TLM103 Phantom Power Phenomenon

A number of years back I used one of my TLM 103s on a custom mic preamp built by a friend. It had an unusually very high current phantom power option and it made the 103 sound phenomenally huge and overall amazing! The only issue (big one at that) is that it caused a bucket of pure hiss!

We’ve always wanted to investigate the cause of the noise to see if we could get rid of it... Anyone have an inkling of where to start though? If successful, I could see building a dedicated, high-current phantom power supply!

Depends what you mean by a high current phantom supply. Phantom power is normally fed to the mic via a pair of 6K8 resistors. Even if these are shorted to ground, the maximum current that can be supplied by a 48V supply is about 14mA.Does not matter how much current the power supply could supply, the resistors prevent it being any more than this.

The only way to increase the current is to reduce the value of the resistors or increase the voltage. Either could damage your mic.

Any chance you can post a schematic of your friend's phantom power supply?

Cheers

ian

I doubt about a schematic, but I’ll ask exactly how much current we’re talking about here; I used to know, but it was too long ago now. He designed his Preamp with options for P48 (standard 6.8k, P48H (2.1k, David Josephson’s denied AES proposal for a new standard), and P48R (his own thing) with a rotary-switch.

Joshenson had warned us of damaging microphones back then as well, but we never broke any!

Most of the time, there never was much, if any, audible improvements between the phantom power versions with the various mics we tried, but to say the sound of the 103 improved with his P48R would be a vast understatement! That noise though! And who knows what kind of damage in the longer-term!

I am not sure what the circuit looks like from the first post

Is it two 2.1k resistors in place of the 6.8K resistors?

Do you have a link for the " David Josephson’s denied AES proposal" circuit?  I would like to read the proposal.
doing a search I found.
http://repforums.prosoundweb.com/index.php?topic=31314.0
http://repforums.prosoundweb.com/index.php?topic=21856.0

I don't like the idea of using two 2.1K resistors instead of the 6.8Ks

They will need to be matched better than 1%. I would want better than .1% match
The regulator circuit in the microphone will be dissipating more heat

I do not know what type of regulation is used inthe TLM103
http://mirror.thelifeofkenneth.com/lib/electronics_archive/Noise_Behaviour_of_Zener_Diodes.pdf

Also look up CMRR in differential  circuits you want the lowest matched drive resistance going to the highest match input resistance for good CMRR.

I can’t seen to find anything, but as I understand it, discussions swirled in the AES Standards Committee for some years after David Josephson did a presentation in 1995.

As for a newer link with Josephson addressing it himself:
https://www.gearslutz.com/board/remote-possibilities-in-acoustic-music-and-location-recording/1103834-rens-heijnis-mic-mods-2.html

By the way, apparently he says 2.2k there.

But really, regardless, I’m really interested in what might be the culprit component(s) that cause the mega bliss of hiss when hit a with a massive amount of current; something they obviously were not designed to handle and may damage over the course of time! But minus the hiss, the sound instantly put us looking at each other in absolute shock!

In synthesizer world noise is added to simple waveforms to add complexity, character, dirt, etc...

It acts as harmonic exciter/enhancer. It could be that the hiss made it sound bigger. And it might be the pre. Maybe it boosted/cutted somewhere and you liked it. I doubt it was anything about the phantom. Maybe higher/lower voltage?

Certainly not a higher/lower voltage. That was one of his big things with his P48R design; it was rock-solid at 48V no matter what. Even had an LED to show if it ever dropped below 48V.

I doubt the preamp itself only because we went through all the phantom power options; as we did with all the mics we tried. The P48R setting with the TLM 103 was the only combo that set itself apart, by far, from the rest.

However, I am open to the possibility of a dither-like noise-shape enhancement. While I heard a story once that the noise of a wood fire enhanced a listening experience by someone, I’d still suspect it to be a subtle to significant enhancement, compared to the massive difference we heard.

But maybe you’re on to something... Maybe it wasn’t the high-current phantom itself. What I mean is, maybe it was the component(s) reacting (along with the noise reaction) and replacing any component(s) with some designed for more current will not only get rid of the noise, but the phenomenal sound as well.

Our API desks 548 channels have an additional series filter resistor of 10k Ohm (and 100uF cap).
This means that certain mics  like the TLM103 aren't happy . So we then use an external mic amp I built based on the same API circuit, but with a 1k Ohm resistor.
I did consider changing all the channels for 1k, but I think this will drastically alter the sound we get! 

It could be as I posted something to do with the TLM103 internal regulator circuit.
I have a 103 that is on loan so if I get it back I will take some measurements and trace the circuit. Without a microphone and phantom supply schematics it is harder to figure out what you are hearing.

