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!

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