Sredna

Re: KM84 style circuit with electret capsule!?
« Reply #20 on: May 19, 2020, 08:48:20 AM »
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).
Hmm... in which direction do the electrons actually go?


abbey road d enfer

Re: KM84 style circuit with electret capsule!?
« Reply #21 on: May 19, 2020, 09:56:26 AM »
Wrap the transformer (or the whole circuit) in copper foil, and connect foil to ground.
A copper foil is no use for magnetic shielding.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

abbey road d enfer

Re: KM84 style circuit with electret capsule!?
« Reply #22 on: May 19, 2020, 10:04:07 AM »
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.

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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.

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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.

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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...

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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.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Sredna

Re: KM84 style circuit with electret capsule!?
« Reply #23 on: May 20, 2020, 04:31:24 AM »
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.
Hmm... in which direction do the electrons actually go?

abbey road d enfer

Re: KM84 style circuit with electret capsule!?
« Reply #24 on: May 20, 2020, 04:33:43 AM »
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.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Khron

Re: KM84 style circuit with electret capsule!?
« Reply #25 on: May 20, 2020, 12:13:29 PM »
... 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
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

abbey road d enfer

Re: KM84 style circuit with electret capsule!?
« Reply #26 on: May 20, 2020, 12:26:04 PM »
... 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.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

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