On 2009-02-08, terryS wrote:
On Feb 2, 6:48Â*pm, "Richard Knoppow" wrote:
"K3HVG" wrote in message

...
Jim, Â*I'm surprised you didn't need an amplifier with that
electret element? Â*That's what the carbon-compatible mics
use in 2-way and avionic installations to get the required

The electret elements from Radio Shack include an amplifier, which
is why you have to power them; but I don't know if it brings the output
level up to that of a carbon mike. The electret T-1 replacement I got
from Mike Sandman obviously has enough amplification built into it
somewhere, since it directly replaces the T-1. But I didn't tear it up
to see what is inside.

Jim Haynes wrote:
On 2009-02-08, terryS wrote:
On Feb 2, 6:48Â*pm, "Richard Knoppow" wrote:
"K3HVG" wrote in message

...
Jim, Â*I'm surprised you didn't need an amplifier with that
electret element? Â*That's what the carbon-compatible mics
use in 2-way and avionic installations to get the required

The electret elements from Radio Shack include an amplifier, which
is why you have to power them; but I don't know if it brings the output
level up to that of a carbon mike. The electret T-1 replacement I got
from Mike Sandman obviously has enough amplification built into it
somewhere, since it directly replaces the T-1. But I didn't tear it up
to see what is inside.

All those electret elements have a FET-IC in them, which is a J-FET with
controlled leakage so that they don't need a grid leak resistor inside them.
Apply voltage across them through a resistor, pick the modulated signal
off through a capacitor. With a little tinkering with series and shunt
resistances you can make one fit right in place of a carbon element.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Scott Dorsey wrote:


All those electret elements have a FET-IC in them, which is a J-FET with
controlled leakage so that they don't need a grid leak resistor inside them.
Apply voltage across them through a resistor, pick the modulated signal
off through a capacitor. With a little tinkering with series and shunt
resistances you can make one fit right in place of a carbon element.
--scott

Very interesting, gentlemen. I'm going to have to give this a try as
I've, heretofore been using the element tken from H-33/PT handsets and
those old single-headest PBX operator's units. Might either of you have
the part numbers? I fear, also, that RatShak may well have deleted this
item as they have hundreds of other components.

de Jeep/K3HVG

K3HVG wrote:
Scott Dorsey wrote:


All those electret elements have a FET-IC in them, which is a J-FET with
controlled leakage so that they don't need a grid leak resistor inside them.
Apply voltage across them through a resistor, pick the modulated signal
off through a capacitor. With a little tinkering with series and shunt
resistances you can make one fit right in place of a carbon element.

Very interesting, gentlemen. I'm going to have to give this a try as
I've, heretofore been using the element tken from H-33/PT handsets and
those old single-headest PBX operator's units. Might either of you have
the part numbers? I fear, also, that RatShak may well have deleted this
item as they have hundreds of other components.

They are all pretty much the same other than polarity... you can grab any
cellphone headset or computer headset from the dollar store and use that.

If you want a nice electret element, try the Panasonic WM-61A, Digi-Key
part number P9925-ND. It's pretty small so be careful soldering, but it
is better made and lower noise than most of the cheapies and still only
costs a couple bucks.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

In article ,
(Scott Dorsey) wrote:

They are all pretty much the same other than polarity... you can grab any
cellphone headset or computer headset from the dollar store and use that.

Over the years, I've tried a number of circuits to replace carbon
microphone elements. None had the output voltage of a good carbon
element, but they sound much better if your system has enough
amplification to compensate.

Polarity could be a problem. A carbon microphone is a series DC circuit
where polarity does not matter. If your equivalent circuit requires
proper polarity, simply use a full wave bridge made of 4 diodes. Two of
the diodes will always be forward biased. The DC source voltage will be
reduced by about 1.4 volts, but that should make little difference.

Fred
K4DII