dmp
Well-known member
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
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