Synthesizer Build part-68: VC DELAY by BMC.

This is BMC 83 the voltage controlled delay using the Princeton Technology (PT) 2399 chip. This is a eurorack friendly project.

Dispite the fact I built over 68 projects I never build a digital delay or reverb, except for project 11 but that was a ready made effects unit. This project takes care of that. It can deliver good fidelity delays of up to 1 second. It can actually do delays of upto 4 seconds but then the fidelity drops fast. The PT2399 wasn't made for such long delay times but shorter times, upto a second, sound really good and with the long times you get some cool distortion, sort of a bitcrush effect.

This was quite an easy project to build. You can find the original article on the Barton Musical Circuits website. There are audio demonstrations on that website so you can hear what the delay sounds like. I also made a demo video myself which you can find at the bottom of this article.

This circuit will work fine on both a dual 12V or a dual 15V powersupply.

The finished delay module

Here's the schematic I used to make my layout from. I changed the opamp numbering to match that of the layout.

I didn't use the 10 Ω resistors in the powerrails as shown on the schematic. But if you have problems with hum you can include them. On the layout below, you could put a 10 Ω resistor from K-3 to I-3 and then lead the red wirebridges from there and the purple wirebridge could be replaced with a 10 Ω resistor for the negative voltage rails.

The diode and 1M resistor in combination with the 100nF cap and the top transistor with collector to pin 6 of the PT2399 make up an anti latch-up circuit that presents a high impedance to pin 6 in the first 400mSec after you switch on which gives the internal oscillator time to warm up and prevents the chip from latching up and crashing which can happen if the resistance between pin 6 and ground is less than 2K at start-up. After start-up this resistance can be much lower but not a straight short to ground. In this module the resistance is then controlled by the second transistor which is opened up by the time control potmeter or external CV input. This resistance controls the delay time.

So there are voltage controls with level potmeters for the delay time and the return amount and the module has an audio output that outputs just the delayed signal and a mixed audio output which mixes the original signal in with the delayed signal controlled by the 'Mix' potmeter. There's also a tone control potmeter which also influences the return time I noticed (see demo video below)

The delay time range goes from 60 milliseconds to 4 seconds but like I mentioned earlier the audio fidelity drops quite a bit with longer delay times, mostly at times longer than 2 seconds but that doesn't have to be a bad thing. It has quite a cool distortion effect. With the longest delay times you do get some clicks and artifacts mixed in the audio but it's not much. The delay times are controlled by the two transistors forming a voltage controlled current sink. The 47 Ω resistor at the emitter of the bottom transistor determins the shortest delay time while the 330K in parallel from the collector to ground determins the maximum delay time.

In my own build I did notice quite some dead space at the beginning (ccw side) of the 'Time' potmeter but lowering the value of the 47 Ω resistor didn't do anything. 

I urge you to download the PDF accompanying the original project. It has a comprehensive description of how the circuit works and what all the components do. 

Here's a block diagram of how the delay works. This is also from the PDF that comes with the build instructions on the BMC website. 

Audio in 2 is the Return input and it has the Direct Output normalled to the socket switch. So if you take the direct output into an external effect module and take the output from that module and connect it to the return input you can have an external effects loop going, creating all sorts of possibilities. You can, for instance, lead the direct output into a lowpass filter and have the VCF out connected back to the return input.

HOW TO PATCH UP THE MODULE:

To get the best out of this module you need to make a synthesizer voice in your modular synthesizer where this delay sits behind the VCA at the end of the signal chain. You can also patch it up so that the delay sits inbetween two VCA's and have the second VCA opened by an ADSR with a slow Release time. That way you get more control over the Delay time, but it's not necessary. The minimum Delay time is 60mSeconds so it won't be able to create flanging or chorus effects. But you can mix in the effect with the clean signal by using the Mix control and the Mix output.

LAYOUTS:

Here are the layouts I made for this project. They are verified as always. I used them to build my module. This was almost another hole in one. I made one little mistake. I had all four non inverting inputs of the TL074 grounded only the last opamp with the direct output must not be grounded. Once I corrected that the circuit sprung to life. Pins 5 and 10 of the TL074 are connected through the strip underneath the chip. The 'Tone' control potmeter has pin 1 not connected. It's important to wire it the way you see in the layout or it won't work properly.

Wiring:

Stripboard only:

Cuts and wirebridges. You know the drill, mark the cuts on the component side using this guide and then stick a pin through the marked holes and mark them again on the copper side. Then cut the marked positions with a hand held 6- or 7mm dril bit.

Don't forget to cut position P-8 underneath the ground wirebridge.

Here is the Bill of Materials. 

It might be a good idea to use a logarithmic 100K potmeter (A100K) for the return potmeter. A lot of changes happen quite early in the throw of that potmeter. However I used a linear 100K myself and that works fine too. But the log type would be more convenient. You could use other value potmeters for all but the Tone Control. That has to be a 10K linear potmeter. The other potmeters are just voltage dividers in this circuit.

