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Delivery time approx. 1-2 weeks.
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HM1 Reference Headphone Amplifier
A reference amplifier with absolutely transparent sound, incredible precision and detail, and immense power reserves. A rich feature set for high-end enthusiasts and professional users.
Reviews
By musical head Wolfgang Hackhausen
(...) this acoustic precision weapon has absolutely no inherent sound. And in my opinion, this is one of the greatest compliments one can pay a music reproduction device (...) pinpoint spatial imaging (...) is capable of both fine and macrodynamic reproduction, effortlessly driving any (ortho-)dynamic headphone on the planet (...)
The full review can be found at musicalhead.de
Special features
Unique: Class A power and a step beyond
- Class A power amplifier in its purest form
- switchable negative feedback (“servo”) while maintaining Class A operation
Crossfade, compare or mix two stereo sources
- Crossfade or switch between two stereo sources (Channel A/Channel B)
- Reference listening comparison through exact level adjustment of the sources before switching
- Creating a mix of two music programs
Three output ports
XLR 4-pin – ¼“ jack – Pentaconn
Sound adjustment and stereo base width setting
Precise, finely tuned tools for sound adjustment. Stereo base width adjustment for adjusting the stereo image (spatiality).
“On the HM1, every headphone sounds the way it should.”
- highest impulse fidelity
- open frequency response
- Extremely low-impedance output for signal-accurate “guiding” of complex loads
The HM1 is manufactured in Germany in a limited edition of 50 pieces per year
- Analog technology in its purest form
- consistent dual-mono construction
- Logic circuits without clock generator
- purely linear power supply with mains transformer in its own housing
- uncompromising selection and selection of components
- complex interior structure
Details HM1
Class A – in HM1
We've equipped the HM1 with immense power reserves, which is evident from its idle consumption of almost 40W. The output power is more than sufficient for any application and is significantly increased in the millisecond range. The output impedance is exceptionally low. We measured the power amplifier's optimal operating point and then fine-tuned it in numerous listening tests with experts, especially mastering engineers. The same applies to the components used, which are naturally carefully selected. This design largely compensates for the potential disadvantages of Class A described below in the theory section.
Class A & Servo – in HM1
To take Class A a step further, we've implemented an idea that's as simple as it is ingenious: We take the best of Class A and negative feedback amplifiers and combine them. Class A operation is given negative feedback ("servo"). Since there is no crossover distortion, there is nothing for the negative feedback to correct. Rather, it only corrects the errors that arise from the interaction between the power amplifier and headphones. The negative feedback path is designed so that no artifacts are created by the correction. The output resistance drops to a value that is generally smaller than the sum of the headphone cables and connectors. This allows headphone systems to be tightly "guided". You could say that with the HM1, every headphone sounds the way it should.
Switching Class A/Class A & Servo
The previous paragraph suggests that the optimum is achieved with Class A & Servo operation. Why then is there a switchable option? Quite simply, because we judge our circuits by ear. Our Class A power amplifier was developed with the highest musical standards in mind. And we believe that, while not identical in sound, it is equally good. Depending on the headphones, music, and personal preference, each user can make their own choice. Even we were surprised that, despite significantly different characteristics and measurement results, the sound quality differs only in subtle details.
Simplified excursion into theory
Class A
Advantages: no crossover distortion on the power amplifier transistors and no artifacts due to negative feedback.
Disadvantages: The internal resistance of the output transistors, and usually also the emitter or collector resistors, are in series with the load. Simply put, the material properties of the output stage contribute to the overall listening experience. And since both the resulting internal resistance of the power amplifier and the resistance of the load, i.e., the headphones, are complex quantities, the result is also complex: It's quite possible that certain combinations of very high-quality amplifiers and headphones may not produce optimal results. Experienced audiophiles know this.
Class AB with negative feedback
Advantage: Any error at the output caused by the effects described above is "corrected" by the negative feedback circuit. As a result, the output resistance of the power amplifier, as long as there is no overload condition, theoretically approaches zero; in practice, it is in the double-digit milliohm range. The complex internal resistance of the headphones is virtually equalized by the low-impedance output. Thus, the headphone system is, so to speak, tightly "guided" by the power amplifier.
