synthgeek's blog

So, after a few attempts with different op amps, and different filter configurations, this is the best I came up with for using the 4007 in a filter:

Note that this will not work well with anything other than signals from logic outputs- inverters, gates, etc.- the stuff Lunettas are made from. I was only able to get the BP input to work, so I removed the other two from the schematic. Other changes include limiting diodes in the feedback path, The use of a TLo8x for the filter amplifier, filter capacitor value changes, and input/output capacitors.

Ultimately, though not a complete failure, this is not what I was hoping for either. I’m sure that my n00bness has much to do with it, but I’m also beginning to have my doubts about the 4007 in this capacity.

Here’s another recording of the BPF, first on a squarewave, then on some more interesting stuff coming from a quad AND/OR select gate:

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Perhaps I will come back to this line of research eventually, for now I think I’ll be looking elsewhere. I am still considering adding this circuit to Loid though, as it does pretty well there I think.

The concept of using a CD4007 for the VC part of a VCF is one I first saw at the Experimental Anonymous archives- specifically, this 4007-based VCF. My attempts with that schematic were unsuccessful, so I started looking for more information on the 4007 itself as a variable resistor.

Eventually my search led me to this filter by Synthmonger. Though the filter in that shematic does look interesting, it’s not what I’m after right now- so I decided to try using the voltage control scheme with a different filter. What I would really like at this point is an MS-20-style SVF, so I’ve decided to try pairing it with something like the first filter shown in this schematic from the EA archives.

I put a 10k resistor in place of the resonance vactrol (note that the pair of resistors here controls the gain of the amplifier block- this is worth playing with I think), and used a 100k pot in place of the 1k resistor between the filter output and the BP input- I tried it the other way around first, but this way worked better. As you may notice, this makes the filter sort of a combination of the two from the above-linked schematic. With this configuration, the BPF works quite well, but not the LP or HP. Here’s an audio example, first showing the BPF, then the LPF, then back to the BPF- you’ll be able to spot the difference:

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Thinking that input overdrive could be an issue, I’ve tried reducing the levels using voltage dividers, but to no avail. However, I have also noticed that the choice of op amp has a definite impact on the sound, so I’ve tried several- TL072, RC4560, RC4580, and MC1458- which are all the choices I currently have in dual op amps (I’m using a dual for easy output buffering). The 1458 provides the best response of the bunch, and is the one you hear in the above recording. I have other choices in different packages (single and quad op amps), so I’ll be trying those as well- they just aren’t as easy to swap out.

Here’s the schematic of what’s on the breadboard as of the time of the above recording:

Note that I did also try powering the op amp from +/-9V,  and though it did sound better, it wasn’t a major difference. I will try it in future tests as well, however, just in case.

If different op amps don’t end up doing the trick, I will probably try a different filter topology. The National Semiconductor application note A Basic Intro to Filters has several basic ideas to choose from- using that and some other filter schematics as a reference, I’m sure something good will eventually come out of this line of research.

Finally got around to a couple of modules I’ve been wanting to try for awhile- the Synthmonger VCO and a CD4069 LFO. The LFO is based on this one, with mods by Tjookum as shown in this thread at TSOL. I also tapped the signal at the + side of the integrator capacitor (the cap between pins 5 and 6 in the schematic), for a square output.

I built the VCO part with 3 oscs initially- but upon going to mount the new module in Loid, decided to disable one, since I’m getting low on panel space, and already have plenty of oscillators available.

Here’s a recording of the two VCO’s being modulated from the two LFO’s, with the VCO’s output being fed into an XOR gate:

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Here’s one with the same LFO-to-osc routing, but with the VCO’s going into 1/3 of a 4053 multiplexer, controlled by several “normal” 40106 oscillators:

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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…

Although I hadn’t planned on doing so at first, I’ve decided to add the MC4024 VCO to Loid. I used the extra space on the board to add a 4015 shift register pair as well. The 4015 is yet another wires-straight-to-pins build.

Decided to do another video this time:

Fixes some errors, and adds some minor enhancements.

Found this thing at a second-hand store cheap- since it’s made out of clear(ish) plastic, I could see a couple of resistors on the circuit board, and thought it would be worth checking out for the price, in case one of them was a pitch control. Score! It was.

Below is a shot (kinda blurry, sorry) of the circuit before bending. The resistor marked “R2″ is the pitch resistor. You can also see the backside of the built-in speaker:

The rest of the circuit wasn’t very exciting, the sound-making part was a blob-type IC. The other possible bend would have been to add external play triggering, but retriggering while it was already playing didn’t interrupt what was already playing, so that wasn’t super-exciting either.

What I ended up doing was taking out the existing resistor, replacing it with a 10k (it was 55k), and adding a 500k pot inline. Here’s a shot of the board after modification, notice the heat-shrink tubing around the resistor for added strength:

I ended up having to add a second 10k resistor to the opposite side of the pot, as 10k wasn’t enough resistance to keep the chip from crashing (easily fixed by cycling the power). Of course, a 20k resistor at the board would have worked too, but I’m fresh out anyway. I also added touch points (using screws) for changing the pitch- they also cause some glitching under certain circumstances, sometimes leading to crashes, which again can be easily remedied by cycling the power.

I also added an output jack (with a 1k resistor on the board end of the wire)- I had to go with 1/8 inch, due to size restrictions imposed by the enclosure. I also had to grind away some plastic which was getting in the way of the jack.

Some shots of the finished piece:

…and here are some sounds. First up, a dry recording, messing with the knob & touch points:

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Next, processed through the LM567 amplitude modulator in Phoenix, then a lowpass filter, and a delay:

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This one is just the Sorry Pawn with delay:

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