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As some of you may recall, I had built an AD633-based analog multiplyer (ring mod) module early on in the first Phoenix chassis, then removed it in favor of keeping that box single-supply- with the intention of putting it back in rotation when I broke out the dual supply again. Well, that time has arrived. I’ve used the same chip, but on a new board. I used another Ray Wilson design this time- namely the MFOS Analog Multiplyer from his “Ultimate Expander” project. I will be adding attenuators to the inputs, other than that, no changes.

On the same board, sharing a TL074 with the multiplyer’s input buffers, are a very simple preamp and noise source.

As mentioned in the notes, the noise source is a little quiet, while the preamp can be excessively LOUD. You may want to follow it with an attenuator if not using it somewhere where you can easily patch it into one. With the gain cranked, and a high enough input level, it’s gonna be as close to rail-to-rail as the TL074 will go. Way more gain available than needed, really, but I figure it’s better that than the opposite problem.

Here’s a recording of the Nick W. VCO (pulse wave) and the MFOS VCO (triangle wave) through the multiplyer, which is in turn put through the LDR filter (bandpass mode) with a little overdrive- the filter cutoff and VCO pulse width are modulated with triangle LFOs:

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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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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.

Once again, this device is named after its housing- in this case, a tin with a “Smokin’ Joe’s Racing” logo from Camel cigarettes:

Here’s a close-up of the board:

Some of the components for the EQ are mounted on the pots. The only additions I’ve made to the circuit from the last posted schematic is to add a feedback loop from the line output to the switched side of the first input jack, so that the connection is cut when something else is plugged in. This feedback loop allows it to act as a sound source in its own right, even without any inputs.

Here’s what the final build looks like- the camera’s flash makes it look blue in these photos, but as the earlier pic shows, it’s actually purple:

You’ve already heard what it sounds like with inputs, so here’s a quick recording of the feedback loop oscillation:

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Yesterday I posted a link to HeadWize, where I had found some good info on EQ design. Figure 1b on that page shows a 2-band Baxandall EQ (high/low shelf), which I have adapted for use in the Mossifier. The EQ section is marked in the new schematic:

This addition uses the two remaining inverters from the CD4069 in place of the op amp from the schematic shown at HeadWize. It works surprisingly well- of course, I’m sure a proper op amp would work better, but it seems to be doing pretty much what it’s supposed to. I put a SPDT switch in the schematic for bypassing the EQ, just because I thought it might be handy- but as it says, that’s entirely optional.

Note that the basic lowpass tone control from the first schematic has been removed. If you wanted some sort of per-channel tone control, that would make a decent option.

That about does it for the design of this bad boy, all that’s left now is to build it on perfboard and house it- and of course I’ll be posting about that too.

While searching out information to aid in the design of an improved tone stack/EQ for the Mossifier, I came across a couple of interesting sites with some great info. Regardless of whether or not I end up using any of these ideas in the Mossifier, I will eventually be using this info for something.

The first one is called Adam’s Amplifiers. There is a bunch of good info and some cool projects here, but the Tone Stacks page is the real treat imo. There are a number of tone circuits of varying complexity (one-knob, two-knob, and three-knob), culled from various amplifier circuits.

The other one is called HeadWize, and is apparently mostly aimed at hi-fi projects- however, there is a page with some great info and schematics for active EQ’s, which is exactly what I was looking for.

I will be adding both of these to the link library.

I recently came across this article: http://www.fairchildsemi.com/an/AN/AN-88.pdf , which details the use of CMOS devices in linear mode. One of the techniques shown uses inverters as amplifier blocks, turning them into something like op amps. This inspired me to try using them as an input buffer/preamp for a 386-based mini amp.

As noted in the linked article, the response is far from linear as the signal swings close to the rails, so anything other than squarewaves will get distorted to the point of hard clipping as the input is driven harder. That means it functions as a distortion device as well as a mixer/amp- in fact, I would almost say it’s more a distortion device that also happens to have an input mixer and speaker output.

Here’s the schematic as it stands so far:

As noted in the red-boxed portion of the schematic, more inputs can be added simply. Per-channel mutes would also be a simple addition, just add SPST switches between the pots and input capacitors. The tone control could also be copied per-input if desired. The next section (U1a-U1c and associated components) is the input buffer/preamp, which is implemented via 1/2 of a CD4069 hex inverter. Note the 47pF cap in the feedback loop- though not specified in the Fairchild article, it is necessary to prevent oscillation and unwanted noise. Higher or lower values did not work.  The 1M pot, however, you can mess with (lower values will give you less drive)- just make sure you leave the 10k resistor there to provide a minimum resistance. To simplify this section, you could find the resistance that gives you the gain you need, and replace VR2 & R3 with a single resistor of that value. Note that removing inverters from the signal path will also decrease gain. Consult the Fairchild article for further information about this.

