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-66: ROLAND TB-303 VCF.

The famous acid house filter from the Roland TB-303. A Eurorack friendly project and a ladder-filter that sounds amazing. 

This is the 14th filter on this website and this is one with a very specific sound so I thought let's make a project out of this because I think that this filter in particular will be of great interest to many people because of it's unique sound. I based my layout on a layout that Jake Jakaan made from a schematic he found online in combination with the original service manual schematic of the TB-303 (TB 303 stands for Transistor Base 303). 

I have to warn you, the filter sounds great in itself but to get that Acid-House sound out of it requires more than just this filter. That specific sound is a delicate balance between filter settings and envelope input and maybe some LFO or offset voltage added. When I first tried this filter I didn't get anything near that classic sound. However, I found some tips and instructions online which helped a lot. I posted a very helpful short video at the bottom of this article below the video demo's that tels you how to get that sound.

I got close in the end though as you can see in the demo video below. My mate Jake Jakaan built a few of these filters and he can really make them sound like a 303 should sound but then he's a professional musician. 

This is really the first filter I ever built that you have to learn how to use. I'm getting there tho ;)

A LITTLE HISTORY:

The TB303 was a bass synthesizer made by Roland and released in 1981. It was supposed to simulate bass guitars but it sounded nothing like a bass guitar and it became a commercial flop. It was taken out of production in 1984 after a run of 10.000 units. These were sold off cheaply by Roland. (If only we knew then what we know now @___@)  However, cheap second hand 303's were picked up by electronic musicians and the twirping, squelching sound became a main stay of electronic dance music genres like Acid House, Chicago house and Techno. There are now numerous clones on the market and original units fetch prices of over $3000,- on the second hand market. Originals were also modified in the 80's, adding distortion and external inputs (Nova mod).

The TB-303 was designed by Tadao Kikumoto who also designed the TR-909 drum machine. It has a single oscillator which produces a sawtooth wave or a squarewave. This goes into a 24dB/Oct lowpass ladderfilter which is manipulated by an envelope generator. 

I have read that it's actually an 18dB/Oct lowpass filter instead of 24dB but I don't know if that's true.

SCHEMATIC:

Here is the schematic. It's a bit low resolution because this was originally a file with a black background and bright green lines. I took it into Photoshop and inverted the image and brightened it up and made it more legible. I also included the transistor pinouts. All transistors on the schematic are NPN 2SC945's except for the two at the bottom marked 733. Those are two 2SA733 PNP transistors.

The capacitors in the schematic are not marked as polarized but the 10µF electrolytic caps are obviously polarized and for the 1µF you can use either type. Polarized or non-polarized. I used polarized caps in the layouts below so that you can see where the minus pole goes if you choose to use polarized caps.

Here's the filter part of the service manual schematic for reference. It has two 2K2 resistors from +12V to T1 and T2 but I think that's a misprint. It needs to be 22K:

The filter does not use any negative voltage. It is powered by +12V and it also needs a +5V powerrails which is provided by the onboard voltage regulator. The +12V goes through a 100 Ohm resistor. I wondered whether or not to include that but I wanted to see how much voltage that resistor takes off from the original 12V and it's only 0.2V so I left it in.

Staying true to the original includes using 2SC945 transistors for the NPN trannies and 2SA733 for the PNP transistors. You can however use other transistors like the BC547 and BC557 but beware when you do because you'll have to redo the layouts. The 2SC945 and 2SA733 have an unusual pinout. It's emitter to the left, collector in the middle and base to the right. I had to constantly keep this in mind when designing the layouts and it wasn't easy but I managed it in a day.

The transistor pairs at the top and bottom of the ladder and the transistors next to it with the common emitter connection need to be matched pairs!! Very important with this filter.

I came to the conclusion that my usual method of matching on Hfe didn't meet the case here so I did it with setting up a differential amplifier on a small breadboard. The method is shown below.

