microphone hiss AKG C12A

[pucho812]

Got a pair of AKG  412's that sound fantastic except they have a huge amount of hiss/white noise.  These have the screw on bases and attach to an external power supply that has polar pattern selector on it.  The capsules don't cut out or exhibit any sort of problems.  From what I can tell the hiss does not change in level when set to different patterns.

Anyone have a schematic or suggestion as to the cause of the hiss?  I don't believe the mic  or power supply has seen a service bench since date of sale.

Edit c12a's

 

[radardoug]

Just telling us they have a whole lot of hiss is meaningless. At what level? Are you using them on quiet instruments? AKG generally dont make crap, so it could be something else.

 

[pucho812]

Just telling us they have a whole lot of hiss is meaningless. At what level? Are you using them on quiet instruments? AKG generally dont make crap, so it could be something else.

o.k. if I plug them in, the hiss is noticeable regardless of what source I put them on. As one cranks up a mic pre to get acceptable recording levels, the hiss increases.  It sounds like hiss, like one would expect to find when playing tape.
Looking for a schematic to at least trouble shoot the power supply as a culprit and to ultimately rule out the capsule as being at fault.

As for the setup,  baby grand piano, to microphones, to console, to pro tools. With the same settings and  different condenser mics or tube mics, there is no hiss.

 

[mjrippe]

AKG generally dont make crap, so it could be something else.

I agree, but these mics are pushing 50 years old.  Hiss could likely be the FET, but I don't know offhand what they used originally.  Someone here will know a good modern replacement.

 

[Gus]

The 412 and 414 look like the same gain stage with the 414 having an extra pattern.

It is a simple SF to EF circuit with a pad in the output to the transformer

You should know the first step disconnect the capsule to follower connection (you can sub in a cap of about the capsule cap from gate to ground  if you want) and test for noise again.

you can still find 30a JFETs or sub in another type  with close to the same specs it is only a follower and you can adjust R3 and R4 values to adjust the operating points of the transistors

a goggle search found a 412 schematic

 

[pucho812]

The 412 and 414 look like the same gain stage with the 414 having an extra pattern.

It is a simple SF to EF circuit with a pad in the output to the transformer

You should know the first step disconnect the capsule to follower connection (you can sub in a cap of about the capsule cap from gate to ground  if you want) and test for noise again.

you can still find 30a JFETs or sub in another type  with close to the same specs it is only a follower and you can adjust R3 and R4 values to adjust the operating points of the transistors

a goggle search found a 412 schematic

thanks gus. 

 

[pucho812]

I'll make some test recordings so we all can hear

 

[Gus]

two links first is the 412
https://elektrotanya.com/PREVIEWS/63463243/23432455/akg/akg_c412.pdf_1.png

look at reply 22 it has part values note how close the circuits are
https://repforums.prosoundweb.com/index.php?topic=36987.15

And this link https://groupdiy.com/index.php?topic=61767.0

Before changing the semiconductors check the parts caps, switches, board traces, solder connections and resistors.

Make sure to check the zener bypass cap if you still have noise. 
Note that AKG used a series resistor to "help" the filter cap bypass the Zener noise.
Zeners have a low internal resistance when working as Zeners and also have noise so the extra  resistor helps.
It is a balancing act of regulation vs noise filtering

I have seen a schematic of a solid state AKG with an external supply for the pattern. I don't seem to have the link any more and a search is not finding it.

Found it it is a C414E1
http://www.benmook.com/tech/documents/Schematics%202/Schematics%201/AKG%20C414e1.pdf
and thishttps://www.historyofrecording.com/AKG-C414.html
and thishttps://groupdiy.com/index.php?topic=63968.0

AND  this is back I thought it disappeared.
http://www.oocities.org/siliconvalley/peaks/5212/akg1.htm

So what microphone part numbers do you have?

 

[Gus]

So what microphones do you have?

The C412 schematic I found does not have an external supply.

The C414E1 has an external supply.

 

[pucho812]

Hello Gus.  The studio owner always refers to them as c412's. But  If you ask me, I would call them c12a's,  So a little hard to say at the moment. Let me snap some photos and post them.

 

[Khron]

The C412 looks to be JFET-based and phantom-powered. The C12A is tube-based and has (and needs) an external power supply.

C412 seems to have a 20dB pad switch on the front; C12A seems to have no switches on the mic body. The mic bodies look very similar, almost identical.

 

[pucho812]

The C412 looks to be JFET-based and phantom-powered. The C12A is tube-based and has (and needs) an external power supply.

C412 seems to have a 20dB pad switch on the front; C12A seems to have no switches on the mic body. The mic bodies look very similar, almost identical.

yes the c12A uses a nuvistor tube...

 

[pucho812]

went into the mic locker today.

the high hiss par in question is in fact a pair of c12a's. They happen to be in a case with a c412, a c412 screw on base and a third c12A base, more on that later.

 

[Gus]

So I looked at the C12a schematic

Have you checked the power supply heater voltage at the tube in the microphone?

It is a constant current supply so it needs to have the correct load on it to measure the voltage or you need to calculate the resistor value that will have the correct voltage across it at the correct constant current.

Curious what the plate/anode and cathode voltages are in the microphone. I am guessing it might be running at 2mA if I am reading the resistor values correctly on the schematic 4.7K, 560ohm and 27K.
Why 2ma?
2mA across 27k is  54VDC
and I read 2.4mA by the pattern control but I cant read the pattern control resistor values

reply 2 has the schematic https://groupdiy.com/index.php?topic=44737.0

 

[Khron]

Might this help?

 

[pucho812]

Si Señor