dbelousov

Microphone preamp impedance and "phantom" resistors
« on: January 08, 2021, 03:57:39 AM »
It seems that a couple of respected high fidelity mic preamp manufacturers don't care about "2k standard" at the level, that they switch P48 on and off lifting supplying resistors.

https://earthworksaudio.com/products/preamps/zdt/zdt-1024/
Quote
Input Impedance: 10KΩ phantom on, 100KΩ phantom off

http://www.gordonaudio.com/specs.htm
Quote
Impedance, differential-mode    2M ohms
Phantom    13.6k ohms

Can it strike back? Some resonances of mic transformers and coils of the dynamics, etc.
Dmitry


ruffrecords

Re: Microphone preamp impedance and "phantom" resistors
« Reply #1 on: January 08, 2021, 08:55:00 AM »
In their defence, to provide phantom power you have to stick a pair of 6K8 resistors in series across the mic input so the input impedance cannot be greater than 13K6. There is absolutely no reason why a mic with a source impedance of up to 150 ohms would need to look into a load any higher than this.

On the other hand, specs tend to be writing by the marketing department who only want to make their equipment appear better than their competitor's no matter how economical they are with the truth. For example the earthworks quotes the following:

Distortion less than 0.0001% without mentioning a load, a signal level or a frequency range all of which make is meaningless.

EIN at 60dB gain -143dBV again without stating the conditions under which this is achieved. With the input shorted, using an average reading meter and a limited bandwidth this might just be possible - but with the input shorted you cannot use it. To put this into context, the noise from a 150 ohm resistor is about -131dBu. By quoting their EIN in dBV they get to add 2.2 to the answer for free so a perfectly noise free amplifier cannot possibly have an EIN better than -133.2dBV.

The Gordon spec is a lot more honest. They state the measurement conditions at the outset and even provide a gragh of EIN versus gain which shows it is at best about 129dBu.

Basically, take manufacturers spec with a pinch of salt.

Cheers

Ian
« Last Edit: January 08, 2021, 08:59:18 AM by ruffrecords »
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

abbey road d enfer

Re: Microphone preamp impedance and "phantom" resistors
« Reply #2 on: January 08, 2021, 08:55:22 AM »
Can it strike back? Some resonances of mic transformers and coils of the dynamics, etc.
One day someone mentioned a particular microphone that went into motor-boating when insufficiently loaded.
For the rest, I have always noted that the higher the impedance the better, although past a certain limit (generally about 5k) there is not much difference.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

EmRR

Re: Microphone preamp impedance and "phantom" resistors
« Reply #3 on: January 08, 2021, 11:21:09 AM »
This is for ribbon mics. 

I wouldn’t call 2K a standard really, other than in a fairly small window of time.  There’ve been preamps about twice as long as there’s been a phantom power standard, and early transformer couple unloaded tube grid preamps have wildly varying input impedance curves with frequency regions much higher than 2k, I’ve seen some over 20k, lots in the 5k-10k range.  Many of these new options are chasing some of the unloaded ribbon sound the older technologies gave, albeit very differently in outcome.   Many are removing the input coupling caps along with the phantom resistors, offering “less in the path”.   
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

dbelousov

Re: Microphone preamp impedance and "phantom" resistors
« Reply #4 on: January 08, 2021, 11:53:27 AM »
Basically, take manufacturers spec with a pinch of salt.

I agree that Earthwork specs look meaningless. About Gordon, I saw a forum post from the man behind it, his name is Grant Carpenter, as I remember. He wrote that specs are so easily manipulated for the sake of marketing, that he publically announces only part of them, but he is always ready to discuss them privately. I understand his position. If you can't beat the liar, just don't play his game.

One day someone mentioned a particular microphone that went into motor-boating when insufficiently loaded.
For the rest, I have always noted that the higher the impedance the better, although past a certain limit (generally about 5k) there is not much difference.