I ordered a hundred of the 2SC945's and made 10 matched pairs and I used those transistors in this project even with the middle trannies in the ladder. I thought I might aswell use all matched transistors but you don't have to do that. You can use other transistor types like the BC547 and BC557 which are used in the Doepfer A-103 VCF6 filter but you'll have to redesign the layout because their pinout is different from the 2SC and 2SA transistors I used.

Two resistors in the circuit have been replaced by trimmer potmeters so we can tune them in to our liking. These are in the Cutoff and Resonance control so they are important to the sound and they do make quite a difference. The way I set them was almost fully open for both (max. resistance).

To make things extra clear I just completed the KiCad version of this schematic:

MATCHING TRANSISTORS.

For this filter I didn't want to rely on just measuring Hfe and matching the transistors on that value. I used the Ian Fritz methode. I took a small piece of stripboard and set up a simple differential amplifier with two transistors. If the transistors are matched then the voltage measured between the two emitters should be zero. Make sure you let the transistors cool down after handling them with your fingers.

For D1 any silicon diode will do. The voltage drop over this diode ensures both transistors get exactly the same Collector Base voltage. Beware this setup requires a dual voltage source of +/-12V. You also need to make sure the two 100K resistors have exactly the same value.

You then need to switch the transistor positions and measure again. I didn't bother with that though. An other method is to leave one transistor in place and change the second one. If you find two transistors that display the same voltage difference from the fixed transistor, those two will be matched.

This method worked very well. I used matched pairs throughout the ladder filter and also for the differential amplifier made up of T1 and T2.

Please read the full article on transistor matching by downloading the article by Ian Fritz. 

Below is a picture of my transistor matching stripboard. I can get them matched to within 1/10,000th of a Volt or 0.1 milliVolt. I cut a DIP8 IC socket in half and connected the top and bottom pin together. I use that as socket for the transistors under test and with this setup I can measure NPN transistors with different pinouts because I have an emitter contact at the top and the bottom. I placed the sockets away from eachother to make it easier to change transistors without influencing the other transistor. I usually accept transistors that measure a difference within 0.3 milliVolt or lower. If you go to extremes with accuracy you'll be measuring until doomsday before you find a match.

LAYOUTS:

Below are the layouts for this project which are verified as ever. I used them to build my filter.

Wiring:

I numbered the transistors that are not part of the ladder, using the same order as in the schematic so you can easily understand which transistor is which when you compare it with the schematic. The light grey transistors are the 2SA733's. I included an extra CV input with the same level control as the Envelope input. 

The transistors in the ladder have the base and collector connected together so they actually function as diodes.

As you can see the envelope and CV input level potmeters have pin 3 connected to a 10K resistor and not straight to ground as is usual with input level potmeters. This is done so that the Envelope input is never fully closed. This is also the case in the original Roland TB-303 because the envelope input is very important for the characteristic sound of this filter. In my own build I did connect pin 3 of the CV potmeter straight to ground instead of the 10K resistor because I wanted to be able to fully close that input. So I leave it up to you how you want to wire that up.

If you look closely at the audio input you can see a 220K resistor on the stripboard that isn't used. I have the audio going straight into the filter through the 1µF cap. Originally that 220K should be in series with that cap but I think the value is too high. I later experimented with a lower value but you can also leave the resistor out.

Stripboard only view:

Cuts and wirebridges seen from the component side. As always, mark the cuts on the component side with a Sharpie or Edding marker and then stick a pin through the marked holes and mark them again on the copper side. Then cut the strips at the marked positions with a sharp hand held 6- or 7mm drill bit.

Bill of materials:

PICTURES:

Here are some pictures of the build proces:

Cuts and wirebridges done:

Everything is soldered in.

Here's the design I made for the panel. Feel free to us it if you want.

And here's how the panel came out:

You can see the colours don't come out as strong with clear waterslide paper as opposed to using white waterslide paper. But I like this effect. The design shouldn't be too overpowering I think.

Here's a look at the finished module:

Side/rear view. I had built a version before this one but it didn't work but I re-used the panel I made so the mounting holes are not positioned where they need to be so that's why the M3 bolt is bent sideways.