PICTURES:

Here are some pictures from the build proces:

I left out the two short wirebridges that connect all 3 ground strips at the eurorack connector together. Instead I soldered them together with some extra solder bridging the gaps.

Stripboard all wired up for testing. I normally only wire things up when I have the panel ready so I can keep the wires as short as possible but with this module I had to be sure first that everything worked. Anyway, it made mounting the board behind the panel easier coz no need for soldering and I was able to stuff all the wiring underneath the stripboard out of harms way.

This is the panel with the waterslide paper applied ready to receive a final thick coat of clear lacquer. The panel is 14hp wide (7CM). the width I normally use because it allows me to mount the stripboard flat behind the panel keeping the depth to a minimum.

Here's the panel design I made in Photoshop just in case you want to use it. It's in A-4 format 300pix/Inch resolution.

Module on the test bench:

The rear of the module. It's 3.8 cm deep so it will fit any Eurorack case.

VIDEO DEMO:

Here's a little demo I recorded showing the module in action.

Here's an interesting look at the inside workings of the PT2399 chip: --- click here ---

Okay, that's it for this one. Hope you like it.

If you have any questions or remarks about this project please put them in the comments below. Remember comments are moderated so they don't appear straightaway. Only after I read them.

You can also post questions on the special FaceBook group for this website.

Synthesizer Build part-54: JOYSTICK CV Controller (Eurorack).

 An easy to build joystick module that outputs 2 CV voltages to control anything from pitch to filter cutoff and anything else that can be changed with a control voltage.

The finished Joystick module installed in a Nifty Case.

Before I started building my modular synthesizer I had a brief try at flying FPV drones. I bought all the gear and some cool drones but the damn things were way too fast for me to control. This was before the stabilized DJI FPV drones were on the market. Anyway.... the gear landed in the coupboard for a few years. I have now successfully taken the hobby up again and now I can fly FPV drones but my transmitter/controller was now outdated so I used one of its gimbals for this project. I found a good schematic on the Mod Wiggler forum.

Close-up of the circuit:

The image below shows the circuitry for one axis. You need two of these circuits to work both axis of the joystick, left and right [X-axis] and up and down [Y-axis].

HOW THE CIRCUIT WORKS:

It's a very simple circuit. Each of the 2 axis of the joystick is assigned two opamps. The voltage coming of the joystick potmeter goes into the inverting input of an opamp and added to that is voltage from the Zero Point trimmer to make sure the voltage is at zero when the joystick is in the rest position (middle). The gain of the opamp is adjustable with the 1M potmeter marked Range. This determines the maximum voltage you get when you push the joystick fully to one position. This goes from 0 to 10V max. when used with a 12V powersupply.

The CV voltage then goes into a second opamp which has an offset potmeter so we can turn the signal into a unipolar one if we want (all positive or all negative voltage) or just give it some offset or even just to make sure the voltage is zero when the joystick is in the middle position. 

This module is meant for Eurorack (dual 12V powersupply) but it will run just as well on a dual 15V powersupply and if you build it for a Kosmo or 5U synthesizer you have more space on the faceplate to accomodate some extra features.

The joystick I used came out of a Taranis QX7 RC controller/transmitter and it has the following resistance values:

When in the middle position (rest) the resistance is 1,31kΩ. Fully right is 2,15 kΩ and fully left is 550 Ω. Same for the up-down potmeter.

The circuit will take a wide range of joystick resistance values so practically any joystick can be used.

I left the springs installed so the joystick always returns to the middle position when let loose.

Ideas for extra features:

The circuit is very bare bones but you can extend it with, for instance, a momentary switch that cuts the CV voltage if you push it, or one that makes contact if you push it and so outputs an extra gate signal.

An other idea that was suggested to me is to have two input sockets with the voltage connected to the socket switches (normalized) but then you can input an audio signal that cuts the voltage and then the joystick controls the amplitude of the audio thus creating a Manually Controlled Amplifier (MCA).

I'll leave all that up to your imaginations. I didn't have room for extra functions on my panel so I left it as presented here.

I did put in two bi-coloured LEDs to give a visual representation of the voltages on the outputs. It glows red for positive and blue for negative voltages. I connected them straight to the output sockets but with a big 10K current limiting resistor so they only glow at their brightest with the full voltage applied and don't pull down the CV outputs. Also to keep the number of components to a minimum. It works like a charm and looks very cool. Their brightness is a good indicator for the amount of voltage present at the output sockets.  They start glowing at around 2V and then get brighter with higher voltages. I mounted the LEDs above the joystick so they are in full view.

