Zähl HM1 Review & Measurements

Zähl HM1 Review & Measurements

Zähl HM1 Reference Desktop Headphone Amplifier

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Disclaimer: I received a small industry insider discount on my HM1 that still puts it very close to MSRP. I purchased this unit months before joining Headphones.com and all thoughts and opinions are my own. I would not have gone home and immediately sold two of my favorite amplifiers to help afford it if I did not think this was something truly special.

Introduction

The Zähl HM1 is a headphone amplifier built by Michael Zähl, who is most well known for his mixing desks used by artists such as The Chemical Brothers, Nils Frahm, and Aphex Twin.

Whilst the Zähl AM1 mixing desk will set you back likely close to $100,000 depending on configuration, the HM1 headphone amplifier costs $7999, which does indeed make it one of the most expensive headphone amplifiers on the market, however, this is far from your ordinary amplifier, and there are a multitude of reasons as to why after hearing it at Munich High-End, I came home, immediately listed multiple pieces of my gear for sale, and placed an order for the HM1 myself….

Design

The HM1 design follows two main principles; everything analog and linear, and no compromises.

That first aspect might sound rather obvious at first, it’s an amplifier right? Why would there be the need for anything digital anyway?

Well, it goes a little deeper than just having no digital components. Not only is absolutely everything down to the logic circuits that control the inputs and features all done entirely within the analog domain, but the design also excludes any form of switching or high frequency components. Many amplifiers use digital or high frequency components in various areas from remote controls to protection circuitry to switchmode power supplies themselves.

In the HM1, all power supplies and regulators are linear, no switching components are used anywhere, and even the remote control (not yet available) will be wired rather than wireless, as using wireless would require digital control circuitry.

As to the ‘no compromise’ aspect, it’s probably best to talk about this as we go through the rest of the design itself.

The power supply transformer for the HM1 is located in a separate chassis to the amplifier, and offers a switch on the front to allow the user to select any regional voltage. This then outputs a stepped down AC to the amplifier, thus keeping the transformer away from the main chassis and helping to reduce any mains leakage (which works well as we will see in the measurements), and then this is rectified to DC in the main chassis itself to keep the actual PSU impedance as low as possible, nice stuff.

But the first aspect of that ‘no compromise’ design is also hidden away here. The transformer unit does not just provide a single AC supply to the amplifier. Zähl wanted to ensure that every single part of the device that could be separated, was separated. There are independent, completely potential-separated windings for the left and right channels, both for the preamp and the headphone amp. As well as another independent winding for logic circuitry and front-panel indicators, so that they do not share power with the main amplifier itself.

The rectification, filtering and voltage regulation for each channel is also completely separate, to ensure that sudden power demands from one channel do not impact the potential amount of energy available to the other channel.

Now we come to the amplifier itself. The chassis is a dark grey, with a gorgeous finish that for the faceplates is a smooth, almost ceramic matte effect, and for the main body and fins is a grey powdercoat with a subtle but welcome sparkle.

On the rear are two sets of inputs, each with a balanced and unbalanced option, and two outputs, one of which is a line output, with again both balanced and unbalanced options. And the other is an RCA ‘A-Thru’ output.

The A-Thru signal is tapped from input channel A, after the input stage (it is not wired in parallel to the inputs themselves), but before any volume or other sound adjustments. This can be used to wire the ‘A-Thru’ output either to another device, or for example into the ‘B’ input to allow you to A/B or even mix the same source but with and without sound adjustments.

The inputs also each have a toggle for 0dB and +15dB gain. You probably want to keep this on 0dB unless you’re using a very low voltage source. The amplifier has an input voltage limit of 11Vrms on 0dB gain (most DACs are 4V or 5V), but 2Vrms on +15dB gain. So don’t go connecting a standard balanced DAC on 15dB gain else you’ll likely encounter some clipping and distortion.