VIDEO DEMO:

This filter has that typical ladder filter quirck where if you turn the resonance up the volume goes down and you get less bass. Most ladder filters have this characteristic. The Moog ladderfilter does it and even the Doepfer A-103 VCF-6 does it. 

Here's demo, trying to get that Acid sound using some of the tips from the YouTube short video below. I came close but it's not quite there. I had a slowly rising sinewave on the CV input and a short pulsing envelope, with just some decay and all the other parameters of the ADSR closed. Instead of an LFO I think an offset voltage alone would be better. You can hear it reaches that "eeeuuurrrghhh" sound as the LFO rises in voltage but then it gets too high and it starts to whistle more. I'm going to do more experiments, using the voltage processor and see where that gets me.

Here's an other short test. Beside the envelope input I also had an offset voltage going into the CV input. That offset voltage came from the dual voltage processor to which I also had a sawtooth LFO connected. I set the processor in such a way that I always fed an offset voltage to the filter but the voltage would swing between about +2V to +4V. You can do this by raising the offset and then using the attenuverter to limit the maximum voltage. A very useful module to have in combination with this filter.

Here's a YouTube short explaining how to get the characteristic 303 sound:

Documentation:

Here's the webpage of Tim Stinchcombe about the TB303 ladder filter.

Here's Ian Fritz's original article on transistor matching in PDF form

PCB Service:

A PCB for this filter is available. It's 6.6 by 9 CM

It costs €10 incl. free shipping inside European union. Use the paypal link below to order. 2 in stock.

(PCB only, no components.) 

That's all for this one.

If you have any questions or remarks about this project then please put them in the comments below or post them in the special facebook group for this website.

Synthesizer Build part-65: YAMAHA CS FILTER w IG00156

 A LP, BP and HP filter in one chip. Not a filter you can just decide to build on a whim though. It uses Yamaha's own IG00156 VCF chip which is very rare and very expensive if you can find one. Like three figures expensive. This projects deals with a Eurorack version of this filter but of course it will work equally well for Kosmo sized synths.

PCB'S AVAILABLE ON REQUEST. Contact me on Facebook messenger. (2 in stock). They are 6 by 8 CM in size. The boards work fine but miss a connection you must make yourself but it couldn't be easier, just connect pins 1 and 9 of the IG00156 chip with a wirebridge.

A good friend of this website, a fellow Dutchman who happens to be an amazing psy-trance producer by the name of Jake Jakaan, signed to a top record label, who uses modules from this website gave me one of these chips. He managed to get hold of a few of them. He states this filter is great for filtering FM sounds from the TH VCO555. It's very low-mid heavy.

It was not easy to find a good schematic for this filter. In fact, all I had was the service manual for the CS-5 and a stripboard layout that someone put together which looked very dodgy and had some mistakes in it (although it did seem to work for my friend but I didn't use it).

I made a completely new schematic for this filter using the original circuit from the service manual and from that I made a new layout, small enough to fit a Eurorack system. The layout turned out to work faultlessly straightaway for which I was very grateful because the IG00156 is not a chip I'd like to blow up. The chip is actually quite robust, I found.

There are two things that are unique to this filter; it has a frequency dependent Q (resonance peak) and a gentle single pole lowpass effect. Resonance (Q) is achieved by damping rather than using a positive feedback loop and because damping will not go to zero the filter can not self oscillate.

This filter is used in all of the Yamaha CS range of synthesizers. Even in their flagship synthesizer, the CS-80. It's a two pole 12dB/Oct. state variable filter. In the CS-80 one chip is used for a lowpass filter and a second one for a highpass filter and in other CS synths like the CS-5, one chip is used for lowpass, bandpass and highpass. The filter I present here has all three functions under a switch although you could have each output go to its own socket but then you have to redesign the output stage or simply bypass the output opamp which I wouldn't advise. You should have opamps with a little bit of gain on the outputs.

