Only one job waiting for me at work this morning - a temperature controller. The note with it had the helpful message 'doesn't work' and 'wanted urgently' (underlined several times). Opened it up and a quick look revealed a fried voltage regulator which had decided not to go alone and took several critical chips with it. Customer was informed that 'urgently' represented a replacement at considerable cost coming from the USA.
So, what to do now? Well, if I'd known, I'd have taken my amp in to work on. However, recently I'd been reminded of a keyboard that I built a long time ago with two transistor tone generators. So, with nothing more 'urgent' I decided to see if I could recreate it. I remembered the general design and also that I used to like using +- 7.5V supplies a lot (dunno why).
An hour or so later I had a nice little parpy sound with no clicks and a scope trace that looked pretty familiar.
Feeling quite pleased with myself I thought I'd see what it'd be like using an OpAmp, as they don't cost an arm and leg these days. I expected the clipping to be much sharper, but to my surprise it wasn't. However with no offset it was pretty symmetrical. The answer to that was to unbalance the supply rails, giving me pretty close to the same result as the transistor version but with fewer components.
A scope and frequency check proved, even with no special filtering anywhere, there was no tendency for generators to lock to each, pitch shift or change waveshape when the second one was started.
The drawing for both versions is
hereThe values shown should produce about 339Hz but the effect of the waveform clipping messes with the phase relationships a bit and lowers the frequency. The fine tune has to be pretty minimal as it unbalances the CR relationships and also slightly changes the gain, output level and waveshape.
So, next was to make another OpAmp generator and tune all three to make a nice major chord - C actually. I padded up resistor and capacitor values until I was fairly close then just used the fine tune preset for the last bit using a guitar tuner to set the pitch
There is a sound clip
here.
E is the transistor generator and C & G are the OpAmp ones. The file is uncompressed so if you like, you can load it into something like Audacity and stretch it right out to see the waveshape and that there really is a smooth attack and decay.
P.S.
The hesitancy isn't bad playing on a keyboard, it's trying to operate three paddle switches just sitting on the bench
P.P.S.
No, I am
not going to make 49 of these for a 'Folderol Special' synth!
