synthgeek's blog

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.