Knif Rauman 845
In his 15 years in thermionic valve based pro audio design Jonte Knif has learned the objective and subjective subtleties of tube amplifier design. In Knif Rauman 845 is the epitome of his craftmanship.
845 monoblock amplifier was designed to provide superb technical specifications like a low damping factor and low distortion and noise, without ignoring more esoteric and hard to measure design principles. Therefore it contains only the highest grade components. These include a huge amorphous core output transformer for extended frequency range, low information and power loss. No electrolytic capacitors are used in or near the signal path. Even the 845 tube anode supply uses a big 1200V 100uF polypropene capacitor from Mundorf.
The topology of the amplifier is fully symmetrical, starting with a differential stage comprising of NOS special quality E80F pentodes which can be run either as pentodes or triodes with a flip of a switch. In pentode mode the open loop gain is 12 dB higher, and this difference is used for global feedback. In triode mode there is no feedback at all. The second stage is also a differential, comprising of a 6H30Pi double triode, which is capable of producing flawlessly the very high signal voltages needed to drive the output stage. Output stage tubes use a fixed bias topology. It is designed to be biased to produce the first 40 Watts in pure class A, sliding very smoothly into class AB for the last 30 Watts. The uniquely designed cast aluminum enclosure provides good heat transfer throughout the whole structure. It protects the output tubes (and fingers) and provides very high mechanical rigidity. The tube cages never get hotter than 70 degrees Celsius, and they are open enough to let the tube cool effectively.
|Tube complement||2 x 845, 2 x E80F, 1x 6H30Pi per channel|
|Enclosure material||Metal casting|
|Dimensions||580 x 330 x 270 mm|
|Frequency response, -1dB points||5 Hz and 35 kHz in triode mode
3 Hz and 55 kHz in pentode mode
|Output noise, A-weighted||-84 dBu in triode mode
-81 dBu in pentode mode
|Dynamic range||105 dB|
|Output power||70 W|
|Harmonic distortion @ 40W||0.6% in triode mode, 0.15% in pentode mode|
|Input impedance||470 kOhms 50 pF|
|Damping factor||2.5 in triode mode
10 in pentode mode
|Design impedance||4 or 8 ohms, factory configurable|