Ringing in preamp circuit - PM1000

mpedrummer · Post by **mpedrummer** » Mon May 11, 2009 5:01 pm

Hey guys -

Recently bought some racked PM1000s from someone on the board, and either they've started a new behavior or I'm just finally noticing - there's a high-pitched noise coming out of them, even if the mic gain is zero, even if there's no mic attached, with or without phantom power turned on.

I hooked it up to a scope - the loudest tone is at ~2230Hz, with clear spikes at 4475, 6705, and 8950, or more likely, actual overtones and I'm mushing the exact center of the spikes.

It seems to be almost entirely independent of the gain - in fact, as I turn the gain up, the spikes lessen somewhat.

From my limited knowledge and googling - I'm guessing it's a capacitor problem of some sort. Any thoughts? Where should I start and what should I look for?

Thanks
Tim

The Scum · Post by **The Scum** » Mon May 11, 2009 9:58 pm

ELIZA mode again: Tell me about your power supply.

And what tool are you using to measure this noise? Are the spikes all of similar amplitude? Are you sure there's nothing at ~1115 Hz?

mpedrummer · Post by **mpedrummer** » Tue May 12, 2009 7:27 am

I'm using Schope, running at 24/96. Not exactly a real oscilloscope, but gets me in the ballpark.

As you can see, the spike at 2230 is significantly louder than the others.

There is no spike at 1115. Here's a zoom in on the lows, you can see that - http://hotelmorrell.com/studio/PM1000%20Issues/Low.png

What's ELIZA mode?

The power supply is certainly not stellar. The pres are racked in a router chassis, and the power supply is the brick that came with the router! I have no idea what happens once it's inside the unit, but the brick provides 48V and 0.38A.

Tim

The Scum · Post by **The Scum** » Tue May 12, 2009 7:56 am

What's ELIZA mode?

You mean you haven't met Eliza?

>> Hello.
How do you do. Please state your problem.
>> my pm1000 makes noise
Your pm1000 makes noise ?
>> yes. 2250 hz.
You seem to be quite positive.
>> I know it's there.
You say you know it's there ?
>> yes.
You are sure.
>> absolutely.
Earlier you said your pm1000 makes noise .
>> I did indeed.
Can you elaborate on that ?
>> it looks like power supply spikes.
I'm not sure I understand you fully.
>> the switching power supply is a piece of garbage
Please go on.
>> it makes noise in the circuit.
What does that suggest to you ?
>> I need a new power supply.
What would it mean to you if you got a new power supply ?
>> no more switching noise.
Are you saying no just to be negative?
>> but it's a positive supply
Do you feel strongly about discussing such things ?

Let me guess, the power supply is very light...no real transformer inside? Then it's a switcher. And it'll induce switching noise in the attached circuits.

Do you have a different supply you can try instead?

mpedrummer · Post by **mpedrummer** » Tue May 12, 2009 8:07 am

Yeah, it's a power supply for a router - not exactly high-end gear.

I know this is a stupid question, but is there a such thing as a "quality" wallwart?

I'll have to go through the Great Box o' Warts tonight and see what else I can try.

honkyjonk · Post by **honkyjonk** » Tue May 12, 2009 4:23 pm

I don't know how many 44V wall warts there are. You need a regulated linear supply, probably a 48V supply that you can modify for 44.

If you PM 'phantom power' here, I believe he has used a Power One 48V supply for the PM1K in this fashion. Power One's are generally really cheap off ebay. Like probably 25 bucks or so for a 48V regulated linear supply.

Or you can get the Five Fish supply kit that is designed specifically for people racking PM1K's.

calaverasgrandes · Post by **calaverasgrandes** » Wed May 13, 2009 9:31 am

I got one of the 5 fish power supplys. It is super easy to build. Some of the best kit directions I have ever seen. Really!
IIRC its about $50 with the transformer and all. You can buy just the PCB and source the parts yourself to save a bit. I opted to buy the full kit becuase I already have my hands full of other projects. The fivefish PS also has 48V on a separate tap for the phantom. Which is a way better implementation than some I have heard of, like running the PM1000's off 48V. (which is a bad idea since lots of the components are 50v max!)
yeah, a computer/router PS like that is always gonna have objectionable noise products. They do regulation on the boards themselves, and even then, SNR isnt terribly crucial in a digital circuit. Esp when operation is in the mhz band. some odd tones in the khz range dont mean diddly.

digitaldrummer · Post by **digitaldrummer** » Wed May 13, 2009 9:52 am

the Power-One and the FiveFish are great supplies but the cheap p/s should work too. One thing I have discovered recently is that by modifying the phantom power circuit I was using I was able to reduce that ring or spike (which I have also seen). The original input modules put 48V through a 3.3K resistor to the center tap on the input transformer. I started changing this to use a "traditional" phantom circuit by using two 6.8K resistors directly into the input XLR. then I used a resistor and cap back at the +48V power source. I found that when I moved the cap closer to the input XLR, the whine or ring was greatly reduced. I was seeing it around 2Khz but was also building with the master/talkback modules instead of the input modules. anyway, now I put the 6.8K resistors into the XLR inputs (pins 2 & 3), then I tied them together and run to the +48v source. I also put a 47uF or larger cap from the ground (pin 1 on the xlr) to where I join the two 6.8K res together - sorry no picture of this right now - hopefully this makes sense. Even when phantom power was not engaged I saw the spike before, but now its much quieter on or off. I think the wire supplying the +48 was acting as an antenna. so hope this helps.

Mike

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Ringing in preamp circuit - PM1000

Ringing in preamp circuit - PM1000

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