KM84 style circuit with electret capsule!?

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Sredna

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You could use a normal resistor for R5 (after you found the right value)

Wrap the transformer (or the whole circuit) in copper foil, and connect foil to ground.

Instead of a pad you could reduce the lower end (C3).
 

abbey road d enfer

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xeawr said:
guys, thanks so much for your valuable feedback. Also sorry for the confusion I caused with the 6.8k resistors, in my most recent schematic I changed them to 2.2k.
As I said earlier, it increases the current draw up to 8mA, pretty close to the maximum (10mA), without any improvement.

As some of you pointed out this is more common and also used in the KM84/U87 circuit
What works in these mics does not necessarily work in another circuit. Slavishly copying without understanding why is dumb.

1. part count is still pretty high (space!)
You could get rid of the xfmr, add a transistor for low impedance output, and impedance balance.

2. the little NTE10-3 tranny lacks shielding, so I'm not sure how reasonable it is to go forward with it (the mic itself has a plastic housing)
The mic may or may not be perfectly usable, depending on surrounding interference. Only a test can tell you. You may want to wrap teh xfmr in mu-metal foil...

-
I would probably need to implement a permanent pad, otherwise I might overload the little transformer. What do you think?
It depends on what you want to do with this mic. You could remove capacitor C4. Putting a pad at the output would be unproductive because the FET stage would clip anyway.
 

Sredna

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abbey road d enfer said:
A copper foil is no use for magnetic shielding.

True, but most microphones I know of doesn’t have shielding for the  transformer (KM84 etc). Except for the housing and that’s mainly brass.
 

abbey road d enfer

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Sredna said:
True, but most microphones I know of doesn’t have shielding for the  transformer (KM84 etc). Except for the housing and that’s mainly brass.
Correct, that's why I said it may work as is.
 

Khron

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... And on the other hand, what's more prone to picking up EMI anyway? The unshielded transformer, or the 1G impedance on the FET's gate? ;D
 

abbey road d enfer

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Khron said:
... And on the other hand, what's more prone to picking up EMI anyway? The unshielded transformer, or the 1G impedance on the FET's gate? ;D
Electrostatic interference is easy to cure, magnetic much less.
 

xeawr

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abbey road d enfer said:
As I said earlier, it increases the current draw up to 8mA, pretty close to the maximum (10mA), without any improvement.
What works in these mics does not necessarily work in another circuit. Slavishly copying without understanding why is dumb.
You could get rid of the xfmr, add a transistor for low impedance output, and impedance balance.
The mic may or may not be perfectly usable, depending on surrounding interference. Only a test can tell you. You may want to wrap teh xfmr in mu-metal foil...

- It depends on what you want to do with this mic. You could remove capacitor C4. Putting a pad at the output would be unproductive because the FET stage would clip anyway.


Ok I somehow saw this coming as I was typing the words "also used in the ... circuit"...
I want to emphasize my thankfulness towards all of you who so generously contribute information, time and effort to other people and their interests and challenges. I'm also really thankful for your feedback abbey road d enfer.

As for the:
"Slavishly copying without understanding why is dumb"

I have a background as professionally trained and working tracking and mixing engineer. Most of my fellow audio engineers frankly don't care much about how these things work, they are familiar with their sonic properties and throw those thing in front of (independent and "major") artists and get the expected results. If anything breaks they give it to a professional company to fix it. In the recent years, my personal interest gravitated towards trying to understand the circuits, the components, the properties of the materials used etc. I don't have people with an EE background that I could just call up to help me understand and solve my problems.
Copying a circuit without understanding might seem like a totally dumb thing to some of you, yet, sitting here drooling over these schematics, and wondering how all of this plays together is often my only approach to gain an understanding of what is going. Again, it might seem like a totally worn out approach to learning things, but honestly, it's one of the very few routes I can take that will allow me to make some progress...