You can use any type of quad opamp for this circuit. I used one of my fake LM324 chips from China and because there are no high frequencies involved it works just fine. You can use a TL074, TL084 etc. They all work fine as long as the pin-outs are the same. It's a good idea to use miniature potmeters for the offset and range controls to save some space on the faceplate. The offset potmeters don't have to be 10K, I used 100K potmeters myself. The range potmeters do need to be 1M otherwise the range of the range will be different ^___^

The trimmers can also be different values. I used 200K trimmers. Afterall they are just voltage dividers in this circuit, so the value is not that important.

CALIBRATING:

The zero point is the point at with the joystick is at rest, right in the middle and in this position the CV outputs must be at zero Volts. You set the zero point with the two multiturn-trimmers.  

The best way to set the zero points for both axis is to have both the Offset and the Range potmeters at the 12 o'clock positions and then connect the CV output to an oscilloscope or volt meter and turn until the voltage is zero. 

Then set the scope or meter to a more sensitive setting and again correct until it reads zero Volts. Try to get it as accurate as you can. After you're done calibrating both channels you don't have to touch the trimmers again. 

Here is the schematic I used for the layouts:

I made a Falstad simulation of the circuit which you can see by clicking here.

Here's one observation I made about this circuit. The voltages from the wipers of the joystick potmeters go through a 51K resistor into an opamp, the gain of which is determined by the 1M potmeter (Range). I noticed that the Range potmeter reaches its maximum at about 1/3rd before the full clockwise position is reached. I think this is due to the 51K resistor. I think it will be better to put in a 91K or even a 100K to get the gain in step with the throw of the potmeter. 

The gain of this stage is determined by the formula: Av = (-Rfeedback/Rin) = (-1M/51K) = -19,6 (the minus simply means the output is inverted). This is too much and that's why the potmeter reaches full gain way before it's turned fully clockwise. With a 100K the gain would be -10 and that would result in the full throw of the potmeter being used. To play it save and make sure you get all the gain you can before you reach the fully clockwise position of the Range potmeter I would suggest using a 91K resistor instead of the 51K on the layout. I've changed the Bill of Materials to include two 91K resistors. However I have not made this change in my own module because I can't access those resistors easily anymore, so I can not guarantee it will fully solve the potmeter throw issue but I can't see why it wouldn't work because the mathematics says it will.

The Falstad simulation doesn't really show this discrepancy so do not rely on it for component values. 

LAYOUTS:

Here is the layout I made for this circuit. It is verified, I used it to build my project. It is small enough to fit flat behind a 14hp Eurorack panel. Beware there are two copper strips underneath the IC that are not cut. They connect the grounded pins together. Pins 3 and 12 and pins 5 and 10. 

There are three 100nF caps visible in the layout but I also put a 100nF cap over pins 6 and 7 of the IC. This is to suppress any voltage spikes or noise. This cap is not visible on the layout and because I had no room for it on the component side I soldered it straight to the pins on the copper side. So there are 4 caps in the Bill of Materials. (I didn't use any bypass caps myself but they are in the layout and B.O.M.).

Here is the stripboard only view. 

Here is the layout for just the cuts and wirebridges. 

As ever mark the cuts at the component side and then stick a pin through the marked holes and mark them again on the copper side. Then you can cut them with a sharp hand held 6 or 7mm dril bit.

And finally here's the bill of materials. It's quite a cheap project if you already have a joystick in stock and anyway, joysticks aren't that expensive if you know where to look. The resistance value of the joystick potmeters isn't that critical. The circuit just uses them as voltage dividers so any value will work. They usually don't go down to zero Ohms. The one I used goes from 550 Ω to 1K3 to 2K15 in the lowest, middle and highest positions.

You can find joysticks on AliExpress for under $20,- for a pair. Just Google: "Radio Rocker Joystick 5K." Those should work just fine.

How to determin which wire is for up and which for down, left or right with a joystick.

Connect an Ohm meter to the middle wire and one of the outer wires of one of the potmeters on the joystick. Say for instance we're looking at the potmeter for the Y-axis (up and down). Now we measure the resistance while moving the joystick up. If the resistance goes down you have the correct wire for the up position. If the resistance goes up that wire should go to the down position on the stripboard, for the Y axis. So if you have the correct wire for a specific direction the resistance between the middle wire and that wire should go down when moving the joystick in that direction, because the wiper of the potmeter moves closer to it. I hope that makes sense.

Here's a screenshot from my oscilloscope. Yellow = X-axis, Blue = Y-axis. In this picture I moved the stick to the outer most positions and you can see both voltages land on exactly 10V maximum with Range turned fully clockwise and no offset applied.

PICTURES:

Here are some pictures I took during the building process:

This is the faceplate I made. Notice the two square holes. I tried fitting two push switches for extra Gate outputs but I came back on that idea because I didn't have enough room to accomodate that.

I made the big round hole with a hand held jig saw.

