Sennheiser MD 421 trafo and non trafo version question

rodabod · Feb 14, 2007

[quote author="clintrubber"]
Does all this different capsule stuff make sense ?
Could I lose the internal TX and still get 200 ?
Or doesn't it really matter much if there's still an internal TX (either in the signal path or attached to the circuit but not transforming)
Who knows more ?
[/quote]

Hi Peter,

Is that picture above from Sennheiser or Beyer? Let's not get confused!

Why does the HLM digram only have a two-position switch? My memory is going again.... Does one switch position offer two impedances depending on which pins you use?

I was working (repairing) a Sennheiser HN microphone earlier today. The 200 Ohms on them is derived straight from the capsule.

I'm not sure if I agree about Beyer using different capsules - they usually use moving-coils which are 200 Ohms and do not use transformers. Therefore, I would guess the transformer would just provide 500 Ohms and Hi-Z as a step-up. As far as I know, there is no point in removing the transformer.

You can test this theory by opening up the mic and checking continuity between each output pin and each side of the capsule coil. If there is continuity then the transformer is being bypassed.

I don't understand the wiring description in the Beyer catalogue - as far as I knew, you just changed the switch on the mic... Wait a second... :idea: ... that's why mine and Hejsan's one hummed - the earth is via the tuchel barrel. I rewired mine.

I'll need to read through this again tomnorrow - I've had a few beers.

clintrubber · Feb 15, 2007

Thanks for the response Roddy :thumb:

[quote author="rodabod"]Is that picture above from Sennheiser or Beyer? Let's not get confused! [/quote]
It's a 'generic' pic I found in an older book about modest mixer-DIY. It does mention a DIN-number so some standarisation might exist here.
The second pic (HL) happens to agree with the wiring in my 'Grundig-MD21', but that could mean nothing w.r.t. the accuracy of the third pic of course.

Why does the HLM digram only have a two-position switch? My memory is going again.... Does one switch position offer two impedances depending on which pins you use?

It's exactly like you said, no case of Bad Brains yet (cool band, I guess many here will remember them).
For the HLM: pins 2+3 give 200, 2+1 give 500 or 25k (switched). 2 = ground.

I was working (repairing) a Sennheiser HN microphone earlier today. The 200 Ohms on them is derived straight from the capsule.

I'm not sure if I agree about Beyer using different capsules - they usually use moving-coils which are 200 Ohms and do not use transformers. Therefore, I would guess the transformer would just provide 500 Ohms and Hi-Z as a step-up.

It'd be nice if the capsule is simply 200 right away.

I was also wondering if there are sometimes more TXs internally, even when just doing a single impedance. I mean, a SM57 has one, but these German mics not, correct ?

As far as I know, there is no point in removing the transformer.

One could think of rather not having unnecessary stuff connected to the circuit, but I guess that as long as the TX won't be loaded it's a high impedance.

You can test this theory by opening up the mic and checking continuity between each output pin and each side of the capsule coil. If there is continuity then the transformer is being bypassed.

Don't understand this completely, unless we're talking separated TX-windings & tests with DC. (The few internal TXs I've seen so far were autoTXs, so DC will still get everywhere)

Not wanting to subject the mic to DC I put my LCR-meter to it. Didn't write down the values yet, I'll redo.
The mic became a speaker BTW, the 120Hz/1kHz was clearly audible and the non-thinking part of me got worried about damaging the mic.

I don't understand the wiring description in the Beyer catalogue - as far as I knew, you just changed the switch on the mic... Wait a second... :idea: ... that's why mine and Hejsan's one hummed - the earth is via the tuchel barrel. I rewired mine.

You touch an interesting question there: how is the hum-cancelling done?(say for a 421)
Another coil further inside close to the capsule ? Or in case of more than one impedance by means of the autoTX as well perhaps ?

I'll need to read through this again tomnorrow - I've had a few beers.

Don't neglect that task for a few old mics :thumb:
Let's find the truth about those various impedances here but take your time.

