Tuesday, 26 November 2019

Synthesizer Build part-5: THE CABINET.

I'm not going to get too deep into this because every individual will make their own cabinet or case to their own taste I think. This cabinet represents the vintage 70's look that I wanted for my synth and I'm very happy with it. Here are some pictures from different stages of completion:



Almost finished, just one more panel to fit but already working very well:



I made no drawings and I measured everything as I was building it. That's the way I usually approach woodwork. The drawing is in my head. I did make two cardboard templates for the side panels to make sure I got those exactly the same. I measured the current draw with all panels that I have build so far switched on. All together it drew 250mA. That's less than an old fashioned bicycle lamp. :) I also installed a temperature sensor that is directly in contact with the heatsink of the LM317 that regulates the 15 volt output and it runs up to about 60°C. That's perfectly fine and normal. It can handle double that and the temperature stays at 60° and doesn't climb.

The panels I use are made from aluminium (or aluminum if you're in the States ^^ ). They are 20 centimeters high so you could say I use the LookMumNoComputer Kosmo format. I bought 2 sheets of 1 meter long and 1.5mm thick and they are powder coated in gray/black. This powder coating is something I can really recommend because it's hard to scratch. If you just spray-paint your panels they will scratch very easily. You can write on the powder coating with a white acrylic pen. The one pen I bought had too wide a tip and I sharpened the tip with a razorblade but in the end it was un-useable. I ordered a pen online with a 0.7mm tip and that works far better. But if you laser-engrave your panels then you don't need all this anyway.
Make sure the panels you are going to use are at least 1.5mm thick aluminium!! If they are any thinner than that they will bend or flex if you put a cable into a socket on the panel. So keep that in mind!
This cabinet is 1 meter and 11 millimeters wide, 38 centimeters high and deep. Now that I'm almost finished I wish I made it a bit longer because I'm running out of space for my modules, haha. Nevermind, I can always put some electronics in the wood panel above all the modules. I think what I'll actually do in the future is to build a second enclosure to house drum modules. That would really open up the musical options ^____^
If you are building your own synthesizer I would be very curious to see some pictures so if you can link to that please leave the link in the comments. That'd be awesome!

March 2020 the second stage:

Here are some pictures of the second stage of the synthesizer. This is a much simpler case and it is 20 by 20 by 100 Centimeters so it sticks out at the back a bit. This was necessary to accommodate the power buss system. I made some trunk locks on the sides so I can clamp the top section to the main synthesizer. This works just perfectly. I did have to solder these locks though because the locks themselves were connected to the main plate with the screwholes by three bits of folded-over metal. So if you put any force on that they would bend and let loose over time. So I heated them with a blowtorch and soldered them from the inside. This worked really well because I used a bit of flux on the metal and this made the solder flow into all the little seems so it is very neatly soldered.





I made the width of the second stage too short by 1 centimeter so I had to use extra pieces of wood to connect the locks to.



Wednesday, 20 November 2019

Synthesizer Build part-4: THE ENVELOPE GENERATOR or ADSR

A very good functioning Envelope Generator with a very simple verified layout and some extra's I added myself. A project I recommend, even for beginners.

The Envelope Generator is generally better known as the ADSR which stands for Attack, Decay, Sustain and Release. These are the four phases a note goes through when you press a key on the keyboard. The attack is the speed of the initial rise of the note, once you press the key. Decay is the time it takes for the note to fall to a constant volume after the initial keypress. Sustain is the level of the note as you keep the key pressed down. It is usually a bit less loud than the first instance of the note being played. Then we have Release and that is the amount of time it takes for the note to fade out once you let go of the key. So the envelope generator produces a signal that determines the volume of the note over time and this signal is being used by the Voltage Controlled Amplifier (VCA) which interprets it as an output level.

I decided to build this ADSR using the chip series that I base the whole synth build on, the AS33xx series of chips. The AS3310 is the ADSR chip and it costs way less then its CEM counterpart. I order all my synth chips from Vintage Synth Parts in Italy and all these chips cost around €6 each there.

So I looked up the datasheet and used the circuit that was presented there. I made the following stripboard layout for it. This layout is verified, I used it for my build. (All potmeters viewed from the front.):


(Last revised: 16-March-2020: Removed direct potmeter connection to 5Volt. Revised potmeter wiring. Trigger when not used shorted to Gate via internal switch in trigger input socket.)

