AT4050 and OSA pre not getting along...

[wing]

Hey there,
I recently purchased an AT4050 mic and I've found that when I plug it in to my OSA pre-amps (in an API lunchbox) and apply 48v, it sounds great for about 5 seconds, and then a huge amount of white noise shows up and the mic itself becomes significantly quieter. Now, the thing is, I also tried plugging it in directly to my interface (MOTU 896HD) and another pre-amp, and I do not have the problem--it sounds great. But on the other hand, the pre-amps do not have this problem with any other mics or cables. To illustrate the chain:

1. AT4050 > OSA > MOTU = tons of white noise

2. AKG 414 > OSA > MOTU = clean

3. MXL V67g > OSA > MOTU = clean

4. SM57 > OSA > MOTU = clean

5. AT4050 > MOTU = clean

6. AT4050 > Bellari RP503 > MOTU = clean

I used the same cables in each experiment. So for some reason, the AT4050 doesn't seem to like the OSA preamps. Could it be an improper voltage issue? I'm mystified.

[antoniosolo]

Check the cable, I brought home an at4033se and with first cable there was noise and I though it was broken. Next cable everything was fine.

[Mudcloth]

Wow. Welcome back to TOMB, wing. It's been a while.

[wing]

Check the cable, I brought home an at4033se and with first cable there was noise and I though it was broken. Next cable everything was fine.

Well I tried other cables, and also the same cables that were fine in other experiments. So it's not that...

Wow. Welcome back to TOMB, wing. It's been a while.

Thanks man! I took a break from recording for quite some time to finish school and move, etc. Just recently got back into it, so I figured I'd stop by here from time to time.


...Anyway, does anyone else have any guesses about this strange happening?

[millzners]

The only thing I can think of is the phantom power. Maybe you could get a DMM and take a look at what voltage is actually being applied to the mic and then see if it's out of spec? Compare that number to the one from the box that works with the AT4050. It is possible the 414 is immune to an issue with the PP and the AT4050 is not.

If you don't have a DMM, maybe try a different PP source like from a mixer or another preamp. Maybe you could take it to a music store and try it out on different things to see if you get the same response. Anyway if you find the mic works with everything but that one pre, PP would be my prime suspect.

[wing]

The only thing I can think of is the phantom power. Maybe you could get a DMM and take a look at what voltage is actually being applied to the mic and then see if it's out of spec? Compare that number to the one from the box that works with the AT4050. It is possible the 414 is immune to an issue with the PP and the AT4050 is not.

If you don't have a DMM, maybe try a different PP source like from a mixer or another preamp. Maybe you could take it to a music store and try it out on different things to see if you get the same response. Anyway if you find the mic works with everything but that one pre, PP would be my prime suspect.

Well, I did use a DMM to test the phantom power on the rack, which is pin 15, and I was getting 48v, so the rack is fine. Perhaps the pre modules themselves aren't using the proper amount of phantom power? Does anyone know how I would test that?

If only I had another type of module in the rack (like an API 512c), that would help. It only happens on the OSA pres, any of my other pres elsewhere do not create this issue.

[The Scum]

...Anyway, does anyone else have any guesses about this strange happening?

Guesses? Sure, I'm full of guesses.

Off the top of my head: that recent solar storm has fouled something up.

Or maybe the recent discussion is Iranian nukes carries more water than previously suspected. They've started by enriching your mic cables.

More seriously: tell us about your power supply. You might also measure Volts AC on the phantom lines.

One thing you can try: plug in the mic cable, without a mic, and measure the phantom between pins 1 and 2 (and also 1 and 3) of the mic cable.

[wing]

You might also measure Volts AC on the phantom lines.

One thing you can try: plug in the mic cable, without a mic, and measure the phantom between pins 1 and 2 (and also 1 and 3) of the mic cable.

Okay, so I tried this, and I was getting a reading of 47.8v with phantom power. Is that normal? Should it be an even 48v, or is that number just usually rounded up?

[digitaldrummer]

47.8v -- that's probably close enough.

is it just one OSA or multiple? do you know which model they are? I have a few OSAs myself but not a 4050. I've used a 4033a and not had an issue though.

couple more things to check...

is your API lunchbox grounded properly? not using a ground lift plug on it, or a power strip it's plugged into, etc.?

if I recall there are a few filter caps on the OSA's +48 and maybe they are old and need replacing, but I would expect it would cause problems with the other mics too..

This is the only 500 series pre in the lunchbox (it is the 6B lunchbox right?). Does moving the preamp to a different slot make a difference?

have you compared the phantom voltage coming out of the MOTU?

tried another cable between the API lunchbox and MOTU?

[The Scum]

We need to verify that there's no noise intrinsic to the OSA's phantom power. Thus the questions:

What do you measure when you look for AC Voltage on the OSA input?

What power supply are you using for the OSA?

Is that normal? Should it be an even 48v, or is that number just usually rounded up?

It's close enough fo me, and probably as much a matter of meter resolution/accuracy as it is of PSU quality. But please look for AC on the line, also.

[wing]

is it just one OSA or multiple? do you know which model they are?

All of them, in various slots. I believe they are the OSA MP1c's.

is your API lunchbox grounded properly? not using a ground lift plug on it, or a power strip it's plugged into, etc.?

I'm not sure. I would think so. But I have not tried a different power strip (I'm using a rackmount power strip). Perhaps I'll try separate power altogether.

This is the only 500 series pre in the lunchbox (it is the 6B lunchbox right?). Does moving the preamp to a different slot make a difference?

Yeah, 6B lunchbox, and yes, they are all the same pre.

have you compared the phantom voltage coming out of the MOTU?

No, but I will...

tried another cable between the API lunchbox and MOTU?

Yes. I've tried various cables, always the same issue...

[Nate Dort]

I had a similar issue with a Tube Tech pre. Turns out that it was fitted with 68.1k phantom supply resistors from the factory instead of 6.81k. Certain mics were starved for current. With no load, the voltage measured fine.

[millzners]

I had a similar issue with a Tube Tech pre. Turns out that it was fitted with 68.1k phantom supply resistors from the factory instead of 6.81k. Certain mics were starved for current. With no load, the voltage measured fine.

Great point, without a load (mic plugged in) you may not get a clear idea of what is going on.

You could open up your pre and probe to see what the voltage on the backside of the XLR jack is when the mic is plugged in. Be careful as this means you'll be working around high voltages from the mains b/c the pre has to be on while you probe. If you're not comfortable doing it find a tech who is, or connect the mic to the pre with a combination of cables and adapters including a TRS male connector so you can unscrew it and touch the probes to the wires...