Mixer phantom power hum.

[k brown]

Just for grins, I picked up a Phonic AM440 compact mixer on eBay (crazy amount of functionality in such a small package - except for not having EQ on the line inputs, pretty much same features as a Mackie 1202 in a package almost half the size); no power supply, but all it is is a 14-0-14vAC trafo, so I cobbled a supply from a Jameco transformer.

No, it's not one of the USB models.

Mixer works flawlessly and is dead quiet, except for considerable hum from the mic inputs when phantom power is on. The pres themselves are fine; silent when an external phantom supply is used, or with a battery electret mic, or a dynamic.

I figured the most likely cause would be bad caps in the phantom supply, but after changing the two output filter caps in the phantom supply, no change. Bad diodes?

Haven't found the schem online.

 

[JohnRoberts]

Years ago IIRC Phonic was the CM (contract manufacturer) for Yamaha, when Yamaha dropped them they just made their own versions.

Phantom power is pretty simple, a relatively clean DC fed to both mic pins 2 and pin 3 through equal value resistors.

For 48V phantom the two resistors are typically 6.81K.

What do you have?

JR

 

[Audio1Man]

Scope the +48VDC and it will have HUM and way below +48VDC

 

[k brown]

Years ago IIRC Phonic was the CM (contract manufacturer) for Yamaha, when Yamaha dropped them they just made their own versions.

Phantom power is pretty simple, a relatively clean DC fed to both mic pins 2 and pin 3 through equal value resistors.

For 48V phantom the two resistors are typically 6.81K.

What do you have?

JR

Haven't traced it out in detail, but the pres look essentially unchanged from the earlier MM1002, except for the switch to SMDs. The pres do differ in that the AM440 has no Mic/Line switch, the Line In is just the familiar pad to the mic pre.

This topo look familiar, John?

I always figured Phonics were sort of copies of Mackie, but looks like more a copy of Peavey! (except for the Mackie-like undersized cap in the gain control circuit).

 

[k brown]

In case the phantom circuit is also similar, here's the one from the Phonic MM1002.

I only replaced two caps, so maybe just my luck that third one is the culprit.

With no mics plugged in, the voltage at pins 2 and 3 measures 54.4v.

 

[JohnRoberts]

54 volts sounds OK... what does voltage look like with a phantom powered mic plugged in? I ASSume the hum is with a mic plugged in?

JR

 

[k brown]

Yes, only with mic plugged in. Volts with mic, ~47.4. Also a great deal of white noise along with the 60 cycle hum - again none of that with an electret mic or external phantom supply.

Thing is so cramped inside, I have to remove the group of four RCAs to get access to the two remaining phantom supply filter caps.

I'm about ready to stick a jack on this thing and supply phantom with 5 9v batts.

 

[JohnRoberts]

You could probably power the phantom with only one 9V battery to test.

Maybe with your VOM measure the resistance from pins 2 and pins 3 to pin 1. Resistances should be identical.

JR

 

[k brown]

You could probably power the phantom with only one 9V battery to test.

Maybe with your VOM measure the resistance from pins 2 and pins 3 to pin 1. Resistances should be identical.

JR

Thanks for your efforts, but looks like it's toast - now the final output has tons of hiss, whether phantom on or not; I think the whole power supply ia frapped.

I probably zapped something poking around while powered up : - {

Without a schem, I'm just dangerously flopping around in the dark . . .

 

[RuudNL]

FWIW: I had a Phonic Helix 24 mixer in the past. 16 inputs and you could record every input AND the stereo mix (so 18 outputs) simultaneous through firewire or USB. I used to work with Studer and Neve at the time, so I never mentioned the brand of this small mixer. But to be honest: the quality was really good and lots of functionality.