Sunday, March 17, 2024

Enleum HPA-23RM Reference Headphone Amplifier


"Enleum" is a combination of the words "Enlightenment" and "Eum" (Korean for "Sound"). You may have heard of Soo In Chae's work under "Bakoon" which was a collaboration between Soo In (founder of Enleum) and Akira Nagai. 


The HPA-23RM (the "R" and "M" being shorthand for Reference and Mobile) is a unique headphone amplifier with two different amplification circuits - a voltage, and current output stage. The voltage stage is a MOSFET output stage based on a simplified version of the JET2 bias circuit seen in the AMP-23R. The current output stage is derived from the HPA-21's bipolar transistor circuit. While this device is not something you would lug around on a regular basis, it is portable enough to be moved around and can be used on-the-go thanks to its built-in battery. Similar in size to a paper-backed novel, the Enleum measures 116mm x 164.5mm x 22mm and weighs 750 grams.

On the front panel, a toggle switch selects gain, while a recessed rotary knob functions as both a volume control and power switch. A 3.5mm socket is mated to the voltage stage, while a 1/4 inch socket is used for the current output stage. A row of LED lights indicate charge status, battery life and operation. 

The inputs are located on the rear panel, with a USB-C socket used for charging, and a pair of RCA sockets and a 3.5mm socket used for the input signal. There is no selector switch, so only one input may be used at a time. If the 3.5mm socket is in use, this is given priority over the RCA input. 

The current-output stage is capable of 1W of power, and half of that into the voltage output (into a 30 ohm load).   


In operation

Rotating the volume knob from its resting position to about 8 o'clock brings the unit into standby mode. The LEDs light up to indicate the battery level. The HPA-23RM uses a discrete resistor relay network to control the volume and further advancing the knob unmutes the unit, with an audible click as you move between each volume level. The manual advises you to bring the volume knob back to the standby position before plugging and unplugging headphones to avoid damage to the unit. 

The unit is specified to operate for up to 5 hours of use in voltage output mode, and 3.5 hours in current output mode, which matched my user experience. Thankfully, the HPA-23RM can be charged while in use.

Sound Quality

For the purposes of this review, I used the Enleum with an iFi Pro iDSD DAC as a source. To cover a variety of IEMs and headphones, I tried the Enleum with the Letshuoer S12 Pro IEM, Beyer DT-880 (250 ohm), Drop HD 8xx and the Audeze LCD2.1. 

As a starting point, Enleum recommends that the voltage output be used with IEMs / headphones that are sensitive, or have a varying impedance curve, while the current output be used with headphones that have a flat impedance curve such as planar headphones. This is not a hard rule, and Enleum does encourage you to experiment. Matching a headphone with a varying impedance curve with the current output does result in a frequency response that tracks the impedance curve, which could be either a good or bad thing depending on your objective. For example, the Drop HD 8xx has an impedance peak at about 100 Hz and a falling impedance in the high-frequencies. Listening to these headphones through the current output gives you added bass and attenuated high-frequencies. Unless you are seeking to alter the tone of your headphones, I found following Enleum's advice to give the best results.

Tonally, the Enleum has a slightly warm and organic midrange, while maintaining an airy and detailed treble. The real shocker is the tightness and power of the bass, which is quite impressive if you consider that this is a battery-powered device. On bright headphones like the Beyer and Drop, this does balance things out a bit, although the warm-sounding Audeze sounded great with the Enleum too. 

The Enleum is highly detailed throughout the frequency range and casts a wide and open soundstage, with a sense of dynamic freedom that eludes most headphone amps. From a performance point of view, the Enleum is easily at the very top, outperforming the headphone output stage of the iDSD DAC, or the Vioelectric HPA V281 that I owned in the past. In my view, the Enleum is really in its element when paired with planar headphones.  

The Enleum does have its weak points. Although there is a gain switch, using either the Beyer or Drop headphones on current mode was tricky, with the volume steps being too far apart. On the positive side, the discrete resistor volume provided precise channel balance even at very low volume levels or high-sensitivity headphones or IEMs. I would also have preferred for the voltage output mode to be paired with a 1/4 inch socket too. 

Battery life is a bit short to make mobile use practical and the repeated charging cycles will necessitate regular battery replacement. Thankfully, the Enleum uses a spring-loaded battery holder for its dual 18650 Lithium-ion batteries, which are widely available. Your unit shouldn't require a return home to South Korea for servicing, and battery replacement should be straightforward. 


Despite the high price tag, the Enleum rewards with top-level sonic performance. It is a must-audition for planar headphone users, and does a fine job with dynamic headphones too. Don't let the mobile moniker fool you though, this amp makes perfect sense for desktop use too - Highly Recommended.

A big thanks goes to Cornerstone Audio for making this review possible.

Price : S$ 4,200

Cornerstone Audio
444 Race Course Rd, S21888 (by appointment only)
Telephone : +65 8890 4885         

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