Wednesday, 18 September 2019

Synthesizer Build part-1: SYMMETRICAL POWER SUPPLY (1,5A) with independent voltage adjustment.

The essential building block for our DIY synth. with outputs for +/- 15V, 12V and 5V at 1.2 Ampères.

I have set myself the task of building my own synthesizer. At the moment I'm slowly buying in the components I'm gonna need, as my budget allows (and that ain't much ^__^) and it'll give me something to do over the winter months.
The first thing I need for this project is a symmetrical power supply that can have different positive and negative voltages independent of eachother. So, for instance, you can set it to +20V and -5V.
I used the LM317 and LM337 for this project because they are easy to work with and fully protected inside against short circuits and over heating. The LM3XX series regulators can deliver up to 1,5 Ampères.
This is the schematic I came up with and it works very well: (click picture for full screen view)


In the schematic above you can see that the output capacitors are 1000µF. They don't have to be this big. 100µF will do nicely too. The caps discharge through the 220 Ohm and 10K potmeter when you switch off.
If you need a power supply that can handle more current, say 10 Ampères for instance, then you can easily adjust this circuit by adding a 2N3055 on the positive side and an MJ2955 on the negative side. You can even put more power transistors in parallel over eachother to get even bigger current specs. Move the capacitors that come after the LM3XX's past the transistors but put an extra 100nF capacitor between the base and the null to suppress transients etc.

But for a synthesizer those alterations are not needed because the individual modules I'm going to build won't draw much current at all. It's mostly in de 20 to 80 milli amps at most. The only transformer that I had that was big enough for this project didn't have a center tap but it had two independent secundary outputs. One at 21V and one at 17V. This was close enough. I connected one wire from each output together to make a center tap and made a circuit board to build the powersupply on. (Make sure when you connect two secundary windings together like this, that you measure the AC output before proceeding with the next steps. If you connect the wrong wires the 2 voltages will try to cancel eachother out. It won't damage the transformer but you won't get any voltage out.) I didn't have etching fluid anymore so I cut different islands on the copperside of the print using a dremmel tool with a milling bit. I had an old 25 Amp. Graetz diode bridge in my collection of components and after I drilled all the holes the build was pretty straight forward.
I used little trimmer potentiometers directly soldered to the print to set the voltage with. Once you set it, you don't have to touch it again but if you want to make it continuously adjustable you can of course use 10K panel mounted potentiometers with a knob. I put some LED's on the output side to indicate that all is working well. The difference in brightness will indicate if the voltages are set differently from eachother. You could also go the luxurious route and use two panel voltmeters to indicate the voltage but as I intend this to power my synthesizer that will not be necessary.
But if you build this as a stand-alone power supply, it would be a good idea to use two meters on the outputs.
The power supply as described above outputs between 1,5 and 25 Volts (dependent on the transformer you use) at a maximum current of 1,5 Ampere. If you want a more powerful version you could use the LM3XX regulators to drive a 2N3055 and a MJ2955 NPN and PNP power transistors as mentioned above and then you can draw up to 10 to 15 Amps. Be aware that the pinouts for the LM regulators differ from eachother. The correct pin numbers are noted on the schematic above.
The ripple is very low on the output. It's actually better than some you buy ready made. Make sure the electrolytic capacitors you use are rated 50 volts or higher. (I used 35V ones and that seems to work fine too but don't go any lower!) and make sure they are oriented the right way. Plus on plus on the positive side and plus to ground and minus to negative voltage on the negative side. The voltage at the secundary directly after rectification can go up to 10 volt higher than the AC voltage from the transformer. Don't present more than 35V to the input pin of the regulators and be sure to use big heatsinks on both of them.
I've measured the ripple and noise of the power supply using the method outlined by Dave Jones from the EEVblog on YouTube in his video EEVblog #594 - How To Measure Power Supply Ripple & Noise
I used my simple home build 'brute force power load' described in the article from januari 2017 and under a load of 1 Ampere the Vrms was 6mV and Vpeak-to-peak was 10mV. Those are very good results.

Here are some pictures of the first powersupply . As you can see there's some room left on the print for extra voltage regulators to get other voltages from the same supply:

I added the inductor coil in series with the 'Null' pole to suppress any high frequency noise. It's just something I added as a test but you can ignore that.

It's a week later and I finished the power supply as I need it for my synthesizer project. It now has -15/0/+15V,  -12/0/+12V and -5/0/+5Volt. I looks a bit of a mess as my projects usually do but it works just fine. Here's a picture of the finished psu:

Obviously you can't plug in all modules in the same holes so I've build a power bus system to which I can connect every module I build. It's a bit crude and I use a lot of hot-glue to stick it all in place but it works just fine and it will all be invisible once the case is ready.

Below you see the connector I build. The pins carry the following voltages: From top to bottom in the pic below, the top 2 pins are ground or 0V. Then I took out two pins and stuffed the holes in the female connector. This is to get an a-symmetrical distribution so you can't put the connector in the wrong way around. Then there's -15, -12 and -5 Volt and then we get +5, +12 and +15 Volt. I kept the plus and minus pins as far away from eachother as possible for safety reasons.

PART TWO. SECOND POWERSUPPLY for stage two of my synthesizer.
So, as I'm writing this we're 6 months on in the synthesizer build and I'm about to add a second stage to go on top of the synthesizer I have already built.
So I need a second power supply. The first design as seen above works so well that I'm repeating it for the second stage with a few minor changes. I'm using multiturn potmeters for the LM317 and 337 voltage regulators so I can set them very accurately. In fact, the one I just built has plus and minus 15.00V that's accurate to 1/100th volt. I'm using all LM3xx regulators for this because I have a lot of them and because their voltage doesn't drop if you pull more current from them which is important because otherwise the VCO's would become out of tune. I'm using the same schematic as above and I made separate prints for every stage of the supply. Here's the stripboard layout:

Bridge rectifier print. (Don't forget to cut the copper strip underneath the fuses and to tin all the copper strips that carry current.)

Voltage regulator print. (Again, make sure to tin all the copper strips that carry current):

Here are some pictures of the finished power supply. Tinning all the copper strips that carry current is important because they get very thin around the holes in the stripboard. Each LM3xx regulator has its own heat-sink and you must make sure these do not make contact with eachother! If you want to mount both LM's on one heatsink then they must be electrically isolated from the metal of the heat-sink.
I've mounted the whole powersupply on a long piece of MDF ready to accommodate the power-bus system I need to build.

I found some very old vintage diodes with a metal case which I think look very cool and they work fine. They are sturdy too because I had some short circuits in testing and the fuse went 2 times but the diodes didn't mind and I use slow fuses too so they did get some current through them.
Use plenty of heat conducting compound on the LM's. The electrolithic caps are 1000µ each; all four of them and that's all the capacitance I put in. 1000µF in the rectifier and 1000µF on the output side of the voltage regulators. The rectifier caps have 10K 1Watt resistors over them to make sure the are drained of voltage when the powersupply is switched off. (It's 2K2 in the picture but they were getting slightly warm so I changed them for 10K's)

Okay that's it for this one. Thank you for stopping by and please leave a comment!!