Cheers :guinness:

Peter

rodabod · Feb 15, 2007

[quote author="clintrubber"]
For the HLM: pins 2+3 give 200, 2+1 give 500 or 25k (switched). 2 = ground.[/quote]

When is pin 2 ground? It can't be if we are using 2+1 for 500 or 25K.

It'd be nice if the capsule is simply 200 right away.

I think this is likely.

I was also wondering if there are sometimes more TXs internally, even when just doing a single impedance. I mean, a SM57 has one, but these German mics not, correct ?

The Beyers generally skip the transformers. The SM57 coil must just not be in the 200 Ohms range, so it requires one. I would rather not have one - it doesn't have much purpose other than obviously impedance shifting and voltage step-up which is unneccesary if the coil is 200 Ohms in the first place.

Don't understand this completely, unless we're talking separated TX-windings & tests with DC. (The few internal TXs I've seen so far were autoTXs, so DC will still get everywhere)

What is an auto TX? What I was describing was attached a meter to one side of the output socket (ie. hot or cold) and then to the corresponding side of the capsule. This will not pass DC through the capsule since a circuit is not formed through it. I don't know how DC could pass via a transformer in this way....

Not wanting to subject the mic to DC I put my LCR-meter to it. Didn't write down the values yet, I'll redo.
The mic became a speaker BTW, the 120Hz/1kHz was clearly audible and the non-thinking part of me got worried about damaging the mic.

DMMs are generally safe in my experience with dynamic mics.

You touch an interesting question there: how is the hum-cancelling done?(say for a 421)
Another coil further inside close to the capsule ? Or in case of more than one impedance by means of the autoTX as well perhaps ?

Most likely a humbucking coil. I don't see how a transformer could help since the induced huim would not be common to both + and - since they would be opposite polarity on each side (and therefore would not cancel).

I need to read up about auto trafos I think....

Roddy

clintrubber · Feb 15, 2007

[quote author="rodabod"][quote author="clintrubber"]For the HLM: pins 2+3 give 200, 2+1 give 500 or 25k (switched). 2 = ground. [/quote]
When is pin 2 ground? It can't be if we are using 2+1 for 500 or 25K. [/quote]
As I think to understand it: the arrangement of the third pic (HLM) seems to make sense, pin 2 always joins the party & is ground each time. The drawback is that you can't have the more desirable floating + & - and a separate ground.
But 200 you have, be it that it's actually in an unbalanced fashion.

The Beyers generally skip the transformers.

That sounds good.

What is an auto TX?

I'd better left out potentially confusing terms like autoTX, sorry. I meant to indicate a transformer with just one winding.

So a ratio is made by means of taps on the very same piece of wire. With such a thing you still can both go up or down in signal-level. I understood this is called an auto-transformer.

You don't get the galvanic isolation of course, but that wouldn't be a problem for an 'impedance-TX' like in that second & third pic.
And for the same reason DC isn't blocked anymore.

DMMs are generally safe in my experience with dynamic mics.

Good to hear (says my non-thinking part).
BTW, the autoranging of my LCR-meter was very obvious to hear thru the mic-that-became-a-speaker (jumps in level).

Most likely a humbucking coil. I don't see how a transformer could help since the induced huim would not be common to both + and - since they would be opposite polarity on each side (and therefore would not cancel).

You're right, this is all compact circuitry.
I put myself on the wrong path by thinking of the humbucking of say an Alembic-bass (active: they simply put a PU without magnet somewhere in the body, subtract that from the signal & trim for least hum).[/quote]

OK, I better get measuring now.

Thanks,

Peter

clintrubber · Feb 16, 2007

[quote author="clintrubber"]OK, I better get measuring now.[/quote]

OK, measurements of the Beyer Soundstar X1 HLM.

*** impedance switch to the left ***

Code:

pins  'Z'	 R_DC R_1kHz

2-3	200	 38	206
2-1	500	 64	540	

1-3	 -	  33	112

*** impedance switch to the right ***

Code:

pins  'Z'    R_DC    R_1kHz

2-3	200	 38	   206
2-1	25k	 6.6k	 39.7k	

1-3	 -	  6.57k	35k

All units are Ohms. 'Z' is the stated impedance (spec).

