The fuse is not really necessary, as the wallwart, if properly constructed, should have a fuse internally, but nice as extra protection. An 670mA fuse for a 700mA wallwart should be perfect as it will blow before the wallwart reaches its maximum capacity.
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Here is my stripboard version of Moritz Kleins compressor. Works super well and not a very hard build. I highly recommend!
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I often come across leaky and noisy PNP germanium transistors while testing them, so after reading Robert Penfold’s article on noise generator circuits some time ago, I decided to put them to good use. Rather than auditioning/abusing silicon transistors in reverse avalanche mode, you can get (better?) noise by using an otherwise throwaway germanium transistor.
Penfold mostly uses a discrete approach with the rather obscure BC650 for amplification. I was able to find BC650’s locally for a few cents, so I just followed his simple common emitter approach that requires only a single supply of 9 volts. I assume that you can replicate his approach to red, yellow, etc noise filtering using op amps, or substitute BC650 for a similar high gain transistor (mine had an hFE of about 820).
The stripboard follows the schematics in the article, but I added a volume control while, in my actual build, I increased the 1.2k resistor to 3.3k as I found the end volume of pink noise too high. You might want to adjust yours accordingly.
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This is a simple voltage generator, attenuator and inverter.
When nothing is plugged into the jacks, the output is a voltage controlled by the potentiometers. The switch selects the voltage range from -5V to 5V to -12V to 12V. When something is plugged in, the potentiometers attenuate in the second half of the pots turn and invert it in the first half of the turn.
I built it as a way to add external control to my ornament and crimes module.
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A sort of two-in-one little module. A sawtooth to triangle converter (according to the Moritz Klein schematic) followed by a Lockhart wavefolder (according to the Ken Stone schematic). The board is rather tight with three components soldered at the underside. If you so wish you can space out the components and/or add extra rows for a 2x5 power header and Shottky diodes. I omitted those because I use simple JST-XH connectors with colour coded cables.
There is no modulation input, but one can modify the layout to add a switch that will separate the potentiometer in order to be able patch in a VCA in its place. The VCA’s amplitude modulation should work as a modulator for the wavefolder.
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I came across Tim Escobedo’s Bronx Cheer (an “envelope waveshaper filter”) some time ago and finally gave it a go. It’s a very simple high gain Darlington transistor fuzz, with a filter attached to it’s simple collector to base biasing. The filter is a simple low-pass “double pi” topology, with its two resistors cleverly replaced by diodes that work as variable resistors that conduct when the signal gets past their forward voltage. The result is a resonant fuzz where the filter frequency changes when one hits the notes harder, and changes again as the notes decay.
I made only minimal changes in the design, replaced the power capacitor to 100μF, and the filter capacitors to 1.5nF, removed the switch that did not make much of a difference, and replaced the MPSA13 that I could not find anywhere locally with the higher gain MPSA14 (the similar MPSA28 works just as good). The quirk of the circuit is that it expects to see the high impedance of guitar pickups at the input. The impedance mismatch can result in the most horrible hum and noise when connected to the output of a synthesizer. Escobedo suggested the trick of connecting the primary windings of a small transformer at the input to mimick the impedance of a guitar pickup. I tried locating the transformer he indicated in more than a dozen local shops and none had anything similar. So I tried different small transformers and inductors (the ones you find in “wall-wart” switching power supplies and fluorescent lamps) and they all worked, with no discernible difference in the sound.
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hmmm, so what is it for then, you think?
The transformer/inductor is necessary to match the impedance of the circuit when one is not using a guitar/bass with passive pick-ups at the input. Impedance mismatch, in this case, results in plenty of hum and noise.
I’m not following this. I’ve made a few passive LC steps for guitars and not had the impedance issue.
What am i missing?
I am missing something here. What are LC steps?
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Adding in a L (inductor) and C (capacitor) passive filters. Step means it’s on the hot wire and not variable. (as in the signal has no option but to pass or ‘step’ through the inductor)
The schematic says ‘any value’ and you have tried several without noticing any difference in the sound. So what then do you mean by ‘they all work’? They can not all lead to a proper impedance correction unless they are all the same and you happen to find the right value being lucky. I’m missing the logic I’m afraid.
