The Ultimate Overdrive

Introduction
I have built and owned a couple of “classic” tube-screamer derivative overdrives and although they are useful for certain applications, I found them to be too compressed and mid-range sounding. They do make good solo boost and tighten low end well, but, I could never make a diode-based overdrive pedal have that open uncompressed raw crunch on low gain settings (think Ritchie Blackmore).
So I decided to give the diode-base overdrives one more chance by building a big daddy overdrive that will let me experiment with as many different clipping options and op-amps as possible to try to find some useful crunch tones and still be able to use it as solo boost. As a starting point I selected Paul C Timmy overdrive which people love for its transparent sound. What also made it appealing to me are the separate bass/treble controls instead of stupid tone control which is a standard on traditional overdrive pedals.
Clipping
Clipping section is the beating heart of every diode-based overdrive. Number and type of diodes determine how much of the signal will be passed through and what will be clipped. Also, different diodes clip in a different manner, so character of distortion will be different. There’s no absolute best clipping arrangement, thus that many different overdrives which share most of the circuit but feature different diodes. So why not make it in a way that would allow as many clipping configurations as humanely possible?
Switchable clipping diodes have been around for a while, most makers put toggle or hidden DIP switches that offer two or three combinations of clipping diodes, but I wanted to take it a step further. My goal is to allow selecting any subset of 7 diodes wired in series…per phase. That would make it possible to do endless combinations of symmetrical and asymmetrical clipping settings. By using seven switches of a 8-pole DIP switch for each side of the waveform we can shunt each diode in the array. The last of the 8 switches of the DIP can be used to disconnect the whole diode array from the signal chain. Without any clipping diodes, we effectively turn the overdrive pedal into a clean booster.
Note that series diode connection is chosen so that we can combine multiple diodes and achieve higher headroom. Less we clip the signal, less compressed and overdriven it is.

diodes

The same concept can be used to create a drop-in module and mod any existing diode-based overdrive, as long as there’s room for two DIPs and all the diodes.

Diode Selection
  • Germanium: clips softly at 300-400mV. I used 1N34A, but bretty much any old germanium diode will work here and produce similar results.
  • Silicon: 1N4148: clips a bit harder at 600-700mV, standard for vast majority of overdrive pedals. I put two of them in the diode array to be able to match stock Timmy setting with two diodes per side in addition to using a single diode. I used 1N4148, but most other silicon diodes can be used with the same performance.
  • Rectifier: these are also essentially silicon diodes, so they perform similarly to 1N4148. I had some UF4007 that I like using for rectifiers, so I used these.
  • Schottkey: some of them have even lower forward voltage drop than germanium diodes. I used BAT46 that clip at 250mV and also got some BAT41 as well so I can mix, those clip at 400mV.
  • MOSFET: there are few different ways to wire a three legged transistor and use it as a diode. The one that’s different than a regular silicon diode uses drain and gate connected together, acting as a cathode and source acting as an anode (for P-channel MOSFETS it will be reversed). MOSFETs clips a bit softer than silicone diodes at higher voltages, usually between 1.5 and 3V. They need another diode in series to polarize them because it has a silicone intrinsic body diode pointing the other way. Unless we use another diode to polarize the FET, the silicon diode will clip the other phase of the signal. I intentionally left it up to the user to choose which of the remaining 6 diodes, if any, will be used together with the MOSFET. MOSFETs (often BS170 or 2N7000) are used by some “boutique” overdrive/dist pedals, like Fulltone OCD. Because MOSFET can clip the signal both ways, we can achieve asymmetrical clipping using only one MOSFET that clips both phases. I used BS250: P-channel MOSFET.
  • LED: different colors have different forward voltages, but clipping characteristics are about the same. They have high headroom due to high forward voltage drop and they clip harder than other diode types. Infra-red can get as low as 1.5V, and ultra-violet as high as 4V. Other colors will fall in between. I suggest getting LEDs with as low voltage as possible, so that we can get some of that clipping at all.
Asymmetrical Clipping

