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34.25 Digital DC Protection
A protective circuit that detects DC voltage at the output of an amplifier, and cuts off the speakers
if it is too high, is standard in power amplifiers. DC voltage not only means that the speaker's
diaphragm does not remain in the ideal center position (instead it is permanently stuck out or
retracted), but also that the speaker is subjected to a larger, constant current flow. However,
loudspeakers (and headphones) are designed for alternating current (AC), not direct current (DC).
RME's Extreme Power output stage delivers enough voltage and current to destroy the connected
headphones in case of a defect. Therefore the ADI-2/4 Pro SE has the protection circuitry of
power amps in hardware. From about 1.4 V DC the headphones are disconnected from the output
stage by a relais. This protection circuit - especially in combination with the overload detection
and the ramp-up of the playback volume - has proven its reliability all over the world.
With DC protection in the digital domain (Digital DC Protection, DCP), RME goes one step further.
DCP detects DC in the digital source signal, both PCM and DSD.
DCP offers three settings. ON activates the digital sensing, mutes the respective output in case
of found DC, and shows a warning screen. If the DC component in the signal disappears, the
output is enabled again after a few seconds. OFF deactivates mute, with the phones outputs still
protected by the hardware DC circuit. Sensing is still active and shows a smaller warning screen.
The option Filter removes DC and infrasound, enabling a playback of even problematic audio
signals in a safe way.
st
Filter adds a special, smooth 1
order high pass to the signal path, which shows zero delay (la-
tency), very low distortion and lower phase deviation as standard digital filters. This filter inten-
tionally not only removes DC, but also attenuates the inaudible, but often to find infrasound junk
through a corner frequency of 7 Hz. The filter operates in all sample rates, but not in DSD mode.
Compared with the hardware circuit, DCP has several advantages:
Much more accurate and lower detection.
An especially steep filter separates music (from 5 Hz) and DC (below 5 Hz). Even with particularly
low-bass music, false triggering never occurs. At the same time, DC detection can be triggered
much earlier than with usual hardware. The XLR output of the ADI-2/4 Pro SE switches off at
0.27V DC (measured at the output). And already at 36 mV at lower ref levels. But only if it is
really DC.
Protection of all analog outputs
While the hardware circuitry only monitors the headphone outputs, DCP can also prevent DC
output at the line outputs.
Protection even with unusual signals
A hardware protection circuit often does not react if one channel is connected to positive DC, the
other to negative DC. In practice this is a very unlikely case - but DCP detects DC even then -
and reacts.
Reference to output level
With DCP the threshold for a DC detection event is coupled to both volume setting and current
reference level. Thus, up to 0.38 V DC is possible at the High Power headphone output, while it
is only 0.068 V at Low Power. High Power headphones are either very robust or high impedance.
In both cases, they can tolerate much more DC than sensitive variants, which play back relatively
loudly even at low levels. Since these are also often very low impedance, comparatively more
current flows. So a lower threshold level is quite advantageous.
User's Guide ADI-2/4 Pro SE - v 1.0
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