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Got a noisy AKG C451E. Condenser Microphones
Old 12th October 2011
Gear Addict

Thread Starter
Got a noisy AKG C451E.

Hey guys,

I've got an old C451E that's getting a little hissy and crumbly. When my MXL did this, I just replaced every conceivable part I could and it came out sounding pretty good.

Anyway, I opened the C451E, expecting to see a million surface mount components, only to be surprised at how many through hole components they managed to pack into that little tube. Not to mention the microscopic wires coming off of the tiny transformer.

I managed to download a repair manual for it and I was wondering, are these hiss/crumbles cause by worn out caps? If not, what should I be replacing?

Old 13th October 2011
Lives for gear
Les's Avatar

We make a similar SDC mic.

Hissing, popping and bacon frying sounds are usually moisture/dirt related.
Electrolytic capacitors are suspect too as older ones had mean lifetimes of 1000 hrs or so. Their failure usually does not result in noise though.

If you could pm a schematic i can have a look.

Or drop it by. I'm just up the road a ways.
Old 13th October 2011
Lives for gear

I'm not going to recommend the oven technique which used to be a common fix for the 451, but you might try putting the capsule in a mic bag with some silica gel.

as these mics age, the electronic parts are going to break down, and they are not repair-friendly. At CB,I we had piles of 451s and 460s, and when something went wrong with them the techs whined a lot.
Old 13th October 2011
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Thread Starter
Old 13th October 2011
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Les's Avatar

Got it.

Pretty standard source follower/emitter follower/transformer circuit.

The electrolytics, particularly C9, are always supect on old gear due to short lifetimes. The FET uses voltage divider bias so if C9 was weak there could be some phantom supply noise transferred to the signal output.

At least it's bypassed with C8.

Most likely is moisture absorbing dirt around the capsule insulator and
FET T1 and it's bias resistor R2 as well as capsule bias resistor R1.

Since those components are mounted on the PCB dirt on the board is suspect as well. For that reason we fly those components...i.e. The high impedance leads are not soldered to the board.

the silica gel thing is fine, but careful cleaning of the components and board mentioned with denatured alcohol might be required.

This early leaded component version looks not so hard to work on unlike later surface mount.

Old 3rd November 2011
Gear Addict

Thread Starter
Thanks for the info. I'll try cleaning the board. Probably with a j/k I'll find something to clean it with and see if that helps.

Would a hair dryer be something I could use after the alcohol cleaning, to remove moisture?
Old 23rd January 2012
Gear interested

Hi, I have a pair of AKG C 451 E for sale. They are used but in best condition. They are in original AKG case and with pair of A 51 adaptor. The price of botha are 400 euro. If you are interested write me on [email protected]
Old 14th April 2018
Here for the gear

NOISY C451E C451EB. Correct dismantling method. I spent some time working out how to dismantle. A previous owner had damaged XLR socket by incorrectly dismantling.
Preamp PCB and XLR socket are held in place by a snapring which locates in a machined slot around inside of mic body. Firstly unscrew capsule assemble and put safely to one side. Next gently PUSH on XLR PINS which should move XLR in by approx 0.5 mm. A small tab on the XLR socket sits in a notch on mic body. Using a safety pin to the right of the small XLR tab , ease the end of the snapring inwards and upwards and remove snapring. Next insert a female XLR lead into mic XLR and then remove gently. Mic XLR and PCB should then slide out. If it does not slide out then the MIC XLR is catching on the mic body machined slot. (XLR tab moving out by 1.5 mm will confirm this.) Then you will need to use some thin steel shim, ( I used feeler gauges about 4 thou) insert in XLR slot and gently ease shim between XLR sleeve and mic body while gently pulling outwards. Once XLR sleeve moves passed the machined slot the whole assembly can then easily slide out.

Noise Issues.----- Like frying bacon / crackling. I checked all PCB voltages, which were correct. Resoldering XLR wires at XLR socket and careful cleaning of XLR socket and PCB with isopropyl alcohol and kids paintbrush cleared the noise problem. Particularly cleaned area around FET , C1 and 1G resistors. If anyone struggling to dismantle I have several pics to help.
Old 16th April 2018
Lives for gear
Another thing that hasn't been mentioned is the capsule itself. Sometimes a dirty connection can cause noise problems. Unscrew the capsule then clean the piece with the gold pin in the middle. Also clean the contact on the mic body. Use either isopropyl alcohol or a little Caig Deoxit. Reassemble and see if it goes away.
Old 17th April 2018
Lives for gear
I used to have that problem with 451/CK1. It was humidity in the capsule. Drying it out, and keeping it stored with desiccant, fixed the thing. No need to open the preamp at all.

What you've got to realize is that, even though this mic can be powered by a 9v volt battery, it boosts the polarizing voltage and is a true condenser. Got to be treated like one.
Old 22nd April 2018
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dfchandler's Avatar

Originally Posted by mbrebes View Post
Unscrew the capsule then clean the piece with the gold pin in the middle. Also clean the contact on the mic body.
Also clean the threads on the capsule and the mic body!

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