Asymmetrical clipping is sometimes used as a way to add some “flavor” to the tone. What it means is that we use different types or different number of diodes for each phase, so signal gets clipped differently on positive and negative side of the waveform. However, I found that arrangements where ratio between the total forward voltages between the two sections is higher than 2 tend to sound ugly. That means that it’s perfectly fine to use 1.5V on one side and 2V on the other, but I would avoid combinations that have say a germanium diode (375mV) on one side and blue LED (3V) on the other. But I encourage you to try it and decide for yourself πŸ™‚

IC selection
Another important part of overdrive character is the op-amp. It amplifies the signal before it gets clipped and then amplifies the signal again for the output. Pretty much any dual op-amp with the standard 8-pin DIL pinout will work here. I installed a wire wrap IC socket to simplify swapping op-amps and ordered a handful of most commonly used op-amps to try:

  • NJM4558D: high gain bipolar op-amp, used in Tubescreamer and countless similar overdrive pedals.
  • JRC4559D: stock chip in Paul Cochrane’s Timmy and Tim pedals. Similar to 4558D, but with a tad more gain and faster slew rate.
  • LM1458N: low power consumption op-amp. Reported to work really well in Timmy, bringing more clarity.
  • TL072ACP: commonly used, low noise and low power consumption JFET op-amp.
  • LF353N: wide bandwidth JFET input op-amp.
  • OPA2134PA: hi-fi audio FET op-amp.
  • NE5532P: bipolar input op-amp, higher current consumption than the rest of the heard.

So far I only really tried two of them. NJM4558D was my first choice and it works really nice, lots of drive and resembles Tubescreamer with that almost nasally sound midrange hump. Then I tried OPA2134PA which is wider band and more neutral sounding and really liked it, so it stayed there.

Parts and Construction
As always I aimed for highest quality parts available and I wanted a “hand-wired” point-to-point construction. The circuit is relatively simple so I thought I could pull off an eyelet board. Other than looking super cool, like a vintage amp, eyelet boards are very sturdy (1/8″ thick) and can take a lot of abuse with soldering, desoldering and playing with components. To make for a more compact layout I got some #35 Keystone eyelets which are smaller than #45 that are traditionally used for tube amps. It also means that you can pull less leads through one eyelet and you should be careful with wire as it can only take thinner wire together with component leads. Other parts include:

  • Dale resistors (cool CMF brownies)
  • Nichicon poly film caps
  • Cornell-Dubilier 1uF poly film caps
  • Panasonic FM electrolytic caps
  • Mill-max wrap IC socket
  • Neutrik jacks
  • Alpha 3PDT switch
  • Alpha 16mm pots
  • 1/8″ thick home-made eyelet board with #35 keystone eyelets. Note that they are smaller than #45 typically used for tube amps.
  • CM 8-pole DIP switches
  • Teflon #20 AWG shielded wire for longer runs
  • Thin #28 AWG ribbon wire for wiring controls

DIP switches are rarely exposed for users to mess with all the time, but I wanted to avoid having to open the pedal each time I wanted to play with the clipping section. This made construction a bit more complicated as it required having two rectangular holes in the enclosure and mounting DIP switches on the back of the circuit board. Having no specialized tools for rectangular holes, we drilled a bunch of small round holes and then filed the hole into shape using small flat files.

As you can see on the layout, 16 DIP pins are connected to only 9 eyelets. Clipping diodes zig-zag from one eyelet to the other and all but the outer two eyelets connect two neighboring pins of the switch. I wanted to cover as much ground as possible, so I hardwired 4 diodes and installed socket pins for the remaining three so I can experiment even more if needed. Every other diode uses sockets, so I only need to have up to one socket pin per eyelet.

Finally, a top tip for playing with DIP switches: use the tip of the pick to flick the mini switches.

Schematic and Layout
Overdrive Schematic
PDF Layout File
DIY Layout File (right click and choose ‘Save As’)
Results
Pictorial

Click on an image to see more details.