The finished face-plate with everything installed but without the stripboard. As you can see the knobs are very close together which isn't ideal so when you design your own faceplate for this module take some time to find out the best places to put these potmeters. If you use miniature potmeters you have more room to move them about to find the best placement.

Below is the stripboard with all components mounted except the power connector. The bottom two strips I later cut away go have some more space for the gimbal to move because when I tried to mount the board behind the panel I needed a bit more space. The bottom two copper strips are not used so I could just cut them off.

Here's how I mounted the stripboard behind the panel. I used some plastic tube as a stand-off. If you do the same, drill a few small holes in the sides very near both ends so the glue can run into those and provide a good grip. Then I hot glued that to the back of the panel, making sure the glue flowed around some of the mounting screws for the joystick, for extra grip. Then I hot-glued the stripboard to that stand-off after the wiring up was all done. I had to be careful not to disrupt the movement of the joystick gimbal, keep that in mind when mounting the stripboard behind the panel. There's almost no place to drill a hole through the stripboard for a normal M3 threaded stand-off so this seemed like the best solution. Works fine.

And here's the finished product. Front view:

Back side. The depth of the module is just under 4 centimeters. It's 14hp wide (7CM):

Finally a little demo video of the module in action in my 'Nifty Case'. This is just a simple patch I put together in 5 minutes. The X-axis CV is controlling the cutoff of the filter in the Doepfer A-111-6 synthesizer voice and the Y-axis CV is controlling the reverb amount from the FX-Aid.

Okay, that's it for this one. Quite a simple build. The only thing I did wrong was that I forgot that the wipers of the offset potmeters connect to the inverting inputs of the opamps so I had the offset potmeters wired the wrong way around. An easy fix. This is a very easy to build module and, I think, a very useful one especially for live performing. It's in fact the equivalent of a synthesizers modulation- and pitch-bend wheels all in one.

If you have any questions or remarks please put them in the comments below or in the special Facebook group for this website.

Synthesizer Build part-50: UTILITY LFO for EURORACK.

This is the Ken Stone Utility LFO, the bigger version of project 47 with an extra feature that I added. I managed to make the stripboard even smaller than the 'Simple Dual LFO' so it fits even the 'Nifty Case' Eurorack skiff. The LFO has the following waveforms: Pulse, Square, Saw to Triangle to Rampwave and a Variable output which has a mix between Square and Triangle/Saw/Ramp waves. I later added a mini-mixer that adds the two variable outputs together to get even weirder waveforms.

An other LFO might not be the most exciting of projects but I think this LFO will be worth it because of the weird waveforms it can make and because I realized, now that I have my own Eurorack system, that modulation sources are important to have. To quote a popular YouTuber: Modulation is what makes Eurorack interesting. I even added a feature of my own later on. It's the ability to mix both variable outputs together to get a variable A+B output. That's at the bottom of this article.

The Dual LFO from project 47 is just the first two stages of the Utility LFO without the mixing stage. Because I actually use the Dual mixer in my Eurorack setup and I loved the idea of having the full Utility LFO available, I thought I'd build the whole LFO this time and try and make it as small as possible. It is 49mm deep and the panel is 9HP wide (4.5 centimeter). This is a bi-polar LFO meaning the waveforms go both positive and negative in voltage.

This LFO does not have a sync option but this is a really useful LFO for a Eurorack system because it has not only the normal waveforms you'd expect but the option to merge waveforms together which makes for some very weird modulation possibilities. And because this is a Dual LFO you could mix the 2 variable waveform outputs together in a Multiple and make even weirder shaped waveforms. That mixing needs to be done outside of this module though. I couldn't fit that inside this design. This LFO would pair really well with the Dual Voltage Processor for that reason.

I managed to get everything on an 18 by 24 hole wide piece of stripboard. Six potmeters and eight output sockets on the panel, and an extra little print for the two bi-colour rate indicator LEDs which I made separate from the main print just like on the Dual LFO and as mentioned before a second little print to mix the two variable outputs together. The panel I made for it is 9 HP wide (4.5 centimeter). You might be able to make it even smaller if you use smaller potmeters in the panel. Even though I included an L-Bracket in the layout, I didn't use one. The stripboard is actually hot-glued in place in between the two columns of potmeters straight to the back of the panel making sure no copper strips make contact with the metal of the panel. I did this to keep the overall depth of the module as low as possible.

I did not include a Eurorack power connector on the print although there is room enough left to put one in. Instead I made a powercord with a Eurorack connector at the end so it always stays connected to the stripboard. This is handier because eurorack ribbon cables take up space too and this is a smaller footprint solution with only three thin wires.

This is mainly a Eurorack project but naturally this circuit will work just as well in a Kosmo sized synthesizer. I optimized the circuit to run on +/-12V but it was originally intended to be run on +/-15V. You will get higher amplitude waveforms when you run this on a dual 15V powersupply so if that's a problem for you, you can change the 1K8 resistors I used on the waveform outputs back to the 1K's you see in the schematic. The original schematic uses +/-15V.

