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.

-- There are now PCBs available for this 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 about a 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 around 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 influences the return time more than it influences the tone, 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 1.5 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 doesn't do well with very high frequencies and over modulated audio. It's advisable to use a level potmeter on the audio input.

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 also noticed the Tone control can add some noise to the output if you turn it up more but that's inherent to the circuit.

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 Extra's No. 2: JOYO Instrument Tuner Hack.

This article explains how to connect a JOYO Instrument tuner to the VCO('s) of your modular synthesizer.

Inspired by LookMumNoComputer's Preformance VCO with built in tuner I bought three of these small Joyo tuners on eBay to see if I could adapt them to work with the synthesizer. I managed to do it and it's not difficult to do at all. Many people seem to do this too going by the Facebook reactions I got on this hack. :-)
The JOYO instrument tuner is a very useful bit of kit that you can buy on eBay or AliExpress etc. for around 5 US Dollars including shipping. It's pretty easy to hack these things so you can connect a jack socket to them and so connect it to the VCO's in your synthesizer. It makes tuning VCO's or Sequencers very fast and very accurate.

Now, before we begin I want to mention that the method below is just the way I did it. I've had lots of reactions to this hack and people do it in all sorts of ways, for instance connecting a jack straight to the piëzo wires without a resistor divider and not having the piëzo mic connected. Using a capacitor instead of resistors and it all seems to work so this is just my way of doing it but it's certainly not the only way, or the best way, to do it! Just so this is clear.

Here's what we do. You'll need to carefully open the tuner case by unscrewing 4 little screws at the back and then take out the two screws that hold the little circuitboard in place and now you can lift out the circuitboard. Leave the actual LCD display safely inside the case and make sure not to get fingerprints all over it. On the back side of the circuitboard, you'll find a very thin Piëzo-electric disk-microphone. Carefully push the white display lighting out of harms way and carefully solder a thin wire to the earth connection, which is the solder blob on the outer part of the Piëzo electric microphone. Solder an other wire to the center of the Piëzo disk microphone. Do not disconnect the red wire from the center of the microphone otherwise this won't work. 
Now make some small notches in the side of the plastic tuner case so you can lead the wires outside. Put the little circuitboard back inside the case first, to make sure where the notches need to go, then mark the right spot and take the circuitboard back out and make the notches in the case. Assemble the tuner and put everything back where it was. Now you can solder on the resistors following the diagram beneath. Solder them straight onto the input socket. Now solder the wires to the input jack socket (3,5mm) and the resistor and then hot-glue the input socket, with the resistors and wires attached, to the side of the tuner case. Make sure when glueing that you don't go past the bottom of the tuner case so the underside stays straight and flat. I myself then hot-glued the tuners to my synthesizer case by putting hot glue on the battery cover, which covers about 3 quarters of the backside, so the tuners are held in place by the battery cover. I later carefully drilled a 2mm hole through the covers into the wooden panel of my synthesizer and put a small screw in each one, that sits flush with the battery cover inside surface. That way it won't fall off at some point.

Powering the tuners:
I soldered two wires to the battery connectors. The spring connector in the middle of the battery compartment is the negative pole and the metal contact at the side is the positive pole. I have a 5V power rails in the powersupply of my DIY synthesizer and I connected the wires from the tuner(s) to that. I put three 1N914 diodes in series into the positive voltage rail to get the voltage down to just over 3V which is what the tuners need.
It's much better to give them their own internal power source because if you run them on the batteries the screen will flicker and vary in brightness. This way is all nice and stable.

SCHEMATIC DRAWING:

Here is the way I wired up the audio inputs. I believe the input of the Joyo tuner is capacitive so a resistor voltage divider like this will work fine and there will be no need to use a capacitor. This resistor voltage divider brings signals with an amplitude of 10Vpp down to 2Vpp.

This is the little circuitboard lifted out of the case. The white panel on the left is the LCD back-light. The gray and pink wires are the ones I soldered on myself:

Here's one of the finished hacked tuners without the battery cover because that is glued to my synthesizer. I blackened the glue with a permanent marker. As you can see I cut off the mounting eye for the clamp and made sure it was all sandpapered nice and flat. The fact that this particular model of tuner has a straight back and sides is why I chose this one. There are models out there with curved sides which make mounting more difficult. Bear that in mind when you buy one of these tuners.

Here are two of the three tuners mounted on my synth above two VCO's:

And finally a little video showing the tuners in action. As you can hear at the end, I was really pleased with the result. It looks very cool too on the synth I think.:

Oh and one final thing. You can use the left over clamps, from the backs of the tuners, to make a so called "Third Hand" to help with soldering :)

Leave the little screws in there and take some thick electrical copper wire (about 50 Centimeters long) and bend an eye at each end and mount the clamps to them with the little screws going through the eyes. Then hot-glue them in place and glue and/or tape that to a heavy enough metal base and with the copper wire you can bend them any which way you need them. And there you have it. Super useful! ^___^

EDIT: 6th of May 2022: Today I did two more of these hacks to have some tuners for my Eurorack system and they work fine :)

These were of a different make but they turned out to be even easier to hack. Here's a picture. I did use a lot of hot-glue to secure the sockets in place but it all works fine. In these I used a 120K resistor and a 6K8 resistor. which reduces a 10V input signal to a 0.54V output signal going into the tuner, making sure it's not damaged by the high synthesizer voltages.

In this case I left the clamps on the tuners because I can then clamp them to the powersupply wire of my Behringer Go Eurorack case. I drilled some holes in that case and connected the powerbrick straight to it so it won't dangle off the case which is very annoying.

Finally I want to show how one of our Facebook members (Martin Nyborg Sørensen) turned this idea into a very cool Eurorack Tuner module. He buffered the in and output so he can route signals through the tuner without any problems. I think you'll agree he did a very professional job.

The other side:

Okay, that's it for this one. If you have any questions please put them in the comments below or post them in the FaceBook Group for this website.
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