Here is the schematic I made and on which I based the layouts below:

Here's a block diagram of the inside of the IG00156 chip:

Source: Yamaha IC Guidebook.

There's an in depth analysis of this filter on the Modwiggler forum: CLICK HERE to read that.

I added an audio level control on the input because this filter is rather sensitive to high volume levels. I also added a gain potmeter on the output that gives you the option to set the gain from 2 to 4 times with a 100K potmeter. That is more than enough, but if you want more gain, put in a 500K potmeter which will give you 12 times gain, or 1M which will provide 23 times gain. It will just clip. There's no use in doing that. I really wouldn't advise it.

The schematic says to use +/-15V but I tested it on a dual 12V powersupply and it works fine. The chip inputs for the cutoff and resonance functions are very sensitive and the complete cutoff range is controlled by a voltage that goes from 0 to 0,25V. Only 250 milliVolt for the full range. This is achieved by the voltage divider consisting of the 22K resistor in the cutoff line and the 470 Ω resistor between pins 7 and 8 (or pin 7 and ground really). The potmeters for Cut-off and Resonance need to be fed with +10 volts so I added 1K8 resistors to the pins where the power comes in to cut off roughly 2 volts. You can actually use other values for these potmeters because they're only used as voltage dividers but then you will have to re-calculate the values of those two resistors to make sure the pots receive +10 volts. For instance, if you use 100K potmeters you're need to use two 18K resistors.

I used this schematic to make my stripboard layout which wasn't that difficult because it's quite a simple filter. There aren't many components in it. It worked straight away although at first I couldn't get it working because I had not wired everything up yet. I thought I had connected all the knobs I needed for testing but I forgot the V/Oct input. That needs to be connected to ground if it is not in use and once I had done that the filter sprang to life. After testing I added a V/Oct section to the stripboard layout as explained in the next paragraph.

VOLT per OCTAVE INPUT:

At first I had the V/Oct socket grounded through the socket switch but then I realized that isn't needed because the V/Oct input is always connected to ground via the 470Ω resistor. 

I applied a voltage divider to the V/Oct input consisting of an 18K and a 470Ω resistor, I went with an octave range of 8 octaves, meaning that the input would get 8 Volts at maximum which would need to be reduced to the same range as the Cutoff input because they all enter the same summer inside the chip (see diagram above). 18K with a 470Ω would give 0 to 203milliVolt which works out perfectly.

Because the filter can not self-oscillate anyway, the filter can not be used as a sinewave oscillator with the resonance fully open. So accurate volt per octave tracking is not an issue and therefore none of these calculations need to be super accurate, it just needs to work so that it sounds good and the filter now tracks nicely up with the octaves.

LAYOUTS:

Here are the layouts I made for this filter. As always they are verified, I used them for my build. As you can see it's a really simple project. There's only 25 resistors, a few capacitors and wirebridges and the chip sockets to put in. The biggest job will be the wiring up of the potmeters and sockets and the making of the panel.

Wiring diagram. Note that the Resonance potmeter is connected the other way around from all the other potmeters, with ground at the clockwise lug. This is usually the case in VCF's.  

Stripboard only:

It's best to use bi-polar or none polarized capacitors for the 1µF caps in the filter outputs and on the audio input. This is because we're dealing with bi-polar signals that go through the zero Volt line. The caps don't need to be this specific value. You can use anything between 1 and 10µF without problems.

As you can see all three filter outputs have a 100K resistor to ground. Together with the 1µF capacitor this forms a highpass filter with a cutoff frequency of 1.6Hz so in effect it keeps DC voltage from passing and all the other frequencies get through. Make sure you use high quality capacitors for the two 1,5nF filter caps. I used Polystyrene ones which always sound the best.

Cuts and wirebridges. As always, mark the cuts with a Sharpie or Edding pen on the component side and then stick a pin through the marked holes and mark them again on the copper side. Then cut the strips at the marked positions with a sharp hand held 6- or 7mm dril bit. Make sure you work accurately!!