Also worth noting that the amplifier left/right inputs are reversed compared to most amps, so check to make sure that you’re hooking it up correctly!

On the front are the volume controls, each independently adjustable for inputs A and B. In-between them are four buttons, the top two are for enabling/disabling each input, so you can shut one off when not in use or not wanted, or you can enable both and mix the two to the output. I personally use this to have my main music playback chain feeding input ‘A’, and then a DAC connected directly to my PC feeding input ‘B’ so that I can listen to music and mix in voice chat from discord for example.

The two buttons labeled ‘DIR’ mean ‘Direct’, and when pressed, will set that input to bypass the sound adjustment features. This allows you to completely remove the sound adjustment circuitry from the signal path if you do not wish to use it, or only apply it to one input but not the other.

Another ‘extra mile’ design element is hidden within the volume controls themselves. When either volume knob is turned down to minimum, a relay shuts off that input entirely to prevent any noise or residual signal level coming through and keep things absolutely silent, regardless of whether the input is actually enabled or not. That alone is a nice little extra, but how it’s done just made me smile. This is done via the use of a linear TKD potentiometer in a logic circuit, attached to the logarithmic TKD potentiometers being used to adjust volume. But Michael Zähl did not just put this linear pot onto the end of the balanced pot, oh no, that wouldn’t be good enough!

Instead, the volume pots are modified to separate the left and right channels, and have the linear pot physically in-between them to provide just that little bit of extra channel separation. Cool stuff!

This wasn’t something mentioned anywhere in the product page, just something that Michael told me about directly. So I wonder how many other interesting design aspects such as this might be hidden in the HM1?

To the left of the volume controls is a knob with two options; ‘Class A’, and ‘Class A + Servo’. The HM1 is a full class A amplifier, and does not use an AB design, nor any form of sliding bias. This means that it is effectively running at full tilt all the time, and any energy not used to drive the outputs is converted to heat. As such, fairly significant cooling is needed as the HM1 does pull around 50 Watts from the wall at all times.

Whilst many would say that Class AB done properly is just as good as Class A but without the drawbacks and challenges associated with the extra power consumption and heat output, others feel that Class A is still the way to go and won’t let a little extra on their power bill get in the way of their music.

Given as the HM1 follows that ‘no compromise’ approach, Class A was the only way to go here. But what’s interesting is that whilst most amplifiers will use negative feedback (a method of error correction in amplifiers) to achieve higher performance, and this is usually inherent to the design and cannot be disabled, the HM1 gives the user the choice.

Selecting ‘Class A’ takes a purist approach. Full Class A, and absolutely no feedback or error correction. Whereas ‘Class A + Servo’ still keeps the amplifier in Class A, but enables feedback for better objective performance.

Whilst some may find that one of these modes is their outright favorite, personally the reason I love it so much is simply that different headphones sound their best on different modes. And I’ve found myself changing this setting depending on which headphone I’m using.

Hifiman Susvara? Servo all the way. But HD800 or ZMF Atrium, no feedback gave me a better experience. And having the ability to switch is fantastic.

Not only that, but in the no feedback mode, whilst there aren’t many no-feedback design amplifiers to compare to, the HM1 seems to perform considerably better than other no feedback options such as the Enleum AMP-23R objectively too. More on this later…..

The toggle beneath the power button allows you to choose whether the amp outputs to the headphones, line outs, or both, and the knob just to the right of the HP outputs allows you to adjust the left/right balance of the output. Each notch adjusts the balance by less than half a dB, so you can make quite fine tuned adjustments with ease.

If you’re wanting to correct for a significant imbalance in your headphones or hearing though, you might need to do it with a different tool like DSP, this is intended for correcting small imperfections in a mix or setup, not for fixing major imbalances.

And then, we come to the sound adjustments.

The first two are bass and treble shelves, which allow you to apply shelves of upto +/- 5dB.

