RSM2 impedance

Advertise with us
Shop deals on ebay
shop deals on amazon
GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
A

abbey road d enfer

Well-known member
Staff member
GDIY Supporter
Moderator
Joined
Jan 22, 2008
Messages
17,077
Location
Marcelland
During a series of tests and evals (see Drawing board) I found that my Thomann-branded RSM2 has an impedance of 2200 ohms! the resonance peak measures 3.7k at 50Hz.
It is spec'd at 200...
It is obvious that this mic needs to see an high input impedance in order to operate properly.
Does anyone here has an RSM2 and could check its impedance?
 
I have several t.bone RB-500 mics from Thomann. One of them has a high ratio transformer (I computed about 1:80), the others have something like 1:27. One way to deal with the hi ratio models is to put in a P48 powered impedance converter. When did you buy your mic? Is it new?

I think these things happen because some people don't know the difference between DC resistance and impedance. The DC resistance of the hi ratio tranny is about 600 ohms. And in those Chinese catalogues they have 200 and 600 ohms versions...
 
Rossi said:
I think these things happen because some people don't know the difference between DC resistance and impedance. The DC resistance of the hi ratio tranny is about 600 ohms. And in those Chinese catalogues they have 200 and 600 ohms versions...
Click to expand...

Also remember the high DC resistance of the primary too though. That'll get reflected over along with the ribbon's impedance.
 
Actually, the dc resistance on the primary isn't too bad on the China ribbon trannies; IIRC the Lundahl and the Edcor ones had higher primary DCR. The dual bobbin low ratio China tranny, at least, is pretty good regarding noise.

What I do find weird is the typical 1.87:1 output tranny in Chinese condenser mics. IIRC their secondary DCR is already close to 200 ohms. And maybe that, too, is because they think impedance is the same as DCR.
 
Rossi said:
I have several t.bone RB-500 mics from Thomann.
Click to expand...
That's what I have. Internally, it's labelled RSM2, consistent with the Nady version.
One way to deal with the hi ratio models is to put in a P48 powered impedance converter.
Click to expand...
Exactly what I did, then...check http://www.groupdiy.com/index.php?topic=37546.0
When did you buy your mic? Is it new?
Click to expand...
About two years ago, new from Thomann.
I think these things happen because some people don't know the difference between DC resistance and impedance. The DC resistance of the hi ratio tranny is about 600 ohms. And in those Chinese catalogues they have 200 and 600 ohms versions...
Click to expand...
I've checked DCR of secondary at 155R.
 
abbey road d enfer said:
I've checked DCR of secondary at 155R.
[/quote

You're right, I misremembered the data. Here's a thread about this very ribbon transformer:

http://www.groupdiy.com/index.php?topic=18764.msg219015;topicseen#msg219015

I didn't know they still sold versions with that transformer. The orignial version had a dual bobbin 1:27; then they had this overwound one for a while, and the last ones I bought (in 2007, I think)  reverted to the 1:27 version.

I have no experience with the Fethead. Is it low noise with this transformer? It doesn't look like it is with low Z ribbons.

PRR once came up with a nice low noise P48 ribbon booster.
http://www.groupdiy.com/index.php?topic=18764.msg219015;topicseen#msg219015

This works very well. It is designed for low impedance ribbons, and is very low noise with the prescribed low Rbb transistors.
You might like a little higher input impedance for the overwound ribbon transformer, though.
Click to expand...
 
Rossi said:
I have no experience with the Fethead. Is it low noise with this transformer? It doesn't look like it is with low Z ribbons.
Click to expand...
As you may see in the other thread (on Drawing Board), I'm not really convinced by the FetHead's performance.
PRR once came up with a nice low noise P48 ribbon booster.
This works very well. It is designed for low impedance ribbons, and is very low noise with the prescribed low Rbb transistors.
Click to expand...
The link is incorrect; I'll try to Search it in the database.
You might like a little higher input impedance for the overwound ribbon transformer, though.
Click to expand...
It should not be a problem, since I'll probably need to adjust the operating current for optimum noise factor.
 
abbey road d enfer said:
During a series of tests and evals (see Drawing board) I found that my Thomann-branded RSM2 has an impedance of 2200 ohms! the resonance peak measures 3.7k at 50Hz.
It is spec'd at 200...
It is obvious that this mic needs to see an high input impedance in order to operate properly.
Does anyone here has an RSM2 and could check its impedance?
Click to expand...

I measured quite a few small EI core transformers from those mics and they were consistent @ ratio around of 1:55. The Pri DCR is 0.02 Ohm, and sec. 76 Ohm.
Pri L=1.52mH and Sec. L=4.3H.
The ribbon is pretty thick, so DCR=0.08 Ohm, for output impedance on 1KHz of 242 Ohm. The actual measurement with HP4274 confirms it.

First, I'd check the clamps--the usual suspect when you see such unusually high impedance. Often, just tightening bolts will do it. Also, the clamps might be contaminated/dirty... you call it... While you are on that check soldering joints... well you know the drill.

If you find the transformer faulty or of too high of the ratio let me know, I have here loads of those...