And here is the bill of materials. It won't be easy to find an IG00156 chip. They are long out of production so your best bet is websites that sell rare synthesizer components. They go on Reverb for $189,- but that is top dollar. They should go for between $70,- a $100,- 

I did not include any bypass caps or extra electrolytic caps for the voltage rails. If you want to include those you need to add them to the list (2 x 100nF and 2 x 10µF/25V). They're not on the layout or schematic either but there's room enough on the stripboard to put them in over the voltage rails.

The two trimpotmeters should really be the normal kind and not multiturn trimmer. There's really no use in having multiturn trimmers because there's no need for that kind of accuracy.

NOTE: There is a Hongkong based listing of the IG00156 chip floating around on the internet selling them for €10,- Don't fall for that, it's a scam!!

PICTURES:

Here are some pictures from the build proces:

All components soldered on:

Testing:

Drilling the panel using a copy of the panel design I made in Photoshop as a dril guide:

Waterslide design applied to the faceplate and now drying on the central heating. All the creases you see will be gone by the time this is dry.

Finished panel ready to receive the pots and sockets etc. After the waterslide design has dried we cut out the holes with a very sharp hobby knife and then apply two more layers of clear acryllic lacquer and let it dry overnight.

Finished module. The module is 14hp wide (7 CM) and 3.7 CM deep.

I only had a 4 way rotary switch, that's why the HP mode on the panel has 2 settings. Lugs 3 and 4 of the switch were connected together.

Side view:

Side and back view:

Here's an oscilloscope screenshot of a squarewave wave in LP mode with full resonance applied:

CALIBRATING THE FILTER:

There are two trimpots on this filter that need to be set.

The first one is the 100K trimpot. You use this to set the throw of the Cut-Off potmeter. Set it in such a way that you get the most resolution of the Cut-Off potmeter.

The other one, the 200K, is used to set the Resonance to maximum. Turn up resonance and turn the trimmer until you're at max resonance. This will probably be around the middle of the trimmer at about a 100K.

It's best to have normal trimpots not multiturn ones. There's no need for those and the normal trimpots are easier to use.

VIDEO DEMO:

Here's the first test I did after finishing the project. This filter literally makes the room shake. If you turn up the volume (with good speakers or headphones) you will hear stuff starting to rattle in the background. It's a very bass heavy filter which really sounds great!

Here's a short demo I made with the X4046 VCO hard synced by the 555 VCO going through the filter.

Here's an other video I found on YouTube dealing with the CS-5 Lowpass filter:

https://www.youtube.com/watch?v=5F8_RfVzO2A

Here's a Facebook video of my friend Jake Jakaan using 3 of these filters in bandpass mode to create a formant filter that makes sounds akin to human speech.

https://www.facebook.com/share/v/1A2Q6Ws1AE/

Okay that's it for this article. Not a filter anyone can build alas but this website is an archive of all the modules I built myself so it certainly belongs here. I also noticed that Yamaha filter schematics that specifically deal with the CS filter are almost non existent on the internet except for the service manuals. So I hope this article will provide at least one good schematic for those looking for it.

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

Synthesizer Build part-63: FASTEST ADSR IN THE WEST by Rene Schmitz.

The fastest ADSR in the West. A simple to build and fully featured Envelope Generator. I thought it was about time for another ADSR project for the website and this one worked out great and it has no trimmers to set. I even added a few extra's to make it even better.

This is an other 7555 based envelope generator like the YuSynth one from project 24. Some people seem to have problems with that one so that's why I choose to do this project now.

This ADSR will work on both +/-15V and +/-12V. At +/-12V the maximum envelope amplitude is just under 8 Volt. If you run it on +/-15V the peak envelope value will be 10 Volt. It's a very fast Envelope Generator. The minimum risetime of the signal is 1mSec or 1/1000th of a second.

