Alter this schematic; what shoud YOU do

[Bernardduur]

Hello all!

I was just repairing the D&M distortion ( Schematic) and now the owner wants me to alter the circuit so it is a bit more usefull to him........

What should YOU do to alter this?

He uses humbucker guitars and love distortion


My thoughts now:
- Remove one of the germanium diode-pairs
- Remove the 33k resistor from the output
- Replace the output pot with a higher value
- Place a 1N400x diode for some asym clipping

More?

[SISKO]

First, ask him what HE wants. Hell tell you, more lo end, more gain.. etc etc.
Then you modifie it for him

Rock on
Bye
SISKO

[Bernardduur]

I asked

He just wanted a distortion pedal

He had it for a long time and never used it......

[RaceDriver205]

You could place a tiny booster prior to the input, this would allow even more gain.
Alternatively, you could pick a simple aditional effect, and have 1 extra switch to toggle between it and the normal distortion.
For example, one could use one of the simple octave up circuits on Tim Escobedos site. Could be done with a small bit of perf, and im sure the owner would find it awesome that his pedal does something new.

[SISKO]

Ohh!! I see.
Well, isnt that just a distortion box?.
If he wants more distortion, i would alter that 33k resistor in the bias path. In fact, i would try to remove it. But it seems that it has a ridiculous gain.
If he doesn't likes that, i would try to remove the soft clippin and add a hard clipping at the output of the OA.

Rock On
Bye
SISKO

[nordine]

i'd add a feedback loop pot for nasty stuff and oscillation (and activate it with the "color" dpdt; would put a common tiny dpdt for the "color" thing)

also would try some simple tone control (cap+pot to ground) , for removing harsh highs (now that it has ultra gain  )... for this, also a small cap in the opamp feedback

cheers

[Mark Hammer]

Though I like the "colour" switch very much (switches between diode type BUT compensates for signal level differences), several things are "wrong" with this design, in terms of having a satisfyingly usable pedal.

1) Desperately needs an anti-pop terminating resistor on the input.  The usual 1M-2M2 range will do nicely.

2) There is no constraint whatsoever upon high frequency content, including objectionable oscillations.  Max gain is set at x228, which can make any interference picked up, or accumulated noise (this IS a 741, after all)  particularly nasty.  Needed is a feedback cap in parallel with the pot+82k, and perhaps something like the SWTC or some other simple fixed OR adjustable lowpass filtering on the output.  A feedback cap of 33-47pf should be adequate.  47pf will get you a 6db/oct rolloff starting roughly around 1.5khz at max gain which is appropriate for a pedal of this type.  If the intent is to include additional tone control on the output, then scale that feedback cap back to 15-22pf.  That way, some sizzle will still come through at max gain and the user can still have some means to tame it if they choose.

If you go with adding a simple treble-cut control, that can be something like a variable resistor and cap to ground as in guitars, which would require no further modification to the circuit.  I suppose something like a 100k pot and 0.1uf cap might be suitable.  If you chose to do something like the SWTC (http://img.photobucket.com/albums/v474/mhammer/SWTC.gif), then the smart thing to do is increase the output pot value to 50k or maybe even 100k.


As for the 33k to Vref, there are things about the behaviour of LM741s (and ua741s) that I don't understand well.  Normally, in an inverting design like this (and note that the output is opposite in phase to the input in this pedal), the noninverting pin (pin 3) will go directly to ground.  On the other hand, we have seen from the Distortion+ that ridiculously low Vref bias currents (the Dist+ uses a pair of 1M resistors to create Vref, where this design uses a pair of 10k)  are suitable to that chip.  Conceivably the 33k Vref resistor IS entirely appropriate to this application, and should not be removed or altered, since it has the effect of reducing bias current to the noninverting input.  Perhaps those more technically inclined would be willing to comment on this.

[petemoore]

You could place a tiny booster prior to the input
 RD, this is the one I'd pick, as it can up the gain, by many X, with a few parts. You'll need room in the box for a board of course...volume pot on it'd be nice. This could make the unit non-inverting also.
 also would try some simple tone control
 Another great option Nordine, a simple HF cut via pot 'n capcan help tame highs that can multiply quickly [w/boosted input] and become too much.
 If you have one...Take an EQ, and try it before/after, try to stay at unity so only frequencies are  added / subtracted, not 'scientific', but may allow you to get a feel for alot of the 'what if's', about what different values of input caps or added voicing components. Then use the boost of EQ also...and notice how the EQ of the chain changes with boosted input of Dist.
 Adding fragments Before and after is easiest, but small test clips may prove useful for 'tacking' alterations inside the circuit also, for example a HF rolloff can be tacked anywhere SP is present in the circuit, and to a ground point.
  Take small value caps on a Ground. wire, and touch chosen values to different points in the circuit where SP is present.
 Booster Before w/volume if possible
 Then a Small HF rolloff cap at Dist Input, and or small value cap across the diodes to smooth clipping.
 Whether there is still HF frequency you would *want to further attenuate at the output or not...using the pot 'n cap adjustable LP filter tacked on the end...yet *another knob?
 

[WGTP]

Just another idea, in addition to the cap in the feedback loop suggested, try increasing the input resistor to 100-220K and decreasing the input cap to 1n.  This will cut the gain some, but give it more of a distortionish rather than fuzz sound, I think.   

Or, you could just reduce the input cap, try a 47-100n cap. 

[oldrocker]

It's a small circuit.  You might want to breadboard this and try some different things.  Then when you have a few ideas have him come over listen to your breadboard and see what he likes before perfing.  Maybe make some MP3 examples for him to listen to and see what sparks his interest.  You'll no doubt want to take Mark H. advice for popping and noise too.

[Bernardduur]

Thanks all as yet; I will try to implement it to the design. Some stuff:

- I can't alter the outside of the pedal; so no extra pots, no LED, no DC adapter
- He wants to keep the "vibe" of the pedal, but with more volume.....

Right now I added a small cap to the feedback loop and replaced one si diode with a 1N400x diode. Next I removed the output 33k resistor.

Sound = great! It is quite gritty to over-the-top grit BUT it responds so well to the volume pot. I never got so much respons from that slight change of tone or volume knob.

Now I have to find some "setting" to put in the color setting