Distortion Pedal Volume Drop (and how hard diode clipping works).

This seems to happen to every guitar player at some time or another. They get a distortion pedal, put it in their signal chain, tweak it to get a good sound, and prepare start rocking out. There is usually a point where they kick on the pedal at a pivotal moment and poof, the guitar seems to almost vanish. The volume knob will be at max on the pedal, but it just can’t seem to keep up.  What gives?

I never personally encountered this issue, but once I became more of a gear head people came to me with this problem. I found a trend. it was usually the combination of super-hot pickups mixed with older style distortion pedals that used clipping diodes to generate the distortion.

Back before the days of super-hot pickups, people were designing very simple distortion boxes. These circuits work by running the guitar signal into a clean(ish) booster of some type, and then running the boosted signal into a passive clipping diode network that places the diodes between the audio signal and audio ground. The diodes cause the distortion that is heard. Some examples of pedals that work in this manner are the MXR Distortion +, the Ross Distortion, the DOD Overdrive 250, the ProCo RAT, and the Boss DS-1.

Let me give a very brief and simplified explanation of what a diode does. A diode will only allow and electrical current to flow through it in one direction, positive or negative.  Which direction it flows is determined by the orientation of the physical component in the circuit.  Let’s say we have a diode that is wired to only pass a positive current. If we try to run a negative current though the diode, it will block it completely. If we try to  put a positive current through it, it will flow just fine, but only once a positive voltage charge has been applied to the diode. This trait is called the diode’s forward voltage. Most silicon diodes used for distortion pedal clipping have a forward voltage of 0.7 volts.

In audio signals, the signal’s voltage corresponds to its loudness. Here is a sine wave, crappily drawn by me.Sine WaveThe center line is zero volts, the peaks are a positive voltage, and the troughs are a negative voltage. Let’s imagine this as the signal coming out of the booster section of a distortion pedal. Following this booster is the clipping section of the circuit. This usually consists of two clipping diodes placed in opposite directions. As I stated above, these clipping diodes are placed between the boosted audio signal and ground. It’s generally known that grounding out an audio signal will mute it. If a straight wire was hooked between the audio signal and ground, the result would be silence (a volume pot on a guitar varies the resistance between the audio signal and ground). Hooking up the diodes between the audio signal and ground will not mute the audio, at least not until the signal voltage reaches the forward voltage of the clipping diode.

If the audio signal never reaches more than +/- 0.7 volts, then the clipping diodes will never reach their forward voltage, they will never be able to bass signal in either direction, and nothing will happen to the audio signal. Should the signal go above +0.7 volts, or below -0.7 volts (and it usually does), that will allow the clipping diodes to reach their forward voltage amount, and the diodes will begin to conduct. Since the diodes are connected to ground, any of the signal voltage that exceeds +/-0.7 volts will be shunted to ground.  I’ve modified my crappy drawing to show the result.Clipped Waveform

So, in short, and what I probably should have said in the first place, the pedals create distortion by amplifying the signal and then hacking off the loudest parts to make a quieter but distorted signal. The amount of level going into the clipping diodes determines the amount of distortion. There is also a passive volume control to determine the desired output level.

Now, as the 1980s progressed, companies began producing louder and louder pickups. These were designed to drive amps harder, to get more sustain, and just sounds generally bigger, louder, and more powerful. This is awesome if that is what is desired.  However, the early distortion circuits were not designed when these very loud and aggressive pickups existed. In a best case scenario, the distortion pedals will never be able to produce an output with a a signal voltage greater than the forward voltages of the clipping diodes. In the case above, this would be a signal of +/-0.7 volts. If germanium diodes are used the result would be a +/-0.4 volt signal. In many cases there are passive circuits in the distortion pedals after the clipping diodes. These passive circuits will take even more level away.

