Line Inputs; Dual Mono Design; The Idea; Realisation Inside The Hm1 - Zahl HM1 Manual

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 amplifier can produce absolutely undistorted output signals.

Realisation inside the HM1

In the HM1, the power supplies for left and right channels are separate, for preamplifiers and power amplifiers.
The mains transformer has independent, potential-separated windings for each channel. Rectification,
filtering and voltage regulation for each channel are individual. For logic circuitry and front-panel indicators,
there is another winding and separate rectification, filtering 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)
DETAILS | P.15