Zähl HM1 Reference Desktop Headphone Amplifier
2025 Shipments now available for pre-order. First batch ships January 2025. 16 remaining!
There are just 50 Zahl HM1s being produced. Headphones.com has secured 17 units for North America in 2025
The Zaehl HM1 Reference Desktop Mixing Headphone Amplifier can only be purchased in North America at Headphonis.com.
Highlights
- Analog-only design
- Regular dual-mono design
- Circuits using logic without clock generators
- A straight-line power supply with an external power transformer
- Perfect impulse reproduction No-compromise component selection No-compromise external and interior structure
- Extremely broad frequency response
- Complex loads are accurately controlled by an extremely low impedance output.
The Zähl (pronounced Zaehl) HM1 is a top-notch, totally analogue, Class A reference headphones mixing amplifier that was created without sacrificing quality to provide an unparalleled degree of transparency and detail that will excite both audiophiles and the pickiest pros.
Additionally, the Zähl HM1 has a stereo base control, two distinct stereo source inputs with a cross-fade, and a precise two-band EQ. The goal of its design is to provide the most accurate reproduction possible.
Two Different Power Stage
There are two modes of operation for the Zähl HM1: Class A and Class A + Servo. With no crossover distortion and no artifacts that produce negative feedback, the first mode produces the clearest sound. In order to address mistakes that arise between the power amplifier and headphones, the second mode incorporates negative feedback into the same Class A circuit. Although both options function better technically, various users will prefer each method.
Mixing Features
Two input sources are included with the HM1 and can be utilized for critical A/B comparisons of various DA converters, cartridges, mixes, or effects. For accurate level matching, a separate volume control is provided for each input. Every input has the option of being gently cross-faded or hard switched.
It is also possible to combine the two sources. Both the headphones and the line outputs, which can be connected to active speakers or a recording system, receive the updated mix.
Sound processing
A delicate two-band EQ with set frequencies and a mild boost or cut is included with the HM1. Instead of altering the sound character, it employs tried-and-true studio circuits to provide delicate tonal accent adjustments, in contrast to conventional tone controls found on amplifiers.
Additionally, the HM1 has a Stereo Base control that allows you to expand the stereo field without sacrificing information in the mid-channels. This control is similar to the circuit used in their IM1 module or AM1 console. The spatiality of a mix that ranges from mono (completely left) to extremely wide can be checked with this processor. To preserve the signal as pure as possible, the stereo base stage can be fully turned off.
Additionally, a balancing control allows you to adjust for any imbalances in the mix, the user’s hearing, or the headphones. Volumes up to 2.5 dB on either side can be compensated for with the 21-position detent.
Dual Mono Design
To prevent inter-channel interference, the HM1 has a dual mono design that rigorously isolates the left and right channels. Both the preamplifier and the power amplifier stage are powered by separate sources for each channel. Additionally, each channel has its own independent, fully potential-separated windings in the Mains transformer, and each channel has its own filtering, voltage regulation, and rectification.
The headphone amplifier has an XLR 4-pin connector and a 6.5mm Jack output. By using the latter, the dual mono principle is guaranteed to be perfectly maintained without any interruption at all.
The power supply has a wide variety of inputs and a strictly linear architecture. The mains input can be configured to run at the following voltages: 100V, 110V, 120V, 220V, 230V, and 240V using one of the best interference filters on the market.
Power Stage
Class A – HM1 realisation
Given that it uses nearly 40W in idle mode, it is already clear that the HM1 power stage has enormous power reserves. For any application, the output power is more than adequate, and it is significantly boosted in the millisecond range. The resistance of the output is exceptionally low. The power stage’s ideal working point was initially determined theoretically, then by measurements on sample units, and ultimately refined by a series of listening tests with specialists, particularly mastering engineers. This also holds true for component selection. The potential drawbacks of Class A, which are discussed in the theory paragraph below, are substantially offset by this careful design approach.
Class A & Servo – HM1 realisation
In order to advance Class A, we employ a concept that is both straightforward and clever: We combine the greatest features of Class A and negative feedback amplifiers to create: Negative feedback is added to Class A operation (“Servo”). The feedback circuitry has nothing to fix because there is no crossover distortion. Instead, it only fixes the mistakes brought forth by the power amplifier and headphones interacting.
The feedback path is set up so that the correction doesn’t produce any artifacts. The output resistance decreases until it is less than the total of the connections and cables for the headphones. Headphone systems can be accurately controlled in this way. Any headphones should sound good when used with the HM1.
Why Class A and Class A & Servo?
One could conclude from the previous paragraph that Class A & Servo operation yields the best results. Why, then, is the option to switch still available? Just because we use our ears to judge our circuits. The highest musical demands were taken into consideration when developing our Class A amplifier. Although it doesn’t sound exactly like the servo version, we found that it still sounds fantastic. The user can choose based on his particular preferences, music, and headphones. Even we were taken aback to discover that the hearing difference is quite slight, even if the characteristics and measurement results are obviously different.