SCHEMATIC:

Below is the schematic I used for my layout. I put in bigger timing capacitors because I wanted one LFO running really slow and the other to about 20Hz. That is more useful for my modulation needs but you might want different values so I strongly advise you to do some testing with different capacitor values to get the LFO in the frequency range you desire.

The types of quad opamps and the one dual opamp used in this project are not critical. The schematic calls for TL074 and TL072 opamps but you can use anything with the same pinout. I used two LM324's and an NE5532 dual opamp. Btw, I did not include the transistor with the rate LED as seen on the schematic but I designed my own circuit for that so I could use bi-coloured LED's. It's just two opamp voltage followers (or buffers) feeding the LED's with the signal from the Pulse output. You can connect them to any output you wish but the pulse is the clearest for the LED's.

Here's the schematic drawing. Beware the opamp numbering does not follow the opamp order I used in the layout.

  

LAYOUTS:

Below are the layouts I made for this project. As ever they are verified. I used them for my build. I changed the order of the opamps used from the order on the schematic. I setup one LFO using all the opamps on the left hand side and the other LFO using all the opamps on the righthand side of the IC's on the stripboard. That made it more compact plus easier to keep the overview. Luckily I did not need to make any changes or do any troubleshooting. I built it and tested it and everything worked first go.

You need four 100K and two 500K linear panel potmeters to this project. Measure and test them before you use them. That can save you some troubleshooting later. If you plan on mounting the stripboard to the panel with hot-glue like I did then try to solder the hook-up wires of the righthand side (the side that's glued to the front panel) as far to the middle as possible so you can still get to them once the stripboard is glued in place. This won't be possible with all wires but try. You could also solder them straight to the back/copper side.

Wiring diagram:

Stripboard only view:

Here are the cuts and wirebridges as seen from the component side! As always, I advise to mark the cuts on the component side first with a black waterproof marker pen and then put a pin through the marked holes and mark them again on the copper side. Then you can make the cuts accurately and you can see where the cuts are when you're soldering in the components.

Bill of materials. 

The timing capacitors listed were chosen for my specific needs. I advise you to go by the schematic (47nF) or else test and determine the best values for your needs. You can use the Falstad simulation linked at the bottom of the article to test different values before building.

No bypass caps are included in this BOM because I didn't use any. They are on the schematic and you can put 100nF caps over the power connections on the IC's to ground if you want to. To do all IC's you'd need six 100nF ceramic caps (you don't have to do the LED driver IC).

This BOM does not include the components for the little mixer I added on further down the article. For that you will need 4 x 100K resistors, a 1K resistor and a 200K trimpot and a TL072 dual opamp.

PICTURES:

Here are some pictures of the build proces and the panel I made for it. I had some fill-in panels left from when I bought my 'Nifty Case' Eurorack case and I used one of them to make my front panel. It was already cut to the right height so ideal for this project ^___^  All I had to do was cut off a piece that was 4.5cm wide (9HP) and spray paint it. I labeled everything with an Edding 400 marker pen.

Wirebridges and cuts. I made some minor changes to the layout after I built this so there are a few discrepancies between this picture and the current layout.

Stripboard ready for testing side A:

 

Panel, not yet labelled:

The finished module with the little 10 by 6 hole stripboard for the rate indicator LED's glued to the back of one of the potmeters. If you look closely at the sockets you can see I soldered all ground connections together with one copperwire going round them all. A wire goes from there to the ground connection on the stripboard.

The finished product:

Here's a little demo video I made showing the LFO in action in a little Eurorack setup. You can hear how the 'Pico Voice' VCO changes in sound as I connect the LFO to it. But then I get distracted by the Pico DSP effects module and the 2hp Freez, LOL. Oh well. You can hear the difference it makes anyway :)

(As you may have noticed, I really suck at making demo videos, LOL)

WAVEFORMS:

Here are some screenshots from the oscilloscope showing some of the waveforms. The one below shows the sharpest a sawtooth wave you can get. Pretty fast rise!

Cursor readings at LFO-B squarewave output:

Here is a collection of variable output readings from Variable output B:

To give you an idea of what can be achieved by mixing the two variable outputs in a simple passive multiple, here are eight images I put together to give you an impression. This should sound awesome with very slow running LFO's which is why I soldered a few more caps in parallel over the timing cap of LFO-B.

You can imagine that some of these waveforms, when put through a quantizer, would generate awesome melody- or bass-lines that can be easily manipulated with the LFO parameters. I tried to capture a little of that in the demo video I posted below.

Here's some technical data from the LFO:

LFO A: freq. range: 270mHz to 24Hz. 

LFO B: freq. range.: 41mHz (one full cycle every 24 seconds using a 650nF cap) to 2Hz.

