Running x0x on 12v DC - why not?

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deep_bias
 
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Running x0x on 12v DC - why not?

Post by deep_bias »

I recently, accidentally, lent a friend my x0xb0x with a 12V DC supply which he has been successfully using. I'm trying to understand whether using it could harm the x0x. From my very basic understanding of regulated power supply circuits, it seems like it actually is fine.

The + DC will just pass through the rectifier and end up on C5. 9v Ac rectified will be 1.41 * 9v (minus .7-1v drop across diode) so about 12V DC is what you should be reading on C5 anyway and within spec of the 16v caps and other parts. The 7805/78L06 regulators should be fine. In the data sheet for LM336 they have a 10v precision reference circuit schematic somewhat similar to the configuration used in the x0x supply which seems to indicate the part is fine being supplied 12 - 36V.

I see in this thread a moderator mentions that the voltage doubler needs an AC input: http://forums.adafruit.com/viewtopic.php?f=13&t=10902. I am not sure why this is the case as the voltage doubler circuit looks like a pretty standard noninverting amplifier which AFAIK should operate correctly with an AC or DC input. Additionally - the 5.333 ref voltage has to be DC as well, which is what it's being fed, right?

Any help is appreciated. I am sure I could have missed some big thing that makes it a problem. Plus - the people must know the truth about their power supply options, right? Thanks!

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mmm
 
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Re: Running x0x on 12v DC - why not?

Post by mmm »

The "voltage doubler" is NOT IC23 (IC23B is just a buffer the also changes the impedance). The "voltage doubler" (or actually a charge pump? I'm not really familiar with this) comes from the way C5 is connected. You will definitely NOT read 12V DC at C5 when using a 9V AC supply; I think it's more like 28V.

The behaviour of the xoxbox when using DC supplies can be described as "erratic" at best. Both tuning and sound are definitely off, compared with the proper supply. The voltages for the digital part and the headphone will be ok, but not those for 5.333V and 12V.

Here's the general rundown:

9V AC: all good
12V AC (and higher): voltage for IC23 is exceeded
9V DC: digital part works, analog part only generates clicks
12V DC: digital part works, analog part is out of tune and sounds funny

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altitude
 
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Re: Running x0x on 12v DC - why not?

Post by altitude »

The 303 originally had a crude switching supply that ran on DC and the voltage doubler used in the x0x was the simplest way to recreate the performance of that circuit. BCbox and the TT-303 both use the original Sumida transformers for their power supplies which are exact copies of the 303. Having fixed regulated rails like a sane person would probably have an effect on the sound

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