The responses are shown below. I’m quite appreciative of the fact that they do not extend too far. Personally I find that bass EQ adjustments which extend too much above 100hz tend to start making things bloated or muddy, whereas more subbass focused adjustments like this are for me quite preferable.

But what is REALLY special, is the knob on the right. This can be described simply as a ‘soundstage knob’.

Many devices have a ‘crossfeed’ feature, which feeds some of the signal from the left channel into the right (and vice versa), but with a slight delay. This is done to try to emulate the effect of a pair of speakers in front of you, where each ear can hear each speaker but with different amplitudes and delays, rather than headphones where the left ear only hears the left channel and the right ear only hears the right channel.

This usually has the effect of pushing things out in front of you a bit and can certainly help with tracks where elements are hard-panned to the left or right, but I wouldn’t call it an ‘increase’ in soundstage and usually isn’t something I find myself utilizing unless I’m listening to a track with hard-panned elements, usually older recordings.

But this knob is not just crossfeed, it’s quite a bit more clever than that, and it does a multi-step process to alter soundstage.

Firstly, it performs a ‘mid/side’ comparison, which looks at what content from the Left and Right channels is similar vs different.

It then adjusts the amplitude of the differences, but leaves the shared content untouched. It also then applies some crossfeed, but ONLY to the differences.

The result is that rather than having things pushed out in front of you like crossfeed, the soundstage is expanded or contracted in an extremely convincing manner. It separates elements in the mix more than they already are and the effect is honestly stunning. Centered elements remain untouched and don’t get smeared or blurred, and elements in a mix that are off at an angle don’t get brought out in front loads like with crossfeed, but simply placed further away, with a slight change in angle depending on how much wider/narrower you’re making things. I did not realize how badly I needed this until I had it, and now find myself listening to almost everything with this set to the +1 setting.

There are two levels of increase available, and two levels of decrease. Allowing you to expand or shrink the stage to suit the mix or keep it in the middle to pass things through untouched. You can also turn it all the way down to output mono.

Whilst I do tend to find myself using the +1 setting for most of my listening now, it’s quite interesting in that this feature really is not one with a ‘best’ setting, and is incredibly track dependent. Very well mastered and produced tracks tend to sound their best with no adjustment at all, and can get a bit overstretched if you push it up too much. Some tracks seem to have somewhat exaggerated soundstage in the first place, and do sound better when you bring things in a bit.

I find myself changing this setting quite frequently and nearly on a track by track basis. It’s brilliant to have this at your fingertips and works VASTLY better than typical crossfeed only implementations. And once again, this is ALL being done entirely in the analog domain, no DSP whatsoever.

If you’re wanting to A/B a sound adjustment setup compared to the original signal, or just avoid any extra circuitry if you’re not using the adjustments at all, you can quickly bypass the whole adjustment suite with the ‘Direct’ button for the input.

Sound

Power:

With the HM1, one of the first things about it that made me so comfortable to go home and sell my main two amps at the time (A Benchmark AHB2 and a Holo Audio Serene), was how well this thing powers really tough to drive headphones.

Getting headphones like the Hifiman Susvara loud enough is easy, almost anything can do it, but to actually have them sound ‘right’ is in my personal experience a fair bit harder, and I’ve gravitated towards much more powerful than usual amplifiers such as the Ferrum OOR, CFA3 and the Benchmark AHB2 for headphones like this in the past. Hifiman themselves has the EF1000, which is actually a 50 Watts per channel speaker amp, to power the Susvara.

When I first heard the HM1 at Munich, it was powering a pair of DCA Stealth, which I’d heard before..but not like this. When I’d heard the DCA Stealth in the past my main complaint was one that seems to be shared by many; dynamics. They just didn’t have much in the way of impact or ‘slam’ despite a lot of the other stuff that was good about them, but on the HM1, that was quite different. They were simply far better here than on any other setup I’d tried. DMS was there at the time, and borrowed a pair of Abyss Diana TC from a nearby booth (thank you Feliks Audio!), and we then gave those a listen too. Safe to say, the experience was simply fantastic, and again, they sounded better there than any other source I’d heard them on.