P48 has two 6.8K (hopefully well matched resistors)
One side of the resistors goes to 48VDC
the other sides go to pins 2 and 3

So with a 3mA current consumption https://en-de.neumann.com/tlm-103#technical-data
The following is with no added resistor in a RC filter after the 48VDC supply(see Walrus's post)
1.5mA x 6.8k = 10.2VDC (1/2 of the current to pins 2 and 3)
48VDC - 10.2 =37.8VDC at pins 2 and 3

DC power = voltage x current
37.8VDC x 3mA = .1134watts

If you change the 6.8ks to 2.2K. The current consumption will go up with lower phantom resistor values because there is less voltage drop across the phantom resistors and a higher DC pin 2 and 3 voltage

So the higher power will most likely be dissipated in the microphone regulator circuit.

Some microphones use the power from pins 2 and 3 to power a voltage regulator set lower than the voltage at pins 2 and 3 with the microphone powered up. Sometimes this regulated voltage powers a DC to DC converter for the capsule voltage.
The extra power is "burned up" in the regulator circuit.
Maybe the power supply noise goes up with more dissipation due to higher heat or current causing changes in the microphone power supply noise.

Or maybe a cap(s) voltage rating is being exceeded with the higher voltage at pins 2 and 3

Have you done this test from the shure web site http://www.shure.com/americas/support/find-an-answer/how-to-test-phantom-power-voltage-and-current

Maybe the phantom supply that you compared the P48H supply to can not supply enough current to the TLM103.

It could be something like a too high a value R in the RC filter in the phantom supply like Walrus posted. This resistor is in series with the phantom resistors. You can figure this resistor value out with the current and voltage measured at pins 2 and 3 with different resistor loading and using ohms law.

I would not like anything greater than 100 ohms as an extra  series resistance in the P48

The voice of reason

Gus said:

I would not like anything greater than 100 ohms as an extra  series resistance in the P48

Click to expand...

Double-blind ABX testing, anyone?  :

Walrus said:

I did consider changing all the channels for 1k, but I think this will drastically alter the sound we get! 

Click to expand...

Khron said:

Double-blind ABX testing, anyone?  :

Click to expand...

OK, drastic is probably exaggeration.  But with some mics less volts will lower headroom etc.

Why would a circuit that wants to see 48vDC draw any more or less current than its nominal current draw once it sees the 48vDC?

I can understand the benefit of a 48vDC power supply that does not sag beneath a 48vDC potential because it has extra current capability, but I do not understand how that would equate to a microphone circuit drawing excess current beyond that needed to maintain the 48vDC.

What am I missing?

Thank you.

I’m only guessing, but I’d imagine  some sort of implementation of a CV/CC supply... Often used in Industrial Design.

You’re right, mic and supply schematics would make it easiest. Actually, I don’t even have the 103s anymore, but as I’ve been building-up my mic-locker over the past few years with custom tube mics, I still haven’t gotten past that sound of that combo! So, I think my next project in the next couple months will be picking up a 103 again and getting with my friend with his preamp to test what the hell is going on... I could even get some samples to show exactly what I’m talking about. I just thought maybe you guys might have some insight before that happens.

By the way, I’m curious why you chose to build a API-style preamp with a different phantom supply rather than simply a dedicated phantom supply only?

Recording Engineer said:

By the way, I’m curious why you chose to build a API-style preamp with a different phantom supply rather than simply a dedicated phantom supply only?

Click to expand...

I was building it (them, I've built a 2 channel, 4 channel and an 8 channel now) for use in our non API rooms. One Neve VR and one SSL.

trans4funks1 said:

Why would a circuit that wants to see 48vDC draw any more or less current than its nominal current draw once it sees the 48vDC?

I can understand the benefit of a 48vDC power supply that does not sag beneath a 48vDC potential because it has extra current capability, but I do not understand how that would equate to a microphone circuit drawing excess current beyond that needed to maintain the 48vDC.

What am I missing?

Thank you.

Click to expand...

The 48VDC is connected to two 6.8K resistors. The other side of the resistor goes to pins 2 and 3. There is a voltage drop across the resistors.  The microphone is supplied with a lower voltage than 48VDC.

If the 6.8k resistors are reduced to 2.2k what happens at the microphone side?

Recording Engineer

Have you checked the phantom supplies using the Shure writeup?

Not aware of the Shure writeup... Which?

Been trying to talk to my friend about it, but we keep playing phone tag!

Link for the Shure phantom power test is reply 8

Oh yeah... Sorry, it had been a while since I had read that link and obviously didn’t remember it was a Shure link.

No, I don’t have access to the preamp at the moment. I still need to connect with my friend again.