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Expo converters are driving me crazy
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Expo converters are driving me crazy
Gemfire
Hi guys! I've been lurking on the forums for some time now and this is my first thread.
So, I've been trying to build an expo converter to go with a classic LM358 tri/square VCO for a few weeks now with no luck. very frustrating
I've tried three or four designs and none of them seem to work for me.
The first one was from All About Circuits article. It kinda worked, I even etched a PCB, but in the end I couldn't make it play 1V/Octave correctly. It seems that no one on the web could do it either. Next I tried a bunch of different designs from around the webs, read a ton of articles on the topic to the point I now understand how the darn thing SHOULD work. But it won't. I tried to breadboard some but they didn't work for me at all. The last one that I breadboarded just now (for the 5th time, probably) is by Rene Schmitz and his 4069 VCO. The darn thing only outputs about 1V max and my VCO goes up to 5-6Hz. That would be a nice LFO, probably, but I don't need one ATM.
So at this point, I'm starting to thing I'm either dumb or missing something. Can someone give me a piece of advice on building an expo converter that would work for my VCO, please, because this is getting ridiculous.
Thanks!
Gemfire
Ok, so regarding this last expo I know it outputs current, so measuring the voltage won’t give a correct reading but still it won’t drive the VCOS higher than a few HZ.
Mungo
There is a comprehensive background on the subject here:
http://www.openmusiclabs.com/2015/03/temperature-compensation-of-analo g-exponential-converters/trackback/index.html
It focuses on the most typical exponential converter design but builds up the operation bit by bit so you can learn along the way.
mskala
You may have already seen my article on exponential converters here: https://northcoastsynthesis.com/news/exponential-converters-and-how-th ey-work/

But that's more "how they work" again and not a specific design.

I guess the best single piece of advice I can give is to figure out exactly what the current and voltage requirements of your VCO core are. The output of an exponential converter is supposed to be very high-impedance, while the input of something that takes current input is always low impedance. As a result, when you connect them, the voltage at the point where they connect will be determined by the input (VCO core or similar) and not the output. But the exponential converter will have limits on the voltage it can handle, and if the core's input voltage isn't within those limits, they won't work properly together. The impression I have from your description is that that may be what's going on.

So the questions to answer are:
* What's the input voltage of the core?
* In which direction is the current supposed to flow?
* How much current does the core want (minimum? maximum? amount per Hz?)

Given that information you're in a better position to design an exponential converter that can meet the specs.
guest
if youre just getting started in all this stuff, a 555 VCO or maybe the MFOS VCO are good places to start:

http://musicfromouterspace.com/analogsynth_new/VCO20090724REV0/VCO2009 0724REV0.html

basically, the exponential converter is just the start of a good VCO. you also need a charge integrator (capacitor+buffer), a comparator, and reset circuitry of some sort. a 555 makes an ok comparator and reset circuit, so all you need to do is add a capacitor and an opamp buffer. the issue with the 4069 and all-about-circuits (AAC) VCOs, is that they dont have good integrators, comparators, or reset circuitry. the AAC VCO converts the expo current to a voltage, and then back to a current through a resistor in order to charge the capacitor. this is horribly inaccurate. the 4069 makes a poor comparator, and the reset circuitry isnt so good either. to make it worse, the exponential converters on the rene schmitz version is not great.
Gemfire
Thanks for all the replies! This gives me something to work with.
devinw1
That a look at the ElectroNotes #129 article. That particular expo in various tweaks has been around a long time and used ALL OVER the place. The exact same circuit is also in the book Electronic Music Circuits by Barry Klein too. I would guess it's so prolific because well.... it works! It's output works very well going to an OTA like a 3080 a 3280 or LM13700. Makes sense as those have a bias current input and your expo generator produces current.

Good luck!
Gemfire
Okay, a bit of dumb question. Does the arrow pointing down on this schematic from EN37 means negative voltage and arrow pointing up means positive? I'm a bit lost here and couldn't find anything in the description of this circuit.




cygmu
Yes, those symbols mean the positive and negative supply voltages.
Gemfire
Great, thanks!
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