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The original Buzz Box was designed by a guy called Hemmo, though I first came across it at the Runoffgroove site. It looked easy enough to try on breadboard quickly, so I went for it. Along the way, I tried a couple of little mods, here’s a schematic of what I came up with:

I went with the red LED simply because it was what I had on hand. I also tried adding a power starve pot, and although it did indeed change the sound, I didn’t feel it was a drastic enough change to be worth it. Others may not agree however, it’s worth trying for yourself.

Here is a recording of a triangle wave from an NE566 being processed with this circuit on a breadboard- first, the dry triangle wave is heard, then the effect. The Buzz Box controls are also swept, first the input drive, then the “fuzz” control, then different drive settings are tried while the fuzz control is swept:

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I’ll definitely be building this, I really like the nasty things it does.

Finally managed to get this off the breadboard, onto perf, and into an enclosure. Here’s a shot of the board before wiring to the jacks, switches, and pots was added:

…and here’s the inside of the box:

This is the finished device- I decided to go with another aluminum Rat Shack enclosure, mostly because the price is right (bad photo, sorry):

I also have a bonus bit of noise- nothing special, just some racket from the Phoenix modular synth being run through this bad boy:

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This project was started as pure experimentation, toying with op amp distortion. Although it takes inspiration from many other designs (nothing entirely new here, just overdriven op amps), it is not a clone or mod of any specific device. Also, though it certainly might work with guitar (and sounds pretty good on my bass I think), it wasn’t designed for it. Its purpose will be as a distortion channel-type preamp for the modular amp project I mentioned in the ReMock+ post.

For the distortion circuit itself, I referenced the article “Cook Your Own Distortion” from the General Guitar Gadgets site, as well as designs by Runoffgroove and Mark Hammer. There are, of course, several things that could be modified- some of them are mentioned in the schematic notes, those and a few others will be discussed here. This is not to say that this post will cover the entire range of things that can be done with an op amp-based distortion unit, just several ideas for tailoring this one to your own purposes. Perhaps the most obvious would be trying different op amps- I tried a few, and preferred the MC1458 for this device- but, as they say, YMMV.

Getting into the rest of the circuitry, it may seem like overkill to have two gain knobs and a drive control- and it probably is- but they are actually all useful.  However, the second gain knob could certainly be left out or changed, while still retaining a good range of different sounds. The configuration shown gives a variable gain factor from about 5.5 (5.45 repeating actually) to 48 in the second gain stage. As mentioned in the above-linked article, the formula for the gain factor is (R1 + R2)/R2, where R1 is the resistance in the feedback path, and R2 is the resistance to ground.  Just as an example, to keep it at the low side of the current configuration, you could change the feedback resistor to 47k, and just remove the 100k pot (leaving the 10k resistor where it is) for a gain factor of 5.7. This would still give a bit of clipping in the second stage.

The capacitors C2, C3, C7 and C8 (along with associated resistors) create simple filters in the feedback loops. Changing these will alter the sound quite a bit. In the current configuration, they act as bandpass filters which act mostly on the lows- the first stage keeps most of the lows intact, while the second stage cuts the lows for an edgier sound. C3 & C8 remove the very high frequencies- around roughly 33kHz.

Getting back to the the drive control, this could probably be left out of you’re building this as a guitar effect, since your guitar’s volume knob would perform the same function. I’ll be going the other way, and leaving out the output volume control, since in my setup it will always be plugged into something with an input volume control- probably a mixer most of the time.

Another thing you could do to simplify things is to remove the LED’s and/or switch from the feedback path of the first stage. You could go the other way and get more complicated here as well, but I personally chose to save that for another project. As mentioned in the schematic notes, one LED I used is some strange multi-color thing that came out of a computer. I only tried it because it was here, it turned out I liked the sound, so I went with it. I also tried two red LED’s, and it was much more subtle.

On to the tone stack: I chose to place it between the gain stages in order to make it like having two distortions in series, with an EQ between them (which is essentially what it is). As a starting point, I used the “Fender” setting in Duncan’s Tone Stack Calculator- first, changing some component values in the simulation to get started, then tweaking values further on the breadboard until I was happy with the sound. My main goal here was to get rid of as much of the mid scoop as possible, while still retaining a decent range of control. The current configuration still gives a slight dip around 400Hz, also known as the “mud zone”, so we can live with that. As is often the case with passive tone controls, there is quite a bit of interaction, though in this case, I consider that a feature. Specifically, what would have been the treble control is now more of a spectrum tilter-turning it one way simultaneously boosts the highs and cuts the lows, and vice versa. The bass control is still a bass control. I replaced the mid control with a fixed resistor- lowering the value here will cut the highs, but if you make the value too low, it will affect the entire frequency range, and become a volume cut instead of a tone control. I would suggest going no lower than 33k, but again, YMMV. Below is the simulated frequency response plot from TSC. The red line corresponds to flat tilt and bass knob settings, the green and pink lines are the two extremes- bass all the way down, tilt all the way to the treble side, and vice versa. The white line is both controls all the way up:

