Zahl HM1 User manual
ZÄHL HM1
Manual V 1.0
HM1 Reference headphones mixing amplier
Reference Amplier with unprecedented transparency and precision,
attention to detail and massive power performance.
Rich feature set for both high-end enthusiasts and professional users.
• perfect impulse reproduction
• ultra wide frequency response
• extremly low impedance output precisely controls complex loads
• pure analogue design
• consistent dual-mono layout
• logic circuits without clock generators
• straight linear power supply with power transformer in external housing
• no-compromise selection of components
• no-compromise interior and exterior construction
Designed and manufactured in Germany.
CONTENTS | P.3
CONTENTS
NOTES, SAFETY INSTRUCTIONS, WARRANTY ....................................................................... 04
SCOPE OF DELIVERY ............................................................................................... 05
SET UP ............................................................................................................. 05
LEGEND ............................................................................................................06
APPLICATION EXAMPLES...........................................................................................08
SPECIAL FEATURES ................................................................................................09
DETAILS.............................................................................................................10
Power Stage.........................................................................................................10
Class A - HM1 realisation......................................................................................... 10
Class A & Servo - HM1 realisation .................................................................................10
Why Class A and Class A & Servo? ...............................................................................10
A little theory.......................................................................................................11
Class A............................................................................................................11
Class AB with negative feedback ..................................................................................11
Mixing Stage..........................................................................................................11
A/B comparison ..................................................................................................11
A/B switching logic ...............................................................................................11
Cross-fade.......................................................................................................12
Mixing two sources...............................................................................................12
Note .............................................................................................................12
Sound Adjustment ..................................................................................................12
Low and high frequency adjustment .............................................................................12
Stereo Base control, a little theory...................................................................................13
Stereo Base control, what it does .................................................................................13
Balance .............................................................................................................13
Headphones Outputs / Balanced Connection........................................................................13
Dual Mono vs. Balanced amplier design .........................................................................14
Line Outputs ........................................................................................................14
Connect active speakers, power amps or other gear ..............................................................14
„A Thru“ Output ..................................................................................................14
Setup your own test bench / experimental bench ................................................................14
Output Select .......................................................................................................14
Line Inputs ..........................................................................................................15
Dual Mono Design ...................................................................................................15
The idea .........................................................................................................15
Realisation inside the HM1 ........................................................................................15
Optimal output connection .......................................................................................15
Power Supply .......................................................................................................16
Worldwide operation .............................................................................................16
Variable cable length .............................................................................................16
TECHNICAL DATA....................................................................................................17
TROUBLESHOOTING .................................................................................................18
CONTACT............................................................................................................18
ZÄHL HM1 MANUAL V 1.0
P.4|NOTES, SAFETY INSTRUCTIONS, WARRANTY
NOTES, SAFETY INSTRUCTIONS, WARRANTY
1. The equipment must only be used for the purpose described in this manual.
2. Keep the manual for further reference. When passing the equipment on, enclose the manual.
3. Do not operate the equipment at
- very high air humidity (>85% relative humidity)
- high ambient temperature (>40°C) or in the vicinity of heat radiating equipment or objects
- places which are exposed to solar radiation
- very low temperatures (<5°C)
4. Ensure appropriate air ventilation.
5. Do not store the equipment at temperatures below -20°C or above +50°C.
6. Do never expose the equipment to environmental conditions which can lead to the incidence of
condensation water.
7. Do not expose the equipment to mechanical stress or shock.
8. Ensure that liquids cannot get into the equipment.
9. Ensure that foreign objects cannot get into the equipment.
10. Only operate the equipment on a safe, legally approved, mains power supply
11. Only clean the equipment with smooth cleaning tissues and soft detergents.
12. Never open the equipment.
13. In case the equipment has been dropped or there is any external or functional damage, do not
continue to operate the equipment. Have the equipment checked at your dealer’s workshop or a
person who is qualied to do such checks.
14. When shipping, use a package which protects the equipment from environmental impact such as
mechanical shock or humidity.