Disadvantage: The crossover distortion occurs first, and is only then corrected. The feedback control (at least theoretically) always lags slightly behind the signal, which can lead to artifacts in the signal. However, linear feedback audio power amplifiers are generally not pure Class AB. Very effective techniques are used to keep crossover distortion low without the need for complex and power-intensive Class A technology. Techniques are also used in the feedback path, so that the "lagging" of the correction plays almost no role.
Mixing stage
With a reference amplifier of the highest quality, the idea of using it for critical A/B comparisons is obvious - be it for the evaluation of different signal sources such as D/A converters or pickup systems in the high-end sector or the comparison of different mixes or sound processing in the professional sector.
A/B listening comparison
For this purpose, the HM1 features two inputs with independent level controls and on/off buttons. Precise level matching is essential for a proper listening comparison. The on/off function can also be controlled via a wired remote control, facilitating a blind comparison.
A/B crossfade
The mixer stage can also be used to crossfade two sources. This provides a very musical alternative to the usual "hard" switching.
mixture
And, of course, the stage is also suitable for mixing two sources. Two music programs create a new one. This new program can be listened to not only through headphones but also via the line outputs on the rear panel—for powered speakers, an audio recording device, etc. This transforms the HM1 into a reference-quality two-channel stereo mixer.
Also important: If only one channel is used, the mixer stage doesn't represent unnecessary "ballast." An unused channel is switched off directly at the input stage and thus cannot negatively affect signal integrity. This switch-off is controlled by a relay and is triggered as soon as a volume control is set to minimum or a channel on/off button is in the off position.
Sound adjustment
This isn't about making any significant changes to the sound. It's safe to assume that the HM1 will be used in conjunction with very high-quality equipment. A traditional tone control isn't appropriate.
However, it can be beneficial to make fine, precise adjustments to the frequency response, depending on the headphones, music program or personal preference.
Adjustment of high and low frequencies
The treble and bass adjustment, each in three fixed steps for boost and cut, is primarily designed for musical performance. We've refined proven circuits from studio technology; the sound isn't distorted, but rather accentuated. The basis is filters with carefully selected components, and all stages are precisely switched with relays.
Professional users use the level, for example, to find out whether a finished mix is balanced in terms of highs and lows or whether it can still be optimized.
Stereo base width, simplified theory
A special feature is the ability to adjust the stereo width. It's well known that the perception of spatiality differs when listening with headphones than with speakers. Instead of a cross-feed circuit, we use our stereo width adjustment. It's based on the mid/side technique proven in professional studios. It opens up additional possibilities.
The stereo signal is converted into a center signal (in simple terms, what L and R have in common) and a side signal (in simple terms, what L and R differ from each other). If you combine the center and side signals 1:1, you get the original stereo signal. However, if you change the ratio between center and side before combining them, you change the stereo base width. In our circuit, we leave the center signal as it is.We only change the side signal. If we lower its level, the stereo image becomes narrower; if we raise its level, the stereo image becomes wider.
Stereo base width setting in the HM1
You have direct access to the perception of spatiality. If a music program sounds too "wide" in your headphones, you turn the control one or two notches to the left. If you perceive a recording as too "dry" (centered), turn the control one or two notches to the right. The far left position is the mono position. This is preferably chosen for early stereo recordings when individual instruments were mixed to the extreme left or right—without any spatial reference.
Professional users use this stage to check the spatiality of their mix and uncover potential problems, especially in the extreme positions.
Also important: The DIR (Direct) button allows you to completely remove this stage from the signal path, thus reflecting the purist approach.
balance
For the balance control, we selected a precision potentiometer with 21 notches. The center position is perfectly calibrated and clearly defined by touch. Left/right volume differences of up to approximately 2.5 dB—caused by the music program, headphones, or your own perception—can be compensated in fine increments.
Headphone outputs/Balanced connection
The 4-pin XLR connector and 6.3mm jack socket are wired in parallel and intended for alternate use. Theoretically, both could be used simultaneously; the power amplifier is powerful enough. However, we do not recommend this for a reference-quality listening experience.
