Jonte Knif of Knif Audio is one of Finland’s most skillful and versatile audio designers. Most famous, also internationally, he is for his designs for pro audio (mastering studios): tubed and passive EQs, compressors, distortion devices, controllers, tube mic preamps etc. – all with Elma rotary switch controls that give the products their unique look.
He’s also known for his fabulous analogue synthesizer, Knifonium, with 26 tubes inside. Not surprising, given that (he’s got a university degree on music technology) his thesis dealt with a unique electro-acoustic clavichord. On top of all, Jonte Knif is a competent hapsichord player himself.
His creations for home audio cover Class A tube/solid-state amplifiers, and dynamic/ribbon loudspeakers. Below is my old review (from 2013) of Knif Audio’s Hybrid Class A power monoblocks, and introduction to a much more recent Knif Rauman 845 Class A tube monoblocks.
Knif Audio’s Hybrid Class A Power Amp
The basic idea behind the amp is not too dissimilar to that of Knif Audio’s Awara Class A double mono hybrid push-pull power amp with inter-stage transformers, Siemens E188CC as the driver, and NPN FETs at the output. The output/power stage of the amp is Class A follower, not particularly sensitive to the speaker load, as long as it’s not terribly low. The reason is that this Class A amp does not know how to move to Class AB. The amp operates on four ohms just fine until the power runs out totally, at which point about 40W comes out of it (the previous design produced about 40 watts to 8 ohms, practically all of that in Class A, before entering into Class AB). Bias currents are set according to 7 ohms: more than that and the amp would become red-hot.
The monoblocks clip at 60W for 8 ohms, and no negative feedback is applied. The little distortion comes mainly from the driver stage. The amp is fully symmetrical. Both speaker poles float. The power FETs are loaded by Knif’s own hand-made coils with an amorphous core. A nice feature of the topology is that the power supply has practically no effect on it, since coupling absorbs almost the same amount of power regardless of the signal level. This applies both to the input and output stage. The power supply caps are – just in case – Siemens Sikorels. Every detail is not just taken care of, but also profoundly justified. Truly impressive!
Listening observations
For the audition I used Brinkmann Marconi preamp. Since the input is floating, almost anything can be connected, but due to the input transformers, the source should preferably be active, and behave correctly with a predictable output impedance. Two speakers participated in the test: a 11 Ohm Voxativ widebander, and a 16 ohm 4-way vintage horn speaker. The ideal speaker load for the monoblocks is 8 ohms but I didn’t spot any ill behavior anomalies with higher speaker impedances.
Irrespectively of the speaker, it wasn’t uneasy to recognize the qualities of the amp, and especially the sound’s overall healthiness, the amazing ”purity” of it. By ”purity” I don’t mean lack of hiss or other such noise factors but rather the ”feeling” that someone came in and mopped up the window of all dirt and interference, mysterious as it may sound. The sound didn’t necessarily become more detailed etc., just cleaner. The amp had an excellent control over the speaker dynamics, whether micro or macro, and it didn’t stop halfway.
Compared to my 6V6 PP power amp, and even more so when compared to a DIY 300B SET power amp, the bass was more extended and better under control. Tonally, the Knif mono monoblocks appeared to be tuned toward the upper end of the tuning range, more clear cut, and especially via the horns, the 6V6 PP was softer. On the other hand, no such difference was apparent when the monoblocks were referenced to the 300B SET. I should have compared the monoblocks to a transistor amp, of course, but didn’t have one at hand.
The main difference between the Knif monoblocks, and the tube power amps, concerned transparency, and the concreteness of the sound. Knif monoblocks reproduced music a tad more homogeneously, as if they had first buffered the program material somewhere, and then delivered it uniformly rather than stratified. I wasn’t surprised: this is the difference I have often experienced between solid state output stages, and ones with tubes. This tine I discovered it eg. with Bartok’s violin/piano piece and that sort of music overall. With some other music I thought that it was the Knif Audio monoblocks that handled the music perfectly right, as it should be. You never know these things beforehand without experimenting and pondering. Hurray for the small differences! All in all, the Knif Audio Hybrid Class A monoblocks reproduced music very boldly and gallantly with no signs of weakness of any sort.
Conclusion
The amp is an original design, genuinely based on clever thinking and fresh solutions. In this respect, I feel confident to say that these monoblocks are likely to deliver more value for money than other such designs in the same category. But as always: they need pairing with proper loudspeakers, technically and sonically. I was left with such an aftertaste that with the right kind of loudspeakers the monoblocks would have even better revealed their true nature and potential.
Knif Rauman 845
As the model name indicates, Knif Rauman 845 is pure tube amp, and 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. Highest grade components include a huge amorphous core output transformer for extended frequency range, low information and power loss. No electrolytic capacitors what so ever. 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 the 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 capable of producing flawlessly the very high signal voltages needed to drive the output stage. Output stage tubes use a fixed bias topology. The amp is biased to produce the first 40 W in pure Class A, sliding very smoothly into class AB for the last 30 watts. The uniquely designed cast aluminum enclosure betters heat dissipation, protects the output tubes and provides very high mechanical rigidity.
I’ve heard the 845 amps performing alive on several occasions, and with superb results, but under such conditions that do not allow a proper review. I’ve heard the amps when they were set to drive Knif Rauman No 1 loudspeakers (the image below), and with ample power reserves of two parallel 845s per channel, the performance was really first-class!
Technical Specifications
Tube complement: 2 x 845, 2 x E80F, 1x 6H30Pi per channel
Enclosure material: Metal casting
Dimensions: 580 x 330 x 270 mm
Weight: 30 kg
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 per channel
Gain: 26 dB
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
