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Replace R1 in the power supply with 200 ohms?
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Re: Replace R1 in the power supply with 200 ohms?

by 3phase on Thu Apr 03, 2014 1:58 pm

ok..he is using a modern dc dc converter..thats the closest thing to a sumida and definitely worth a try. Thanks for the tip.
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Re: Replace R1 in the power supply with 200 ohms?

by guest on Fri Apr 04, 2014 6:41 pm

i went through and re-looked at the 303 powersupply when i did the x0x-heart. i dont have the exact numbers with me, but its around 15V output, and switches at 150kHz. the actual droop effect happens rarely, and only under heavy current draw conditions (lots of notes with accents). but, it does have a great sound when it does happen.

the sumida transformer is not really the important thing, as the output impedance of the switching supply is much lower than the 100ohm resistor. so its mostly the 100ohm/47uF combo that determines the sound. the switching aspect isnt really important either. 150kHz is above the audible range, and by the time it goes through the 47uF capacitor and gets out the other side of the opamp, you dont see any of that anymore.

the best thing to do (on the x0x) is to use an lm317 variable regulator before the 100ohm/47uF combo, and then put a pot on it, so you can turn it down, simulating dead batteries. you can also build a simple switching regulator (like the mintyboost ada sells) and put a pot on that. that way you can actually run it from batteries. the powersupply on the x0x-heart switches at a similar frequency, and outputs a similar voltage to the 303, but its all SMT, and not the easisest to build.
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Re: Replace R1 in the power supply with 200 ohms?

by 3phase on Tue Dec 16, 2014 3:25 pm

Hi, i ve experimented a little around and the 47uF/100 ohm thing was easy to apply on a xox since its basically the same circuit except that the caps are 100uF. seems to get a little less edgy this way but didnt had the same fx as the original 303 power supply. Ist it possible that the ground is behaving different on the original powersupply and somehow floating under load?
Anyway.. dont know if the hazzle of rebuilding the original power supply is really worth the little difference you get, but its getting closer to a real 303 this way. To quantify the difference i would say its similar to the diference between a original 622 vca or the newer one. It gets smoother and more pumping/singing with the original power supply circuit. A little.. some might even prefer the tighter more edgy sound of the original power supply, but not me ;-)
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