Diode Clipping / Gain Recovery question

[Colonel Angus]

Okay, so I am building a Franken-Rat pedal off madbeans slo loris board. Instead of using the diode selector on the board I wired a breakout board with a 6PDT and six clipping sections to choose from (don't ask why, it seemed like a good idea... also a chance to play with the new dremel).

As advertized, the GE sections ( 1 symmetrical, 1 asymmetrical) have much lower output. I had read about the difference, but the number 7 is less impressive to read than the sound of 7db volume difference (which is substantial, come to find).

My question is this:

Which would be easier/make more sense : A gain recovery stage for the GE modes, or attenuating the SI and MOSFET stages to the lower level of the GE?

Thanks in advance, this is going to be a super pedal for my good friend and band mate, he loves his PROCO Rat soooooo much and this is the only thing I could think of to top it 

[Bill Mountain]

Okay, so I am building a Franken-Rat pedal off madbeans slo loris board. Instead of using the diode selector on the board I wired a breakout board with a 6PDT and six clipping sections to choose from (don't ask why, it seemed like a good idea... also a chance to play with the new dremel).

As advertized, the GE sections ( 1 symmetrical, 1 asymmetrical) have much lower output. I had read about the difference, but the number 7 is less impressive to read than the sound of 7db volume difference (which is substantial, come to find).

My question is this:

Which would be easier/make more sense : A gain recovery stage for the GE modes, or attenuating the SI and MOSFET stages to the lower level of the GE?

Thanks in advance, this is going to be a super pedal for my good friend and band mate, he loves his PROCO Rat soooooo much and this is the only thing I could think of to top it  

Well...how would you go about doing either?

Would this need to have enough output to hit the front end of a tube amp?

I've often thought of putting the gain recovery stage from the BMP after Ge diodes in an Distortion + style circuit.

Part of the charm of different clipping diodes is the volume differences so, I wouldn't put too much effort into volume correction.

[Colonel Angus]

I guess that was question #2... How would you do either of these things ? I didn't want to presume that anyone would help my noob ass .

I agree there is a charm to the differences, however this pedal is bound for someone that prizes practicality ( read: technologically retarded) soooooo..... in the spirit of actually getting him to use the thing, it would be desirable to have the various clipping stages at roughly the same output.

I just have this mental picture of him grooving on the GE section with his AC30 all dialed in, then switching to the LED and suddenly the amp explodes.

[Bill Mountain]

You'd have to do some math and then have a switch that can select diodes and gain stage adjustments (either boosts or cuts) at the same time.  I've used a 2P5T rotary switch that could do something like that but the math involved would make my head spin!

Here's how I'd do it:

Boost:

You could make an opamp boost and switch in different gain setting resistors with each diode.  Set the highest voltage diodes with a jumper as a buffer and then increase the resistors to taste with each different setting.

Or Cut:

Set up a voltage divider and swap shunt resistors with the switch.  The lowest voltage diodes would be an open circuit.

[Bill Mountain]

The math would be calculating the clipping voltage of each diode pair and the setting a minimum or maximum and boosting or cutting each pair by the amount needed.

Remember that Ge diodes may have a lower voltage than the guitar by itself so you'll need to decide how hot you want the output of the pedal.  Personally, I would boost because I'm not a fan of pedals that are quieter than my bass.  But I also hate how much output LED's have.  Most of it is unusable for me because I don't use tube amps.  So after I boosted to match diodes, I would cut back to about 1 volt of output.

[Colonel Angus]

Sounds like cut would be best. Shunt the SI and MOSFET to the level of the ge then opamp boost everything to get the gain close to the original spec ?

Thanks a lot for the input. Looks like I've got a bit of reading and math to do

[Jdansti]

This might be too simplistic and I don't know if it would work on your Rat, but it would be easy to test on a breadboard.  You could place a trimmer pot in series with each pair of diodes and adjust the trimmers so that you get the same volume from each.  One disadvantage might be that the end result would change the amount of distortion on each pair to something that is undesirable.  If I knew more I would know whether this would work or not, but as a hacker, I would have to use the trial and error approach.

[Colonel Angus]

I agree. Some quantity of trial and error will be required. I'm much better at errors than reading and math

[Bill Mountain]

This might be too simplistic and I don't know if it would work on your Rat, but it would be easy to test on a breadboard.  You could place a trimmer pot in series with each pair of diodes and adjust the trimmers so that you get the same volume from each.  One disadvantage might be that the end result would change the amount of distortion on each pair to something that is undesirable.  If I knew more I would know whether this would work or not, but as a hacker, I would have to use the trial and error approach.

