changing KM84 to 50Ohm or removing the pad?

[Wiz]

Been searching can't find the info...


Has anyone changed the impedance of their microphone from 150 Ohm to 50 Ohm

You can solder/unsolder connections on the transformer inside, to set it to either value.

Mics shipped new (mainly to Gotham Audio in the USA) in this configuration, will have a red dot next to the serial number.

On another discussion board, a very well seasoned engineer mentioned to me that he had a "aha" moment when coming across KM84s of 50 ohm input impedance back in the day. It was in reference to a discussion where I had mentioned that my KM184s were much much quieter than my KM84.

I am not über keen to do anything to my KM84 and can't really find any reference online to anything definitive to his topic. I have found the Neumann PDF on the KM84 and it shows the circuit and talks about how to set the impedance etc


cheers

Wiz

[Alastair Reynolds]

The question I would ask is why do you want to change it ?Do you have a specific preamp with a 30-50 ohm input impedance? In earlier days many preamps in consoles did have taps for 50 or 200 ohm for mics,then bridging 10K for line and sometimes a switchable 600 ohm termination. A moving coil dynamic like a SC 55 A Shure had a connection directly from the voice coil,the B had a 50 to 200 transformer and the C had a 50 to 100k transformer that allowed it to feed the grid of a tube directly. A 4021 STC (Ball &Biscuit) had an output impedance of about 30 ohms and would require a 50 ohm tap on the transformer.The early KM 84 may have been supplied tapped at 50 ohm to suit the commonly used mixers/preamps of the day so as to be interchangeable with existing dynamics.
Impedance matching and loading will change the sound.there is a good article in Recording mag online re SM57 loading.just Google "SM57 loading" and have a read.
Can anyone add any opinion to this ?

[rick]

Yes ill kick in
Leave you km series the hell alone internally
I have broken more of them then i care to own
Up to just having a bit of a looksee and a fiddle
Gunter told me most of the km repairs he has
Ever had were mics with nothing wrong except
Somebody opened the up for a look and
Busted something

That said i swapped the impedance around one day
On a km 83 expecting something remarkable
I have nothing to remark either way .

I have a bunch of broken kms that i figure i
Will pay somebody to fix one day
Fiddly great little mics

[Wiz]

Yes ill kick in
Leave you km series the hell alone internally
I have broken more of them then i care to own
Up to just having a bit of a looksee and a fiddle
Gunter told me most of the km repairs he has
Ever had were mics with nothing wrong except
Somebody opened the up for a look and
Busted something

That said i swapped the impedance around one day
On a km 83 expecting something remarkable
I have nothing to remark either way .

I have a bunch of broken kms that i figure i
Will pay somebody to fix one day
Fiddly great little mics

I looked at the little screws..... Then came to T Rock.....

Advice will be heeded

Thanks

Cheers


Wiz

[Wiz]

The question I would ask is why do you want to change it ?Do you have a specific preamp with a 30-50 ohm input impedance? In earlier days many preamps in consoles did have taps for 50 or 200 ohm for mics,then bridging 10K for line and sometimes a switchable 600 ohm termination. A moving coil dynamic like a SC 55 A Shure had a connection directly from the voice coil,the B had a 50 to 200 transformer and the C had a 50 to 100k transformer that allowed it to feed the grid of a tube directly. A 4021 STC (Ball &Biscuit) had an output impedance of about 30 ohms and would require a 50 ohm tap on the transformer.The early KM 84 may have been supplied tapped at 50 ohm to suit the commonly used mixers/preamps of the day so as to be interchangeable with existing dynamics.
Impedance matching and loading will change the sound.there is a good article in Recording mag online re SM57 loading.just Google "SM57 loading" and have a read.
Can anyone add any opinion to this ?

Thanks Alistair

[rob]

The mics output impedance is set by the strapping on its transformer.
you'll get 6dB more level out the mic set to 200 ohms into a modern bridging impedance preamp.
it won't change the signal to noise performance of the mic itself.
I've restrapped the output impedance of many older mics. It is no big deal just requires due care and the right tools.

[Wiz]

The mics output impedance is set by the strapping on its transformer.
you'll get 6dB more level out the mic set to 200 ohms into a modern bridging impedance preamp.
it won't change the signal to noise performance of the mic itself.
I've restrapped the output impedance of many older mics. It is no big deal just requires due care and the right tools.

Thanks Rob


Cheers

Wiz