Now this is roughly the same setup as in the previous 7555 ADSR from project 24 but it does work a lot better, especially when using Schottky diodes. The whole problem with the DC offset voltage left on the ADSR output comes down to the forward voltage drop of the diodes. That's why Schottky's are so useful because they have only a third of the voltage drop of silicone diodes. Now what would be even better is to have diodes with no voltage drop. To achieve that we have to put the diodes inside the feedback loop of an opamp. That's what happens in the Kassutronics Precision ADSR which I recently also added to the website as project number 67.

So this ADSR and the Yusynth 7555 ADSR have become a bit obsolete now, eventhough they work just fine for use with VCA's and for Filter CV signals. I would advise anyone wanting to build an ADSR to go to project 67 and build that one.

SCHEMATIC:

Here is the schematic for this circuit. I've redrawn it from the sketch posted on Rene Schmitz website.

The opamps are numbered in the order they are used on the stripboard. All diodes have been replaced with Schottky diodes which work much better in this circuit.

Click here for a FALSTAD SIMMULATION of this circuit.

I've added some extra's to this design. The original design only uses two opamps but I wanted to include a LED and also an inverted output so I decided to use a quad opamp, the TL074, and include an attenuverted output where you can have in inverted envelope signal with the potmeter turned fully counter clockwise, attenuation with zero signal when the potmeter is at the 12 o'clock position and a normal envelope when the potmeter is turned fully clockwise. I took the design from the AD/AR attenuverter mod from Ole Stavnshoej design (project 44). This is a great option to have when you use the ADSR with a filter. Turning the attenuverter will give the filter some very cool resonance response.

I decided to adapt the design to more run of the mill parts, like for instance I used 1M potmeters instead of the 2M2 ones in the original schematic. I changed the 220Ohm resistors to 100Ohm types and the capacitor from 2µ2 to 4µ7 to keep the original timebase intact. This is all explained in the text underneath the original schematic on the Schmitzbits website.

The circuit is relatively simple so I was able to build it up on a very eurorack friendly sized piece of stripboard. It's only 21 strips by 33 holes. Although I left the V,W and X strips on the board, they are not populated. You can use them to connect L brackets to mount the board to a panel. The three transistors on the Gate input represent the same setup as we've seen before in the Yusynth 7555 ADSR only there he had no resistors at the base of transistors 2 and 3. It works as follows: the first two transistors make up a schmitt trigger which turns any input signal into a sharp gate signal. That signal now goes through a capacitor that turns any long gate signal into a short pulse. That pulse is inverted in the third transistor stage to make it acceptable for the 7555 chip, going in at pin 2. 

Once the ADSR has been triggered the sustain level for that cycle is frozen. You can not add sustain while the ADSR is in its cycle, unlike the Digisound ADSR which can do this. Not that that's important. it's just something I noticed while testing the circuit.

The 1M resistor, in red on the schematic and in purple on the layouts below, can be added to provide for some input hysteresis. This will improve triggering on slowly changing waveforms. In the layout below, the purple 1M resistor on the left indicates where it should go if you want to include it. I left it out. Only include it if you really think you're going to need it. If in doubt, Leave it out.

LAYOUTS:

Here are the layouts I made for this project. As always they are verified. I used them to build my module. It's important to use logarithmic 1M potmeters for Attack, Decay and Release. The time based parameters. Otherwise it will be much more difficult to set these parameters accurately. It will work with linear types but get logarithmic pots for these. Sustain is a level control so that can and should be a 10K linear type potmeter.

I used Schottky diodes throughout this design because with 1N4148 diodes there's a DC offset voltage present on the output. Using Schottky diodes helps to prevent that.

Wiring diagram:

Stripboard only:

Again, leave out the purple 1M resistor unless you're going to feed this ADSR with slowly changing Gate input signals. Nor likely so leave it out.

Cuts and wirebridges seen from the component side. You know the drill by now; mark the cuts on the component side with a Sharpie or Edding pen and then stick a pin through the marked holes and mark them again on the copper side. Then cut at the marked places with a sharp, hand held, 6- or 7mm drill bit.

