Sennheiser MD 421 trafo and non trafo version question

I have 2 MD 421 mics here.
Both grey , N types I believe .
One has a trafo on board with a rollof filter, other one is going from capsule straight to the connector.
Could that make any difference in sound ?
What's the purpose for this trafo?
the rollof switch is broken, so I want to bypass that thing.
O rcan I completely bypass the trafo and switch and connect it like the other one ?

What are the exact model numbers?

Some MD421s have different output impedances.

The HN or HL models have two optional output impedances.

I am currently trying to sort my HN version. They have a different pinout at the tuchel connector.

The transformer will affect the sound. It is there to step up the output level / output impedance to ~ 200 Ohms.

Yes, The MD-421 HN and HL version have two impedance choice on the connector. (HN= Hoch/Niedrich ?? in German and HL= High/low)

If I remember right, pin out for the Tuchel connector was: pin 3= Lo impedance (some hundred ohms), pin 1= "Hi impedance" (some kohms), pin 2=common for Hi and Lo, and the connector body/shield is ground.

So if it is a HN or HL, you can bypass the transformer and connect the real balanced Lo impedance capsule output to the connector.

--Bo

I own a HL / HN MD421 and first found out about this last night.

I read elsewhere that pin 2 is "common", ie. it is not ground. It is one terminal from the secondary of the transformer.

Pins 1 and 3 provide either ~ 200 Ohms or ~ 1.5 KOhms when used with common.

But where is ground? Ground is the shield on the Tuchel connector.

I have checked this - Pins 1, 2 and 3 are not connected to ground - I will need to make a special cable.

Does anyone know how to remove the bass roll-off thing?

Under the serial number plate, bend it up at one end and there's a "secret" screw in there.

...that's how it it on all of our (later model) Black ones, at least.

Keith

Ahhh...

Cheers Keef.

I never knew about that one.

I'm wondering if I should remove the transformer from the barrel that you pull out and rewire it as per a "normal" low impedance mic.

But I could just end up with a broken transformer and the remains of a perfectly good mic.

I wonder how many people buy this mic and think that the capsule is shagged by not knowing that it has a different pinout. It sounds crap if you use a normal cable (obviously).

The new ones are different. The "transformer" is mounted right at the capsule end. The body is a big empty chasm. Some have longer bodies, some shorter, some have cast pot metal liners inside the plastic of the case, others don't.

However, all the ones I've seen have the three terminals available, though two are "super-low", (a couple of Ω) and one is 'normal' low impledance. (say 300Ω) but wired in a comparable arrangement. I wonder if the capsule is similar in yours and the "second" transformer is 'bypassed' if you wire the low impedance output?

..and did they add the large lump of pot metal to some of them to keep the 'balance' of the mic similar to the ones with the transformer in them?

Questions... questions... imponderable questions!

Keith

t is possible to buy an XLR insert for the mics instead, there are wery cheap if you can live without the locut function.

All the capsules are the same -N -U -HL etc. only difference is output connector and impedance, but that all being done in the connector insert.

You've raised some good points there.

I wonder if you might be right about there being a second step up transformer to provide high-Z.

I have two original script-logo 421s. One is HN, the other is normal.

I will try and find the differences.

Both have an aluminium barrel which is drawn out as part of the bass filter. On the HN model, I can see a transformer at the end of this barrel.

I can't get the barrel on the normal version to come out far enough yet - I don't want to break the wires to the capsule!

The later versions of the MD421 are quite different. As you may well know, Sennheiser changed the design to make it cheaper to make as the original had lots of expensive parts.

Also, the earlier versions have a different voice-coil on the capsule. There are Aluminium voice-coils and Copper voice-coils.

As a guess, I would imagine the aluminium ones may have a better transient response.

I can't remember if it is the later ones that have the Aluminium coil or the earlier ones. Either way, they are definitely different.

.
I was wondering about this mic also. Checking around for another diaphragm. This MD421 is old script-logo style with 4pos M/S switch... all I could find for docs was Gyraf's site mech drawing. But impossible to tell scale from the doc.

Pin 1 is gnd on mine. Checked resistance across pins 2 and 3, and found it changes with the position of switch:
S = 50 ohm
- = 105
- = 128
- = 157
M = 218 ohm

More questions.
2. Rodabod, sounds like you might have sources I didn't find. Could you PM any helpful links?
3. What's proper technique to remove the windscreen section?
4. Anyone know the capsule type/size? I've emailed Transound about this also.

thanks, Paul
.

[quote author="pmroz"].
Checked resistance across pins 2 and 3, and found it changes with the position of switch:
S = 50 ohm
- = 105
- = 128
- = 157
M = 218 ohm

More questions.
2. Rodabod, sounds like you might have sources I didn't find. Could you PM any helpful links?
[/quote]

The bass-filter will affect impedance. As you switch to "S", the filter network will allow some signal to flow to earth. Measuring at the "M" position is fine - yours is fine.

I will try and find a link for a parts diagram and some other stuff. Can you PM me your email address - some of my documents are PDFs on my computer.

Roddy

all my MD421 have 3 wires coming from the capsule. red, blue, black.
I'm pretty sure the black one goes to ground, on all models.

need to find out how to connect the other two wires for "normal " operation on a xlr connector.

Use your multimeter to check that black is connected to earth.


So assuming that red is +ve and blue is -ve,

connect black to pin 1,

connect red to pin 2,

connect blue to pin 3.

Standard XLR.

Then check output impedance / resistance. It should be roughlt the same as your other MD421.

