The HM1 Reference Headphone Mixing Amplifier by Zähl , Designed and manufactured in Germany.

The HM1 Reference Amplifier with unprecedented transparency and precision, attention to detail and massive power performance.
Rich feature set for both high-end enthusiasts and professional users.

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
Class A power
HM1 mixing, cross-fading or comparison of 2 stereo sources
Cross-fade, compare or mix two stereo sources
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
Sound adjustment and Stereo Base control
Precise, finely tuned tools for sound adjustment.
Stereo base control for adjusting the stereo image (spatiality).
perfect impulse reproduction
ultra wide frequency response
extremly low impedance output precisely controls complex loads
Manufactured in Germany in a limited edition of 50 units per year
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
Precision comparison
2-channel mixing console
Effect/Test

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. Then sources are alternately activated 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. The mix is present at the line out outputs and may be fed to an audio recording device.
By using the "A Thru" output, a sound processing or effect device can be "looped in". It is fed from "A Thru" and its output is applied to Input B. The ratio between original and effect unit is set at the A/B volume controls. Instead of the effect, any device to be tested can be looped in. The listening comparison is then made by carefully matching the volume controls and then pressing the A/B on/off switches.

Conventional
Preamplifier in minimalist system
Example for a conventional setup as headphones amplifier: Your reference DAC at Input A and an analogue source at 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 to HM1 Input A, turntable (with built-in or external RIAA amplifier) to Input B, active speakers to Line Out.

More Details in the Manual.

Owner's manual