I used a simple DMM for the R_DC measurements and
my LCR-meter for measuring R_1kHz (so I got the
'impedance' right away).

The combination of pins 1-3 isn't defined for
use but does something anyway of course.

Thoughts & observations:

schematic

So the third pic in one of the previous posts seems indeed
a valid representation of the situation for this mic.
As already stated in the picture, only the first drawing
is 'true sym.' The rest will do of course, but it's not the same.

Looking at the third pic I still don't understand yet how the
200 can be done without aid of the internal TX, but OK, if it
does 200 and it sounds OK then it sounds OK.

I was hoping to find more info & background w.r.t. DIN 45594 but didn't find much.

rewiring

So I intend to keep the internal wiring unmodified and make
a dedicated DIN-to-XLR-cable for this mic. So it'll be different from
the adapter-cables for 'regular 200-only DIN-equipped mics'.
The Beyer can be opened: the DIN-3 is easy to remove, but internal wiring
is short and digging further inside seems tricky.

So the intended cable for 200 Ohms:

Code:

DIN-3 (signal+)	to XLR-2 (signal+)
DIN-2 (ground)	 to XLR-3 (signal-)
DIN-edge (case)	to XLR-1 (ground)

Feel free to shoot!

Cheers,

Peter

rodabod · Feb 17, 2007

[quote author="clintrubber"]
As I think to understand it: the arrangement of the third pic (HLM) seems to make sense, pin 2 always joins the party & is ground each time. The drawback is that you can't have the more desirable floating + & - and a separate ground.
But 200 you have, be it that it's actually in an unbalanced fashion.
[/quote]

Hmmm..... I'm not convinced!

200 Ohms unbalanced? Never! I think Pin 2 is a common (-) and you need to use the tuchel barrel as a separate ground.

Otherwise the mic body will be floating and noisy, and it would be unbalanced.

I'm still not sure about the transformer passing DC - the secondary is isolated from the primary, is it not? Maybe I need to look more closely.

You're right, this is all compact circuitry.

Not that compact - the HB coil mimics the voice-coil and will be of similar dimaensions - often larger actually (eg. 3 or 4 cm). Same principle as a dummy guitar pickup.

So the third pic in one of the previous posts seems indeed
a valid representation of the situation for this mic.

But, how come moving the switch did not affect the 200-Ohm setting's impedance? The circuit is changed by the switch.

As already stated in the picture, only the first drawing
is 'true sym.'

I don't agree with this - the rest look symmetrical to me. Ground is via the tuchel barrel in most cases.

Pin 2 is not connected to the chassis as far as I can see, so I would not regard this a ground - am I incorrect?

I hope I am not going to cause confusion...

clintrubber · Feb 17, 2007

Hi Roddy, thanks for the RE :thumb:

[quote author="rodabod"][quote author="clintrubber"]
As I think to understand it: the arrangement of the third pic (HLM) seems to make sense, pin 2 always joins the party & is ground each time. The drawback is that you can't have the more desirable floating + & - and a separate ground.
But 200 you have, be it that it's actually in an unbalanced fashion.
[/quote]

Hmmm..... I'm not convinced!

200 Ohms unbalanced? Never! I think Pin 2 is a common (-) and you need to use the tuchel barrel as a separate ground.

Otherwise the mic body will be floating and noisy, and it would be unbalanced.[/quote]
You're right, whatever they called it & intended it for back then, we could of course still use it balanced. What do you think of my intended wiring below?

[quote author="clintrubber"]
rewiring
So I intend to keep the internal wiring unmodified and make a dedicated DIN-to-XLR-cable for this mic.

So the intended cable for 200 Ohms:

Code:

DIN-3 (signal+)	to XLR-2 (signal+)
DIN-2 (ground)	 to XLR-3 (signal-)
DIN-edge (case)	to XLR-1 (ground)

[/quote]

I'm still not sure about the transformer passing DC - the secondary is isolated from the primary, is it not? Maybe I need to look more closely.