15. Manufacturer’s warranty covers the equipment to be free from defects of quality at the time of
delivery for a period of 36 month presumed that
- the equipment was treated properly according to its intended use
- all information and safety instructions given in this manual have been followed
- the equipment shows no external damage
- the equipment is shipped to the manufacturer or to an authorised repair-shop free of charge
- a proof of purchase is supplied
- a detailed failure description is supplied
The manufacturer takes over cost of parts and labour incurred by repair. Unless otherwise
agreed any other costs including shipping and packaging will be charged.
16. We expressly exclude any liability for incidental or consequential damages which might arise from
operating the equipment, including failure of the equipment.
17. All information in this manual has been carefully reviewed. It has been updated at the time of pas-
sing for press. Nevertheless we do not take over any liability for sufciency or errors.
18. EEC Declaration of Conformity: The equipment applies to applicable EMC rules 2014/30/EU
SCOPE OF DELIVERY & SET-UP |P.5
SCOPE OF DELIVERY
• HM1 Main Unit
• HMP1 Mains Adapter
• HMP1 Connection Cable
• Mains Cable
• Spare Fuses
• Manual
SET-UP
• First read the chapter NOTES, SAFETY INSTRUCTIONS; WARRANTY in this manual.
• Remove the HM1 from its packaging: insert your hands into the foam recesses on the front and back of the unit, reach
your ngers slightly under the unit and then lift it upwards. Do not lift the unit by the knobs or the connectors.
• Remove the HMP1 from the packaging: insert your hands into the foam recesses on the sides of the unit, reach your
ngers slightly under the unit and then lift it out upwards. Do not lift the unit by the connectors.
• Remove the HMP1 connection cable from the packaging. It is equipped with 12-pin connectors on the ends.
• Plug the end with the female connector into the socket on the rear panel or the HM1.
Make sure that the locating peg shows upwards.
• Turn the strain relief nut clockwise. Push the connector slightly after a few turns.
The connector is seated correctly when the rubber seal on the socket is no longer visible.
• Plug the end with the male connector into the socket on the rear panel or the HMP1 Mains Adapter.
Make sure that the locating peg shows upwards.
• Turn the strain relief nut clockwise. Push the connector slightly after a few turns.
The connector is seated correctly when the rubber seal on the socket is no longer visible.
• Check whether the mains voltage setting on the HMP1 Mains Adapter front panel complies to your local mains voltage. If
not, set the voltage selector to the correct voltage. Use a coin or something similar to operate the mains voltage selector.
• Make sure that the mains switch on the rear panel of the HMP1 is in the off position.
• Insert the mains cable.
• Turn the mains switch to the on position.
• On your HM1 unit, press the power on/off button, the power button will illuminate.
• After several seconds the operating indicator (logo sign) right to the power button with illuminate.
Now your HM1 is ready for operation.
Note: The surface of front and rear panel is black anodized with a special matt nish, the lettering is lasered.
All this makes it very durable. Fingernails or other abrasive objects can leave marks on the matt surface.
These can be wiped off very easily with microbre cloth or similar lint-free cloth.
IMPORTANT! Always follow this order when connecting, otherwise damage may occur!
1. Install HMP1 connection cable
2. Install mains cable
3. Switch on, any order: power switch on HMP1 rear panel and power button on HM1 front panel.
ZÄHL HM1 MANUAL V 1.0
P.6|LEGEND
LEGEND
LEGEND | P.7
ZÄHL HM1 MANUAL V 1.0
P.8|APPLICATION EXAMPLES
APPLICATION EXAMPLES
Conventional
Preamplier in minimalist system
Precision comparison
2-Channel Mixing Console
Effect/Test
Example for a conventional setup as headphones amplier:
Your reference DAC at Input A and an analogue source at Input B
Headphones connected to amplier output, speaker amps or active speakers
to the Line Output. Toggle between headphones and speakers. Example of a
minimalist setup: Streamer to HM1 Input A, analogue source to Input B, active
speakers to Line Out.
Two sources connected to Inputs A and B, precision level meter connected to Line
Output. Source levels can be exactly matched using the level meter and the volume
controls. Activate the sources alternately by the Chan on/off switches.
