AKG C12 pattern switch resistor values - clone builds mistake? Confused...

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ln76d

ln76d

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I found the difference in resistors values between N12 and N12K schematics.  10 times on each resistor - 500kohm or 50kohm.
I'm guessing that this not depend on PSU circuit because difference is mostly in type of rectifiing parts.
Could this be a simple mistake in writing? ...
R21-R29? Hmm...  Most of people use 8 resistors? Drawing looks like 8? Why 9?

Popularized clone circuit use 0,4Mohm resistors...

Now back to the original circuit.
0,4Mohm lin. (0,05Mohm / Wid.) - guessing - 0,4Mohm linear (0,05Mohm / Widerstand (which means resistor))

Logic tells me that this means 0,4Mohm linear potentiometr and in the parentheses is equivalent circuit for resistor stepped switch.

Since resistors are connected in series:

8 x 0,05Mohm = 0,4Mohm - best match if i'm right
8 x 0,5Mohm = 4Mohm
8 x 0,4Mohm = 3,2Mohm

Best will be get the original AKG switch box :)

Someone have it?




 

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Yeap! I was right  ;)

it need to be eight pcs of 0,05Mohm resistors!
There was also AKG C 24! I forgot to check it before...
Even if there were few things changed in the circuit of C24,  pattern switch (S12) was the same only it was dual (S24) for both capsules.
I don't know how the difference in resistors values may affect on the circuit working or if it was discovered earlier on the forum, but i think it could be useful :)





 

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ln76d said:
Now back to the original circuit.
0,4Mohm lin. (0,05Mohm / Wid.) - guessing - 0,4Mohm linear (0,05Mohm / Widerstand (which means resistor))
Click to expand...

Except it reads; 0,5M Wid

Aside from that, you may be on to something.

Regards,
Mark
 

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Biasrocks said:
ln76d said:
Now back to the original circuit.
0,4Mohm lin. (0,05Mohm / Wid.) - guessing - 0,4Mohm linear (0,05Mohm / Widerstand (which means resistor))
Click to expand...

Except it reads; 0,5M Wid

Aside from that, you may be on to something.

Regards,
Mark
Click to expand...

Yep, but if you will look on my attachments, you will see what i mean.
0,5Mohm was on the first (available on the net) schematic PSU N12K (made by WSW) from the 1959r. N12 schematic is from 1960, which is updated and i believe that on the N12K schematic was fault with 0,5Mohm.
C24 from the sixties (second attachment) have same 50k/0,05M ohm value.
Eight 50kohm resistors in series on rotary switch will give you 0,4Mohm (almost) linear potentiometer what would suggesting original description "0,4Mohm lin." and the equivalent resistor circuit "0,05Mohm Wid." I doubt that they mean in 1959r - 0,4Mohm linear resistors.
 
Just throwing this out there:

This resistor string terminates into a 30M+ bias resistor, the other side of which is sitting at AC ground:  so the actual value of this "virtual potentiometer string" really doesn't matter.  Whether the resistors are 50K or 500K won't change the actual bias voltage (as seen by the capsule).

I would use whatever you have.  ;D
 
Hi Matador!

Thanks for reply and explanation of circuit operation!!!
That's why i called topic title "clone builds mistake" :D :D :D
I was hoping that it trigger a small discussion, before the topic will sink into the other topics questions :)

Cheers


 
I found good (i think) source of explenation how the polar pattern works:

http://www.foxaudioresearch.ca/Polarpatterns.htm
 
That's a good explanation.

The other trick:  there is no DC current through the bias network.  The capsule looks like a ~70pF cap to the bias network, which is already sitting at DC ground.  There is no current in the actual bias resistor (other than minuscule leakage currents), except when the system is adjusting itself to new bias voltages (right after turning the switch).  This is why it takes 10 seconds or so for the pattern to "change" after the switch is moved.

Also despite the fact that that page describes Fox's m251 equivalent, the real ELA M251 capsule's polarization scheme is quite different.  I figured it out a few months ago and can dig it up if you are interested.
 
Matador said:
That's a good explanation.

The other trick:  there is no DC current through the bias network.  The capsule looks like a ~70pF cap to the bias network, which is already sitting at DC ground.  There is no current in the actual bias resistor (other than minuscule leakage currents), except when the system is adjusting itself to new bias voltages (right after turning the switch).  This is why it takes 10 seconds or so for the pattern to "change" after the switch is moved.

Also despite the fact that that page describes Fox's m251 equivalent, the real ELA M251 capsule's polarization scheme is quite different.  I figured it out a few months ago and can dig it up if you are interested.
Click to expand...

yeah dig it up please mate
 
Matador said:
Here is the 251 capsule biasing mechanism:  FC = front capsule, FB = front backplate, RB = rear backplate, RC = rear capsule

ModeFCFBRBRC
Cardioid+60VGND (via grid)floatfloat
Figure 8+60VGND (via grid)GND+60V
Omni+60VGND (via grid)short to FB+60V

Signal is tapped from the FB connection.  Figure-8 and omni look suspiciously close together, however the secret between them is:  for figure 8, the +60V connection is isolated from the +60V supply to the FC via a low-pass filter (the resistor and cap in the capsule assembly).  Thus the capsule looks like two, "2-terminal" cardioid capsules stuck "back to back".  However in omni, the FB and RB connections are shorted together, and the FC and RC nodes are shorted together.  So now it looks like a 3-terminal capsule in a regular omni-biasing scheme.

Very different from how U87 does it!
Click to expand...
 

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