Video Clips

Click on a thumbnail to play the video on YouTube.

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Scoping the Output
Curious to see how different diodes clip, I made a simple (and almost free) setup that allows me to pass a 1KHz sine wave through the pedal with gain knob maxed, bass and treble cut at zero, and analyze the output (check out this article for more details). Of course, it’s far from professional scopes, but is enough to give some insight into the world of clipping. Since diodes have different forward voltage, each of the option produces output of different loudness. For example, LED produces output that is 10 times as loud as Ge diodes. I tried to normalize output of each clipping diode using pedal’s level pot trying to get them all to the same volume, so it’d be easier to compare the wave forms. Below is the result.

Clean sound with no active diodes, pretty much perfect sine wave.

Clean sound

Germanium diodes (1N34A). Much lower output and very soft clipping. The resulting waveform still resembles the starting sine wave, it’s just a bit compressed (both visually and sonically).

Germanium diodes

Schottkey diodes (BAT46). Low output, similarly to Germanium, but slightly more compressed.

Schottkey diodes

Silicon diodes (1N4148). Even more clipping and much higher output.

Silicon diodes

Rectifier Diode (UF4007). Very similar to silicon diodes, which is to be expected, as rectifier diodes are also silicon composition.

Rectifier Diode

Output op-amp stage clipping without any clipping diodes active. This proved to be the biggest obstacle when analyzing diodes with higher headroom (LED and FET), as they let more signal pass through which in turn overdrives the output gain stage. We are trying to analyze the output signal of the pedal, but op-amp clipping will “pollute” diode clipping that you are trying to capture. If we turn down gain to avoid output stage clipping, we’ll also drop below the clipping threshold of the diodes, so output signal will be pretty clean. That’s why I also had to analyze the output of the first gain stage, before it gets clipped by the output op-amp. As you can see, op-amps clip hard and sudden.

Op-amp clipping without any diodes active

FET (BS250) and Schottkey (BAT46) diode in series. Noticeably more compression and distortion. As you can see, this wave form resembles op-amp clipping and that’s exactly what happens here.

FET + Schottkey

LED (5mm RED). Again, output op-amp clips the signal and turns it almost into a square wave.

5mm RED LED

Comments
31 Responses to “The Ultimate Overdrive”
  1. blackcorvo says:

    You should try using Infra-red LEDs instead of common red LEDs. IR’s have a lower voltage drop (1.2v while red LEDs usually are over 2v). Might be interesting to put them in place of one of the silicon diode types you used, since they’re both so similar anyways.

    • Rod says:

      Have all parts now, have built a DOD250 and an MXR+ Dist. (almost same circuit), for practice while I waited for some parts. I have white 1.2v infrared, I have others too with higher drop voltages. I am going to breadboard this, test, then solder on good ol perfboard like the 2 I built above. Will post pics of the Ultimate when I am done, and if it works of course. I am a newbie. πŸ™‚

  2. Rod says:

    Im so interested in this build, I am going to buy the parts and try it right away. I know some basics, am good at soldering, and have never built a pedal. Going to build this as a prototype before I cram in a pedal. This will be my first build, oh boy! πŸ™‚

    Will update when I am in testing phase.

  3. Aniket says:

    Hey…..great circuit. Few questions though. Can you tell me what does the vref does in your circuit? And the inverting loop voltage divider has no ground connection….can you please explain the inverting loop. I’m a newbie to electronics…..so any kind of help is appreciated. Thanks in advance

    • bancika says:

      Vref is reference voltage for opamp, commonly set to 1/2 of the main voltage. We need it to avoid bipolar supply. You can either power opamps with bipolar power supply (+ and – 9V) and use ground as a reference, or we can power it with 9V supply and use 4.5V as a reference, thus faking a bipolar +- 4.5V supply. That’s about as much as I know πŸ™‚
      Cheers

      • Aniket says:

        Thanks. One more thing, the inverting loop has no connection to ground. Don’t you think that is necessary? Because i think the circuit will not work without it.