Interestingly the HM1 does not actually have a higher RMS power output capability than some other amps like a Singxer SA-1 for example, BUT, the subjective sound and level of authority you get with hard to drive headphones really is a step up here. And objectively, particularly in servo mode, the HM1 seems to simply shrug off difficult loads and does not show almost any change in performance in relation to load impedance.

But not only can it drive the really tough stuff without restriction, IEMs and sensitive headphones work excellently too. The next day, I tried my friend Skedra’s pair of Lime Ears Anima IEMs on the HM1, which I’d spent several hours the previous evening listening to on a couple different sources, and once again, it was simply put, outright better. I was already thinking about what I’d need to sell to afford one by this point….

And now, a fair few months down the line and with plenty of listening time at home, with various headphones such as the Hifiman Susvara, Final D8000 LE, Raal CA1A and SR1B, ZMF Atrium and Caldera and the 64 Audio U12T, I’m confident in saying that at present, the HM1 is overall the single best amplifier I’ve heard.

Synergy:

Gear synergy is something which can lead to all sorts of debates about which product is better/worse than another, and in many instances it comes down to what else you’re using it with.

The Hifiman Susvara sounds excellent on the Benchmark AHB2, and sounded soft and somewhat mediocre on the HeadAmp GS-X Mini. Whereas the ZMF Verite Closed sounded wonderful on the GS-X mini but too sterile and slightly aggressive on the Benchmark AHB2.

Sometimes headphones sound best on a slightly warmer amplifier and sometimes they want something more neutral, and the feedback toggle on the HM1 is a big reason as to why I’ve found it to work so well with absolutely everything I’ve thrown at it.

With feedback off, the presentation is a little warmer and ‘meatier’. A slight added sweetness to the treble that tames headphones prone to sibilance or harshness such as the HD800, and also a bit more weight and body to many elements which fits well with headphones such as the ZMF Atrium with it’s wonderfully powerful low-end response.

And then with headphones that could for the most part be described as more neutral such as the Hifiman Susvara or the Final D8000 Pro LE, enabling feedback gives you a slight increase in resolution and control. Soundstage is a bit more airy and open and it feels like the amplifier has absolute ‘grip’ over the drivers at all times.

Leading edge immediacy and force of impact is excellent, leaving absolutely nothing missing there compared to any other amplifier I’ve tried.

Having both of these options is incredibly welcome and allows the amplifier to fit well with all the headphones I’ve tried. It’s not an enormous change, you’re not fundamentally replacing the amplifier, but depending on your headphone, a presentation with absolute detail, grip and control may fit better, and for others, a little bit of sweetness and added body works wonders.

Performance:

As to the actual performance of the amplifier, it’s incredibly difficult to describe it without sounding like an ad. There is so much about this that is so good that I’m struggling to think of anything I can also bring up to balance out the negative and the positive. (Other than the price of course…)

The main drawback I’ve found is that with some very sensitive IEMs there is a slightly audible noise floor, not at all high enough to be a bother, but if you’re in a quiet room and listen with nothing playing you might be able to hear it if your IEMs are sensitive enough. Noise level is only 4dB higher than a Chord Mojo 2 for example, so if you can’t hear the noise on a Mojo 2 with your IEMs you won’t hear it on this either most likely.

I also would have liked to see a remote control included so that it can be used as a preamp without being in arm’s reach, though I understand why this can’t be done given the ‘analog only’ approach. And it’s also fairly big, but then so are the Holo Bliss, CFA3, HeadAmp GS-X MK2, SPL Phonitor and plenty of other high end amplifiers so that’s not uncommon.

