synthgeek's blog

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.

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…