Ideally, the 4-pin XLR connector is used in conjunction with balanced headphones. This ensures that the signal streams from the two power amplifiers are not mixed, preventing mutual interference. The HM1 can achieve its full potential (see also "Dual Mono Setup").
Comparison of dual mono versus symmetrical power amplifier
Thanks to its dual mono design, the HM1 has all the advantages of a balanced power amplifier when driving balanced headphones and avoids their disadvantages.
Disadvantages of a balanced power amplifier include the significantly increased number of electronic components – a balanced output contains two output stages per channel, operating in antiphase. More components mean an increased possibility of nonlinearities, and two output stages double the output impedance. These effects are avoided in the HM1.
Easy connection of headphones with jack plugs
Another advantage of the HM1 architecture is that even unbalanced headphones with a 6.3mm jack plug can be operated without any problems.
(for this topic see also chapter “Dual Mono Construction”)
Line inputs
Balanced (XLR) or unbalanced (RCA) signals can be connected to Line Input A and Line Input B. However, XLR and RCA signals cannot be used simultaneously on the same input. The Bal/Unbal switches independently set the inputs to balanced (Bal) or unbalanced (Unbal) mode.
Each input has an additional switch that increases the input gain by 15 dB. This allows even low-level signals to be processed without any problems.
Line outputs
The signal at the line outputs corresponds to the signal in the headphones, except that it does not pass through the headphone power amplifier, but rather through balanced or unbalanced line output stages and is routed to rear connectors.
Connection options for power amplifiers, active speakers and more
Balanced and unbalanced outputs can be used simultaneously to drive power amplifiers, active speakers, level meters, or other components. Connecting a recording unit is also conceivable. When the HM1 z.BIf a comparison is performed between two stereo sources, the results can be documented there. The same applies if the HM1 is used as a mixing console.
A Thru Exit
The "A Thru" signal is coupled from input channel A. The coupling point is located after the input amplifier. The switchable 15 dB gain at input A is therefore effective. The Thru output has an unbalanced output stage.
Invitation to experiment and test
A signal present at input A can thus be routed to another application without loss. A conceivable setup is one in which "A Thru" is routed to a sound processing device or effects unit, whose output then feeds line input B. The A/B volume controls can be used to adjust the ratio between the original and processed signal. This setup is also ideal for testing any device: Channel A delivers the original signal, while channel B delivers the signal through the device under test.
Dual mono construction
The basic idea
The strictly separate design of the left and right channels is intended to eliminate any external influences in the circuit stages. The terms "channel separation" or "crosstalk" don't really address the issue. Music signals generally have certain left/right correlations, so an extremely low crosstalk value—moreover, usually determined using sine waves—doesn't mean much. However, if individual pulse power is required in the channels, it is essential that no coupling can occur via a shared power supply or via ground lines carrying signal current. Only in this way can an amplifier produce absolutely undistorted output signals.
Implementation in HM1
In the HM1, the power supply for the left and right channels is separate, namely for the preamplifier and power amplifier. The mains transformer has independent, fully isolated windings for each channel. Rectification, filtering, and voltage regulation are performed separately for each channel. Logic circuits and indicator lights have an additional winding and their own rectification, filtering, and voltage regulation.
The zero potentials of the two channels are connected to each other at one point, but this is done purely as a potential equalization, it is impossible for signal currents to be mixed.
Optimal headphone connection
If the XLR 4-pin output is used in conjunction with balanced wired headphones, the dual mono principle is fully implemented.
When using the jack output, the ground wires of both channels are connected to the jack socket or jack plug of the headphones. Headphones with a jack plug are therefore not quite the ideal application. However, the HM1's optimized ground routing ensures that adverse effects are minimized as much as possible.
(for this topic see also chapter “Headphone outputs/balanced connection”)
Power supply
Mains voltage and mains transformer are always potential sources of interference – consequently, we have outsourced them to the HMP1 mains adapter.
The power adapter also features a standby power supply that delivers a stabilized low voltage. This voltage is applied to the power switch on the front panel of the HM1 and activates the mains transformer via a relay.
