The black Benchmark HPA4 is a 2 RU rackmount reference headphone/line amplifier designed to deliver pristine amplification of analog sources for audiophiles, mixing and mastering engineers, and music aficionados in homes, recording studios, and mastering suites. Resulting from Benchmark's collaboration with THX, the HPA4 features a THX-888 headphone amplifier for an output signal with high voltage, current, and power complemented by low distortion, noise, and crosstalk. Designed, assembled, and tested in the USA, it boasts a sturdy brushed aluminum housing with a milled faceplate and milled sides.

The rear-panel I/O includes two balanced stereo XLR inputs, two unbalanced stereo RCA inputs, XLR and RCA stereo outputs, and a mono XLR output. The front-panel offers two headphone outputs, one unbalanced 1/4" TRS jack and one balanced XLR 4-pin jack. Volume adjustments are executed via the 256-step balanced relay gain control, which uses increments of 0.5 dB. The integrated touchscreen allows convenient access to common functions such as input selection, dim, mute, and more. Built-in protection systems guard against potential damage from DC, short-circuit, over-current, over-voltage, and thermal issues.

THX-888 Headphone Amplifier    High Voltage, Current, and Power    Low Distortion, Noise, and Crosstalk    256-Step Balanced Relay Gain Control    Balanced and Unbalanced Line I/O    DC, Thermal & Short-Circuit Protection    Touchscreen Control    XLR 4-Pin Balanced Headphone Output    1/4" TRS Unbalanced Headphone Output    Designed, Assembled, Tested in the USA

The collaboration between Benchmark and THX features THX Achromatic Audio Amplifier (AAA) technology, which reduces harmonic, intermodulation, and crossover distortion by 20 to 40 dB to guarantee a realistic and fatigue-free listening experience. It accomplishes this using a patented feed-forward topology to null conventional distortion and noise mechanisms, resulting in extreme linearity. THX AAA allows the amplifier to reach its maximum output power and sound pressure level without producing the distortion that normally accompanies increased output levels in traditional amplifiers.

The THX-888 amplifier is driven by a Benchmark line amplifier that features relay gain control, relay input selection, and relay muting. The line amplifier incorporates premium gold-contact relays. The gain control has 256 steps in 0.5 dB increments. It also features a balance control. Relay closures are precisely timed to deliver smooth volume changes. The HPA4 includes four independent 256-step attenuators: two for the headphone outputs and two for the line outputs. To make this all happen, the HPA4 includes a total of 64 precision relays.

The HPA4 is 100% analog and should be driven from an external D/A converter or analog source. It eclipses the performance of typical high-end preamplifiers by achieving much lower noise and distortion.

The THX-888 amplifier can cleanly deliver high voltage into low-sensitivity, high-impedance headphones and it can cleanly deliver high power and current into low-impedance headphones. The HPA4 delivers 11.9 Vrms into 300 Ohms and 6W into 16 Ohms. Given these extreme capabilities, Benchmark has incorporated several protection systems to protect expensive headphones and the amplifier itself. These include DC, short-circuit, over-current, over-voltage, and thermal protection.

The bandwidth of the HPA4 extends from 0.01 Hz to over 500 kHz with a drop of only 3 dB. The 10 Hz to 20 kHz spectrum boasts a maximum variance of only -0.003 dB. This means that the HPA4 delivers very precise amplitude and phase accuracy over the entire audio bandwidth. Wave shapes are accurately reproduced, and the entire spectrum is delivered with precise timing.

The headphone output of the THX-888 amplifier features a 1/4" TRS and an XLR 4-pin. While both outputs deliver the full performance of the HPA4, the XLR4 is most ideal due to its lower contact resistance. The HPA4 features two balanced stereo inputs via XLR 3-pin jacks and two unbalanced stereo inputs via RCA jacks. The line amplifier provides one balanced stereo output, one balanced mono sum, and one unbalanced stereo output.

The touchscreen provides easy access to advanced features such as balance control, input level offsets, input names, screen dimming, remote control, and function locking. Help screens explain the special functions.