So, why is there sometimes volume loos with distortion pedals?  The distortion pedals simply have maximum voltage output levels that are lower than the levels that are able to be produced by very hot, modern pickups. The voltage of the guitar signal when the pedal is bypassed it going to be hotter and stronger than the highest possible output of the pedal.

There is no real elegant solution. Everything seems like a compromise on some level. The first solution is to change pickups or put some kind of permanent level pad before the distortion unit. Another workaround is to put a clean volume boost pedal after the distortion pedal and only switch it on when the distortion pedal is on. The classic distortion circuits can be modified to clip at a higher threshold, but the tradeoff is that the higher output level sacrifices the saturation amount. There are also many modern distortion pedals that have adapted classic circuits and added more output level. All of these solutions are either clunky or incur some kind of expense. Quite frankly, the people who have come to me in this scenario approached me because they were unable to throw money at the problem.  Still, people love the classic distortion circuits, and people love hot pickups, so we’re always going to encounter situations where these kinds of mismatches can occur.

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Busy Busy Busy

I do not post here enough.

However, I started a Facebook page.
https://www.facebook.com/benadrianaudio

Where I put my small thoughts.

I finally got all of Jason Pace’s amps back to him. I also touched up the original KWB prototype. Scott Evans may be right; the Si diodes may be the best sounding. I change my mind all the time, so it’s a good thing I put the diods on sockets.

I finally, finally, finished Steve V’s “Explode” pedal. Steve playes guitar and yells in this wonderfully harsh band called Blacks. The pedal is a fun little fuzz circuit that is being driven with an insane booster. After the fuzz I tried a big muff style tone stack, but with separated bass and treble controls. It works better than I expected. I think Steve will like his one-off pedal.

I have a lot of pedals to build now. I’m a bit overwhelmed. Still, I worked all weekend, I’m making progress, I’m solving problems, and things seem to be falling into place. I hope to just have bigger and better nows as time goes on.

Also, buy a Cartographer record.

Tech Log 2011-03-09

Replaced a switch and battery clip on a 1986 RAT for Jeremy H.

Diagnosed Jason P’s Traynor. Looks like it’s just the wacky standby switch. Now, where do I get one of those?

Tuned up Jon G’s Fuzzy Bunny.

Experimented with the new fuzz circuit.

Worked on Trey P’s Bunnydrive; almost finished!

Pinched the hell out of my finger in a mic stand.

Pedal building resumes.

For a while I’ve been repairing gear, fine tuning my rig, and enjoying my new guitar. The benadrian pedal company has kind of taken a back seat to everything else.

Today was a nice step, though. I finished two Kowloon Walled Bunny pedals: #005 for a member of the Electrical Audio forum, and #PT2 (prototype 2) for myself! Finally, I own a KWB pedal for Cartographer use.

Here’s number 005

I feel good. It felt like I was working working working to just get back up to parity. It’s nice to be productive again.

Time to order parts!

Electrical Guitar Company & Cartographer Amps

Long time, no speak, friends of my nerdery. January was a very, very busy month. I all but stopped making pedals due to band, recording, and of course day job commitments. the heavy lifting has passed. Now, let me play catch up on some of the stuff that did happen.

On February 5th, after about six months of waiting, my Electrical Guitar Company “Custom 500” arrived.

Yes that is an aluminum neck and fretboard. In fact the neck goes all the way through the body and the bridge bolts directly into the neck. Here is the guitar with the body removed.

You can see more and hi-res photos here.
http://www.flickr.com/photos/benadrian/sets/72157623243036439/

This is just an amazing instrument. Rather than type a whole new description, I’ll just copy what I wrote to the builder.

—–
The sound is amazing! On tour in late 2007 I played Rich’s all aluminum offset one, and then Colin from Fun Ender’s guitar. The feel wigged me out at first, but after about 10 minutes I just couldn’t put them down. I never played them plugged in, though.