  4. Rick says:

    Actually your right, it dies not squelch or squeal when the box is enclosed and placed in line in my pedal beard. I realize why it was happening is because I have a small ZT lunchbox amp as my reference on my workbench, and I had the pedal directly in front, causing some sort of loop I’m certain.

    I do have one oddity. When non of the dip switches are selected, I get no signal passing through. Is this the correct function?

    Thanks for this design by the way, I learned a lot along the way. I’ll be making a donation soon friend.

    Cheers,
    Rick

    • bancika says:

      Glad to hear the squeal is gone. As for the DIP. As long as the last switch is OFF, the clipping is disabled, so selection of the previous switches doesn’t matter. There should be sound when all of them are OFF. When you disable clipping, you turn the first stage into a clean boost. Maybe you meant when all of them are ON? In that case we are effectively shorting the feedback loop and it makes it the same as if the Gain was turned all the way down to 0.

      Cheers

  5. Rick says:

    Hi Bancika,

    My build was long being my first pedal, but I was able to come away with relative success! πŸ˜€ I only had 10 dip switches avalible and a few different diodes than your list. Everything functions fine though I had to flip flop a few wires to get the pots from acting backwards. I guess if looking at the diagram you must discover if the drawing is viewed as front or back of pots, lol. All working now. Only one issue is on a few selections like say Ger or Sil, I get a very high pitch squeel sound, and can correct it by rolling off a bit of the highs. My guess is this is caused from me using strip board and braided wire as opposed to your build which is far more solid. None the less I’m happy with ;). I have pictures if interested but not sure how to upload here

    • bancika says:

      Cool, thanks for reporting. I have recently fixed the wiring on one of the pots. All pots are drawn as seen from the back. Note that bass and treble pots should be wired to cut, not to boost, so they work opposite of conventional EQ pots. As for the squeal, it shouldn’t happen, this is a relatively modest gain pedal. Did you try other IC?
      Cheers

  6. Felipe says:

    Well, I am giving this one a shot. I’m a newb from Costa Rica (well, I am not originally from CR, but live here!). I found most of the components locally (well, the type; the quality of the components is up to debate, as I have no idea what brand they are!). I got all the components ready except for the 47 nF capacitor… Could not find it locally. Any ideas what I could use as a replacement for it, if any?

    I also did not like the button I had to get (not much choice locally!) or the box (plastic; no metal available), but those I figured I could change later on. I am also using a breadboard instead of the eyelet board as keystones are not locally available… We’ll see how it does at the end of the day! But thanks for posting!

    • Felipe says:

      One more question; is the layout corrected? If not, what is the correct way of connecting the 50K Bass pot?

      • Bancika says:

        the layout is corrected. You can use two 22nF caps in parallel or two 100nF in series if you can’t find 47nF…although it’s standard value, it should be available everywhere…or you can use some similar valued cap, input cap is not crucial here.

        • Felipe says:

          Thanks a lot! I decided to order some stuff from the web, because I wanted to make the pedal a bit more durable. Among the things I ordered are the metal case, the button, the plugs (both 1/4 inch and the DC) and the missing capacitor. I also decided to order board material and turrets (I know you used eyelets, but I figured if I ordered turrets, they would work for building an amp also; besides, worse case scenario, if they are too tall for the case, I have the tools to cut them down). I am now waiting for all these parts to arrive.

          Anyway, I was wondering if you have a diagram of the lower part (switch, battery, DC, input and output); I apologize if this seems to be obvious, but I am a noob at this, and want to make sure I get it right!

  7. Rickygene says:

    Hi Bancika,

    Thanks for submitting this build, I’m starting it today! I was wondering a few quick things though. I’m new to reading schematics and I noticed the Bass 50k pot lugs where differently wired than the drawing, in which resistor goes to the center lug. Since I cannot see it in the pictures I’m left to imagine the schematic is the incorrect one, and the drawing correct?