The volume control knob features a high-quality optical encoder that is rated for heavy use. An acceleration feature makes it easy to move through the 256 volume steps while maintaining 0.5 dB/step resolution. A press of the control knob toggles between headphone volume, line out volume, or both.

Two bidirectional 12V trigger ports can be used to link the HPA4 with external D/A converters and power amplifiers. The trigger signal controls the power-up and power-down sequencing of the entire audio system. The trigger ports are compatible with most industry-standard trigger input or output ports, but can communicate bidirectionally with other Benchmark products.

The HPA4 includes independent on-screen mute buttons for the headphone and line outputs. Both can also be muted with the volume knob or with the remote control (available separately). An on-screen -20 dB dim button instantly reduces the level by 20 dB, providing a temporary volume reduction and an easy return to the previous listening level. This control makes it easy to transition between a normal listening level and a background level. The dim function is also accessible from the remote control.

Inputs may be renamed and unused inputs may be disabled. Input levels can be trimmed to provide input-to-input level matching. Screen brightness is adjustable and timers can be set to dim or shut off the display. The screen can be locked to prevent access to advanced features.

The case features a milled faceplate and milled sides. Top, bottom, and rear panels are made from thick aluminum and feature a brushed texture. The HPA4 was designed, assembled, and tested in the USA.

Related Questions and Answers

Q: 1. Can anyone compare the sound quality of the RS 175 to the Sennheiser HD 450BT wireless (bluetooth) headphones?

A: I am not seeing much reviews on the differences in Sound Quality. They should be very similar. The one main difference between the 2 will be the connectivity. The RS175 has a Base that has Analog and Digital Optical Audio Input to which it transmits the audio to the packaged headphones over Radio Frequency (RF). A System like this is mainly used with a TV to transmit the audio. The HD450BT connects with Bluetooth which is meant to connect with a portable device like a Smartphone or Tablet.

Q: 2. I've read some reviews on Amazon that Whibal cards aren't made the way they used to. Are they still made of solid plastic where the neutral grey color goes all the way through? Thanks

A: The WhiBal G7 White Balance Reference Gray Card is plastic and physically rugged--waterproof and scratchproof (color goes all the way through, not just a coating on top).

Q: 3. will this work with high quality pro mics like the RODE lavalier for iPhone mics?

A: Yes. This is compatible with that Rode, and most mics that do not require phantom power.

Q: 4. Is this an 18% gray?

A: According to Michael Tapes the designer, he states that it is an 18% gray.

Q: 5. Is this connector compatible with iPhone 11?

A: Yes, the Apple Lightning to 3.5mm Headphone Jack Adapter BH #APLAADAP is compatible with all iphones that use the Lightning connection which is the connection on the iPhone 11.

Q: 6. Is this case good for Senal SMH-1000 ?

A: I have Senal SMH-1200 and it fits good: not too tight or loose. Based on the picture of the Senal SMH-1000, it looks identical. I would think it would fit based on the picture. Sorry , closest i can come up with.

Q: 7. We cannot get a sound out of the headphone. The headphone have been charged and connected as instructed???

A: On the back of the right ear piece is a small power button. You need to hold it down for about 3 seconds until the green light above it turns on. This will also activate the base unit to turn on. Wait about 5 seconds and sound will come out of the headphones. Be sure that the volume control on the side of the right ear piece is turned up. Good Luck!

Q: 8. I want to use the RS 175 system to monitor the output of an analog mixer that is part of a home recording setup. How much latency can I expect between the time the signal appears at the mixer and the time it reaches the headphones? I can't find any specs on latency.

A: As it operates on RF (Radio Frequency) it should provide none to very minimal Latency. Unfortunately the Sennheiser website is not providing any addition information on the Latency.

Q: 9. Will this fit Sony WH-XB900N?

A: Yes, these should be fine for this headphone.

Q: 10. I'm looking to hook up external microphone to my iPhone 14 pro. Would this Apple adapter work for me ? Microphone Is Sunpak

A: Yes, the Apple Lightning to 3.5mm Headphone Jack Adapter BH #APLAADAP would be the adapter needed to connect a 3.5mm mic.