Let me describe my observations of this headphone amplifier from McIntosh.
First about the positive aspects:
- attractive appearance, combined with classic notes of audio equipment of the past, black metal chassis, chrome top, beautifully executed transformer boxes.
- fully balanced XLR connections for inputs and outputs.
- excellent overload capacity of the input, operating without distortion in the range from 2 Vrms to 16 Vrms (!).
- automatic (relay) switching of transformer outputs in accordance with the position of the headphone impedance selector knob.
- compact footprint for a tube device.
- sufficient output power reserve for most headphones
- low level of the output noise, absolute silence in the headphones during pauses between music tracks. Measurements show a value of about -108 dB for the noise floor.
- low (for tube devices) harmonic distortion, driven mainly by the value of the 2nd harmonic, which, according to some information, has a pleasant sound for listeners. I can't hear them, with a value of 0.05% at my listening volumes in Sennheiser HD650 headphones.
- transparent sound. Frequency response measurements show that the amplifier has a uniform gain in the entire frequency range from 20 Hz to 20 kHz, and the imbalance between the channels is minimal (0.2 dB), by ear the left and right channels sound equally smooth.
- absence of any hint of warm tube sound. The sound is completely analytical. I will attribute this to the pluses, since the device does not introduce improvisation into the reproduced sound. In a blind test, I would not distinguish this tube amplifier from a high-class solid-state amplifier.
Now about the negative aspects:
- the device is susceptible to EMI from nearby Wi-Fi routers, mobile phones and Bluetooth devices. In general, everything that works in the 2.4 GHz and 5 GHz frequency ranges. I had to move the amplifier away from my Wi-Fi router, otherwise, at rest, digital communications modulations were heard in the headphones. Some searching on the Internet revealed that this drawback is inherent in many tube amplifiers, and is caused by the design of the vacuum tubes, the internal electrodes of which act as mini-antennas for interference, then amplified by them. I had to reinstall the metal mesh covering the tubes as this significantly reduced the amplifier's susceptibility to electromagnetic interference, but at the same time reduced the aesthetic appeal of the tubes' glow.
- with such a high price, there are manufacturing quality control flaws. One of the 12AT7 tube sockets is soldered to the mainboard at an angle and sits slightly below the surface of the top case cover. This causes the tube to sit crooked when fully inserted. To make it look even with the rest of the tubes, I had to pull it out at a slight angle. I'm guessing the mainboard and tube sockets are not made in the US and import quality control is lame.
Another quality issue was with the screws that hold the mesh tube cover in place. The threads are in the stamped cover itself, which is thin metal and only has 1.5 threads. It is weak and the threads were stripped on one screw when the new unit arrived.
- everyone hates the volume knob on this amp. It's loose, very inconsistent in its steps, and way too sensitive in the most useful volume range. Basically, the most useful volume range is from the 12:30 to 2 o'clock position of the knob. My measurements of the output signal showed that the value jumps between predefined output volume levels. This does not look like an analog volume potentiometer, but more like a rotary encoder. My guess is that it goes to an analog input on the board's microcontroller, the potentiometer position is digitized, and then fixed gain settings are applied to the tube circuit. This works poorly, and is one of the major design flaws of this amp.