No, not necessarily isolated. At least my Grundig GDM21 has a single winding impedance-TX. One winding with three connections. It follows exactly the second pic (HL).
And I expect a single winding-TX ('autoTX') for the Beyer as well. That assumption would be in line with my R_DC measurements & the third pic (HLM).
It'll be cheaper and the galvanic isolation doesn't seem necessary here.

So the third pic in one of the previous posts seems indeed
a valid representation of the situation for this mic.

Click to expand...

But, how come moving the switch did not affect the 200-Ohm setting's impedance? The circuit is changed by the switch.

The switch selects what is seen on DIN-pin #1: either 500 or 25k. The 200 appears on DIN-pin #3 so indep. of the switch-setting.

That third diagram also shows the 200 (#3) to be coming of a tap ot the TX, so suggesting some ratio between the capsule & the impedance as seen at the output (between 2 & 3).
But true or not, it IS resulting in 200 in some way and if I can wire it up and sounds fine then we're done. I guess it's just for some peace of mind that I rather have had no TX in-line, but OK.

As already stated in the picture, only the first drawing
is 'true sym.'

Click to expand...

I don't agree with this - the rest look symmetrical to me. Ground is via the tuchel barrel in most cases.

You're correct, the rest is (or at least could be used for) sym as well. Maybe we could call the first 'true & nice & neat sym' because the pic can nicely be mirrored and the second & third pic just 'sym' since they're not mirrorable when the TX is joining for a certain impedance setting.

Pin 2 is not connected to the chassis as far as I can see, so I would not regard this a ground - am I incorrect?

No you're correct. While it is 'common' for all HLM-impedances that still doesn't make it ground.

I hope I am not going to cause confusion...

No ! Thanks for sparring :thumb:

Cheers,

Peter

rodabod · Feb 17, 2007

Peter, it looks like we are in agreement, although I would personally refer to the mic chassis as "earth" or "ground". Would that be technically incorrect?

Anyway, I will pose this (perhaps stupid) question:

Generally speaking, what load does a transformer primary present when the secondary is unloaded? (how long is a piece of string....)

What would be happening here:

Have I shorted out the coil with the primary winding?....

Roddy

clintrubber · Feb 19, 2007

Oops Roddy, I missed your answer, sorry.

[quote author="rodabod"]Peter, it looks like we are in agreement, although I would personally refer to the mic chassis as "earth" or "ground". Would that be technically incorrect?[/quote]
I used 'case', didn't I? I'm sure that one was at least incorrect :thumb:
The rest sounds definitely better. From memory I guess the nicest descriptions would be 'signal ground' & 'chassis earth', but again, that's from memory.

Generally speaking, what load does a transformer primary present when the secondary is unloaded? (how long is a piece of string....)

For DC the R_DC will be there and for AC (so above some frequency other than 0 Hz) it'll be high since the TX is in principle transforming infinity Ohms with N*N so still infinity.
I'm not sure how non-idealities would spoil this pretty picture. I guess one would prefer to have not-needed stuff keeping its hands away from the signal-path, but I expect it won't hurt (too much).
Could imagine that it has an influence since it's at the beginning of the electrical signal chain, before amplification. Who knows more ?

What would be happening here:
http://img409.imageshack.us/img409/4518/hlmwiringkz0.png
Have I shorted out the coil with the primary winding?....

No, I think it'll be fine as long as the 500/25k output isn't used.

Bye,

Peter

clintrubber · Feb 19, 2007

FWIW, that book from which I scanned those connecting-diagrams
had some on auto-TXs as well (Fig.8a).

Interestingly, here the 200 Ohm (N) 'feeds' the auto-TX [b
and[/b] also goes straight though. This is unlike 'the third pic'
above but is in line with the 200 Ohm capsule that you expect
to be used.

The 'H' & 'M' switch can now simply be thought of a selector after the
auto-TX.

So both diagrams realize the same functionality, but this last one looks
more like it might be there inside that mic.

Bye,

Peter

clintrubber · Mar 13, 2008

- - - edit: info replaced - - -

rodabod · Mar 13, 2008

Who said that?

clintrubber · Mar 13, 2008

--- edit: message gone ! :shock: ---

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