As a mixer, the HM1 can merge any two sources at Inputs A and B to create
a new music programme - or simply cross-fade from one source to the other.
A device is fed from output „A Thru“ and its output is applied to Input B.
The ratio between the original and the „looped-in“ signal is set at the
A/B volume controls. Or switch between the two.
SPECIAL FEATURES | P.9
SPECIAL FEATURES
UNIQUE: CLASS A POWER
AND A STEP BEYOND
CROSS-FADE, COMPARE
OR MIX TWO STEREO SOURCES
SOUND ADJUSTMENT
AND STEREO BASE CONTROL
• Class A power amplier in its purest form
• switchable negative feedback („servo“) while
maintaining Class A operation
Precise, nely tuned tools for sound adjustment.
Stereo base control for adjusting the stereo
image (spatiality).
• Cross-fade or switch between two stereo sources
(channel A /channel B)
• perform a precise, critical A/B comparison with
independent volume compensation
• create a mix from two audio sources
ZÄHL HM1 MANUAL V 1.0
P.10 |DETAILS
Power Stage
Class A - HM1 realisation
The HM1 power stage provides massive power reserves, which is already evident from the fact that it consumes
almost 40W in idle mode. The output power is more than sufcient for any application, and in the millisecond
range it is increased substantially. The output resistance is remarkably low. The optimum operating point of the
power stage was rst specied by theory, then by measurements on the sample units and nally ne-tuned during
numerous listening tests with experts, especially mastering engineers. The same applies to the selection of
components. This meticulous way of design largely compensates for the possible disadvantages of Class A
described in the theory paragraph below.
Class A & Servo - HM1 realisation
To take Class A one step further, we implement an idea which is as simple as it is ingenious: We take the best of Class A
and the best of negative feedback ampliers and put it together: Class A operation is supplemented by negative feedback
("Servo"). Since there is no crossover distortion, there is nothing for the feedback circuitry to correct. Rather, it only corrects
the errors which result from the interaction between the power amplier and the headphones. The feedback path is
designed in such a way that no artefacts are created by the correction. The output resistance drops to a value which is
smaller than the sum of headphones cables and connectors. In this way, headphone systems can be controlled precisely.
One can say that with the HM1, any headphones sound as they should.
Why Class A and Class A & Servo?
The previous paragraph may lead to the conclusion that the optimum is achieved with Class A & Servo operation So why do
we still have the switching option? Quite simply because we judge our circuits by ear. Our Class A amplier was developed
with the highest musical demands in mind. We experienced that is does not sound the same as the servo version, but it
sounds equally outstanding. Depending on headphones, music and personal preference, the user can make his choice.
Even we were surprised to nd that despite clearly different characteristics and measurement results, the hearing
difference is rather subtle.
DETAILS
DETAILS | P.11
A little theory
Class A
Advantages: no crossover distortion at the output transistors and no artefacts which under certain circumstances
can be caused by negative feedback.
Disadvantages: The internal resistance of the output transistors and usually also the emitter or collector resistors
are in series with the load. Simplied, one can say that the material properties of the output stage components
substantially determine the sound. And since both the resulting internal resistance of the output stage and the
resistance of the load, i.e. the headphones, are complex items, the result is also complex: it is quite common that
certain combinations of even very high-quality ampliers and headphones do not produce optimal results.
Experienced audiophiles know this experience.
Class AB with negative feedback
Advantage: Any error at the output caused by the effects described above is "corrected" by a negative feedback
circuit. And furthermore, the output resistance of the power amplier, as long as there is no overload condition,
theoretically approaches zero; practically it is in the 2-digit milli-ohm range. The complex internal resistance of a
headphone system is almost equalised by the low impedance output. A headphone system is therefore tightly
“guided” by the power amplier.
Disadvantage: The crossover distortion occurs rst, then it is corrected. Thus, negative feedback control (at least
theoretically) always runs a little behind the action, which can lead to artefacts in the signal. However, modern
negative feedback audio power ampliers are usually not purely Class AB. Very effective techniques are used to keep
crossover distortion low without having to use the costly and power-intensive Class A technique. Certain techniques are
used in the negative feedback signal, so that the "lagging" of the correction almost no longer plays a role.
