A properly made power brick!

[Th3_uN1Qu3]

I found a good deal on a used pro DJ mixer (but more on that later on). It needs a bit of love (new faders, and the chrome on the faceplate is badly worn, i'll see what i can do about that), but it is technically OK for the most part.

The mixer called for a 15v DC supply and it has a 7812 onboard. (yup, no split supplies, the whole thing is AC coupled, fortunately they used high quality ELNA caps). The supply it came with is a 12v, 1A switcher so it's naturally not enough. After the 7812 i get roughly 10.8v.

The mixer works fine with it but i noticed a 15kHz whine that gets annoying when the volume is cranked. I disconnected all inputs from the mixer and the whine was still there, so i've narrowed it down to the power supply. I cracked it apart to have a look, and was surprised pleasantly by what i saw. Take a look at the attachments. The caps are Teapo and Su'scon, but all are good, there's a full transient filter on the input and a pi filter on the output, and a BIG varistor wired straight to the mains plug. So there's nothing wrong with it.

It's an universal input supply (100-240v) and my mains is 230v, so a feedback loop adjustment should be all that's needed to bring it up to 15 volts. I'm hoping that bringing the 7812 back in regulation will give enough ripple rejection to get rid of the whine. Otherwise, it's probably calling for a linear... 15v at 500mA isn't a whole lot, and i have a suitable transformer lying around, but i'd have to ghetto a case for it, while the switcher looks nice so i'd rather make it work if possible.

[kikkoman]

Re: A properly made power brick!

why do it the hard way?
out with the 7812!

[Th3_uN1Qu3]

Re: A properly made power brick!

If anything awry happens, the 7812 fails short and protects the rest. If i remove the 7812 and something awry happens... i blow all the opamps.

Anyway. Upped the voltage. No change (except blowing up the protection zener of the switcher). Exchanged with a (wimpy transformer) linear. A healthy dose of mains hum... and the whine is still there. It's not coming from the power supply but from the mixer board itself.

There are some mods on the LED level meter, traces cut, a resistor and some bypass caps installed... the whine could be coming from there. The meter is driven by a BA6822s IC, i'll take a look at it now. Edit: Yup, the BA6822 uses an oscillator and has a switcher internally. The recommended oscillating frequency is 4kHz... eek. Yup, i think i found my problem.

[Th3_uN1Qu3]

Re: A properly made power brick!

My bad, the mods were on the FX input section (which i don't need anyway). And i was barking at the wrong tree... the whine is coming from my soundcard.

[Wizard]

Re: A properly made power brick!

This SMPS wall wart is horrible especially for audio stuff. No smart IC and under volted. In a pro DJ Mixer. Tsk tsk. Clean power is critical.

The mixer you will find the oscillation in one of circuit leaking out somewhere, find it with scope! Using a linear power supply and get hum means filtering circuit on the DJ boards are poor, improve them.

Cheers, Wizard

[Th3_uN1Qu3]

Re: A properly made power brick!

This SMPS wall wart is horrible especially for audio stuff. No smart IC and under volted.

No longer undervolted. And it doesn't have to be smart - it has to be clean.

Using a linear power supply and get hum means filtering circuit on the DJ boards are poor, improve them.

Or it means the transformer i had on hand was too small to handle the load (it was a 200mA trafo), and all the ripple got thru everything because the voltage dropped and the 7812 went out of regulation. The filtering is adequate in the mixer. With the switcher the only hum is picked up via the cables and it's a minute amount, and the whine only starts when i connect my soundcard, and it is very faint. It is never audible under normal conditions.

Connecting the rec out to the line input of my soundcard gives the following levels on the spectrum analyzer of Sound Forge: 50Hz at -102dB, 150Hz at -101dB, 15.631Hz at -96dB. Overall noise floor is -71dB peak, -85dB RMS. With a mike connected and the mike gain maxed noise floor is -62dB peak, -75dB RMS and for the most part this is whatever the mike picks up from the room. The 15.631Hz tone goes up to -90dB. Still nothing to worry about, and like i said, it can be easily notched out if any processing done to the recording happens to bring it up. And like i said, this tone appears ONLY from my soundcard (Audigy SE, unfortunately i fried the onboard Realtek HD a while back), and not with any other source.

I don't think it gets any cleaner than this on a piece of equipment where all I/O is unbalanced. The only thing is still needs is a little more oomph on the headphone amplifier - it is loud, but it distorts very easily on bass. And there's one more little thing - the PFL bus needs some tweaking. The master PFL switch needs a pad because it comes in too darn loud compared with everything else, and the PFL switches on the channels need to be rewired post-EQ. Otherwise, if i forget i flipped that bass cut switch, i've done the booboo, coz i can't hear it in the headphones, but everybody else can...

[Wizard]

Re: A properly made power brick!

15.6KHz noise needs to be found and fixed, not notched out. By the way 15.6 is very close to TV NTSC frequency (15.7KHz) or 625 line freq.

Cheers, Wizard

[Th3_uN1Qu3]

Re: A properly made power brick!

I don't have a TV in my room. Maybe there is one in my soundcard.