Duty cycle of the pulsewave output goes between 2% and 98%. I made a mistake and used a 100K potmeter for the Shape but it needs a 500K potmeter. I only had one 500K pot so I put it in LFO B and the screenschots above are from that LFO. You can use a 100K for shape but that will significantly lower the range of the duty cycle (20 to 80%) also the slope of the Saw-/Rampwaves will be slower rising. It will also speed up the overall frequency of the LFO.

Maximum current draw = 20mA for both the positive and negative side.

The maximum amplitude of the waveforms is about +/-5V except for the Variable waveform output. When that potmeter is set to the mid point the output amplitude drops to about +/-2.5V because the potmeter acts as a voltage divider so in the mid position the amplitude is half of what it is when it is fully clockwise or counter clockwise. (We effectively have two 50K resistors on either side of the wiper of the 100K potmeter.) These may seem like low voltages but keep in mind that an LFO is usually attenuated anyway because if the changes are too big it just doesn't sound good on a VCO or in most patches. If you really need higher voltages you can make the 1K8 resistors on the outputs even bigger.

Here's the link to the Falstad simulation of this circuit. This is the Dual version with one speed potmeter at 100K and the other at 500K as it is in my LFO. You can change these variables by right clicking on them and choosing 'Edit':

--- CLICK HERE --- 

EXTRA FEATURE:

Okay as I write this it is four days since I posted this article and the cool looking results from the mixed variable outputs kept going through my mind. I really wanted to incorporate that into this design and now I found a way. I used an other small piece of stripboard on which I soldered a dual opamp and wired it up like the mixer in article 17 with 2 inputs. I knew there was no way to put an extra output socket on my panel so I sacrificed the squarewave output of LFO-B. That was the least useful of the outputs because I can get a squarewave anyway from the pulse output if I need one and I still have the squarewave of LFO-A. So I carefully soldered in the little mixer print which was just as big as the little LED driver and wired it all up and I re-labelled Square output B into 'Vari A+B'. I used a 200K trimmer to go over the inverting input of the second opamp and the output so I could adjust the gain and make it a little higher than the Variable outputs. I placed the little print above the sockets and used hot-glue to stick it in place. It is actually glued to the little LED driver board. This works perfectly! Now I have an output with two variable waveforms mixed together coming out of it.

Here is a picture showing how I added the mixer. I used two rigid copper wires to tap the signals from the sockets and lead them into the mixer. The non-inverting inputs of the opamps are connected to ground on the copper side:

Here's the layout of this little mixer. Mine is even smaller than this layout, but this will work fine. All resistors are 100K:

Here's the schematic for this little mixer:

DEMO VIDEO:

Finally I want to show you a little experiment I did using the new output. I put the Variable A+B signal through a quantizer (the 2hp Tune) and from there into the Pico Voice Wavetable Oscillator. The audio then went through the Pico DSP for some added reverb. You can really generate the weirdest melodies with this although this setup would benefit from the Voltage Processor because the negative phase of the signal does not produce any notes so it needs a positive offset voltage and the signal can do with some attenuation to get the notes closer together but I think you get the idea watching this short demo:

Okay, that's it for now. Article 50, wow I can hardly believe it. This journey started for me in October 2019 and I knew nothing about synthesizers then, but I was determined to get to grips with it and learn as much as possible. And what better way to learn then to build your own modular. So here we are more then 3 years and 50 projects later with a cool collection of builds helping hundreds of people to do the same. I'm really proud of what, not only I created but also of all the people who helped so much along the way with comments and directions. I'm not gonna name names but you know who you are, all of you. Thank you!! So many people told me they find the site a great help in their hobby and that's the biggest reward I could wish for. I am however going to wind down the DIY aspect of the hobby because having now built my own system and also haven gotten into Eurorack, I need to spend more time actually using it and figuring out how to use it all together. But I will remain available on Facebook and here to answer questions and help as much as I can to ensure you have a good experience using this website and get as much enjoyment out of it as I did.

PCB Version.

I recently made some Eurorack friendly PCB's for this LFO including the Variable output mixer I added on.

Here's the KiCad schematic I made for this LFO:

To get your PCB, goto the Menu and click on the top option called 'PCB Service' to find these circuitboards.

If you have any questions please put them in the comments below or post them on the special Facebook Group for this website where there are some awesome people willing to answer your questions.

Synthesizer Extra's No.3: THE GRISTLEIZER.

An effects unit made famous by British Electronic Industrial Band 'Throbbing Gristle' from the late 70's. The band used it on everything, from guitars to synthesizers and even microphones and it will take all the different input levels without problem.

HISTORY:

I built this as a stand-alone effects unit, as it originally was. There are now numerous variations of the Gristleizer (pronounce as: Grissel-eye-zer) available. Even as Eurorack modules with lots of extra functions but I wanted to build the original one as used by Throbbing Gristle back in the day. Band member Chris Carter built the unit from an article in Practical Electronics Magazine (July 1975 issue). 

