[Scott Dorsey]

Phil Allison wrote:
Scott Dorsey wrote:

4. All the GkTB equations we were forced to memorize in school are useful at
RF but not useful in a mismatched low frequency world.

** False. Johnson noise is dominant in any resistive source situation.

Resistive source? An SM-57? I think maybe this is where one of the problems
is located.
--scott


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

[Phil Allison[_4_]]

Scott Dorsey wrote:
---------------------



4. All the GkTB equations we were forced to memorize in school are useful at
RF but not useful in a mismatched low frequency world.

** False. Johnson noise is dominant in any resistive source situation.

Resistive source? An SM-57?


** Run an impedance test yourself.

From a few hundred Hz up to 15kHz, it is nearly flat.

There is a broad peak around 120Hz IIRC.

Essentially a 270 ohm source.

The internal auto-tranny just ups the value by a factor of 20 from the 13 ohms capsule.

Reason why makers use a simple resistor for pre-amp specifications.

And the type does not matter !!



.... Phil







I think maybe this is where one of the problems
is located.
--scott


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

[[email protected]]

On 7 Nov 2019 13:43:57 -0500, (Scott Dorsey) wrote:

Ralph Barone wrote:

I was under the impression that electromigration was only an issue in
modern microprocessors and other ICs built using nanometer scale
transistors. I would assume that an op amp is built with huge transistors
(and huge traces) in order to achieve low noise, and would therefore be
relatively immune to electromigration damage.

I think it's a matter of scale and voltage. So it's an issue at low voltages
with tiny transistors, but at higher voltages (and really 15V isn't that
high) with larger transistors. I think it's also a matter of the shape of
the metallization traces, with sharp angles promoting migration.

That said, if it were that big a problem I'd be seeing LM709s and LM301s
starting to fail by now, right?
--scott

Current density and temperature are the important factors. The properties
of the metalization alloy is highly significant and it is this area that has led to
major improvents in time to fail, allowing smaller interconnect widths. AL
dominated ICs until this century when it was figured out how to make a Cu alloy work
Interconnect shape and vias to other metal levels are weak points but these are addressed in layout.