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.

Advertisements

3 thoughts on “Distortion Pedal Volume Drop (and how hard diode clipping works).

  1. The last paragraph has many solutions. I would hesitate to call anything here a fix because nothing is actually broken. Everything is working exactly as intended, but the gear in use just does not play well together. The “fix” is to get a quieter pickup or a louder distortion pedal. There is a mod that I have done to get more level, but it will change the sound of the pedal with regards to maximum saturation. This involves breaking the connection between the clipping diodes and ground and inserting a resistor between the diodes and ground. I would try a 4.7k, 10k, 22k, or 47k and see what sounds best. A pedal nerd could actually build something like an Electro Harmonix LPB-1 circuit and install it at the end of the distortion circuit, but that seems really clunky and I’d not want to do that to any old or potentially collectable pedal.

    To reiterate, there’s no real fix because it’s more of an unfortunate combination of gear choice. If I had a 15 watt amp that could not get over my drummer, the fix would be to either get a bigger amp or make the drummer play less loud. There’s nothing I could do to the 15 watt amp to make it the right amp for the job.

    Thanks for reading!

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s