Print only:



This design works very well and does the job it needs to do. It has a few little quircks though. The potmeters for instance. I used normal linear type potmeter and that works but it would be better to have reversed logarithmic or anti-logarithmic potmeters because the difference between 1 second and 10 seconds on the Release for instance is only a few degrees of turning the knob. But once you're used to this it's not really a problem. The Sustain potmeter is at its maximum at about 2/5th of its maximum throw. If you turn it further the Sustain level rises but the attack won't be able to reach it. So if you have Attack set to, for instance, one second, it will rise normally and after one second it will suddenly jump to the Sustain level. I've got some oscilloscope pictures below to illustrate this.
The external trigger input is normally shorted out through a switch in the Trigger input socket. So if there's no trigger cable attached, the trigger for the chip is provided by the Gate signal through the 3nF capacitor. The AS3310 needs a simultaneous gate and trigger signal to function. So if you plug a cable into the trigger input but you don't provide a trigger signal, the Attack parameter of the ADSR will not work! So this is not a malfunction, this is how it's supposed to work.

I added a few extra's to this design. First there is the option to output a signal that is twice the voltage of the normal output (10Vpp instead of 5Vpp). You can use this, together with a passive attenuator in the mixer panel, to drive a filter's resonance or other parameters of the synth. Then there's also an inverted signal output, to add to the options of driving parameters of the synth. This goes from 0V to -10V.
All this takes place at the opamp on the lower left of the circuitboard. You can use the old favourite TL072 for this or the TL082. Pins 1,2 and 3 together with the two 100K resistors form the 2x amplification. You can use other resistor values as long as you use 2 resistors of the same value. Then the input signal is split at the non inverting input and goes, via a 100K resistor to the other side of the chip where the opamp is set up as an inverting buffer. Both opamp outputs have their own output jack socket. The normal 5V ADSR output is a separate socket (of course).

And finally I added a manual trigger option, at first I added it so I could put a gate signal on the gate input for test purposes, but then I thought this would be handy to have anyway so I added the switch to the final panel. I decoupled the manual trigger signal from the gate input socket with a Schottky Diode so no voltage goes into the circuitry that delivers the normal gate signals so as not to damage it (although this is probably not necessary). I used a Schottky diode because their voltage drop is only 0.2 Volts so it doesn't detract too much from the usual 5 Volt gate signal.

Here's the E.G. mounted in my synth. You can see that I doubled the output sockets. There's 2 outputs for normal 5Vpp ADSR and 2 for either 10Vpp or Inverted -10Vpp I also installed a Gate output and a Trigger output. The Trigger output is connected to the Gate output via a 3nF capacitor and the Gate output is simply switched in parallel over the Gate input. I will however install a opamp buffer for the gate output in the near future.:



Here are some oscilloscope screenshots showing the function of each variable:
This is the normal envelope CV at 10Vpp.


Varying the Decay time:



The picture below shows the quirck I mentioned earlier where the Sustain level is set higher than the Attack can reach and so after the Attack cycle has finished the Decay is skipped all together (because there is no Decay if the Sustain is too high) and the envelope jumps to the Sustain level. You can clearly hear the jump in volume in the audio. You can use this to your benefit though because it sorta has a percussive quality to it. Anyway, if you don't want this, just turn the Sustain down a bit. Problem solved. You can also limit the maximum resistor value of the Sustain potmeter by adding a resistor or trimmer to pin 3, but you'll have to experiment to find which value works best.



Lowering the Sustain level:



Increasing the Release time:



And finally switching between the inverse envelope (which was set to 0 to -5V in my ADSR but to 0 to -10V in the stripboard layout) and the 10Vpp envelope.



Okay, that's it for this one.
I hope you enjoyed this article and leave a comment please if you found this helpful! Much appreciated! Also, any questions? Put them in the comments or contact me on Facebook. I'm a member of the 'Synth DIY' Facebook group and the LMNC Discussions FBgroup.
Stay tuned for more build reports and click 'follow' to be notified of new posts to this website.

Friday, 15 November 2019

Synthesizer Build part-3: TRIANGLE TO SINEWAVE CONVERTER

After finishing the VCO build successfully I wanted to expand it's possibilities by adding a sinewave output. As I had used the LookMumNoComputer design for the VCO I thought I'd go with his design for the Triangle to Sine converter too. I didn't have a schematic so I did something I normally never do, I just used the stripboard layout that was available on the internet. Luckily it is a good design only I couldn't get a good sinewave out of it at first. I think the trouble was that I used the design 'as is' with the +12-0--12V power supply but the signal I put in came from a VCO that uses dual 15V power. Also that signal is multiplied by two in the output buffer opamp of my VCO to get to 10V peak to peak. The original design is for a signal that is 5V peak to peak. So I must have overwhelmed it a bit. Anyway, I decided to alter the design a bit to suit my needs.
This is the original stripboard layout from the LookMumNoComputer website:


What I got at first was a triangle wave that was cut off at the top and bottom and also not symmetrical. I tried different things to get the signal level down to get it to form a good sinewave and after some trial and error I managed to get it to work. I made several changes to this design and adapted it so it takes + and - 15 Volt because all my other modules use those voltages and so it can handle 10V peak to peak input signals.