I found that "modern classic" Chinese cathode followers sound rather dull into 1.8-2k. 3.5-4k is fine for them. 4-5k looks like a sweet spot for a universal mic preamp for me.
Dmitry

ruffrecords

Re: Microphone preamp impedance and "phantom" resistors
« Reply #5 on: January 08, 2021, 04:06:33 PM »
I found that "modern classic" Chinese cathode followers sound rather dull into 1.8-2k. 3.5-4k is fine for them. 4-5k looks like a sweet spot for a universal mic preamp for me.

Which can presumably be backed up by some free field frequency response curves into different load impedances. Should not be hard to do. Just feed some white noise into a speaker, stick the mic a metre away and take an FFT of the output signal with different loads. Most DAWs can create white noise and FFT and if not there is always the free software REW to do it for you. I look forward to someone producing some objective measurements.

If this turns out to confirm the dullness of these Chinese mics then it would be worth while reverse engineering them to find out why?

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

abbey road d enfer

Re: Microphone preamp impedance and "phantom" resistors
« Reply #6 on: January 08, 2021, 04:58:52 PM »
If this turns out to confirm the dullness of these Chinese mics then it would be worth while reverse engineering them to find out why?
Although I confess knowing close to nothing about cheap chinese mics, I would not be too surprized to learn that their transformers are not of the highest quality. In particular, higher leakage inductance often results in loss of HF, that a higher load impedance can help with.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

dbelousov

Re: Microphone preamp impedance and "phantom" resistors
« Reply #7 on: January 15, 2021, 10:59:32 AM »
Ok, there is a test record. Mics are a couple of CF. The first clip was recorded into ~10K impedance for both mics to make clear, that they sound similar. For the second clip, I reconnected the second mic into the same preamp with ~1,8K impedance, the first one was still in 10K. The difference is very audible to me.

https://yadi.sk/d/p7yJemnmgzX5tw
Dmitry

abbey road d enfer

Re: Microphone preamp impedance and "phantom" resistors
« Reply #8 on: January 15, 2021, 11:19:21 AM »
The difference is very audible to me.
No one can deny there's a difference, and it can be backed up with objective measurements.
Most microphones (except those with xfmr-less outputs) are inductive by nature. The more they're loaded, the less treble. In addition, decreasing the load (increasing the load impedance) reduces distortion.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

dbelousov

Re: Microphone preamp impedance and "phantom" resistors
« Reply #9 on: January 15, 2021, 11:29:00 AM »
And files from REW. Conditions were not perfect, the mic was between my screen, rack, and console, hence lots of reflections. But we are interested in the relative result, so I don't think it matters that much.

https://yadi.sk/d/c2uaiaWQl3ZY1Q
Dmitry


abbey road d enfer

Re: Microphone preamp impedance and "phantom" resistors
« Reply #10 on: January 15, 2021, 12:15:08 PM »
And files from REW.
I don't see any noticeable differences... :o
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

dbelousov

Re: Microphone preamp impedance and "phantom" resistors
« Reply #11 on: January 15, 2021, 12:49:07 PM »
Me neither. But they don't sound the same in a real application.

I found this when I tested one of my mic preamps against another one. Both were clean "wire-with-gain" types, but one of them was an early prototype without P48. So I used tube mics for the initial "real" test, but then I found, that the difference was too large, it was impossible for those two circuits to produce so different sound. After some analysis and experiments, I found that the input impedance was the key.

Alas, I have no equipment to make some more complex measurements for mics.
« Last Edit: January 18, 2021, 10:13:24 AM by dbelousov »
Dmitry

Gus

Re: Microphone preamp impedance and "phantom" resistors
« Reply #12 on: January 16, 2021, 09:18:02 AM »
Higher matched input resistance should help CMRR

EmRR

Re: Microphone preamp impedance and "phantom" resistors
« Reply #13 on: January 16, 2021, 11:07:38 AM »
I restored some early 70’s preamps recently that didn’t have phantom power, but did have primary center tap brought out to the connector, and a secondary load resistor.  When I considered phantom resistor loading, it dropped input impedance about 10-15% if I recall and it was already on the low side, so the option was to change the secondary load resistor or feed phantom through the center tap. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

aazaa

Re: Microphone preamp impedance and "phantom" resistors
« Reply #14 on: January 18, 2021, 08:25:30 AM »
Good idea to feed phantom through the center tap!