But other than that? Honestly, this is simply the best I’ve heard. Resolution and instrument separation is absolutely top tier and only limited by your headphones themselves. Timbre is as lifelike as can be especially in the Servo mode, or with a welcome added lushness in the no-feedback mode, both of which work wonderfully with different songs and headphones. But what is perhaps most impressive is how this manages to present such incredible resolution, unrestricted dynamics and impact, and impressive neutrality, without being at all fatiguing.

The amplifier that is closest to the HM1 for me is the Holo Audio Bliss, which really does get incredibly close to the sound quality alone of the HM1, and shares some aspects such as being full Class A, having excellent objective performance, and massive power on tap, but the main difference in sound between the two is that the Bliss is just ever so slightly, if only by a hair, warmer than neutral.

Not at all close to other ‘warm’ amplifiers such as the Rebel Amp, GS-X Mini, Enleum AMP-23R or Hifiman EF1000, but when comparing to the Ferrum OOR for example the Bliss’ treble in particular is juuust a tad less sterile or dry. (Not that the OOR is in itself sterile or dry, it’s almost perfectly neutral in my view and amps like the Topping A90 or Benchmark HPA4 are more sterile and not as enjoyable as it is). The HM1 though is perfectly neutral in Class A + Servo mode, immensely dynamic, separates and stages wonderfully, and does all of this without ever feeling the slightest bit fatiguing. You can have your cake and eat it too.

But when you add the sound adjustments on top, it really is icing on that cake. The EQ is certainly nice to have and I feel is quite well done with the profile of the shelves not extending too far, and the 1.5dB increments being small enough to allow fairly fine grained control, but it isn’t necessarily anything super special. EQ is available on various other products and can be done digitally too. So it’s mostly a convenience aspect of having them at your fingertips.

But what really is unique and a genuinely fantastic improvement that you cannot replicate without a fair bit of DAW/VST setup, is the soundstage adjustment. Having this really does make you realize how much better so many songs could sound if only they were just a little bit bigger or better separated in the stereo image, or how much better some songs could sound if they weren’t quite so overly exaggerated in their staging or diffused.

Michael Zähl said it himself, that this is a mixing tool, not something that will get left on maximum all the time. If a song is properly recorded and produced, it should sound its best with no alteration, and I’ve found that to be the case, with albums like Daft Punk’s ‘Random Access Memories’ sounding the best with the knob at 0. But there is plenty of stuff especially in the age of loudness wars where a little bit more openness goes a long way, and the +1 setting on this adjustment has become something of a default for me other than for very well mastered tracks.

Measurements

Measurements Include:

  • THD+N (SINAD)
  • Bandwidth
  • THD+N vs Frequency
  • Power
  • Output Impedance
  • CMRR (Common Mode Rejection Ratio)
  • Power On/Off Behaviour (Safety Test)
  • Volume Matching vs Level of Attenuation
  • Noise (20hz-20khz, 20hz-96khz and 20hz-1Mhz)
  • Intermodulation Distortion (IMD)
  • Dynamic Intermodulation Distortion (DIM)
  • Crosstalk
  • Multitone
  • Square Wave Output

Additional measurements and test information available in the full report

Tests conducted with standard 4V line level XLR input, and tested at 4V/Unity-Gain output, 700mV (Headphone Level) output, and 50mV (IEM Level) Output.

Test Setup

  • Audio Precision APx555 B-Series analyzer
  • Measurement setup and device under test are running on regulated 230V power from a Furman SPR-16-Ei
  • HM1 was warmed up for 12 hours prior to testing
  • ‘Class A+Servo’ mode used unless otherwise specified
  • Exact analyzer/filter configurations for each measurement are detailed in the full reports
  • CH1 (Blue) = Left, CH2 (Red) = Right

Full Measurement Reports

Full Report

THD+N / SINAD

Let’s start by looking at performance at the output level specified by Zähl. The spec sheet lists the THD performance at 7.75V into a 30Ω load, which is 2 Watts continuous. My dummy load doesn’t have a 30Ω setting but does have 32Ω, so I’ll use that:

1khz Sine, 4V input, 7.7V output (32Ω Load, Class A + Servo):

114dB THD+N and all harmonics below -120dB. Very nice!

