out of an MXR Distortion plus with LED clipping diodes?

I picked up a real Dist + a while ago. I did a true bypass mod.
I reused the original switch, which is DPDT, so I decided to use LED clipping diodes instead of Germanium, so one could function as the on/off indicator. The problem is, the LEDs aren't very bright. So, the clipping threshold is higher now (and so is the volume), also there isn't that much distortion.

I made a clone a while ago with switchable diodes, following Brian Wampler's advice, and it had good gain even with LED clipping.
https://www.premierguitar.com/gear/mxr-distortion-plus-mods

I'm using one 3mm, and one 5mm, for a little bit of asymetrical clipping. What should I change? Could it be the distortion pot value which is 470KC? I changed R2 to 1K, as Wampler suggested, but it didn't have the intended effect. I can't really move the placement of the gain pot without completely rebuilding the whole thing as a clone.

[antonis]

An easy fix (for gain increase) should be to lower R2 value more..
Brian's mod calls for 1 + 1000/1 gain (1M/1k plus unity) so for 470k pot, 470R for R2 should be fine..
(of course, you'll have to double C3 value - about 470nF..)

>I changed R2 to 1K, as Wampler suggested, but it didn't have the intended effect.<
I presume you also changed C3 value to 220nF..
If not, you've raised HPF corner frequency from 720Hz up to 3380Hz, resulting into diminished gain for frequencies lower than 3.38kHz..

P.S.
Could you plz post a schematic of your particular modded circuit to have a discussion reference..??

[Ell]

I didn't change C3! Thanks for the tip Antonis, I will try that out and see if it gives the desired effect!

I'm not sure the formatting is right, but it's the schematic from his Premier guitar article at the bottom, only with a 5 way switch controlling hard clipping diodes. They all sounded pretty good, no diode, LED, asymmetrical silicon, germanium and symmetrical silicon.

There are a couple of "issues" with the stock Distortion plus. Low volume, and high noise. I was thinking that changing to a different type of diode would help reduce noise as well.


That's the schematic from the premier guitar article, and here was my layout based on it.

[antonis]

There are a couple of "issues" with the stock Distortion plus. Low volume, and high noise.

Volume relates to clipping diode(s) forward voltage drop..
Noise could be related to lack of NFB branch capacitor..
(forming a LPF with respective feedback resistor..)

P.S.
I shouldn't use 1M resistors neitrher for voltage divider nor for 741 biasing..

https://www.analog.com/media/en/technical-documentation/application-notes/AN-581.pdf?doc=AD7699.pdf
(focus on first 3 pages, especially about various filter bandwidths and items values..)

[Mark Hammer]

1) Let's differentiate between "gain", "clipping", and output level.  The forward voltage of the diodes used in this configuration (i.e., going to ground) sets the ceiling on what maximum output level can be.  Diodes with higher Vf will allow for greater maximum output.  That is related to, but separate from, "gain" and clipping.

2)  The LM741 used in the classic circuit has limits on headroom, when used with a 9V supply.  As a result, when gain is greater than around 50x or so, the chip itself is clipping, irrespective of whether there are any diodes at all.  With diodes, this essentially makes it a double clipper (once in the chip, and once more by the diodes).

3)  Gain can certainly be increased, by tinkering with the feedback resistance of the op-amp.  One could probably bring the amplified level out of the chip up to the Vf of a pair of LEDs, but the question is whether at that point you're hearing more of the LED clipping or more of the LM741 headroom-limit clipping.

4)  One of the quirks of the Dist+ is that it is designed to roll off bass as the gain is increased.  So much so that at max gain, the rolloff starts around 720hz.  Keep in mind that pickup output is a function of how much ferromagnetic material you're wiggling over the pickup.  So thicker/wound strings will provide more output than thinner/unwound.  Applying the same gain to a hotter input signal will result in greater clipping.  So, if you changed the cap in series with the Gain pot to a higher value (e.g., 220nf), there would be more bass in your signal and whatever diodes you have in there would be more likely to clip.

5)  Part of the noise issue is not just the use of an "unquiet" op-amp lie the 741, but the fact that little attempt is made to restrain the hiss and fizz.  You can reduce (though never eliminate) the audible hiss/noise in the final output by altering the value of two capacitors.  First, use a 47-68pf cap in the feedback of the op-amp, in parallel with the 1M resistor.  Second, change the 1000pf cap in parallel with the clipping diodes for something in the 3300-4700pf range.  Those changes will still retain the bite, but attenuate a lot of the noise and fizz.

6)  Several schematics of the circuit are floating around.  When MXR initially produced the pedal, they foolishly used a 10k volume pot.  This was a bad choice.  A 50k-100k log pot is a much better choice.  And while I know that a 500k reverse-log pot is normally used for the Gain control, the original pedal was more or less designed to be able to go from a semi-clean boost to a fuzzier sound.  Reverse log passes through the highest pot resistances quickly, such that low gain settings comprise only a small portion of the pot's rotation.  But that's really a holdover from the days when a "pedalboard" might consist of maybe 2-3 pedals on the floor, with short patch cables that could be easily unplugged after playing, in order to maximize battery life.  Does the average player really need to use the Dist+ as a sort of clean booster anymore?  Not really.  So I just use a 50-100k linear pot to set gain.  The tone never gets clean, but then that's not what I use the pedal for.

7) I'm not a big believer in "asymmetric clipping", since it really requires specific conditions to occur, and we don't always get those.  But playing with forward voltage can yield useful outcomes.  So, consider keeping the germanium or silicon diodes, but adding one more in series with one of the diodes in use (direction doesn't matter as long as the two in series are "pointing" the same way).  This will clip one half cycle more than the other, but more importantly will result in a hotter maximum output level.  Using a pair of LEDs will do that as well, but this will achieve a better balance of diode clipping vs op-amp clipping, and provide more volume.

[aviherman5]

Very cool insight, Mark!

7) I'm not a big believer in "asymmetric clipping", since it really requires specific conditions to occur, and we don't always get those. 

Could you please elaborate on the conditions required for asymmetric clipping to occur?

Thanks!