ruffrecords

Re: Microphone preamp impedance and "phantom" resistors
« Reply #15 on: January 18, 2021, 12:53:35 PM »
Good idea to feed phantom through the center tap!
Not always. It means dc current will flow in the primary windings which some mic input transformer manufacturers say you should not do - presumably because the transformer is not designed to take it. Usually the two windings can be designed to be balanced such that the magnetic field caused by the dc in each half cancels. If the transformer is not designed to do this the unbalance can cause the core to saturate much earlier than you would want.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

EmRR

Re: Microphone preamp impedance and "phantom" resistors
« Reply #16 on: January 18, 2021, 01:04:36 PM »
It is in the phantom spec - If a center tapped transformer isn't designed properly to handle it, it's not an in-spec design, and also not a hum-bucking transformer with the hum advantages that brings.  Most older things with center taps are designed to do this.    Go all the way back to double button carbon mics, several hundred mA of current flowing! 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

aazaa

Re: Microphone preamp impedance and "phantom" resistors
« Reply #17 on: February 22, 2021, 03:55:55 AM »
I powered the phantom of my pp UTC Tamura preamp by using the center tap of the UTC LS12, as suggested by EmRR. It works perfectly.
Thanks for the tip, Doug!

Paul
« Last Edit: February 22, 2021, 05:25:55 AM by aazaa »

aazaa

Re: Microphone preamp impedance and "phantom" resistors
« Reply #18 on: February 23, 2021, 08:28:21 AM »
I tested 3 Shure sm81 microphones  in my preamp to know what phantom powering trough the center tap connection of the primary does in reality with DC unbalance.

UTC LS transformers have a very good reputation.
Nevertheless measurements of Rdc show little differences in both halves of the primary. Is this a problem when feeding phantom trough the center tap?

UTC LS12, Transformer 1:
Rdc primary: 18.55 Ohm + 17.98 Ohm=36.53 Ohm.
Unbalance caused by connection of the 3 microphones measured 0.0004V / 0.0003V / 0.0003V respectively.
0.0004V/36.53=0.00001095A
0.0004x0.00001095=0.00000000438VA
With the least performing microphone DC unbalance has a power of 0.00000000438VA to cause magnetization of the core.

Transformer 2 was measuring less difference in Rdc but surprisingly it performed slightly less with one of the microphones. The bizarre result of combination of unbalances in microphone and transformer. Here unbalance has a power of  0.00000000679VA to cause unwanted core magnetization.

Transformer 2 was performing better with both other microphones.
Measurement showed 0.0000V, so 0.0000?V, values beyond the reach of my meter.
The result is a power to magnetize<0.000000001VA.


If these values are causing fear, then don't do it! Fear never is a good companion.
But even then, it is good to know that for the slightest deviation of the 6k8 resistors or unbalance in the microphone, current will correct this by choosing the easiest path, the path with the least resistance:
It will flow partially through the primary of the transformer.


Paul

« Last Edit: February 23, 2021, 11:23:40 AM by aazaa »

abbey road d enfer

Re: Microphone preamp impedance and "phantom" resistors
« Reply #19 on: February 23, 2021, 01:59:49 PM »
I tested 3 Shure sm81 microphones  in my preamp to know what phantom powering trough the center tap connection of the primary does in reality with DC unbalance.

UTC LS transformers have a very good reputation.
Nevertheless measurements of Rdc show little differences in both halves of the primary. Is this a problem when feeding phantom trough the center tap?
DC magnetization is caused by different turn number, not DC resistance differences. It is perfectly normal for windings to have slightly different DCR even when the turn number is the same. Different average diameter.
However, it may result in some differential DC voltage, which the xfmr in the mic may not like.

Quote
With the least performing microphone DC unbalance has a power of 0.00000000438VA to cause magnetization of the core.
That's probably less than the earth magnetic field.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

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