Now let’s do the usual figures.

1khz Sine, 4V input, 4V output (300Ω Load, Class A + Servo):

111dB THD+N here.

This is in ‘direct’ mode, without the sound adjustments included. Given as some analog EQ products have been shown to somewhat drastically reduce performance, how does the HM1 do here when we take it out of ‘direct’ mode and put the sound adjustments in the signal path? (I also adjusted all three knobs to make sure that stuff was actually being done)

1khz Sine, 4V input, 4V output (300Ω Load, Class A + Servo, ‘Direct’ mode OFF):

A 2-3dB change in THD+N when you use the sound adjustments. Hardly anything. Quite impressive to have this level of performance with so much circuitry beyond a typical amplifier being included! For comparison, the Schiit Lokius whilst certainly quite a nice product especially for the price, did in ASR’s testing reduce THD+N from 121dB in bypass, to 94dB when enabled but neutral, and 79dB when one of the controls was maxed. So it seems that the adjustments on the HM1 are significantly more transparent (other than the actual adjustment effect itself of course).

Now, what about if we turn feedback off?

1khz Sine, 4V input, 4V output (300Ω Load, Class A):

As would be expected, we see a rise in THD, particularly in 2nd order harmonics. But still performance is pretty excellent, and far ahead of other no-feedback amplifiers such as the Enleum AMP-23R, which for the same 300Ω load, with 2V in 2V out (as it is RCA), achieved 62dB THD+N, nearly 90x higher in absolute levels.

1khz Sine, 4V input, 700mV output (Headphone Level, 32Ω Load, Class A+Servo):

1khz Sine, 4V input, 700mV output (Headphone Level, 300Ω Load, Class A+Servo):

At headphone listening levels like this, load impedance has no impact on performance.

1khz Sine, 4V input, 50mV output (IEM Level, 12Ω Load, Class A+Servo):

With IEMs the HM1 works very nicely, this is similar performance to a Mojo 2 or Ferrum OOR for example, BUT, on very sensitive IEMs the noise is slightly audible when music isn’t playing, and there are amplifiers like the Singxer SA-1 with about 93dB SNR @ 50mV so you may find those to work better for really sensitive IEMs. Also, worth noting that these measurements are taken with the PSU box off to the side of the amp. If you stack the PSU box on top of the amp you may get more noise.

Bandwidth

The HM1 has an extremely wide bandwidth. Zähl says that the -3dB point is at 500khz! I unfortunately don’t have a sine signal generator which can go that high so cannot test this directly, but can show that up to 200khz it’s completely flat.

THD+N vs Frequency (96khz capture bandwidth)

(96khz bandwidth used on the analyzer. Don’t compare this directly to standalone audible band THD+N measurements as the measurement setup is not the same.)

Power

32Ω: 3.5W

300Ω: 0.4W

THD vs Measured Level in dBV (Output voltage):

The HM1 seems to show almost no change in distortion with more difficult loads.

Maximum output is limited by voltage not current.

Additionally, due to the 10.6Vrms output limit, and the +6dB gain at the headphone outs, if you have a DAC with an output level of anything up to 5.3V, you can run it with the HM1, completely max the amp volume and be certain that the amplifier is not reaching any limits or struggling to drive your headphones no matter what they are.

THD vs Measured Level in Watts:

This is quite a good example of why the maximum power spec of an amplifier alone does not necessarily give you much information about how well it will power difficult to drive headphones.

A ‘10 Watt’ amplifier may be worse for driving difficult headphones than a ‘2 Watt’ one.

How can this be? Well let’s explain:

The maximum power of an amplifier is given as the max power it can output before reaching 1% THD+N. It does not mean it will behave or perform the same all the way up to that level. Usually even if an amplifier can supply a lot of power such as 10 Watts, it’s often showing noticeable increases in distortion at lower levels like 0.5W.