Best, M
 
Oh, Mark, I still have that STC 4038 transformer here if you ever are in the mood to look at it.

And a bunch of other transformers, half of which I can't remember where they came from...
 
Marik said:
I measured quite a few small EI core transformers from those mics and they were consistent @ ratio around of 1:55. The Pri DCR is 0.02 Ohm, and sec. 76 Ohm.
Pri L=1.52mH and Sec. L=4.3H.
The ribbon is pretty thick, so DCR=0.08 Ohm, for output impedance on 1KHz of 242 Ohm. The actual measurement with HP4274 confirms it.
Click to expand...
The one I have is really different; the secondary inductanec is ca. 40H! (I don't have an HP LC bridge at home, so I have to do with the generator and meter, but I'll check as soon as I go to the lab where I have a Racal-Dana LC bridge).
First, I'd check the clamps--the usual suspect when you see such unusually high impedance. Often, just tightening bolts will do it. Also, the clamps might be contaminated/dirty... you call it... While you are on that check soldering joints... well you know the drill.
Click to expand...
Thanks for the tips! Yes, that's what I'll do... I definitely think something's wrong here. [/quote]  If you find the transformer faulty or of too high of the ratio let me know, I have here loads of those...

Best, M
[/quote] Thanks a lot for the offer! I'll let you know...
 
rodabod said:
Oh, Mark, I still have that STC 4038 transformer here if you ever are in the mood to look at it.

And a bunch of other transformers, half of which I can't remember where they came from...
Click to expand...

Hey Roddy,

Thank you! Since then I actually serviced a couple of 4038, so I measured those in and out. But would love to play with ANY other transformers. Please let me know what you have.

Best, M
 
abbey road d enfer said:
Marik said:
I measured quite a few small EI core transformers from those mics and they were consistent @ ratio around of 1:55. The Pri DCR is 0.02 Ohm, and sec. 76 Ohm.
Pri L=1.52mH and Sec. L=4.3H.
The ribbon is pretty thick, so DCR=0.08 Ohm, for output impedance on 1KHz of 242 Ohm. The actual measurement with HP4274 confirms it.
Click to expand...
The one I have is really different; the secondary inductanec is ca. 40H! (I don't have an HP LC bridge at home, so I have to do with the generator and meter, but I'll check as soon as I go to the lab where I have a Racal-Dana LC bridge).
First, I'd check the clamps--the usual suspect when you see such unusually high impedance. Often, just tightening bolts will do it. Also, the clamps might be contaminated/dirty... you call it... While you are on that check soldering joints... well you know the drill.
Click to expand...
Thanks for the tips! Yes, that's what I'll do... I definitely think something's wrong here.
Click to expand...
 
If you find the transformer faulty or of too high of the ratio let me know, I have here loads of those...

Best, M
Click to expand...
Thanks a lot for the offer! I'll let you know...
Click to expand...

Something is definitely wrong there. To get 40H on that tiny core needs LOTS of Sec. turns, but then for the Sec they would need to use very light wire and get much more than 155 Ohm DCR. If the ribbon is 0.08 Ohm (as it supposed to be) then 2.2K gives us transformer ratio of 1:165, which seems unlikely, but then, who knows with those Chinese mics...

Could you measure that transformer, incl. ratio, as well as Pri inductance and DCR?

Also, if you could put the ribbon on a bridge (or use a DMM with Kelvin probes and nulling function, something like HP 3457A) that would also tell a lot, as for what's going on there.

Best, M
 
Marik said:
Could you measure that transformer, incl. ratio, as well as Pri inductance and DCR?
Click to expand...
I go to the lab this afternoon, I hope I'll have time to do that.
Also, if you could put the ribbon on a bridge (or use a DMM with Kelvin probes and nulling function, something like HP 3457A) that would also tell a lot, as for what's going on there.
Click to expand...
I'll try, but I'm not sure the Racal-Dana bridge has enough resolution to give a useful indication.
 
Ok, I have measured the transformer:
Primary L=4mH DCR <0.01ohm
Secondary L= 45H R=155ohms
So the ratio is actually a tad over 100, i.e. 40dB!
The annoying problem is that in spite of this high ratio, the output level is miserable and the source impedance so high it makes most of my preamps noisy (the ISA430MkII in particular - the 737SP fares a tad better as well as the SPL ProMike [INA163-based, 12k input Z!]).
I couldn't measure the ribbon, it's out of the resolution of the racal LCR bridge.
I reckon I could change the xfmr, it wouldn't change much the problem that the ribbon seems to be extremely weak. I checked the connections, they all seem to be unquestionabkle, but the magnets seem to be very weak. I have some extremely strong Nd magnets; I'll try and see if I can increase the gap induction.
As far as I'm concerned, this mic is a piece of useless crap; I don't care much, it's not expensive and I use it as a prop on promo shots.
Still I believe it's an interesting technical subject.

@Rossi: You have quite a collection of these mics; it may be interesting to compare them. I'll be at Frankfurt Pro Light & Sound 24th & 25th of March. Maybe we could try to meet there...? maybe silly, you may as well be at the opposite of the country...Germany's vast...
 