On Saturday I got to plug it in. A few friends who were practicing down the hall came by to check it out. The range of tones available is very diverse. The RW/RP of the pickups is a great bonus. Mainly, though, what everyone noticed what the immediacy of the response. I never got that before when playing the guitars unplugged. The think I’m telling my friends is that it has the attack of a Tele, but the level drops off less for the
sustain. It’s like a Fender response with more sustain, or a Gibson response without the weak, mushy attack . It’s just a totally unique ADSR curve (synth term)

The other thing I told my friends is that it kind of sounds like an angrier, more responsive version of my Ric 360.

The pickups, holy shit! they sound great, yeah, but I had my amp running at full tilt, standing right in front of it, and there was no squeal at all. I couldn’t get microphonic feedback at all. So great!

So yeah, I’m blown away. Fit, finish, sound; I played it all weekend in every little open moment.
—–

So yes, new favorite guitar by far.

I also finally settled on my Cartographer amp setup. I finished the Traynor. I left the tone caps the way they were, but I redid the phase inverter circuit to classic Marshall values. Instead of a 2-12, I managed to grab an inexpensive but good 4-12 from my friend Jeremy. the other amp is half of my old bass amp from Replicator. It looks very intimidating.

That’s the full catchup. Look for more building and repairing soon.

The Kowloon Walled Bunny

Well over ten years ago, my band played a show with Pound, WI. Russell Hall, the guitarist (who now plays in the spectacular United Sons of Toil), was using a Ross Distortion. I thought it sounded great. Sometime around 2001 or 2002 I picked one up at a secondhand guitar shop. I really loved it. I still own it and I still love it.

Sometime soon thereafter, when I began to research pedals for fun, I discovered that the MXR Distortion +, the Ross Distortion, and the DOD Overdrive 250 were all essentially the same circuit with only slight differences. After looking at the schematic, I had a lot of ideas for possible mods, but I never got around to trying them.

Let’s jump to 2007. My friend Scott Evans of Kowloon Walled City had a number MXR distortions. He had one that he really liked, and sounded different than all the rest. We had a little shootout.
Scott's Pedals

What we discovered is that the one that Scott really liked had a broken clipping diode. Essentially, it was as if a diode was removed. It sounded great, just rough edged, loud, and rude. I kind of wanted to do this to my Ross, but it was a nice old pedal, so I refrained.

This year Kowloon Walled City released their second album, which is astoundingly good and can be downloaded for free! I figured I would force them into an endorsement deal while simultaneously doing something I’d wanted to do for a long time; make my ultimate version of the Distortion + circuit. I pretty much just declared that I was making this pedal, and Scott and the other guys in the band were very excited.

So, what did I do? Not much, actually. I guess that’s a bad thing to say given that I want to sell these. I started building pedal with the belief that I wanted to be open and share information, so here goes.

Op amp:741. This is the classic MXR op amp. The Ross uses a 4558, but yeah, the 741 does sound better. People say it’s because it has a shitty slew rate and is just kind of crappy by technical standards. I’ve never measured, I just know I like the sound.

Resistors: I pretty much use the stock value from the Ross.

Caps: I almost use the stock values from the MXR. There is no 10pf in the negative feedback loop of the op amp, since it’s the 741. the one big Change is that I have raised the value of the input coupling cap from .01uf to .1uf. Since Kowloon Walled City tunes down to stupid, this will allow a bit more of fundamental to get through. The risk is that the distortion can get farty with too much low frequency input. I don’t care; I’m not looking for a tight metal distortion.

Clipping Diodes: I put in sockets so that the user can choose and change diodes whenever and customize their sound. The version I built for Scott has two Germanium 1N34s, which can be kind of quiet. However, I added a knob to the pedal called the GSE knob. That knob adds resistance between ground and one clipping diode, simulating Scotts broken pedal. Personally, I prefer one Ge diode and one LED. The pedal ships with a variety of diodes for user tweakability. It can also be run with no diodes and will act as a boost pedal.

So, here is the first production model.
KWB

Thanks for reading!