A - First I made the gain of the input buffer adjustable by changing the 15K resistor for a 10K potmeter over the negative input and the output of the opamp. This makes for a very useful adjustment point as I found out when I tested this. I works really well.

B - Then there is a voltage devider over the inverting input of that opamp which in the original design is made up of a 30K resistor coming from +12V and a 10K going to ground. I changed the 30K into a 10K because after the first alterations I still had a wave that was flatter at one end than the other. I thought it would be a good idea to get the offset voltage to mid range to level things out. This did the trick nicely.
I had also made the input level of the triangle wave adjustable so I could test different levels but this proved un-necessary. The best setting was full level anyway so that idea was scrapped.

After having done all this I was able, by turning the potmeters, to get a fantastic looking sinewave which only got the tiniest little distortion at the bottom of the wave above 8.5kHz

C - The last alteration I did was to put 10µF electrolytic capacitors on the in- and output lines. This I did because I noticed that the in and output had a -5V DC voltage on them and that's not something you want to tolerate normally. The capacitors took care of that. So now it all works perfectly.

Oh and one final thing. Make sure to ground the audio in- and outputs to the ground of the power supply of this converter. I got 50Hz waves that were 100V peak to peak on my scope from only connecting the output to the scope. Adding a grounding wire from the 0 Volt rail to the audio ground turned that to zero!

I made a stripboard layout of the converter with all the alterations I added.


This is a little sketch I made of the circuit with the alterations I made:



Here are some images of the sinewave from my scope and also an FFT image which shows how low the distortion is when you look at the harmonics which are negligible really.




The bottom image shows the tiny distortion at the bottom of the wave at frequencies above 8.5 kHz



When I build this, I didn't know where in the synthesizer I would put it but I had an off-cut of aluminium that was 3 cm wide and I thought, 'let's use that for the converter.' I added a noise generator made from the MM5837 noise chip (that is also used in the 'Prophet 5', my favourite synth.). The panel being so small meant I had to mount the pcb's at a 90° angle to the panel so I made some L brackets from some copper sheeting I have and I made some rings from plastic to make sure I didn't get any short circuits and so I put it all together. So now it looks like this:



I accidentally drilled one hole too many but luckily it was only a 3mm hole so I put a little LED in that to fill it up. Looks nice too so problem solved haha.

Okay, that's it for this one.
More synthesizer build articles to come. Stay tuned!  Oh and leave a comment while you're here, please!

Thursday, 7 November 2019

Synthesizer Build part-2: THE VCO

A word in advance: this article deals with the first VCO I've ever built and is based on the datasheet schematic combined with the LookMumNoComputer lay-out for the CEM or AS3340 chip. I personally had great trouble getting this VCO tuned over a wide range of octaves.  I could also never get really deep notes from this design. I have since found a much better VCO design so if you want to build a simple but excellent working and tunable VCO on stripboard I refer you to Synthesizer Build part-18: A Really Good VCO design.

Here's the original text for the first VCO build:
After having constructed the power supply and the power bus system it is time to move on to the next step. The Voltage Controlled Oscillator. I'm not going to go into details as to how it works etc. There's plenty info online about that.  In order to make this a complete build, not just the circuit board I needed something to mount the knobs and in- and outputs on. So I ordered a sheet of Aluminium, 200 x 1000 X 1.5 mm and powder coated gray/black on one side. That is fantastic stuff to make panels out off and I highly recommend it. You can saw off panels of the right width using an electric jigsaw with a fine toothed metal saw. Make sure you guide the saw with a straight piece of wood or metal to get nice straight panels.

For my VCO I chose the AS3340 chip which is a complete 1 volt per Octave VCO in a chip. It's a clone of the CEM3340 which were used in the 80's in synths like the Prophet 5 and many others.
The VCO we're building here will have almost all the options that the AS3340 chip has to offer and those we didn't include are not worth having anyway ;)
The schematic I used is pretty much just the schematic that comes on the datasheet.

This is the one I used:


I used the layout made by Sam Battle, from LookMumNoComputer and did a few enhancements on it. (Look to the one on the right).