Here’s a demonstration; below is the HM1’s THD vs Output level graph in dBV, and I’ve added the THD vs Output level results for another amplifier that has about 30% more power than the HM1. 30% is a lot! And so more power means it should be way better for hard to drive headphones right? Well, not quite.

As you can see in the graph, despite the fact that the 2nd amplifier can reach a higher MAXIMUM output, it begins current limiting and rising heavily in distortion far earlier. So it might have more maximum power, but driving difficult loads at higher levels, the lower power amp will actually be better here. The power spec of an amp is a hard maximum where it will either shut off or have huge levels of distortion, a little deeper inspection is required to see how well it is likely to drive difficult to power headphones.

As to why this is shown with dBV instead of Watts on the X-axis, it’s because if Wattage itself is used as the X-axis unit, then the traces shift left/right depending on BOTH output level and impedance, not just output level alone, making it extremely difficult to see how an amplifier’s performance changes for the same output but a different load unless you have a calculator handy. It gets quite messy.

The same graph as above, but plotted using Watts on the X-axis would look like this for example:

With dBV it’s also incredibly easy to see how an amp will perform for a given volume with YOUR headphones.

For example if you have HD600, that has a specced impedance of 300Ohm, and sensitivity of 105dB(1Vrms).

1Vrms = 0dBV, so just look on the 300 Ohm line, at 0dBV on the X-axis, that's the distortion your amp will have driving the headphones at 105dB SPL.

If you listen at 95dB SPL, just look at -10dBV instead. Easy!

We will be providing power graphs in dBV for the above reasons going forward, though will continue to provide graphs with Watts on the X-axis as well.

Output Impedance

Class A: 0.8Ω

Class A+Servo: 0.03Ω

(Output impedance vs Frequency graphs will be provided at a later date, I am currently waiting for Audio Precision to update the utility.)

CMRR

100hz: 71.3dB (HP Out), 71.2dB (Pre Out)

1khz: 71.2dB (HP Out), 71.2dB (Pre Out)

10khz: 71.8dB (HP Out), 71.8dB (Pre Out)

Power On/Off Behaviour

This test shows the output of the device when turned on/off, to check whether the output may put connected headphones at risk. Regardless of what this test may show, please ALWAYS disconnect your headphones when turning any amplifier on/off unless the manufacturer explicitly instructs you to do otherwise.

Turning on (from cold, 300Ω load):

Turning off (from warm, 300Ω load):

DC Offset is low, and no transient spikes above 3mV when turning off or on. So connected headphones are safe. Always good practice to disconnect headphones before turning any amplifier on/off though.

Volume Matching vs Attenuation

This test adjusts the amplifier to different levels using the volume knob, and shows the level difference in dB between the two channels.

Some amplifiers particularly at lower volumes will have channel imbalance. This test checks how closely the two channels of an amplifier match as volume is reduced.

+6dB (Max) = 0.03dB

0dB =0.18dB

-6dB = 0.44dB

-12dB = 0.61dB

-26dB = 0.19dB

-32dB = 0.08dB

-38dB = 0.49dB (50mV IEM Level Output)

1dB channel difference reached at -43dB

Volume matching is very good, keeping under 0.61dB all the way down to 50mV output (IEM Level) from 4V input.

Noise

256k FFT, 3 Averages

7.7uV upto 20khz (-114dB referenced to 4Vrms)

1M FFT, 3 Averages

68uV upto 1Mhz (-95dB referenced to 4Vrms)

No squelch/idle-mute circuit detected

IMD

Dynamic IMD (DIM)

Crosstalk

Crosstalk is higher than ideal, though this is likely unavoidable given the stereo base adjustment feature which requires channel comparison.

Multitone

Square Wave Output

Thanks to the extremely wide bandwidth, square wave output is extremely clean.

Zähl HM1 Reference Desktop Headphone Amplifier

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