Recently I had the pleasure of working on an Air Tight ATM-300B tube amplifier, a piece of high Japanese craftsmanship that perfectly embodies the single-ended Class A philosophy with the renowned 300B tubes. This amplifier is famous for its impeccable construction and refined sonic performance, which make it a cult object among high-fidelity enthusiasts. The single-ended configuration with 300B tubes is a deliberate choice to achieve warm, detailed musical reproduction, with an output power of about 9 watts per channel.

The amplifier arrives in the lab

The customer brought me this unit after purchasing it second-hand, complete with a full set of tubes that he wanted checked to verify their condition. He asked for a thorough inspection, both of the tubes and of the amplifier’s overall performance, to be sure everything was working as it should.

I started with measuring all the tubes using my trusted uTracer 3+. The 300B tubes were in excellent condition, with emission and operating characteristics perfectly in line with factory specifications. The signal and rectifier tubes were also in very good shape, a pleasant surprise considering this was a used purchase.

Instrument measurements and functional checks

After checking the tubes, I moved on to a series of instrumental measurements to verify the amplifier’s performance:

• Output power: the unit delivers about 7 clean watts; it reaches 9 watts, but with a clear onset of heavy clipping.
• Bandwidth: with minimal feedback, I measured a frequency response from 10 Hz to 25 kHz at -1 dB. With the selector set to maximum feedback, the bandwidth extends up to 65 kHz, an impressive figure for a single-ended amplifier.
• Distortion: at 1 watt output, total harmonic distortion (THD) always remained below 1% in all feedback configurations.

The damping factor selector

One nice feature of this amplifier is the three-position selector for the damping factor. This control acts directly on the amount of feedback, allowing the amplifier to be matched to the type of loudspeakers used. And here comes the part that makes me smile with satisfaction, much to the dismay of many extremist audiophiles: this amplifier, which sells for prices starting at around 22,000 €, uses negative feedback. The designer clearly understands that a certain amount of feedback is always necessary to achieve balanced, controlled reproduction. You cannot simply eliminate feedback entirely and expect the amplifier to sound good, and Air Tight proves this with a selector that lets the user choose the behavior best suited to their system.

Conclusions

All things considered, the Air Tight ATM-300B confirmed itself as a top-class amplifier, with high-level instrumental performance and a flexibility of use that few other units can match. All the tubes were in perfect working order, and the measurements confirmed the quality of the design. So much for those who claim that negative feedback is the absolute evil.

Below you will find all the measurement graphs

Bandwidth:

Selector on 1	Selector on 3

THD:

Selector on 1	Selector on 3

Square wave video in the 3 steps…