Mixing Stage
A/B comparison
Concerning the reference quality of the HM1, the idea of using it for critical A/B comparisons is most obvious -
be it for evaluating different sources like D/A converters or cartridge systems in the high-end range or comparing
different mixes or sound processing options in the professional range.
For this purpose, the HM1 has two inputs with independent level control and on/off buttons. Exact level matching
is essential for a critical A/B listening comparison. By the way, the A/B switching can also be operated via a cable
remote control, which facilitates a blind comparison (Future function, details will be published when available).
A/B switching logic
Toggle: After power-up, both A and B On/Off switches are in the off state. If you press A or B, the corresponding
channel is switched on. If you then press the other button, this channel is switched on while the one that was on
before is switched off. So you can toggle between A and B.
Individual: Pressing a button repeatedly will switch the respective channel on and off and on and so on.
Hold: If you hold down one button while pressing the other, both channels are switched on.
ZÄHL HM1 MANUAL V 1.0
P.12 |DETAILS
Cross-fade
The mixing stage can also be used to crossfade two sources. A very musical alternative to the otherwise usual
"hard" switching.
Mixing two sources
And of course this stage is suitable for mixing any two sources. A new programme is created from two music
programmes. This new programme can then not only be listened to in the headphones but is also available at the
line outputs on the back of the unit, which can be used for connecting active speakers and recording units.
This turns the HM1 into a 2-channel stereo mixing console with reference quality.
Also important: If only one channel is used, the mixing stage is not superuous "ballast". An unused channel is
switched off directly at the input stage and thus cannot negatively inuence the signal integrity. This switch-off is
realised via relays and is triggered as soon as a volume control is set to minimum or a channel on/off button is in
the off position
Note
The HM1 volume controls are precision conductive plastic audio potentiometers that have an additional linear taper.
When a volume control is set to the full counterclockwise position, this linear taper provides a control voltage to cut
off the audio signal at a suitable point in the signal path by relays. This provides far better signal attenuation than
would be possible with a potentiometer alone.
Sound Adjustment
This stage is not designed to massively affect the sound. As a rule, all audio gear connected to the HM1 is of
extremely high quality, a classic tone control would not be appropriate.
However, it can be advantageous to make ne, precise adjustments in the frequency response domain,
depending on the headphones, music programme or even personal preference.
Low and high frequency adjustment
The low and high frequency adjustment, each in three xed steps as boost and cut, is primarily designed for
musical purposes. We have further developed proven circuits from studio technology to achieve a stage which sets
subtle accents rather than changes the sound character. The basis are lters with selected components, of course
all circuits are precisely switched by relays.
Professionals use the functionality, for example, to nd out whether a mix is balanced in terms of low and high
frequencies or can still be optimised.
DETAILS | P.13
Stereo Base control, a little theory
A special feature is the possibility to adjust the stereo width. The perception of spatiality when listening with
headphones differs from listening with loudspeakers. Instead of a cross-feed circuit, we use our stereo base width
setting. It is based on the mid/side technique which is commonly used in professional studios. It provides
additional benets.
The stereo signal is converted into a mid signal (in simple terms, what L and R have in common) and a side signal
(in simple terms, what distinguishes L and R). Summing mid and side signals in a 1:1 ratio will result in the original
stereo signal. However, if you change the ratio between mid and side before summing them, you change the stereo
base width. In our circuit, we leave the mid signal as it is. We only adjust the level of the side signal. If we lower the
level, the stereo image becomes narrower, if we raise the level, the stereo image becomes wider.
Stereo Base control, what it does
You have direct access to the perception of spatiality. With a music programme which seems too "wide" in the
headphones, one will turn the control one or two steps to the left. If you perceive a recording as too "dry" (centred),
turn the control one or two steps to the right. The leftmost position is mono. This is the preferred setting for early
stereo recordings, when individual instruments have been mixed to the extreme left or right - without spatial
reference.
Professional users check the spatiality of their mix with this function and may discover possible problems,
especially in the extreme positions.
Also important: With the DIR (Direct) button, this stage can be completely switched out of the signal path.