(The above link will lead you to a downloadable PDF of the entire magazine. The article starts on page 29.)

The circuit was based on a design by a then 15 year old Roy Gwinn. Alternatively, you can get just the relevant article in PDF form in the 'Files' section of the EDDY BERGMAN Discussions FaceBook Group.

I first became aware of this sound effects unit when I watched the documentary 'Synth Britannia' on YouTube. (Click here to see the part about Throbbing Gristle).

Here's a picture of the original unit built by Chris Carter. This was Cosey Fanni Tutti's original unit used until 2009 when it stopped working:

Here's a look at the inside (low resolution):

Below is a link to the website of Chris Carter which has tons of links on it to all sorts of Gristleizer related webpages. It starts with a lot of videos but if you scroll down you'll find links to the various webpages.

Chris Carter's webpage about the Gristleizer:  -CLICK HERE-

The Gristleizer on Boing Boing.net:  - CLICK HERE-

Here's the website of Throbbing Gristle.

SCHEMATIC:

The schematic below is a modern update of the original one from the article in Practical Electronics, and this is the one I used to build mine. The two diodes in this circuits are 1N34A Germanium diodes but I used 1N4148 and this works just as well. I did later change the 1N4148 diodes for Germanium type 1N60 diodes but it didn't have any effect on the signal.

This circuit runs on a dual 9V powersupply but I used a dual 12V powersupply built into the case. It's not an issue because the opamps can easily take it. In the picture above you can see the original used two 9V batteries to create a dual 9 Volt powersource. The circuit itself is really rather simple. The only thing that can be confusing is the wiring of the 4 way switch. Luckily I got that right at the first try.

In the version I built, all the potmeters are mounted on the front panel, even the ones initially intended to be trimmer pots that you only set once, like Shape and Offset, I thought it might be handy. 

Please ignore the 'Max1044' chip in the schematic. This is a voltage converter chip which is not used in my layout because I use a custom built powersupply.

WHAT IT DOES:

It's difficult to describe the effect this unit has on any sound you put into it. It has a built-in LFO with a choise of 4 waveforms, that gives a tremelo effect to the sound but that can be cranked up all the way into the lower audio frequencies which gives a cool sort of filter sweep sound combined with distortion. See the demo video below to get an impression of what it can do. 

I did some internal measurements and I measured an LFO frequency from one cycle every 70 seconds to 115 cycles per second (115Hz).

It also has a resonant filter that is mixed into the sound. but you can't set any Resonance like a normal VCF. Watch the demo video below to see what it does. It's quite unique. I can see why it fitted the music of Throbbing Gristle so well.

One cool thing about this circuit is that it can handle a really wide range of input signal amplitudes. You can feed it line level signals or signals at the synthesizer level of -5/+5Vpp or 0-10Vpp. It's fortunate that there is an output level potmeter because the output can be very loud because this unit can amplify the audio, but if you turn the Bias down the audio level can go down too and with the output level potmeter you can crank it up again.

EDIT: I have tried it with my Steinberger Spirit XT-2 bass guitar which has passive pickups and the signal gets through quite loud. In many ways this works mostly as a distortion and also as tremolo effect. If you turn the rate up you get a nice beating of 2 frequencies against eachother as the bass frequency comes near the LFO frequency. When they used the Gristleizer on Genesis' voice they must have has it in VCA mode. You can hear that in songs like 'Hamburger Lady'.

Overall I was not very impressed with the sound it produced when using it with my bass guitar. I think synthesizers sound best. 

CV CONTROL:

I have not experimented with CV control for any of the parameters but I don't think putting Vactrols over the potmeters will work very well, but you'll have to experiment if you want CV control. I am however reliably informed that you can put an external signal (LFO) on the wiper connection, pin 2, of the Depth potmeter. You can use a toggle switch to switch between internal LFO or external CV or use an input socket with built in switch normalled to the internal LFO. Then you can connect and external ADSR and use it as a weird sort of filter. I have not tried this myself however.

LINE LEVEL SIGNALS:

When I first tested my unit with line-level input signals I could hear a ticking noise mixed in with the audio. I first thought this was a power supply issue but then I found out that I had forgotten to put in capacitor C6. This is the 100nF one on the left of the print from the cathode of the diode to ground. I soldered it in and, because I had an ON/OFF switch on the front panel that I hadn't wired up yet, I used the switch to turn on or off capacitor C6. You can just hear the effect it has. It smooths out the audio a bit. I can see why the cap is there. If I turn the capacitor C6 off, the sound is slightly rougher.