This would give each pair a similar threshold and would partially eliminate what makes each diode different (compression, etc.).

[Mark Hammer]

A post-clip gain recovery stage is preferred, if one is unconcerned about complexity.  Why?  Because many of our most desirable sounds come from feeding an amp a hot signal that already has a bit of lower-order harmonic content in it.  If one's strategy is to attenuate higher signal levels from a different diode set to achieve equal loudness, relative to a different diode complement, then you've already made it difficult for yourself to achieve high output levels with modest gain settings.  You want to be able to get not only higher-distortion/high-output signals from the pedal, but lower-distortion/high-output level signals.

[Colonel Angus]

Thanks alot Mark! I took all of the advice here and made something pretty cool, I think. After measuring (carefully with my ears, wish I had a scope) the 6 stages : LED, 1n914(stock), 1n36a, NOS Ge Assym, Si + Ge, and MOSFET ; I found that all the germanium clipped stages were pretty close in overall output. I used the other half of the DP6T rotary to either 1) send the board output to the output jack (LED and MOSFET) or 2) send the board output to a simple 2n5088 booster with a 100k trimpot and then the output jack. Not terribly scientific but it achieved the goal of having all the stages close to the same output. There is still some variation but it is acceptable, I don't think any amps will explode while playing with the box. This thing was a bitch to fit into a BB on account of having 2 breakout boards, but everything worked!

I really appreciate all the advice!

[nek314]

Couldn't you simply put two Ge diodes in series for each leg of the clipping section? That way, their threshold voltage is doubled, making it close to the Si diode threshold. I would think that it's the material properties of Ge and not the low forward voltage that makes it sound the way it does.

[Mark Hammer]

Nope.  Material properties probably do play some role, but at least 80% of it or more is the change in threshold.  Remember when you have a signal that changes in both amplitude and source harmonic content as much as guitar does, the quality of distortion heard is more a function of when you clip than it is how you clip.  If guitar delivered a steady-state signal like an oscillator, then I'd say you have a potential case for an audible effect of materials...but not under real-world conditions.

I certainly wouldn't disregard the role of the material itself completely, but it is a very small player compared to the simple shift in forward voltage and how much of the signal diode conduction pertains to.  Think of it like that little bit of extra seasoning you put in the soup.  The stock, the tomatoes, and the salt comprise most of the flavour, and somewhere way off in the corner is a little pinch of savoury or saffron that the recipe called for.  You may notice the difference if it is there or not, but damned if you'd be able to identify it amidst everything else competing for your tongue's attention.

[Colonel Angus]

Here is the item, I didn't get a pic after I boxed it. Finished just in time for rehearsal where the "Rat King" went right onto the guitar player's pedal board. I would agree with the seasoning comparison; after playing with the Rat King for a few minutes to test, all the clippers delivered a ProCo RAT sort of sound, but with different flavor sauce. SI+GE is like a chainsaw, MOSFET is detailed and harmonically bright with dark fuzzy undertones, 1n60 is like a fuzzy blanket, LED is like bombs going off. I think all the modes will be useful to someone who loves the RAT.

The Ruetz mod ( I like Dano's name for it "lube") is subtle but useful, really does lube the sound.

I guess the consistent character comes from the way the LM308 makes distortion?

[nek314]

Nope.  Material properties probably do play some role, but at least 80% of it or more is the change in threshold.  Remember when you have a signal that changes in both amplitude and source harmonic content as much as guitar does, the quality of distortion heard is more a function of when you clip than it is how you clip.  If guitar delivered a steady-state signal like an oscillator, then I'd say you have a potential case for an audible effect of materials...but not under real-world conditions.

I certainly wouldn't disregard the role of the material itself completely, but it is a very small player compared to the simple shift in forward voltage and how much of the signal diode conduction pertains to.  Think of it like that little bit of extra seasoning you put in the soup.  The stock, the tomatoes, and the salt comprise most of the flavour, and somewhere way off in the corner is a little pinch of savoury or saffron that the recipe called for.  You may notice the difference if it is there or not, but damned if you'd be able to identify it amidst everything else competing for your tongue's attention.

I see. Thanks for the knowledge.

[teemuk]

I guess the consistent character comes from the way the LM308 makes distortion?

Or maybe from everything else in the circuit, except diodes, being the same. 

[Colonel Angus]

I meant more in regards to the different diodes adding flavor but the overall soup ( to borrow a metaphor) still tastes like rat. I played it next to the proco rat and while there are noticable differences and lots of variability with six clipping stages, it still sounds like the rat. Versus putting it next to a bmp or ff, where the sounds will be very different.