Thanks for your measurements. I didn't have an inductance meter at the time, so 1:80 was no more than an estimate based on gain loss with various preamps.

I'm not sure, though, 12k is correct for your preamp's input Z unless you measured it that high. Those chip preamps are usually pretty low Z, often 1.2k rather than 12k (printing error?). At usual mic preamp impedances of 1-2k, the hi-Z version sounds dull and due to impedance mismatch you also lose most of the tranny gain. But if there's nothing wrong with your ribbon motor, you should be able to get decent performance out of it with an active impedance converter that presents the tranny with 22k; I would actually aim for 50k or more.

I bought most of my RB-500s for an article I wrote about improving them.

http://www.soundandrecording.de/artikelsuche.php?Suchtext=pimp+my+ribbon&Rubrik=*&Ausgabe=value&Jahr=*&=suchen&Magazin=SR

To be honest,  I hardly use my RB-500s these days, as I now own a number of Beyer ribbons. They are nice mics for the money, though.

I will be at the Frankfurt Musikmesse, but I'll be working as a reporter, which means three days of constant talk and hustle between booths. If last year is any indication, I'll be hard to squeeze in more than a few minutes...
 
abbey road d enfer said:
Ok, I have measured the transformer:
Primary L=4mH DCR <0.01ohm
Secondary L= 45H R=155ohms
So the ratio is actually a tad over 100, i.e. 40dB!
The annoying problem is that in spite of this high ratio, the output level is miserable and the source impedance so high it makes most of my preamps noisy (the ISA430MkII in particular - the 737SP fares a tad better as well as the SPL ProMike [INA163-based, 12k input Z!]).
I couldn't measure the ribbon, it's out of the resolution of the racal LCR bridge.
I reckon I could change the xfmr, it wouldn't change much the problem that the ribbon seems to be extremely weak. I checked the connections, they all seem to be unquestionabkle, but the magnets seem to be very weak. I have some extremely strong Nd magnets; I'll try and see if I can increase the gap induction.
As far as I'm concerned, this mic is a piece of useless crap; I don't care much, it's not expensive and I use it as a prop on promo shots.
Still I believe it's an interesting technical subject.
Click to expand...

Abbey,

Is it a two bobbin transformer, or a small EI? In the last case I have very hard time understanding how they managed to squize those specs.

The magnets in those mics are Neodymiums and should give about 35000Gauss (0.35T). The only way for Nd to lose magnetics is to heat it. I highly doubt the mic would withstand it without showing the traces... unless they installed them this way.

You say it gives very low output, do you have some other ribbons to compare to? Obviously, something going on there. If you get stuck with this mic, would you mind sending it here for me to check it? You really picked my interest with this one!

Best, M
 
Rossi said:
I'm not sure, though, 12k is correct for your preamp's input Z unless you measured it that high.
Click to expand...
Yes, this SPL pre measures at 12k,  no typo here...
  At usual mic preamp impedances of 1-2k, the hi-Z version sounds dull and due to impedance mismatch you also lose most of the tranny gain.
Click to expand...
Yes, that's what happens with most of my pres (430MkII, 737SP, Classic Channel,...)
But if there's nothing wrong with your ribbon motor, you should be able to get decent performance out of it with an active impedance converter that presents the tranny with 22k; I would actually aim for 50k or more.
Click to expand...
The sonic quality is actually significantly better with FetHead (22k input Z), but still noise is an issue.
I bought most of my RB-500s for an article I wrote about improving them.
http://www.soundandrecording.de/artikelsuche.php?Suchtext=pimp+my+ribbon&Rubrik=*&Ausgabe=value&Jahr=*&=suchen&Magazin=SR
Click to expand...
I'll have  a look, but I'm afraid my German is terrible, I can only read the pictures  ;)
To be honest,  I hardly use my RB-500s these days, as I now own a number of Beyer ribbons. They are nice mics for the money, though.
Click to expand...
I have an M160, which I like a lot (in particular on violin) but don't use so often (because I don't have many sessions with violin). I like Beyer's anyway; M88 is my favorite on e-guitar and Leslie.
I will be at the Frankfurt Musikmesse, but I'll be working as a reporter, which means three days of constant talk and hustle between booths. If last year is any indication, I'll be hard to squeeze in more than a few minutes...
Click to expand...
I know what it is, I've exhibited at AES, APRS, NSCA, MusikMesse for 20 years. I still have a booth there but I just go to meetings.
 

Similar threads

Adam Frandsen
New speaker design by NOOB
6 7 8
Replies
140
Views
8K
JohnRoberts
JohnRoberts
B
Design of a sound card "Front end" for audio test set
2
Replies
30
Views
973
thor.zmt
thor.zmt
trobbins
Vintage Mic Transformer for MC step-up - gain-phase and impedance measurements
5 6 7
Replies
130
Views
7K
Cqwet Dbdfte
Cqwet Dbdfte
Rusan
OPA Alice Failure
2
Replies
34
Views
3K
JeffKall
JeffKall
capnrefsmmat
Charge amplifier for piezoelectric sensors
Replies
10
Views
2K
abbey road d enfer
A

Latest posts

Back
Top