I did try adding a 1M resistor to ground on the input and plugging in my bass, but wasn’t impressed with the results. If you would like to be able to use this device with hi-Z instrument-level inputs, you could try messing around with that some more. I no longer have a guitar available to try it with, so I didn’t go any further down that road myself.

The next part is the power amp section of the amplifier, provided by an LM386. This section is somewhat similar to, and was inspired by, the main amplifier part of the Beavis Audio Noisy Cricket design. If you wanted to make this section simpler, there are a couple of things you could do. The easiest way would be to remove the 1k audio taper pot between pins 1 and 8, and the 100nF capacitor from pin 7 (Bypass) to ground (this capacitor is only needed when using higher gains), leaving all 3 of those pins unconnected. This would remove the option for added gain/distortion from this section, but it’s a bit OTT anyway, so that’s no big deal. Another option would be to leave the Bypass cap in place, but replace the pot with a SPST switch and a 1k resistor inline, thus having a low gain/high gain switch. It may seem counter-intuitive that adding resistance here increases gain, but it’s because the 386 has an internal 1.35k resistor between pins 1 and 8, so adding a resistor or pot there actually decreases the resistance, due to parallel resistors.

The final sections are the outputs. The 10 Ohm resistor (R5) sets the output impedance for the speaker output. The other branch provides a buffered line output- R7 and R8 form a voltage divider to tame the level. Volume controls could be added to both outputs by replacing R7 and R8 with a voltage divider via potentiometer, like the inputs- pin 3 would connect to the output caps, pin 2 to the output, pin 1 to ground- and adding the same thing between the speaker output and its capacitor.

You may notice there are two unused inverters, I have some ideas for making use of them, so I didn’t delete them from the schematic in the editor yet. As per proper CMOS practice, these unused inputs should be tied to ground if you build this as it stands.

It can be powered from a 9V battery, though I don’t know how long it would last. I would guess battery life should be decent, comparable to the Noisy Cricket and similar designs, but that’s just a guess. I’m currently using a 9V wall-wart supply. It should be possible to run it at anything between 4 and 12 volts, as both chips are capable of that operating range. However, the information in the Fairchild article suggests better performance at higher voltages, as that gives more headroom for the CMOS amplifiers, keeping the input from “hitting the rails” and going into hard clipping. I chose 9V simply because I have an adapter handy that fits the bill, and it makes for easy portability. Also, it fits with the original spirit of the mini-amp devices which inspired it.

Here’s a shot of the breadboard build:

…and here’s some audio- first, a sine wave from the K2000 (first dry into a different mixer, then into the mossifier at lowish gain, then at high gain):

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This is what the high gain part looks like on a waveform display- as you can see, the clipping is not symmetrical:

The next one starts with a 40106 osc, starting at lowish gain, then sweeping into higher gain, and back down around the mid point. Then it switches to an actual patch from Loid being played through it, first with just one input, then a second is brought in later:

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The speaker output is actually pretty loud- definitely louder than the Dirty Cow amp. I successfully drove both a 6″ bookshelf-type speaker, and a 12″ Peavey PA speaker with good results.

Before calling this finished, I plan to try to improve the tone stack. I have ideas for using the two unused inverters there, for an active filter setup. Failing that, I may attempt an inverter-based version of the passive buffered BPF from Loid. More to come…

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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My friend Kris has given me a bunch of interesting materials for use in my electronics experiments, so I figure the least I can do is build him something. His main instrument is drums, but he also plays bass, so I’ve decided to build him an LM386-based portable bass amp. I have a small Candy Land lunchbox that will make a perfect enclosure:

The circuit is cobbled together from ideas snagged from three other designs: the Little Gem and Ruby designs from Runoffgroove, and the  Noisy Cricket design by Beavis Audio Research. Ultimately they all lead back to the Smokey Amp by a guy named Dave Stork.

The controls are pretty simple- on/off switch (with LED), and volume & gain knobs. The silver jack is the input, the black one is the output. I used a switching jack for the output so the internal speaker turns off when you plug it in to a seperate cab.

Here’s an example of how it sounds- for this recording, I had the output going to a 12″ speaker, close miked. Bass is an Ibanez active 5 string with its volume set low, volume on the amp is cranked, gain about half (I think you’ll recognize the tune):

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