The schematic includes a bypass switch for the tone stack, which could also be left out. Speaking of the schematic, here it is:

With the tone stack between two distorting gain stages like this, it acts more as a way to adjust the character of the second stage’s distortion than an equalizer for the actual sound coming out of the effect. While this is done by design, you could certainly move it to the end of the circuit if you prefer. I had actually considered including a simple lowpass tone control at the end- but there’s already so many knobs, and as part of the larger project, I plan to build at least one EQ-only module.

Here’s a recording of a sine wave from the K2000 being processed, with various controls being swept/switched:

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We’ll follow up with a few bass riffs… first, both stages cranked, with the LED disengaged:

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Now, with the second stage backed off (minimum gain), first one still maxxed out:

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This time, both stages backed off, but not quite at minimum:

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The next two have the LED engaged, first lowish gain on both stages:

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Now with both stages cranked:

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I finally managed to get this built and boxed up…

There were some errors in the last schematic, here’s the fixed version:

As you can see, I decided not to include a bypass switch, because I don’t need it for my uses, and that makes it a little cheaper to build. Including one would not be difficult, if you need help, see this article by R.G. Keen. For my final build, I also decided to leave out the output volume control- again, because it’s not necessary in my setup. When it comes to the modular amp/preamp/distortion device thing I mentioned last time, it would be especially redundant.

Here’s a couple shots of the internals:

A few changes were made since that photo was taken, most notably the wiring for the “softer mode” diodes- I moved the switch wires to a position which more accurately represents the schematic, and changed some jumpers on the bottom which you can’t see in the photo (oops- that was a silly mistake on my part).

Final build notes: I built the perfboard version with the 100pF cap in place on U1a (you may remember I had to remove it on the breadboard build to make the RC4560 work). My thoughts were that it would make trying other op amps easier, and if I had to swap out the 4560 for another TL072 or whatever in the final build, so be it. As it turns out, it works just fine this way, even with the 4560. I have some theories as to why this is, but will let more educated folks tackle that question via google.

The final build is also much less noisy (which makes sense), and perhaps because of that, at least in part, I’ve also noticed much more difference in the sounds between op amps in the distortion section. The differences are just slightly more noticeable with “softer mode” engaged. The TL072 had a, well, softer sound (in either mode) than the RC4560- I suspect due to lower output level, and thus, less clipping. I suppose one could get really crazy here, and make a version that switches between chips- but I think I’ll just keep the 072 in mind for another device entirely. As you can see in the photo, I mounted the board in such a way that I can easily swap out chips, should I change my mind later. Also, since I have no single-row headers to use, I used a 16-pin IC socket so I can also swap out the clipping diodes (there are jumpers on the bottom side connecting the two sides where needed). I’m a little iffy about them holding in place with much jiggling around though, so I may have to think of some way to hold them a little better, without changing the mounting- electrical tape is my first thought. :)

For the audio examples this time around, I plugged by bass in, with the output of this unit going into the Dirty Cow amp, which in turn is plugged into a 12″ Peavey PA speaker. The speaker is then miked with a Sennheiser e835 dynamic- the sound is just a tad dark, I didn’t really get very technical about it.

For the first several recordings, I kept the gain all the way down and the input level lowish on the Dirty Cow, in order to get as little extra distortion from it as possible. I also kept the tone towards the low-boost side.

For the first one, the ReMock+ is set to “dark”, with harder mode on, lo EQ boosted (not quite all the way), high EQ cut:

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In the next one, it’s set to “bright”, harder mode, lo EQ cut, hi flat:

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For the next one, we go back to “dark”, but switch to softer mode, and set the EQ flat:

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The next one is the same as above, but set to “bright”:

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…and for the last one, we go back to harder mode, keep it set to “bright”, boost the lo EQ, then also crank the amp gain and input level:

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In this setup, the difference between “harder” and “softer” modes was barely noticeable- I suspect this was partly due to added distortion from the amp.Also, I really didn’t play with the input level of the ReMock itself, which can make a pretty big difference actually.

All things considered, I’m very happy with this device- big thanks to Runoffgroove for the original inspiration!