Right. This is not the case of an LC filter. The transformer (used here as an inductor as we only use the primary winding) just mimics the impedance of a passive guitar pick-up. The circuit, just like many other fuzz circuits and some wah effects, expects a high impedance input and will misbehave if presented with something else. Jack Orman has written a little article about this quirk of fuzz effects, and devised a guitar pickup simulator to that effect. The particular circuit, like several others designed by Escobedo, uses this particular trick.
I tried it on guitar and bass and works just fine with and without the inductor/transformer. No differencein the sound. When fed the low impedance output of a keyboard, synthesizer, or another guitar effect you get a lot of noise and hum, however. Remember, this circuit uses a transistor with a gain of 10,000 min and will amplify any artifact. Hence the need of that transormer/inductor at the input. My 10c contribution is that you don’t really need that obscure trasformer listed by Orman and Escobedo. Any little transformer or inductor salvaged from a broken CFL or a mobile phone charger could likely do the job!
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Here is another simple project. With a little help from the people in this forum (thanks @illucynic), I managed to get a working layout on stripboard for the Korg MS-20 VCA. It follows closely the schematic of the MS-20 service manual for the VCA in the modular section of the synthesiser (labelled MVCA in the manual). The schematic calls for an 4558 op amp, but I changed that to an TL072 and I also changed the transistor to a commonly available 2N3904. Other than that I had to adjust the values of the resistors around the second op amp to fit the (low resistance) LDR I had available at the time. I tried a couple of different LEDs and a 5mm green LED gave the best response. You might need to play around with these components, but I found that there are several combinations that work. I also made the output AC coupled, but this is entirely optional and you can remove the resistor and capacitor without altering the layout on the board.
Now, I don’t have an MS-20 but there are a couple of videos of the MS-20 on youtube that sound similar… In any case, I found this VCA to be very usable, especially since it does not require any fancy or expensive parts.
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Congratulations, nice project. I’ve been curious about this VCA for some time.
I have some questions:
What type of green LED did you use? Diffuse or super bright type?
What is the procedure to select resistor and potentiometer and match them according to LDR?
Thanks.
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I used an LED from my parts bin, so I can’t tell for sure, but it certainly looks like the diffused part. It’s not very bright at all. I got a similar response with a 5mm diffused red LED, but brighter LEDs did not work as they overwhelmed the LDR.
To find the right values for the resistor and potentiometer, I worked backwards from a very crude measuring of resistance of the LDR that I had in hand. I measured something like 1k light and 33k dark resistance.
- With a 2,2+1,2=3,4k resistance at the op amp feedback loop I could get a response ranging from gain of 3 to 0.1.
- The corresponding figures with the potentiometer turned down were about 0 to 0.04.
I thought this was a decent range, going from the CV fully attenuating the signal to giving it a slight boost with a turn of the potentiometer, which, by the way must have been a trimmer in the MS-20 because I cannot see anything on the patchbay VCA panel.
When I tried an LDR with a much higher dark resistance, I scaled up the values of the resistor and potentiometer and I got a similar response, without changing the LED. I figured that it should not be difficult to get something decent with pretty much any LDR, as long as you played around with the resistance around the second op amp using very common parts.
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Note that these are not antonyms. You can get super bright diffuse LEDs, or low brightness non diffuse ones.
I haven’t done much vactrol making yet but I plan to soon. I have some green super bright, non diffuse, flat top LEDs I will be trying, in accordance with some of the recommendations in the discussion starting here.
I certainly would expect this would depend on the LED current determined by the series resistor. I’ve used super bright LEDs as indicators where they would be blindingly bright at full current, but with large enough resistors they are comfortable to look at and draw far less current.
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I did not change the current limiting resistor when I tried on the, visibly, brighter LED. The response I got did not sound anything like a typical VCA.
I checked some LDR’s that I have here and I see that the resistance measurements found are very close to yours.
I’ll put this on a breadboard and run some tests.
Thank you so much.
Thank you for clearing that up.
By the way, I don’t know about this low brightness non diffuse ones.
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