Worldwide operation
The standby power supply has a wide-range input, is purely linear in design and complies with current standby regulations.
The mains input is equipped with one of the best available noise protection filters and can be adjusted for operation on the following voltages: 100V-110V-120V-220V-230V-240V.
The power switch on the back of the HMP1 can be used to completely switch off the device.
Connection cable length selectable
The connecting cable between the power adapter and the main unit carries only isolated low voltages. The cable length is widely variable and can be specified by the user when ordering.
Technical data
Inputs/Outputs
Line inputs balanced – impedance 20KΩ – max. level +23dBu
Line outputs balanced – impedance 47Ω – max. level +23dBu
both behave like transformers: a- or b-wire can be bridged to ground
Line inputs unbalanced – impedance 20KΩ – max. level +23dBu
Line outputs unbalanced – impedance 47Ω – max. level +23dBu
Headphone output
Impedance Class A 0.8Ω, Class A & Servo 0.045Ω
max. level +23.5dBu (+/-16.5V)
Power amplifier output power per side
RMS 4W/30Ω; 7W/15Ω
Peak 7.6W/30Ω; 11W/15Ω; 12W/10Ω (18W for 1.5ms)
Frequency response
10Hz ... 30kHz -/+0.05dB
1Hz ... 500kHz better than -3dB at the headphone output at +6dBu
Basic gain
Line/Line – 0dB – +15dB additionally switchable
Line/Headphones – +6dB – +15dB additionally switchable
External voltage
20Hz ... 20kHz unweighted RMS/A or B active
XLR/RCA output -102dBu
Headphone output -97dBu
Headphone output THD
Class A 0.07% – Class A & Servo 0.0005%
(typ. at +20dBu/7.75Veff at 30Ω load corresponding to 2W RMS)
Power supply
Mains voltage – AC 50-60Hz, switchable 100V/110V/120V/220V/230V/240V
Power consumption – typically 40W, max. 60W
Dimensions and weights
HM1: WxHxD approx. 225 x 90 x 300mm, approx. 5kg
HMP1 (power adapter): WxHxD approx. 170 x 60 x 185mm, approx. 1.2kg
Applications
Conventional
Preamplifier in a minimalist system
Example of a conventional headphone amplifier application: Your reference DAC on input A and an analog source on input B.
By connecting power amplifiers or active speakers to the line out outputs, the HM1 can be used as a preamplifier. Example of a minimalist setup: Streamer connected to HM1 input A, turntable (with built-in or external RIAA amplifier) connected to input B, active speakers connected to line out.
Precise listening comparison
2-channel mixing console
Effect/Test
Two sources at inputs A and B, precision level meters at the line output. Both sources are precisely adjusted in level using the level meter and volume controls, and then alternately activated using the channel on/off buttons.
As a mixer, the HM1 can combine any two sources at inputs A and B into a new music program—or simply crossfade from one source to the other. The result can be tapped from one of the line outs and recorded to a recording medium.
Using the "A Thru" output, a sound processor or effects device can be "inserted." It is fed from the "A Thru" and its output is routed to Input B. The balance between the original and the effect/processed signal is adjusted using the A/B volume controls. Instead of the effect, any device to be tested can also be inserted. The listening comparison is then performed by pressing the A/B on/off switch.
story
Why HM1?
There has been enormous development in headphone technology over the last 10 years.
Headphones in the professional sector
Top-of-the-line products now enable applications in the professional sector that were previously unthinkable.
While headphones were previously used primarily during recording, sound engineers now also use them for reference listening during the final mix. A pristine reproduction that doesn't embellish or obscure anything is essential, as this forms the basis for the mix and any necessary sound processing.
High-end headphones
Demanding high-end enthusiasts generally have music enjoyment in mind. So why shouldn't headphones or an amplifier shape the sound a little more pleasing than the actual recording? Anyone who has delved deeply into the subject will know the answer: Unfortunately, such "whitewashing" is unreliable. What is advantageous for one recording can be a disadvantage for the next. Even within a musical recording, a sound coloration that enhances one instrument usually comes at the expense of another or the overall balance. If the recording is really good, faithful reproduction is always the best option.