    Also, the diode for the 9V in resembles a silicon diode. Does it matter which one? Should I use say a 1N4001 or something like that?

    Thanks for any response help in advance :).

    • Bancika says:

      Good catch, the layout is wrong. I updated it. And yes, 1N4001 should work ok as a polarity protection diode.

      Cheers

  8. Alex says:

    Hi,

    Nice circuit, very interesting, it’s worth a try.

    I need some help on finishing it, mainly the true bypass part and how it’s the circuit switched off, unpluging the power or simply by unpluging the guitar jack. I’ve seen both ways of doing it but I’m a confused newbie.

    When you say:”BS170: MOSFET with source leg connected as anode and drain and gate connected together and acting as a cathode. Clips softer at around 600mV but needs another diode in series to polarize it. I intentionally left it up to the user to choose which one (or more) of the remaining 6 diodes will be used together with the MOSFET. Used by some β€œboutique” pedals.” What do you mean? That both, MOSFET and diode, count as 1 diode and are wired as 1 single diode? If so, bypassing 1 only switch, must bypass both diodes, is it correct?

    Thanks.

    • Bancika says:

      yeah, both the MOSFET and diode form a single diode. If you do not include a series diode, MOSFET will actually have another internal silicon diode in the opposite direction that will clip the other side of the waveform. We do not want that. We just want the MOSFET to clip one side of the waveform.

      For bypass refer to tonepad website, it has all the info you need.

  9. Dan says:

    I don’t understand how the diods are wired… are there 7 or 14 diods? Are some of them hardwired? Could you please explain?

    • Bancika says:

      there are 14 diodes, 7 per waveform phase (positive and negative). They are wired in series, 7 for positive and 7 for negative phase of the signal. They are all in series but DIP switches can bypass any particular diode. Ones that are not bypassed are active and will form a big clipping diode for that side. This allows us to do various combinations:
      * no diodes at all (8th switch on the DIP open)
      * the same diodes, aka symmetrical clipping (all switches closed except for the one that corresponds to that diode)
      * asymmetrical clipping (either select different diodes per phase, or combine multiple diodes on one or both sides for more headroom and less clipping)
      Anything else you can think of πŸ™‚

  10. Kevin says:

    Hey,

    Which IC did you use? I can’t read te number really well.

    Cheers

    • Bancika says:

      I got at least a dozen different ICs to experiment with. Haven’t went through all of them yet, it’s a matter of taste for the most part. The one in the pic is a classic tubescreamer “jrc 4558d”, but others may or may not sound better to you. Lower noise, more clarity, etc. Depends on what you’re after.

  11. Bryden says:

    hey dude,
    Is there any way that I will be able to get the exact amount of components needed, as well as the other side of the circuit board? Is there any YouTube links or pictures you could share with me? I am wanting to do this for a Systems Engineering project and it would be absolutely amazing and very helpful to know all the parts needed.
    I am really looking forward to building this, as it looks like a great design and concept

    Cheers

    • Bancika says:

      I don’t have the list, but you can go though the schematic and write down all components in less than 5 minutes.
      Cheers

  12. xixiviii says:

    Hi Bane,

    Thanks for the link, this is very interesting to me as well.

    I am currently having a really good experience with the flexi clip, so this one should be fun too.

    SC

    • Bancika says:

      Cool. It’s very cool being able to stack diodes in series and increase headroom while mixing the clipping properties of different diode types. A lot of room for experimenting πŸ™‚

  13. Jon says:

    I have never been more excited to build a pedal!!! Exactly what I have been searching for!

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    The idea behind this site is to share my experience with Do It Yourself approach to guitars, amplifiers and pedals. Whether you want to save a couple of bucks by performing a mod or upgrade yourself instead of paying a tech, or want to build your own piece of gear from scratch, I'm sure you will find something interesting here. Also, this is the home of DIY Layout Creator, a free piece of software for drawing circuit layouts and schematics, written with DIY enthusiasts in mind.