Btw, you can connect a dynamic microphone straight to the input of the Gristleizer and experiment with the effect is has on the human voice. Should you experience noise or weird sounds in the line-level audio then turn down the voltage from the powersupply a bit (if you can). I just did a test with dual 9V and dual 12V and a lower voltage is definitely better for line-level signals. I made my powersupply with LM317 and 337 regulators so I can easily adjust the voltage. Dual 12V is better for synthesizer level signals.

Btw, when I say 'line-level' I mean signals that come straight out of a guitar or microphone. Signals in the range of 100 milliVolts to 1 Volt peak to peak.

LAYOUT:

Here's the new (verified) layout I made for it. The previous version that was up here until the 16th of November 2022 had a mistake in the layout. I had forgotten to ground pin 5 of the TL074. Now that that's all corrected the Gristleizer works like a charm. I included a bypass switch in the layout which I thought might be handy to have. (The weird thing in my unit is that if the Level potmeter is fully clockwise or fully counter clockwise, the bypass signal doesn't get through.) 

The Bias potmeter is wired the other way around from what you would expect. I thought this was the best way but it doesn't really matter which way you wire it up. The layout below has the supply voltage at +/-9V but I run mine on +/-12V which works fine too. (There's an alternative layout with trimmers for the Shape and Offset potmeters further down the article if you want to build it like the original unit but I find putting those potmeters on the front panel much better.)

Wiring Diagram:

Be very accurate with soldering up the 2 pole 4 way rotary switch. Solder the resistors and jump wires straight to the switch first, before you wire it up to the stripboard.

You can connect together all the potmeter lugs that need negative voltage and all the ones that need positive voltage with hookup wire and then power them all at once with two wires coming off the power rails on the stripboard. That way you keep the wiring to a minimum.

Here's the 'stripboard only' view. There are no bypass capacitors in this design but in my own build I did put a 100nF cap over the plus and minus rails near the TL074, but it's not necessary, especially not if you feed it with batteries:

Cuts and wirebridges, component side view:

Cuts only, copper side view:

Here's the Bill of Materials:

ALTERNATIVE 4 POTMETER LAYOUT:

Here is an alternative layout which lets out the Shape and Offset panel potmeters and replaces them with trimmers on the stripboard. This is the version as used by Throbbing Gristle. Beware that the stripboard is a little bit longer (43 holes in total) and that it has two extra cuts in the ground strip next to the trimmer potmeters, so the wipers don't connect to eachother or to ground.

Once again, be very, VERY accurate when wiring up the switch. You must get that right before doing anything else otherwise it won't work right.

Here are some pictures of the stripboard and the wooden case which I also built myself from 3,5mm plywood. I forgot to take pictures of the stripboard alone during building. In the pictures capacitor C6 is not yet put in. I had forgotten it first and after I put it in I could notice that it smooths out the rough edges of the sound, so to speak. In certain settings it acts in the same way as the Bias control, adding a little more Bias as C6 is turned on. But it's not necessary to use a switch. Just solder in C6 as it normally should be. It's not a useful feature to have this capacitor switchable.

The picture below shows the powersupply print glued to the inside top of the case:

This is the front panel: 

The finished product:

DEMO VIDEO:

Here's a new demo video I made of my Gristleizer directly as I tested the revised version. After correcting the mistake of forgetting to ground pin 5 or the TL074 everything works much better as you can see and hear. You might have to turn up the volume a little.

If for some reason the video doesn't appear then here's the direct link to YouTube: https://www.youtube.com/watch?v=akE6kQEMy28

The effects are I think very useful. You can get a tremolo effect going and turn it up into audio range and then mix it with a resonant filter so you get that filter sweep sound. Very cool I think. And I haven't even tried it with other sound sources like guitar or a (dynamic) microphone.

To close off this article I'll show you some screenshots from the oscilloscope.  These are the different LFO waveforms in each of the four switch settings:

The next one shows waveforms created in VCF Mode with an audio Sinewave as input:

This is in VCA mode. The LFO shuts or opens the VCA giving a Tremolo effect.

The above screenshots give an impression of the main effect the Gristleizer produces. There are of course numerous shapes in between with all the different potmeters you can set in so many ways. 

I want to leave you with a video of Roy Gwinn, the inventor of the Gristleizer circuit, which I recently found on YouTube. Hear the man himself talk about the circuit and how it came to be. The video was uploaded in 2017 and it mainly deals with the Eurorack Gristleizer that had just come out but it's very interesting to watch. He had no idea his circuit was used and made famous by Throbbing Gristle until 2007 when he learned about it.

If you can't see the video then click this link: -- Roy Gwinn Video --

I made a Falstad simulation of the Gristleizer circuit which you can view by -- CLICKING HERE --

Here's a build guide for the Eurorack module Gristleizer from Instructables.com. It has a schematic and a good description of the function of all the changeable parameters.

Okay, that's it for now. If you have any questions please leave them in the comments below or post on the EddyBergman DIY Projects Facebook group.