music, guitars & other obsessions

SMPS

A Productive Day

Got a bunch of projects done that have been piling up…

1) Put in the Cabinet Sim/Boost & Speaker Mute switch for the Vox VT15, added the external speaker jack as well. What a difference! The 8″ in combo absolutely blows, now plugging in the Ragin’ Cajun actually make the amp sound half decent – still no match for true tube amp though…

2) Completed the wiring for grounded wall wart power supply that will be used by the SMPS inside the Octal Amp. The SMPS needs to be both referenced to chassis and earth ground, otherwise the amp generates very loud squelch when volume is turned pass 4-5. Cobbled together the wiring using old S-Video cable and 3-pin DIN plug/jack, so no one can plug in a regular wall wart by mistake. The amp is still a bit noisy for my liking, but when it is cranked up, you don’t hear the noise at all.

3) Back up the HTPC, I think for the first time in 3-4 years! The XP machine is rock stable, so it will be used til who knows when, if it ain’t broke, don’t mess with it! Who needs Vista or Win7?!

4) Received a bunch spreadsheets for plotting loadlines, biasing and Spice modeling from the kind souls at diyaudio and antique radio forums, we are a gang of geeky brothers ;p Now I just got to learn to use these wonderful tools.

A good day indeed!


SMPS Test

After reading some threads on diyaudio, diystompboxes and AX84, I decided to try some DC-DC converters to if they would work in guitar amps. Functionally, of course, they would but how would they sound, that’s what I wanted to find out… The biggest complain from users (or just speculators), was SMPS are noisy as the switching noise gets coupled into the audio circuit, well we are about to find out yea or nay.

I ordered two DC-DC converters, one 25W and the other 120W, both use TL494 PWM for control, MOSFET push-pull switching, step-up transformer, pretty much text-book design. The switching frequency is 70KHz. The larger unit has the MOSFET mounted on medium size heat-sink, the smaller one has none.

So how did they work? Quite well, actually. I removed the PT HV and the tube rectifier from the Otto amp, and hooked it up to the converter, first the larger one then the small one. Below are some shots of the units in action.

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Both converters worked just fine, I was surprised the smaller one worked without breaking a sweat, the switches did not even get warm after half an hour of playing. The noise level was about the same as the old supply, I was not able to detect any switching induced noise even when I put my ear right next to the speaker. Since the test was run with the lab supply, I could not try the amp in the studio, so further tests will be done after I get some wallwarts to supply the voltage.

Some shots of the converter to give an idea of their sizes.

120W Converter

25W Converter