Now, here’s a bonus bit of noise, with the Modutronic Messmaker prototype being run through the ReMock+ (starts off in softer mode, with the input level set low- first tweak is the input being turned up, then various controls are swept & switched throughout):

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EDIT: I have updated the schematic, R17 should come before the output volume pot.

This is a standalone distortion unit, which also marks the beginnings of another project. I haven’t come up with a name for this one yet, but the idea is a modular amplifier and distortion unit. There will be a number of preamp, tone/EQ,  and power amp modules, focusing on circuits suitable for bass guitar and synthesizers- my main instruments. I will probably also make at least one mic preamp for this project eventually. Nothing is set in stone yet, but there will of course be at least one 386-based power amp module- and of course, the circuit I’ll be discussing here (which is a preamp/distortion unit). I expect the going to be slow on this project overall, because I will be continuing with modules for Loid and Phoenix as well.

The distortion circuit itself is a modification of the Mockman V2.0 by Runoffgroove (the name ReMock+ is a nod in the original’s direction, as well as a play on “remix”, with the “+” added to indicate the addition of an EQ). I replaced the 47pF feedback capacitors with 100pF’s to reduce noise, and added a switchable 2-diode “softer drive” mod. I also added a 10k audio taper pot to the input, which allows for something approaching a clean signal, as well as control over the amount of clipping.

The second part of the circuit is a simple 2-band (hi/lo, or treble/bass) Baxandall EQ. If you’ve been paying attention, you may notice that the R/C network which makes up the filters is the same as the one used in the Mossifier, only this time we’re using an op amp instead of inverters for the amplifier part.

Here’s the schematic:

This schematic is suitable for a standalone distortion box, I may or may not break it into seperate distortion and EQ modules for the modular amp I mentioned. In the meantime, I’ll be building one to use while I gather resources for the big project.

Here’s a breadboard shot:

The version in the photo is using a TI RC4560 for the distortion op amp. For this particular op amp, I had to remove the first 100pF feedback capacitor (C4 in the schematic)- no other changes were needed. I also tried a TL072 and an MC1458, and they both worked best with C4 in place. As far as the sound of the distortion, I didn’t notice a huge difference with any of the different op amps, other than slightly lower noise with the RC4560. I also played around with the values of the input cap and the caps in the “bright/dark” section, and ended up going back to the original values. Feel free to change them to suit your own needs though.

I’ll need to wait until I can make another parts order to finish this build, at which time I will be making another post (mostly need more jacks and switches for this particular project). For now, here’s an audio demo of a plain sine wave from the K2000, playing a simple arpeggiated sequence- first dry, then through the ReMock+:

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When the distortion first kicks in, you’re hearing “harder” mode, with the EQ roughly flat and the input gain cranked. At about 0:25, I crank the high EQ, then back it off a little, and so on, messing with the two EQ controls. At around 0:51, “softer” mode is first heard. Thereafter, it’s just a bunch of various knob tweaking.

I also gave it a quick run-through as a bass distortion with the Dirty Cow as the amp, through my Peavy 12″ speaker, and it sounded good. Didn’t feel like putting a mic to it today though, sorry- next time.

Another 386-based lo-fi bass amp build, again inspired by ideas from Beavis Audio Research and Runoffgroove. This one is a bit more complicated, incorporating power supply filtering (via 220uF capacitor), buffered input (via 2N5951 transistor), and a tone control (from the Beavis Audio article on the Big Muff Pi tone stack, inserted between volume control and 386 input pin).

The input section (including buffer) is like the one from the Beavis Audio Noisy Cricket design, but with the BMP tone stack instead of the one from the Cricket. Also, I used a single 100nF cap in place of the 47nF specified in the Cricket schematic, and omitted the second input cap. The rest of the design is inspired very much by the Grace Overdrive design from Runoffgroove, with a larger output cap.

Perhaps you’re wondering where the name came from… well, like the Candy Land Combo, the Dirty Cow gets its name from its enclosure:

The jack with the red plug in it is the input, output is on the back. There is a power switch and LED, and the knobs (from left) are: gain, tone, and volume. Their placement is a bit misleading, since I went with a “non-master” volume setup (the volume is on the input, not the output). That also means the volume and gain knobs are interactive, allowing for a more adjustable response. Compared to the Candy Land amp, this one is a little louder, but is also capable of more sounds approaching “clean” territory.  Here’s some audio (kinda sloppy playing, sorry)- first up, a cleanish tone (same active Ibanez bass as last time) volume just over 3/4, gain lowish):

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and here’s a more distorted one (volume and gain around 3/4):

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…and some even sloppier guitar playing, gain and volume cranked (passive-pickup cheap Strat copy):

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