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(More customer reviews)Though I'm the satisfied owner of a Fiio E9 headphone amp, I own Ultrasone PRO750 headphones which are much like Grados only with more robust bass. The Ultrasones sound dry on some recordings, so when I found the Bellari HA540 Class A tube headphone amp at a steep discount, I purchased it. Since my integrated amp has two tape loops, I can connect the Fiio and the Bellari simultaneously.
The Bellari is manufactured in the USA by a division of Rolls. Readers might be familiar with Rolls through their HA43PRO headphone amp, which is often used to connect to the RCA outs of a TV. The Bellari uses one tube, a 12AX7, which is very common and easily sourced. Rather than use the stock tube, I purchased a Mullard. I'll only use the stock tube to fill in when I need to get another.
The Bellari is a Class A amp, but is barely warm to the touch. It's an OTL design: there's no output transformer. In practical terms, that means your headphones are providing the load on the tube. Headphones with too low an impedance could overdrive the amp. A headphone amp is the perfect application for an OTL design because headphones rarely have impedances as low as loudspeakers, which are mostly in the 4 to 8 ohm range.
The lowest impedance headphones I own are the Denon AH-D2000 at 25 Ohms, and the Bellari can handle it--but that's about as low an impedance as I'd want. The Ultrasone PRO 750 isn't very efficient, but the Bellari has lots of gain--it can drive the Ultrasones into pain territory without trying. It has two gain settings: a low setting for iPods, and a high setting for typical line out devices like CD players or the tape loop of a preamp, integrated amp, or receiver. The low and high designations refer to the input voltage, iPods having lower output than CD players or tape loops.
My expectations for the Bellari were more than met: it adds liquidity to the Ultrasones. I'm not into tubes per se, but it's a synergistic combination. This isn't a fuzzy warm tube sound--the Bellari has some real kick, with subterranean bass. The OTL design has a very clear, open sound, and if you've never heard an amp like this, your first impression will be how different it sounds from solid-state designs.
Apart from upgrading the tube, the only other change I recommend is to remove the tube cage (unless you have children or pets). The four small sheet metal screws that secure the top of the case are a pain. The tube cage is held in place by two Allen bolts underneath the top of the case. Removing the tube cage allows you to replace the tube without opening the case.
My only criticism of the Bellari is that the headphone out jack is located on the rear of the amp, but you can reach it easily, so it's not a problem. If you want to experiment with a tube amp, you may find the Bellari HA540 a perfect introduction.
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