So sorry if these inquiries are/were somewhat annoying and thanks again for all your help so far! I really appreciate it, and for my part, I try to remain in the "helper zone" and pass on to other people whatever knowledge I have  :)
 

xeawr

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by the way I was able to wire a circuit together and stuff it inside the mic and it actually works - used it on an amp once. Pretty "ok", not overwhelming (most likely the capsule).
But now the mic is so full of components that if I ever have to fix/open it again I think I will just gut it all and try to find a decent dynamic capsule - problem solved  :D

thanks again!!!
 

Vac11

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Hi,

Im testing this circuit now. Microphone basicaly works. My schemo with all measured voltages is in attachement. Ive used cheap Aliexpress 3USD electret capsule without internal FET and Beyerdynamic 7:1 transformer and also different FET. I have some questions.

1) Why R3 value in xeawr´s schematic is 20K? This resistor basically set the overal gain of amplifier, ok? Original KM84 have there 56K+10K+47K resistors, but hey with different capsule...

2) Question about 1nF capacitor between the capsule and gate of FET. Ive tried mic without this capacitor, so capsule wire directly to fet and i didnt find any changes in sound. What is exact funktion of this capacitor?

Sensitivity of this microphone is rather higher than I want.

3) Is there easy way to decrease the sensitivity? Pad capacitor?

4) Is there any reason why in original KM84 schematic was zener diode instaled between two resistors and not directly after two 2k2 resistors like in attached schematic?

Any help will be much appreciated

M.

km841.jpg

 

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Gus

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Copy the 84 circuit without the capsule charge circuit and drain to gate pad caps
 

kingkorg

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Vac11 said:
2) Question about 1nF capacitor between the capsule and gate of FET. Ive tried mic without this capacitor, so capsule wire directly to fet and i didnt find any changes in sound. What is exact funktion of this capacitor?


km841.jpg

It's blocking polarisation DC used to "activate" regular non electret capsule. Since you are using electret, you don't need it.
 

Vac11

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Gus said:
Copy the 84 circuit without the capsule charge circuit and drain to gate pad caps

Yes, copy is one way... I only wanted try something different and learn something new....thanks.
 

abbey road d enfer

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Vac11 said:
1) Why R3 value in xeawr´s schematic is 20K? This resistor basically set the overal gain of amplifier, ok? Original KM84 have there 56K+10K+47K resistors, but hey with different capsule...
Don't sweat too much over this schemo. It is full of design errors due probably to desire to use less components, actually less than needed for good performance. Over simplification...

2) Question about 1nF capacitor between the capsule and gate of FET. Ive tried mic without this capacitor, so capsule wire directly to fet and i didnt find any changes in sound.
Maybe no difference in sound, but probably in sensitivity and headroom.

What is exact funktion of this capacitor?
I believe it's already been answered, but it allows adjusting the gate voltage independantly of the capsule's voltage.

3) Is there easy way to decrease the sensitivity? Pad capacitor?
I'm not a fan of "charge amp". Use a different arrangement like voltage follower instead of common-source.

4) Is there any reason why in original KM84 schematic was zener diode instaled between two resistors and not directly after two 2k2 resistors like in attached schematic?
Zeners are very noisy, that's why they must be filtered in both directions with RC decoupling.
 

RuudNL

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abbey road d enfer said:
I believe it's already been answered, but it allows adjusting the gate voltage independantly of the capsule's
The gate is at ground potential.
The electret capsule is a capacitor in itself, so there is no need for a capsule-to-gate capacitor.
 

Gus

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Vac11 said:
Yes, copy is one way... I only wanted try something different and learn something new....thanks.

Look at what is different from the KM84 schematic and your schematic.
There are reasons for how Neumann designed the 84 like schematic.
Often it is "better" to understand why a circuit is designed as drawn in a schematic before you change something.


 

abbey road d enfer

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RuudNL said:
The gate is at ground potential.
The electret capsule is a capacitor in itself, so there is no need for a capsule-to-gate capacitor.
that's correct, I didn't realize it's a FET-less capsule there.
 

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