What model numbers are your MD421s?

Ok, after a long time searching the net, I have found these useful, "official" instructions:

http://edanders.home.comcast.net/diy/md421.htm

"INSTRUCTIONS FOR DISASSEMBLY AND RECONSTRUCTION OF MD421:

Remove serial tag by lifting tang [the lil nub on the tag] and sliding to the rear.


Remove flat head screw.


SLOWLY twist and pull out rolloff/connector assembly.


Remove 7mm nut inside housing. (dig through silicon seal material with nut driver [if necessary]).


"Work" the capsule/basket assembly out of the front of the housing.


Basket may be removed or replaced with 4 screws now exposed.


Assemble in reverse fashion except first knock out washer in housing (covered with sealant) and remove old sealant. Common silicone RTV sealant may be used to seal the hole where the capsule wiring passes through the housing. LET SEALANT CURE BEFORE MOUNTING ROLLOFF/CONNECTOR ASSEMBLY."

Quoted from this (great) site:

http://edanders.home.comcast.net/mics.htm

.
Thanks Rod! Great link, how come google didn't give it to me?
I'll PM you with my email if you have anything else.

Andre, if you're trying to connect one with a M|||S switch, here's how mine is connected:
If you take the screw out and remove the bottom barrel/trafo assembly, there is a circuit board. Rotate your view so the wires are along the top of the circle. The 2 here are wired from left to right: black, blue, red to give the output above in ohms (low impedance). No ohms from pin1 to 2 or 3, BTW.
It's been said that these mics vary, so don't know if it will help. Hope it does.

Is that scribble on Gyraf's drawing a Beyer trafo #? Looks like it.


.

[quote author="pmroz"].
Thanks Rod! Great link, how come google didn't give it to me?
I'll PM you with my email if you have anything else.
[/quote]

Google is funny, I can't remember how search engines work, but I don't get a hit even if I search for particular phrases on that site.

Eventually, I tried "Reslo shaver"(!) I don't know how I remembered that one.

Anyway, I have one more PDF. It is a Sennheiser pinout diagram for the variations of the MD421, eg. XLR, Tuchel, Gross Tuchel.

It might not be much use. If you want it, just PM me.

What was that you were saying about a Beyer trafo?

My models are the N type.
I discovered a neat trick to mount an xlr on the mic.
Take everything out the cylinder housing, glue the inner 3 pin plate of a Neutrik male connector inside the cylinder with two component stuff.
et voila.

Just as a reference for future Sennheiser HN / HL owners;

"The HN models allow you to get a both a high impedance output (1500 Ohms) and the standard 200Ohm by wiring them differently.

Pin 2 is common.
Pin 1 is + and 1500hms out.
Pin 3 is + and 200Ohms out.
Casing/body is ground/shield.

If I remember correctly, HN is german and stands for Hoch-Niedrich (High-Low). The US models were labeled HL. "

Quoted from some Swedish guy at PSW forums. I can confirm that this is correct.

[quote author="rodabod"]Just as a reference for future Sennheiser HN / HL owners;

"The HN models allow you to get a both a high impedance output (1500 Ohms) and the standard 200Ohm by wiring them differently.

Pin 2 is common.
Pin 1 is + and 1500hms out.
Pin 3 is + and 200Ohms out.
Casing/body is ground/shield.

If I remember correctly, HN is german and stands for Hoch-Niedrich (High-Low). The US models were labeled HL. "

Quoted from some Swedish guy at PSW forums. I can confirm that this is correct.[/quote]

A GDM121 / MD21 that came in with attached 'DIN-3' cable has this dual-imp as well, so the above info came in handy, thanks.

As it turned out the internal TX (likely a Beyer Dyn. BTW) has just a single winding, with the mic-capsule-signal entering the tap and going directly to pin#3. I was wondering whether the low-Z went thru-TX or not, so as it appeared it isn't.

I measured the Rdc of the capsule itself, with TX disconnected it's around 200 Ohms. With TX connected the Rdc between #2 & #3 is around 45 Ohms.

I'm not sure leaving the TX in will hurt much, but since I won't be using the Hi-Z output (#1) much, I figure can better remove that internal TX altogether. And if I need it I can put it in an external box.

Bye,

Peter

And now we have yet another mic here and as it happens it does even three impedances.

The mic is a Beyer Soundstar X1 HLM (so not the X1 N), so has High (25k), Medium (500) & Low (200) impedances and a small switch.

More here:
http://www.el-me-se.de/pdf_files/Beyer SOUNDSRAR X1.pdf (145kB, and yep, there's a typo in the link)

or this Beyer-catalog has more info as well, on page 3:
http://www.moshier.net/catalogues/Beyer_1969.zip (5MB)


OK, on now to my question.

For that we need this pic:



The Sennheiser that Roddy described in this thread will be the HL-pic. If you want 200 Ohms, I understand the internal TX could be skipped. Nice & easy.

The Beyer will be using the third diagram from the left: HLM.
(BTW, I think the 'M' & 'H' at the impedance switch in the pic will need to be swapped)

If I'm only interested in using this mic with a 200 Ohms source impedance (L, or N) and skip the other two settings (H, M) I figure I still will be needing the internal TX.
Since as I understand it, the 200 is derived from a tap on the internal TX.

This would also mean that the X1-HLN mic would be using a different capsule than the X1-N version:
say a 500 Ohms capsule i.s.o. the 200 Ohms capsule of the X1-N.

So finally the bunch of questions:
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 ?

Thanks!

Peter