For one, his layout is meant for the CEM3340 which uses a 10K pulldown resistor on pin 4, the squarewave output. For the AS chip, that's supposed to be a 51K resistor although I'm reliably informed it doesn't make a tad bit of difference what you use here. There was also a mistake in his design, namely the 10K resistor in the bottom left near the TL072. It is switched in parallel with the 10K on pin 4 making the overall resistance 5K. Just leave the bottom 10 K resistor out.
The 10K trimmer potmeter at the top left of the 3340 needs to be a multiple turn potmeter so you can set it very accurately.

Sam's layout doesn't include the High Frequency Tracking but you really need to include it in your VCO. I first build it without and at first it seemed to work fine but after having completed the whole synthesizer I couldn't get really deep bass tones out of it. That is until I included the High Frequency Tracking. Seems a paradox that something meant for High Frequency adjustments can have so much influence on the bass notes but if you look at the schematics you can see that it pulls the CV voltage on pin 15 down to ground a bit through the 20K potmeter. I kept out the 360K resistor between +15V and CV input because that kicks the VCO into really high notes. I don't know why that resistor is there but it really screws up the frequencies. I left it out but maybe I should have experimented further with that resistor in place. Anyway...
The HF adjustment pot only adjusts about half a note over its full throw so when you first test it it might look as though it doesn't work but it does when you start tuning the higher octaves of the VCO.

Furthermore I gave the buffer for the triangle wave a gain of 2 by adding two 100K resistors to the TL072. That gets the level of the triangle output up to 10V peak-to-peak, in line with the output voltages of the other two waveforms. Btw, you can use any resistor value between 50K and 1M for this purpose as long as both resistors have the same value.

I also added the Positive and Negative Hard Sync options from the Digisound 80 Modular design so that's also available on this VCO.

Here is the layout that I drew and used:


So there we have it. It's become quite a comprehensive VCO with lots of options.
I added a 100K resistor to the +15V input of the Pulse Width Modulation potmeter to get it to work over the complete throw of the potmeter and I added a switch to have the ability to decouple it from the PW Control Voltage if you have PWM controlled by an LFO for instance. You don't have to decouple it but the option is there.

I tested the finished print and everything worked as expected but there was a funny quirck in the squarewave output. Below 1.35kHz there was a strange triangular wave ringing on the downward slope of the square-wave. Here's some pictures of that from my scope:



I opened a discussion about this in the Synth DIY Facebook Group and there were many suggestions but I still haven't figured out the cause. It's not a de-coupling issue anyway.
I suspect that leaving out the High Frequency Tracking I mentioned earlier may be the cause. (Note: I did some more tests and it turns out that it does have a big influence on this issue. Including HF Track with the 360K resistor to +15V almost gets rid of the problem but on low frequencies there still is a bit of ringing on the downward slope but not nearly as much as now.
But as I mentioned before, the 360K resistor really screws up the frequency response so it can not be included. I have heared that there might be batches of chips that have this fault, so it might be the chip. I don't know and don't really care because you don't hear it and everything works fine.

[Edit: In the second VCO I built and now use (see article 18) this ringing is still there but it is much less then in this design. The new VCO has at most 3 spikes in the downward slope of the squarewave. Anyway it has proven to be not a problem what so ever. You can't hear it and it doesn't affect the working of either VCO in any way.]

Although the connection is there in the layout, I did not use the Soft Sync input on my final build. I don't think I'll need it. I did use the FM input. You can connect a second VCO to that for instance.
Here's a look at the finished product, panel and all. The powder coated Aluminium was a great choise and looks so cool. It doesn't scratch easy at all, it's perfect for this project.




I'll explain what's on the panel.
We have the FM input at the top left. The big knob at the top is the Coarse Tune potmeter, below that on the left are the CV1 and CV2 inputs and on the right are the Triangle-, Ramp- or Sawtoothwave and below that the Squarewave outputs. ( I always put inputs on the left and outputs on the right.)
Then there are three inputs to the left of the blue knobs. Those are the Pulse Width Control Voltage input, the blue knob next to it controls its level. Underneath that are the Positive Hard Sync and Negative Hard Sync inputs. The bottom blue knob is the Manual Pulse Width control if you don't use a control voltage. The switch with the diode symbol let's you choose to put a diode in the external Pulse With input line which de-couples it from the internal PWM control or to bypass that diode and get more range on the PWM control knob. [edit] I have since scrapped this idea and I took out the diode. The switch is now used to turn off or on the manual Pulse Width Modulation potmeter as described above. (It's not necessary but the switch was there so might aswel use it for something). I added one more output which isn't in this picture and that is a "CV out" function to connect the second VCO to the first one. It's simply switched in parallel with the CV-1 input.


Okay, that's it for this one. If you have questions or suggestions please write them in a comment. Next part will be about a filter, probably the Prophet One Low Pass filter.
Stay tuned!