A LITTLE THEORY
Class A
Benefits include the absence of crossover distortion at the output transistors and negative feedback-induced artifacts.
Cons: The emitter or collector resistors are often connected in series with the load, as is the internal resistance of the output transistors. To put it simply, the sound is mostly determined by the material characteristics of the output stage components. It is also normal for some combinations of even extremely high-quality amplifiers and headphones to not yield the best results since the internal resistance of the output stage and the resistance of the load, or the headphones, are both complex objects. This experience is familiar to seasoned audiophiles.
Class AB with negative feedback
Benefit: A negative feedback circuit “corrects” any output fault brought on by the effects mentioned above. Furthermore, the power amplifier’s output resistance is in the 2-digit milli-ohm region in practice, but theoretically it approaches 0 when there is no overload problem. The low impedance output of a headphone system nearly equalizes the complicated internal resistance. Thus, the power amplifier firmly “guides” a headphone system.
A disadvantage is that the crossover distortion happens first and is then fixed. Therefore, theoretically, negative feedback control always lags a bit behind the action, which might cause signal artifacts. Modern audio power amplifiers with negative feedback, however, are typically not strictly Class AB. Crossover distortion is kept to a minimum using very efficient methods instead of the expensive and power-hungry Class A technology. The negative feedback signal employs certain strategies to virtually eliminate the “lagging” of the correction.
Mixing Stage
A/B comparison
CRegarding the HM1’s reference quality, it is most evident that it should be used for crucial A/B comparisons, whether these comparisons are between professional-grade mixes or sound processing options or high-end sources like D/A converters or cartridge systems.
The HM1 includes two inputs with separate level control and on/off buttons for this purpose. A key A/B listening comparison requires precise level matching. A blind comparison is made easier by the fact that the on/off function can also be controlled with a cable remote control.
Cross-fade
Crossfading two sources is another purpose for the mixing step. An extremely melodic substitute for the more common “hard” switching.
Mixing two sources
Naturally, any two sources can be mixed at this step. Two music programs are combined to form a new one. This new program can then be listened to through the headphones as well as the line outputs on the rear of the device, which can be used to connect recording devices and active speakers. As a result, the HM1 becomes a reference-quality stereo mixing console with two channels.
Importantly, the mixing stage is not unnecessary “ballast” if only one channel is utilized. Since an unused channel is turned off right at the input stage, it cannot compromise the integrity of the signal. Relays are used to achieve this switch-off, which is activated when a channel on/off button is in the off position or a volume control is set to the lowest setting.
Sound Adjustment
The purpose of this step is not to significantly alter the sound. Since all of the audio equipment that is often attached to the HM1 is of very high quality, a traditional tone control would not be suitable.
However, depending on the headphones, music program, or simply personal choice, it may be beneficial to make small, exact adjustments in the frequency response domain.
Low and high frequency adjustment
The main function of the low and high frequency adjustment, which are each in three set stages as boost and cut, is musical. To create a stage that adds delicate accents rather than altering the sound character, we have further improved tried-and-true circuits from studio technology. Filters with certain components serve as the foundation, and relays, of course, perfectly switch every circuit.
Experts utilize the feature, for instance, to determine whether a mix is optimized or balanced in terms of high and low frequencies.
Stereo Base control, a little theory
The ability to change the stereo width is a unique feature. When listening through headphones as opposed to loudspeakers, the sense of space is altered. We utilize our stereo base width setting in place of a cross-feed circuit. The mid/side approach, which is frequently employed in professional studios, is its foundation. It offers extra advantages.
The stereo signal is transformed into a side signal, which just indicates what separates L and R, and a mid signal, which simply indicates what L and R have in common. The original stereo signal can be obtained by adding the mid and side channels in a 1:1 ratio. Nevertheless, the stereo base width is altered if the mid-side ratio is altered prior to their summation. We don’t alter the mid signal in our circuit. All we do is change the side signal’s intensity. The stereo image gets narrower when the level is lowered and wider when the level is raised.
Stereo Base control, what it does
The perception of spatiality is directly accessible to you. One will move the joystick one or two steps to the left when listening to a music program that appears to be excessively “wide” in the headphones. Turn the slider one or two steps to the right if you think a recording is too “dry” (centered). The position on the left is mono. When individual instruments have been combined to the far left or right without any spatial reference, this is the ideal condition for early stereo recordings.
Professional users can use this option to verify the spatiality of their mix and may find potential issues, particularly in the extreme positions.
Importantly, this stage can be fully turned off from the signal path by pressing the DIR (Direct) button. The purist approach is considered in this way.
Balance
For balancing control, a precision potentiometer with 21 detents is available. The center point is haptically distinct and precisely tuned. Left/right volume discrepancies of up to around 2.5 dB, which can be brought on by a music program, headphones, or personal perception, can be adjusted in tiny steps.
