Understanding v672 zero impedance preamp input

[dmp]

I'm trying to better understand the input to the Telefunken v672 preamps...
Resistors set the input impedance to the preamp, where in the manual (pg3), it indicates that the impedance of the preamp alone is less than 5 ohms - (but I've only found the manual in german so I'm stuck using google translate...)
It seems like the input transformer connects to the grid of the first transistor similar to other preamps. I'm not sure what makes this one special so the input impedance is so low?
I've racked up a pair of these with a five position switch selecting Re pairs, using 47 ohm, 300 ohm, 600 ohm, 1200 ohm, 6800 ohm. I haven't noticed a tonal change between these, but a big difference in gain. Higher Re reduces the gain significantly and consequently raises the noise floor of the track.
A bridging input argument would say Re should be at least 600 ohms, but in practice the low Re seems fine. And for quiet sources, like room mics, it is necessary to prevent excessive noise.

Schematic here:

Manual can be found at:
http://audio.kubarth.com/rundfunk/index.cgi

 

[dmp]

I measured the input transformer as giving 6dB gain, so it is apparently a 1:2 ratio. I had expected a high ratio step up transformer, so perplexed by this. But after more reading on input impedance of transistors, I think the series inductors between the transistor's emitter and ground is what gives a low input impedance to the transistor. The transistor gain * emitter impedance determines the transistor input impedance. The impedance of the inductors is frequency dependent, but apparently low. Interesting design.

 

[abbey road d enfer]

No no...google "zero field input.
The low impedance is brought by the particular NFB arrangement.
NFB is not created in the usual manner by substracting a fraction of the output voltage to the input voltage, it's done by substracting a current issued from the output to the input current.
The principle is identical to virtual-ground in opamps.