Faulty P4 ?

[bluesnoz]

Hi Ted,

I recently purchased a P4 dual preamp. A really nice sounding little preamp but it has an issue with hum. I’ve tried a couple of mics (DPA 4061 and Heil PR35) and they both exhibit the same problem which I’ll try and describe in detail.

Without touching the microphone there is significant hum present, holding the mic or XLR connector case reduces this hum significantly and holding the P4 with the other hand simultaneously eliminates it completely. It’s like there is something missing with regard to earth connections. I wondered if the power supply sold with the unit was the correct one with regards to earthing ? Or if some earthing connection inside the P4 has come lose ?

It was rather annoying when I was recording recently, but I discovered a small piece of foil wrapped around the XLR casing and P4 case did the same trick as my hands eliminating the hum….but hardly a permanent solution !

Can you shed any light on this fault ?

I have some experience in electronics and don’t mind using a soldering iron…. if I know what I’m looking for !

Any help greatly appreciated.

David

[Ted Fletcher]

I’m checking this one out in the lab this morning….. It’s not a fault I’ve seen before, but I will try to get to the bottom of it!

[Ted Fletcher]

OK, I have checked a P4 in the lab using an SM58 and there is absolutely no hum under any conditions.

The most likely cause is that the ground of the PC board has become disconnected from the outer metal chassis.

Take the cover off….. by undoing the 5 screws on the back and the 2 screws at each end, this exposes the PC board.

There should be a wire link connecting the ground of the PC board to the body of the input gain pot on channel 1… and the fixing nut for this pot should be good and tight, and there should be a shakeproof washer under the nut.

Alternatively, if that is all correct, then there could be a discontinuity in the mic cable… the ground (pin 1) not being connected to the outer case of the XLR connectors.

Check all these and you should find the problem.

One other thought just struck me….. are you using the correct power supply?? This has to be 12VAC. If a DC power supply is used the P4 might just about work, but it would certainly hum, and phantom power would not work.

I think I have covered all the possibilities! The P4 is a very fine little mic amp…. I use one as a precision amplifier for acoustics measurements because of its extreme frequency response and low noise. Originally it was designed as a recording channel for ‘voice-overs’; the limiter compressor is perfect for good punchy voice sounds.

[bluesnoz]

Bingo !

Thanks Ted, problem solved.

Indeed when I looked inside the P4 there was no earth connection from the PCB to the gain pot on channel 1…. and no evidence there ever was one, I have the pictures to prove it 😯 Somehow this got missed in manufacture ??, my serial number is 112382 for your reference.

The difference after the fix is significant, no hum issues at all and just as clean as my Allen heath zed 10fx mixer. In fact P4 does sound slightly more open than the Zed, I’m very pleased.

Many thanks for your advice.

PS the powersupply is 12VAC so no problem there

[Ted Fletcher]

Glad we have fixed that one…..

That particular P4 was one of the first to be manufactured; it’s likely that the ground link was introduced as a modification after the first batch.

Anyway, glad it’s fixed!

[bluesnoz]

Hi Ted, me again with a new problem on the same P4 preamp.

Just got myself a pair of pencil condensers for recording guitar and tried them out with the P4. Unfortunately on applying phantom on the P4 with both mics plugged in there is a very loud low frequency hum…. This goes away when the phantom power is switched off. There is also very little hum with only one mic plugged in and phantom on….until the volume on the single mic is increased, then the hum returns temporarily.

I’m guessing that perhaps a capacitor on the phantom power regulation has dried out and can no longer deliver clean power over a few mA ? The mics (Thomann SC140’s) themselves are fine and have been used on a couple of other preamps with no problem.

Any advice appreciated ?

Cheers

[Ted Fletcher]

Hmmm…. yes, that is certainly a problem with the power supply circuit; we did have some current capacity problems with the earlier P4 units and there were some modifications…. changes to component values.

The problem might be cured by replacing C21…. originally this was a 47uF 100V capacitor. On later versions I increased this to 100uF or higher.

If you still have problems, send me the unit and I will fix it free of charge…; no guarantees as I don’t know the current demand of those particular mics. But I would only charge just the cost of returning it to you.

(If you do change that capacitor, it’s worth changing the regulator transistor Q4 (2N5550) as well.)

[bluesnoz]

Hi Ted, thanks for the reply. I’ve just had a look at my stocks and I don’t have the right components. If you’re still willing I’d like to take you up on your offer and post the P4 to you for an inspection and service if possible. Perhaps you can email me.

Cheers

David

[bluesnoz]

Hi Ted, thanks for repairing the P4, I’ve just been testing it and it seems to work a lot better now thanks.

Cheers

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