Headphone Outputs + Balanced Connection
The ¼-inch jack socket and XLR 4-pin connector are wired in parallel and have different uses. In theory, if the power amplifier is powerful enough, you could utilize both simultaneously. However, we do not advise such use for a reference-quality listening experience.
The XLR 4-pin should ideally be used with wired headphones that are balanced. Mutual interference is impossible in this situation because the signal currents from the two power amplifiers are not combined. The HM1 can reach its maximum potential (see also “Dual Mono Design” chapter).
Dual Mono vs. Balanced amplifier design
The HM1 avoids the disadvantages of a balanced output stage while still offering the same benefits thanks to its dual mono architecture.
Because a balanced output stage has two output stages per channel that operate in reverse phase, it has the disadvantage of having a much higher number of electronic components. The output impedance doubles with two output stages. Non-linearities are more likely to occur when there are more components.
¼ inch jack compatible
Unbalanced headphones with ¼ inch jack connections can also be used without any issues thanks to the HM1 architecture.
Line Inputs
Line Inputs A and B can be linked to either balanced signals (XLR) or unbalanced signals (RCA). It is forbidden to utilize RCA and XLR on the same input at the same time. The inputs are independently set to either balanced (Bal) or unbalanced (Unbal) mode by the Bal/Unbal switches.
An additional 15dB gain is added by each switch that each input offers. This guarantees that even low-level signals can be processed without any issues.
Line Outputs
The signal at the line outputs is identical to the signal in the headphones, with the exception that it is routed to connectors on the back via balanced or unbalanced line output stages rather than the headphones amplifier.
Connect active speakers, power amps or other gear
Power amps, active speakers, level meters, and other devices can be driven concurrently via balanced and unbalanced outputs. One of the outputs may be used to record the outcome, for instance, if the HM1 is used to compare two stereo sources. When utilizing the HM1 as a mixing console, the same holds true.
“A Thru” Output
The post-input amplifier’s input channel A is used to tap the “A Thru” signal. The “Thru” signal is impacted by the switchable 15dB boost stage at Input A. The output stage of the Thru output is not balanced.
Setup your own test bench / experimental bench
As a result, a signal that is connected to Input A can be lostlessly sent to another application. As an example of an application, “A Thru” is sent to an effects unit or sound processing device, whose output is then sent to Line Input B. The A/B volume controls allow you to adjust the ratio of the original signal to the processed signal. Naturally, this configuration is also great for testing any device: The original signal is supplied by channel A, while the signal from the equipment to be tested is supplied by channel B.
Dual Mono Design
The idea
Strictly segregating left and right channels is intended to prevent interference from entering the circuits. A phrase such as “crosstalk” fails to capture the essence of the issue. Since music signals typically contain left and right references, a very low crosstalk value—which is also typically tested using sine waves—does not always indicate much. However, music causes extremely powerful signal pulses, which in turn cause the output stages to demand enormous currents on an individual basis. In these situations, it is crucial that there be no coupling through ground lines carrying signal current or a shared power source. An amplifier can only generate output signals that are completely distortion-free in this manner.
Realisation inside the HM1
Preamplifiers and power amplifiers in the HM1 have independent power supplies for the left and right channels. Each channel in the mains transformer has its own windings that are fully potential-separated. Each channel has its own voltage regulation, filtering, and rectifying. There is an additional winding and independent rectification, filtering, and voltage control for logic circuits and front-panel indicators.
Signal currents cannot be mixed since the ground potentials of the two channels are solely coupled to one another for the sole purpose of potential equalization.
Optimal output connection
The dual mono idea is fully applied when balanced wired headphones are used in conjunction with the XLR 4-pin output.
Both channels’ ground wires are plugged into the jack socket and the headphones’ jack connector while the jack output is in use. Therefore, using headphones with jack plugs is not the best option. However, the HM1’s optimized ground routing guarantees that the negative impacts are minimized.
(See also Chapter: Balanced Connection / Headphone Outputs.)
Power Supply
We have outsourced the mains transformer and mains voltage to the HMP1 mains adapter because these are always possible sources of interference.
A standby power supply device that provides a stabilized low voltage is also included in the mains adapter. This voltage is applied to the HM1’s front panel Power switch, which uses a relay to turn on the mains transformer itself.
Worldwide operation
The standby power supply is fully linear, has a wide variety of inputs, and conforms to standard standby power standards.
The mains input can be configured to operate on the following voltages: 100V, 110V, 120V, 220V, 230V, and 240V. It also has one of the best interference filters on the market.
The HMP1 can be fully de-energized by turning off the mains switch on the rear of the device.
Variable cable length
Only potential-separated low voltages are carried by the connection cable between the main unit and the mains adapter. When placing an order, the user can specify the cable’s length, which varies across a broad range.