Here is the Bill of Materials. I altered the diode types to Schottky diodes because they will work much better in this design. I put in the BAT4* series (like: BAT41, BAT42, BAT43 etc) because they work really well and have good availability in webshops. Any Schottky diode will do though.

OSCILLOSCOPE IMAGES:

Here are some screen shots from my oscilloscope to give you an impression of what the signal looks like. All testing was done with a +/-12V powersupply:

Here's the normal envelope output. The envelope signal does have a small positive offset voltage of 400mV I noticed. But this won't cause any VCA to stay open so it's of no consequence. 

However I changed the diodes for Attack and Release into Schottky diodes and that reduced the offset to just 16mV (16 thousandth of a volt) which is the same as 0V to me. The offset voltage is the result of the fact that the 4.7µF cap has to discharge through a diode and a diode has a voltage drop over it of about 0.6V (with silicon diodes). So the lower that voltage drop the better. With Schottky diodes the voltage drop is only about 0.2V which allows the cap to discharge as good as fully.

Here's the normal output in yellow and the inverted in blue coming from the attenuverter mod I added on myself. It works like a charm.

Here you can see, in the blue trace, the attenuverter in action. I'm turning the potmeter as the trace goes from left to right.

This is the signal at a pretty high rate at almost 3Hz. No problem for this ADSR.

In yellow you can see the pulse as it comes out of the third transistor and into pin 2 of the 7555. It's a inverted pulse, triggered by the gate signal, that starts the ADSR.

This time the yellow trace is the gate signal at the input. This was measured after the 10K input resistor. The gate signal was a +/-5V pulse wave from an LFO.

PICTURES:

Here are some pictures from the build proces.

Stripboard with cuts and wirebridges done.

Finished stripboard ready for wiring up.

All wired up ready for testing

I decided to use this ADSR for my DIY Kosmo synth and not for Eurorack so I took the YuSynth ADSR and replaced the stripboard with this one. I had to widen one hole to fit the attenuverter potmeter to which I added a bi-coloured LED to fill up another hole where a switch had been. I used a 4K7 resistor to connect it to the attenuverted output socket. I also re-used the manual trigger button that was already present in the panel. I took two 47K resistors and made a voltage divider so when I press the manual trigger it sends 7.5V to the gate input. (My DIY synth runs on +/-15V mostly).

Backview of the panel:

Here's what it now looks like mounted into the synth. My ADSR module has two Gate inputs each with a Schottky diode in series with the socket (soldered straight to the socket). This is to prevent +7.5V entering the Gate socket when I push the manual trigger button.

Luckily I could re-use the potmeters, which were 1M logarithmic types with a 10K linear pot for the sustain, the same as in this project.

You can see the blue LED underneath the attenuverter potmeter. The hole I had to fill up was 6mm and this LED is only 3mm so I used hotglue and made a sort of white blob that lights up red or blue. Worked out pretty well :)

I kept the dual gate inputs from my previous ADSR because I think it's handy to have. The gate inputs have Schottky diodes on them so that when I push the manual trigger button I don't get 7,5 Volt pushed into the gate patch cable(s). It's a safety feature I advise you to copy if you are going to include a manual trigger button.

Troubleshooting tip: If your Decay and Sustain are not working then the most likely cause will be a broken Sustain potmeter. It happend to me when I built it into the panel I used for the YuSynth 7555 ADSR and it turned out it had a broken Sustain potmeter all the time.

DEMO:

Here's a video I found on YouTube of someone demonstrating this ADSR in action. He's using it on the cutoff of a lowpass filter. Sounds pretty sweet. If he had the version with my attenuverter mod it would have sounded even better LOL ;) 

So that's another one done. I thought it was about time for a new ADSR project on this website, especially since some people seem to have problems getting the YuSynth 7555 ADSR of project 24 to work right. That's weird though because I always rated that one as near perfect but I think this will make an excellent alternative especially with the extra's I added. I'm really chuffed that it worked so well. Okay, I hope you will enjoy building this one. 

If you have any questions or comments about this project then please post them in the comments below of on the special Facebook group for this website.