In this way, the purist approach is taken into account.
Balance
A precision potentiometer with 21 detents is available for balance control. The centre position is perfectly
calibrated and haptically clearly dened. In ne increments, left/right volume differences of up to about 2.5 dB -
caused by music programme, headphones or individual perception - can be compensated.
Headphones Outputs / Balanced Connection
XLR 4-pin connector and ¼ inch jack socket are wired in parallel and are intended for alternative use. Theoretically,
you could use both at the same time, the power amplier is strong enough. However, for a reference quality
listening experience, we do not recommend such use.
Ideally, the XLR 4-pin is used in conjunction with balanced wired headphones. In this case, the signal currents of
the two power ampliers are not mixed with each other, mutual interference is impossible. The HM1 can unfold its
full quality (also refer to chapter "Dual Mono Design").
ZÄHL HM1 MANUAL V 1.0
P.14 |DETAILS
Dual Mono vs. Balanced amplier design
With its dual mono design, the HM1 features the same advantages as a balanced output stage but avoids the drawbacks.
The drawbacks of a balanced output stage are the signicantly increased number of electronic components -
a balanced output contains two output stages per channel which work in reverse phase. Two output stages result
in doubling the output impedance. More components result in an increased risk of non-linearities.
¼ inch jack compatible
Another advantage of the HM1 architecture is that unbalanced headphones with ¼ inch jack plugs can also be
operated 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
headphones amplier, but through balanced or unbalanced line output stages and is routed to connectors on the
rear.
Connect active speakers, power amps or other gear
Balanced and unbalanced outputs may be used simultaneously to drive power ampliers, active speakers, level
meters or other components. If, for example, the HM1 is used for comparing two stereo sources, the result may be
recorded using one of the outputs. The same applies when using the HM1 as a mixing console.
„A Thru“ Output
The "A Thru" signal is tapped from Input channel A, post input amplier. The switchable 15dB gain stage at Input A
affects the “Thru” signal. The Thru output has an unbalanced output stage.
Setup your own test bench / experimental bench
A signal connected to Input A can thus be routed to another application without loss. Application example: "A Thru"
is routed to a sound processing device or effects unit, the output of this unit is fed to Line Input B. The ratio
between original and processed signal can be set by the A/B volume controls. Of course, this setup is also excellent
for testing any device: Channel A supplies the original, channel B the signal coming from the unit to be tested.
Output Select
The output select toggle switch - located on the front panel above the XLR headphones output - is very useful if
you are using your HM1 to drive your headphones as well as to drive a power amp or active speakers at the line
outputs. In the upper position only the line outputs are active, in the lower position only the headphones output is
active. In the middle position both are active, which makes sense in case you have a recording or metering device
(or other) connected to the line outputs.
DETAILS | P.15
Line Inputs
Balanced signals (XLR) or unbalanced signals (RCA) can be connected to Line Input A and Line Input B. XLR and RCA
must not be used simultaneously on the same input. The Bal/Unbal switches set the inputs independently to
balanced (Bal) or unbalanced (Unbal) mode.
Each input provides another switch which adds an extra 15dB gain. This ensures that even signals with low levels
can be processed without any problems.
Dual Mono Design
The idea
The idea behind strictly separating left and right channels is to keep any interference away from the circuits.
A term like “crosstalk” does not get to the core of the matter. Music signals usually have left/right references, so that
an extremely low crosstalk value - moreover, usually measured with sine waves - does not mean so much. But with
music highly energetic signal pulses occur, and then high currents are individually demanded by the output stages.
It is extremely important that in these cases there is no coupling via a common power supply or via ground lines
carrying signal current. Only this way an amplier can produce absolutely undistorted output signals.
Realisation inside the HM1
In the HM1, the power supplies for left and right channels are separate, for preampliers and power ampliers.
The mains transformer has independent, potential-separated windings for each channel. Rectication,
ltering and voltage regulation for each channel are individual. For logic circuitry and front-panel indicators,
there is another winding and separate rectication, ltering and voltage regulation.
The ground potentials of the two channels are connected to each other at one point only, but this is done purely as
potential equalisation; it is impossible for signal currents to be mixed.
