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KR Audio 300B:
The ATM-300 arrived with Electro-Harmonix 300B EH but could also be ordered with current production Western Electric for a total price of 36.000zł. Polish importer Soundclub also supplied me with KR Audio tubes at 1900zł/pr. Those changed the sound but not to the extent I expected. After a shorter listening session, their character was quite similar to the Harmonix. Over the longer haul, the KR Audio was slightly softer, not as edgy as the EX, albeit not as a function of less lit-up treble but a different handling of transients. At first glance, this made the Russians more vivid. In truth, the Czechs were more open and resolved in the upper midrange and treble. Even though transients on a whole were gentler, the extra information filled out the treble.

As I wrote earlier, the ATM-300 is not a dark or warm amplifier. The Czechs confirmed this but were less mechanical than the Electro-Harmonix, which had a very clean but less saturated top end. I mentioned a slight lack of body with the Russians which the KR Audio valves made disappear to some extent. It remained a characteristic of the Air Tight sound but there now was more of everything to make it all clearer. The only element I liked better with the Russians was the bass. The Czechs rounded it over a bit and went softer. This wasn’t pronounced but audible in the lowest reaches.

Create Audio 300B + 12AU7 plus a preamplifier in the mix:
Changing from the KR Audio to the Chinese Create Audio valves moved the tonal balance upwards. Resolution clearly heightened and previously unheard elements showed up. Yet I wasn’t sure if this went in the right direction. To my ears the ATM-300’s strong and clean treble with the EH or even KR was on the verge of being tonally correct. Adding more made the amp clip sooner and the tonal balance moved up into the higher midrange. I became curious how the Japanese amplifier would behave in this situation with a preamplifier in the signal path. Usually the sound with an active preamp gets fuller and smoother. This wasn’t the case here so I returned to the KR.

Adding a preamplifier rather complicated the case than solve it. With the preamp, I did not get any increase in dynamics, space or vividness while resolution diminished. This is why I recommend using the volume controls Air Tight provides. Their input impedance is so high that even a CD player’s standard 2V output will be sufficient for high in-room levels. It would be better even if a CD player had higher than standard output voltage. Some decks by Ayon and Wadia are flexible in that regard.  For power tubes, I would stick to carbon-plate and not mesh types like the Sophia Electric Princess 300B/c; or order the amplifier with Western Electrics.

Air Tight’s ATM-300 is nominally a power amplifier with regulated input sensitivity. Because the latter is quite high (230, 300 or 450mV) and coupled to a pot, we can treat it as an integrated amplifier with single input. The machine is fully tube, with the input handled by two 12AU7A (ECC82 EH) double triodes whose halves work in parallel. These tubes also carry the Air Tight logo and are marked “Platinum Selection” to indicate being measured and paired with tighter tolerance. When I talked about such tube selection with Canor, they told me how they reject about 80% of their valves. From the Air Tight’s input tubes the signal proceeds to the 12BH7A control tubes also from EH. The output stage runs 300B triodes, one per channel. The ATM-300 can be ordered with different glass. The review loaner was fitted with all Electro-Harmonix from current production. The power supply uses a 5U4GBEH rectifier also in a Platinum Selection version.

The amplifier adopts a classic valve gear look with a low-profile chassis and all tubes and encased transformer on deck. The chassis is made from extremely well-crafted aluminum anodized grey. It looks terrific and I wish every manufacturer went to such precision. On the front panel we have three knurled knobs – two for dual-mono volume control, one for bias. A small voltmeter next to the golden company logo assists with bias adjustments. Otherwise there’s only a power switch with indicator light. The connectors are not placed on the back plate but behind the transformers on top. We get a pair of RCA inputs and two pairs of gold-plated WBT 0735 loudspeaker terminals plus the IEC power socket. The speaker terminals are fixed impedance but the output transformers have secondaries for 4, 8 and 16Ω. They get wired up according to a customer’s needs by the importer. The power transformer is made by Air Tight while the F-2007 output transformers are wound by Tamura Seisakuscho Ltd., a known Japanese iron specialist. Between the tubes sits a three-position damping selector which sets feedback level from 0 to 4 and 6dB. This also affects input sensitivity. The higher the feedback, the lower the sensitivity and circuit gain. The unit is supported on solid feet made from stainless steel and micro rubber.

The bottom plate is copper plated to double as RFI shield. A similar smaller plate serves as mounting base for the tubes and other parts. Internal connections are point to point with added supports. The circuit employs expensive Sprague capacitors nearly exclusively, with orange polypropylene units coupling the individual stages. The feedback loop and signal path use NOS resistors. The 300B cathode resistors are heatsinked and from Dale. The anode current is tube rectified, then goes to a classic Pi filter with a resistor in the first stage and a large in-house wound choke in the second stage which supplies the output stage. Tube heater currents are rectified and stabilized, output tube bias is automatic. Everything looks very orderly. Finally, the signal travels via shielded flying-lead wire from the back to the front where the Alps Blue Velvet potentiometers are placed.

Technical data (according to manufacturer):
Tubes: 2 x 300B, 1 x 5U4G, 2 x 12BH7A, 2 x 12AU7A/ECC82
Output power: 8W + 8W (8Ω)
THD: < 1% (6W)
Input sensitivity: 230, 300, 450mV (selectable)
Damping: 0, 4, 6dB (selectable)
Dimensions: 430 x 275x 245mm WxDxH
Weight: 24kg

The French magazine Son & Image reviewed this amplifier in their May/June 2006 issue. Their measurements were as follows: "The parameters varied with feedback. At 0.9% THD, output power in position 1 was 3.5W RMS per channel, in position 2 5.8W and 6.2W in position 3. In all positions the frequency response was 20Hz-20kHz (+/- 3dB) and ideally flat in positions 2 and 3. Clipping occurred from 7.1 W and at 8W THD equaled 7%. At 1W THD was 0.16%." That review can be found here.

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