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All listening sessions occurred with non-stock input tubes - E88CC SQ Philips and 12BH7A RCA. The standard Electro-Harmonix aren't bad and you can start with them but the other two are far better in resolution and timbre. Another case were the rectifier and output tubes. The distributor and a friendly audiophile allowed me to compare the standard TJ 300B with current Westrex Corporation WE 300Bs, then compare the standard (though Art Audio selected) Chinese 274B rectifier to the Mullard and Telefunken GZ34.

The 300B comparison was fascinating. The Western Electrics have a much better treble and midrange at least in terms of timbre. In general it is lower and fuller. After plugging those tubes in, everything gains weight and dignity. Cymbals, reverb and most of all space are phenomenal. The "Pyramid" cut by The Modern Jazz Quartet is based on a constant snare drum rhythm base in front of which a vibraphone occupies the right channel, a piano the left. With the American tubes the distances between instruments were much clearer, the snare drum sat farther behind the vibraphone. With the TJ, those elements were more crowded. Ditto the instruments on e.s.t.'s Leucocyte. They became fleshier, more believable and exuded incredible, intense expressiveness. The WE treble is less 'metallic' than TJ's yet the latter isn't metallic at all. Still, WE creates the treble in a more natural, softer yet more vivid way. Over the Chinese tubes, the cymbals and vibraphone stiffen up a bit to appear less resolved and natural. Well, resolution. The WEs have a less thorough bass that the TJs. The latter also reaches lower. But if I had to choose, I would take the WE. The cymbals from Lontano were electrifying in their presence, timbre and weightiness. Yet there is one 'but'. When I listened to Stanko's trumpet, I had no doubt that the TJ better showed the relationship between mouthpiece and funnel. I know this sound from many attended recording sessions and the TJs better resolved this aspect. The WE are slightly less resolving in the central midrange. TJ there illuminates more and deeper. But the overall impression is that the American tubes are closer to the goal though TJ still has something to say.

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While WE was a step forward compared to TJ, the rectifier swap from Mullard to Telefunken was a sideways move. The German tubes give a brighter, more to-the-point sound. It gave interesting results with the WE 300B but upset the ideal balance I had before. Pyramid's snare drum was closer to the vibraphone and the latter lost some of its height. It was still in the same place but you could not 'see' that it had height. The cymbals were stronger over the Telefunkens but again I thought that the Mullards were the golden means. The German GZ34 are splendid, much better than the Slovak JJs (too bright) and Polish Telamps (too unresolved). But the depth and fullness of the Mullards impressed me the most and my opinion wasn't changed even by Stanko's mouthpiece. The 274Bs however were weak. They lacked the saturation of the Mullard or the resolution of the Telefunken. It is counter intuitive how rectifier valves affect the sound. Once we remember that the loudspeaker's voice coils feed off the current supplied by those tubes, the effect perhaps becomes somewhat more understandable.

Headphone amplifier: This was to be a small addition to the main review, below even the tube rolling. And although sequentially, I did arrange it that way, the listening time over the Conductor as a headphone amplifier prolonged. For a long time now, I have used the integrated Leben CS300 amplifier for my headphones. This is a brilliant device, very competent as an integrated amp and even better as a headphone amplifier. With headphones, I only heard it bettered once by the expensive Cary CAD-300-SEI. I believe the Conductor to be as good and in certain regards better than the Leben and even the Cary. Compared to the first, the British preamplifier shows an even more vivid but, in the good sense of the word, slightly softer picture. Its stage is visibly deeper, instruments are better separated and distinguished by dynamic shifts the Leben no longer tracks. The CS300 is still the high-end of headphone amplifiers and probably the best price/performance proposition. But in absolute terms, the Conductor presents an even more believable sound. As I already wrote, it is at once more resolved yet devoid of sharpness and brightness. The effect was overwhelming on my AKG K701s which, being a bit soft and recessed in the treble, simply sang. The Japanese contestant bettered the Brit only in the bass. It extends lower and the infrasonic range is tauter. In the first week of listening to the Conductor I thought its bass a little compressed, sounding slightly clipped as though the unit could not supply sufficient current. After some time the effect disappeared but ultimate tautness and kick are not something this heaphone amplifier can be proud of. Regardless, you must explore this function. Even if it exceeds your purse, you will know in which direction you should look if you really love headphones...

Discs used during listening sessions: e.s.t., Leucocyte, ACT 9018-2, CD | The Modern Jazz Quartet, Pyramid, Atlantic/Warner Music Japan, WPCR-25125, CD | David Gilmour, Live In Gdańsk, EMI, 2354882, 2 x CD | Galahad, Sleepers, Avalon Records, GHCD4, CD | Tomasz Stanko Quartet, Lontano, ECM 1980, CD | Pat Martino, Starbright, Warner Music Japan, WPCR-13183, SHM-CD | Radiohead, Kid A, Parlophone/EMI, 27753, CD.

The Conductor preamp is one of the newest products from Art audio. This is a quite hefty device with a chromed front plate and a few knobs. The first one powers it on, the second raises the gain -- handy when listening to headphones -- the third manages the volume (this and mute are available from a remote), the fourth selects inputs and the fifth assigns the source for the tape outputs. There is also a headphone socket. On the back we see a row of sockets, to the far left those reserved for a turntable but here just another line input. Then we have seven line inputs including two tape loops and two variable pre outputs. There also is a heat sink - all voltages are stabilized.

Inside we see a lot of specialized PCBs. Three larger ones are horizontally mounted, with the amplifying circuitry on them, a big one for the power supply and a smaller one for the motor-driven pot. A vertically mounted input selector PCB sits near the front panel and two others near the back, one for the control logic, the other with the relays. The circuit is based on two tubes per channel, 6922 EH Electro-Harmonix on the input and 12BH7 Yugoslavian EI output buffers. The coupling capacitors are Wima polypropylenes. The potentiometer is a blue Alps connected to the circuit with unshielded cables. There are lots of cables inside for audio signals and grounds running from each PCB to a central point to create a star ground. The power supply uses a medium size transformer and plenty of Vishay/BC capacitors. The headphone socket is not gold plated. The manufacturer states that there are no coupling capacitors on the output as that would create high output impedance and limit the frequency response. Since this is a tube circuit, such caps can be eliminated by only two means - an output transformer or a DC servo as in most CD players and solid-state preamplifiers. A DC servo loop is probably on the PCB above the motherboard.

The Diavolo power amplifier is a 300B SET (here with TJ 300Bs but KR VV32Bs are optional), Philips E88CC SQ inputs and RCA 12BH7A drivers. The power supply runs one 274B rectifier per channel that can be swapped for a GZ34 or CV378. Behind the tubes we see big and very heavy power and output transformers. On the back we have RCA input sockets, speaker terminals with separate 4 and 8-ohm taps and two heatsinks that get very warm during operation. I almost forgot - we also get a balanced XLR input.

The circuit is split between two separate PCBs. The whole is truly minimalist and most parts sits on small PCBs used for the output tube bias setting. The power supplied is rectified and stabilized for the input tubes and heaters. The power supply runs many capacitors including splendid BHC unit from Denis Morecroft. There is also one choke per channel. The coupling between tubes is via Hovland Musicap polypropylene capacitors. The balanced input is de-symmetrized by small potted transformers. Similar to the preamplifier, all grounds reference to a central star ground.

Technical data (according to manufacturer):
Output power -10/13W (300B/V 32B)
Input sensitivity - 350 mV
Input impedance - 220 kΩ
Frequency response 20 Hz – 20 kHz (+/- 1dB)

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