Optimal output connection
If the XLR 4-pin output is used in combination with balanced wired headphones, the dual mono principle is
implemented in full consequence.
When using the jack output, the ground wires of both channels are connected in the jack socket and the jack plug
of the headphones. The use of headphones with jack plugs therefore is not that ideal. In the HM1, however, an
optimised ground routing ensures that the disadvantageous effects are as low as possible.
(also refer to chapter: Headphones Outputs / Balanced Connection)
ZÄHL HM1 MANUAL V 1.0
P.16 |DETAILS
Power Supply
Mains voltage and mains transformer are always potential sources of interference - consequently, we have
outsourced them to the HMP1 mains adapter.
The mains adapter also contains a standby power supply unit which delivers a stabilised low voltage. This voltage is
applied to the Power switch on the front panel of the HM1 and serves to switch on the actual mains transformer
via a relay.
Worldwide operation
The standby power supply has a wide-range input, is a purely linear design.
The mains input is equipped with one of the best interference lters available and can be set for operation on the
following voltages: 100V-110V-120V-220V-230V-240V.
The mains switch on the back of the HMP1 can be used to completely de-energise the unit.
Variable cable length
The connecting cable between the mains adapter and the main unit carries potential-separated low voltages only.
Standard length is 0,65m. Shorter and longer versions are available. Custom length upon request.
TECHNICAL DATA | P.17
TECHNICAL DATA
Headphones Output
Class A Impedance 0,8Ω max. level +23dBu
Class A & Servo Impedance 0,045Ω max. level +23dBu
unbalanced / dual mono design
Output Power per Side
RMS 4W@30Ω / 7W@15Ω
Peak 7,6W@30Ω / 11W@15Ω / 12W@10Ω (18W for 1,5ms)
Headphones Output THD (Total Harmonic Distortion)
Class A 0,07%
Class A & Servo 0,0005%
(typ. @ +20dBu/7,75Veff / 30Ω load / 2W RMS)
Frequency Response
10Hz ... 30kHz -/+0,05dB
1Hz ... 500kHz better than -3dB @ +6dBu output level
Line I/Os
Inputs Impedance 20KΩ max. level +23dBu
Outputs Impedance 47Ω max. level +23dBu
balanced I/Os: transformer-like behaviour: hot or cold may be connected to ground without loss
Gain
Line/Line 0dB, additional 15dB switchable on rear panel
Line/Headphones 6dB, additional 15dB switchable on rear panel
Noise
XLR/RCA Output -102dBu
Headphones Output -97dBu
20Hz ... 20kHz at RMS / A or B active
Power Supply
Mains Voltage AC 50-60Hz, 100V,110V,120V,220V,230V,240V
Power Consumption typ. 40W
Mains Fuses 5x20 mm, rating 0,63A time-lag (2x)
Measures & Weights
HM1 WxHxD approx. 225 x 90 x 300mm, approx. 5kg
HMP1 (Mains Adapter) WxHxD approx. 170 x 60 x 185mm, approx. 1,5kg
ZÄHL HM1 MANUAL V 1.0
P.18 |TROUBLESHOOTING & CONTACT
TROUBLESHOOTING
Zaehl Elektronik-Tontechnik
Am Stadion 18-24
D-51465 Bergisch Gladbach
Telephone +49(0) 2202 - 863 901
Mail inf[email protected]
www.zaehl.com
CONTACT
Checking mains fuses
In case your HM1 does not show any function although it is correctly connected and switched on, you may check the mains
fuses on the Mains Adapter HMP1.
• Unplug the power cable.
• Find the fuse holder drawer on the mains input assembly, between mains input connector and mains switch.
• Use your thumbnails or a small at-blade screwdriver to unlock the latches at the top and bottom of the fuse drawer.
The drawer will pop out slightly.
• Take the fuse drawer out completely. It contains two identical fuses, size 5x20 mm, rating 0,63A time-lag.
• If broken replace them with the same type only.
• Put the fuse drawer back in place.
• Repower your unit.
• If it still does not work properly, contact us or your local dealer in order to get it serviced.
Table of contents
