synthgeek's blog

Original circuit here: Q&D VCF

This circuit is a resonant twin-t filter. In this circuit, its purpose (according to the creator), is to give a “decent fake lowpass response”, which it does fairly well. Tim’s schematic shows two variations, one with self-oscillation, and one without. I built the one with oscillation. Since I was breadboarding it with a dual op amp (an NE5532, to be exact), I tried adding an overdrive-able input buffer/preamp, which worked pretty well, so I went with it.

I had a plastic box sitting around with no lid, so I’ve temporarily built it into that for use with Loid. It will eventually become part of the piece I mentioned in the last post. Pots from left to right are: gain, cutoff, resonance.

Here’s an audio demo, with the Q&D filter being fed with an XOR pseudo-ring mod, which in turn is being fed with two squarewaves from Schmitt trigger oscillators- at 1:06, the input is removed, and you can hear just the filter’s self-oscillation (modulation via 4069 LFO):

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

These circuits currently exist on Loid’s breadboard extension. As I’m out of space in the containers, and am also reaching the limit of the power supply I’m using, Loid will be getting another piece added soon. In addition to the modules discussed here, I also plan to add a 4017-based sequencer, a noise source, a couple of VCO’s, and most likely some other stuff. If all goes according to the current plan, the result will be a sort of utility/”brain-box”, which will also function as a take-away sequencer and beat box.

The simple VCA is in this series of pages, towards the back of the “book”. This is from a project Ray Wilson was involved in before MFOS came into being, called the WP-20 mini-synth.

Logic-only VCA and AREG at this thread. I will only be using the EG. I used an LM358 for the op amp. There is also a similar AREG in the WP-20 book which is based on the LM3900 and uses a 555 as a gate generator.

The following simple recording shows the AREG being used as a highly controllable LFO, using a slow squarewave as the trigger/gate source, modulating the frequency of a CD4046 VCO. At first, a 40106-based square LFO is being used directly as the gate, later I switch it over to a divider for even slower modulation.

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

No recording of the VCA, but it just controls the volume, nothing super exciting. I can, however, say it works quite well with this simple EG. My plan is to build a quad version using all 4 op amps in the MC3401 I’m using, which is a relative/equivalent to the LM3900.

This box is based on the filter from the Weird Sound Generator by Music From Outer Space, with a simple op amp-based drive circuit after it. Its main purpose in my setup will be processing sounds from Loid. The idea came about while I was messing around with the WSG filter on a breadboard, and decided to see what would happen with a simple drive circuit placed after it. The results are of the “quick & dirty” sort, like Loid himself, as well as the WSG from whence the filter comes. I didn’t necessarily mean for it to go this far, but once it was working, I realized I liked it enough to build it.

Here’s a shot of the WSG filter board:

Here’s a schematic of the overdrive/shaper circuit, and a shot of the board:

I decided to build it so that the drive circuit can be switched out of the audio path, because it changes the sound of the filter’s “resonance” quite a bit. It actually almost seems to tame it completely as it’s turned up, but also does something else to the sound which I thought was pretty cool (audio example below).

The case is a cheap aluminum Rat Shack job, which I suppose is actually a step up from my usual recycled cases. Here’s a shot of the inside of it, before the boards & pots were put in:

And here’s the final product, which I’ve decided to call the WSD (Weird Sound Destroyer):

It uses its own power supply (a 9V wall wart I had laying around), because Loid is starting to push the limits of his. I used a switching supply jack, so an alternate battery supply would be easy to add (if I can squeeze one in there).

Time for the audio. Here we have a Schmitt trigger oscillator- first dry, then through the WSD. At first, the drive circuit is switched out, then at about 0:43 it gets switched in, then back out around 1:39:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

This came about while messing around with a CD4093 on the breadboard- I patched it up to an existing 3-input NAND gate already existing in Loid, along with a momentary switch for triggering- and cool stuff started happening.

The basic idea is based on the NAND gate in the 4093, which allows you to create an oscillator which can be gated, manually or via another logic signal. Having one gated oscillator is fun, but what happens if you use the old diode-gate trick on top of regular old gating. That was pretty cool too. Purely by chance, I hooked the two up to a 3-input NAND gate as shown in the schematic, and thus the Trigpulser was born.

Since the 4093 has 4 NAND Schmitt triggers, and a CD4023 has 3 gates, that meant the module could have two “sides”, and a final NAND gate on the output. I decided to build it so that the two sides could be used independently, or together through the final gate.

Here’s a recording of one side being used alone, triggered with two squarewaves from “normal” Schmitt trigger oscillators:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

…and here’s one of both oscs being used with the final NAND gate (the pauses are naturally occurring from the material used as trigger inputs):

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

…and here’s a bonus video of a single side in use:

I will also be designing a stand-alone version of this, with a 40106 providing material for the trigger inputs.

This module is based on a basic op amp “building block” circuit- the differential amplifier. In a nutshell, the output is equal to the difference between the voltages at the inputs, multiplied by the gain factor. In this case the gain factor is 1, so the signal at the “-” input is subtracted from the signal at the “+” input. Any dual op amp should work.

Although it works fine as-is for my purposes with Loid, adding buffers to the inputs would also be a good idea. Just copy the output buffer (U1b) once for each input, and place them before R1 & R2.

On audio sources, as one might expect, it has a ring modulator-type effect. Here’s what it sounds like when used on two CV sources, which are then fed into the CV input of a Synthmonger VCO- first, with two 4069 triangle LFO’s, then a triangle LFO and the output of an R/2R being fed with some crazy stuff to give a pseudo-random source:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

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:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

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:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

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:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

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:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

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:

I recently ordered a couple of CD2024 dividers from Jameco, but got Motorola MC2024‘s instead. Last time they sent me Motorola substitutions, it didn’t matter- but this time, the chip is entirely different: a dual VCO, as opposed to a divider. Turns out these might be useful, so I won’t bother sending them back, but I do plan to point it out to them.

This chip is designed to operate at radio frequencies (MHz), and a chart in the datasheet shows that the noise performance gets worse as you go lower in frequency. However, I didn’t let that stop me from having a go at it on the breadboard. Although it’s not perfect (the noise/distortion gets noticeably worse as you go into low frequencies), it is fun to play with.

Below is the circuit I came up with for using it as an audio-frequency oscillator. Note that the datasheet recommends +5V operation, so I also put that in the schematic- however, the version I built on breadboard uses +9V, and it seems to work. That is to say, it doesn’t seem to overheat or anything like that. Also note that the function of SW1 is to switch between CV modulation of the 2nd osc along with the first one (thus making it like a unison osc if using a dual-ganged pot, which I represented in the schenatic as VR2a & VR2b), or modulation of the 2nd osc from the first (thus making it a pseudo-FM osc).

Here are some recordings of my tests- the first one shows the oscs in “unison” mode, with CV modulation from a Schmitt trigger LFO:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

The next one shows the oscs in “FM” mode, with osc 1 being modulated from the same LFO:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

The third and last one is a combination of both modes, with the modulation source’s rate also being tweaked, often going into audio rates:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.