Definition of EIN here: https://www.sweetwater.com/insync/ein/ KH
Thanx Klaus, let me try to explain EIN / Equivalent Input Noise more simply:
EIN is the noise of an amplifier looked at, as if all of its noise is an external voltage applied to its input.
This takes the amp’s gain out of the equation, as gain doesn’t change this input signal.
Caused by thermic movement of atoms, even a resistor itself generates noise.
200 Ohm generates 0.26 uV = -129 dBu noise voltage, the lowest EIN possible with a normal dynamic microphone, even if the amp is cooled with helium.
Quadrupling the 200 Ohm resistor doubles the noise.
(Noise adds “geometrically”, not “arithmetically”, as noise is “uncorrelated”).
Unfortunately a transformer transforms:
• the voltage by its ratio, minus the losses by the windings resistances,
• but the resistance by the
square of it’s input to output ratio, plus the losses on primary and secondary windings.
The Lundahl LL1538 wired 1:5 e.g. transforms:
• Voltage 4.6 times (incl. transformer loss)
• Resistance 35 times. (incl. transformer winding resistances), and therefore noise 5.9 times.
This means, at a close to perfect mic-pre, the transformer does worsen the noise situation.
Only with a bad pre, or a pre not made for mics, the Lundahl transformer has an advantage.
EDIT: Had to correct the noise calculations, to include transformer resistance losses.
Calculations based on 200 Ohm microphone, loaded by 1 kOhm preamp input.
