Violectric DHA V590² Pro Headphone Amplifier, Preamp, and DAC - Manual (Page 27)

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The potentiometer has nothing to do with the analogue signals from DHA V590 PRO but generates a
control voltage which is transferred into the digital domain by an analogue-to-digital converter and
fed to a micro controller which generates the digital word to control a 256 step relay attenuator.
The relays are very special “Reed Relays”. Here, the switching contacts are situated in a glass tube
filled with a protective gas. The contacts are actuated by a magnetic field.
The 256 steps are realized with 8 relays per channel because 2
8 = 256.
Each step is defined as 0.4 dB which totals in over 100 dB attenuation range. The contacts of the
relays are switching between resistors with an accuracy of 1% or 0.1% to attenuate the signal. So
best channel matching and minimal crosstalk is realized – and the relays will never scratch.
Why a low output impedance is essential?
When actuated, electro-dynamic systems respond with a counterforce. When the voice coil of a
headphone has been displaced by the signal, an (error-) current will be induced when it swings back
to its initial position. This current must be suppressed as far as possible, which is affected best if the
amplifier's output impedance is the lowest possible. The damping factor describes nothing but the
ratio between the output impedance of an amplifier and a given load.
Since there is no known technical specification, we define the load (voice coil impedance) as
50 Ohms. With DHA V590 having an output impedance of < 0.3 ohms in balanced mode and
< 0.15 Ohms in unbalanced mode this results in a damping factor of 160 (balanced) and a
damping factor of 320 (unbalanced).
Due to general recommendations the output impedance of an amp shall not be higher than 5 % of
the headphone impedance, that means a minimum damping factor of 20.
Why are high supply voltages essential?
A headphone doesn't really require high power, but from the equation P = U
2 / R we can see that the
square of the supply voltage determines the power into a given load resistance. The higher the
headphone's impedance, the more voltage will be needed to achieve high listening levels. But this
deals with the achievable loudness to a limited extent only: Technically spoken, music lives on fast
transients which put high demands on signal processing. And thus, a fast transient can easily push
an average amplifier with +/-15 volts supply to its limits (95 % of all headphone amps in the market
are operated with these or even lower supply voltages). Due to the high supply voltage and the
balanced operation mode of DHA V590 you will benefit from far over two times more output voltage
swing capability compared to single ended amps with “standard” supply voltage.
Why we are making our amps in such a way.
They are made with transistors and operated with +/- 25 V supply voltage because it is senseful to
do so. But a headphone amp must not be as powerful as a speaker amp.
Our “power“ stage consists of eight transistors, four small ones, four bigger ones, all of them very
fast. They are driven by an op-amp in non-inverting mode, the gain is set to 0 dB. In balanced mode
the gain equals +6 dB as two power stages act in push-pull mode.
For high impedance headphones a very high output voltage of 21 V RMS is achieved - while low
impedance headphones will profit from a power over 6 Watt into 50 Ohms per channel.
Under all operating conditions noise, distortion and dynamic range is on the edges of physics.
Why does a output relay make sense when switching power?
Amplifiers generate unwanted output signals when applying or removing power, which can damage
the connected headphones. The relay breaks the connection between amplifier and headphone for
some seconds after power-on and thus protects the latter until electrical conditions have stabilized.