7
3.4. Subsonic filter
This filter cuts off inaudible frequency components which when amplified suppose a
risk of damage to the low frequency speakers as they generate excessive excursions of the
woofer's diaphragm. The APAs feature a switchable, Butterworth shaped Subsonic Filter
located inside the unit (see figure 1) with a 25Hz cut-off frequency and 18dB/oct slope.
Subsonic filters are very important when playing back vinyl records because the
phono cartridges are specially sensitive to feedback and acoustic coupling at very low
frequencies.
APA amplifiers are delivered with switched ON Subsonic Filter.
CAUTION: Changes on the Subsonic Filter have to be performed by a qualified
technician.
3.5. Limiter circuit
This system is an always active protection inside the APA series of amplifiers. The
ANTICLIP circuitry constantly analyses harmonic distortion caused by excessive signal
excursion at the power amplifier's output and automatically reduces the input level in order
never to exceed 1% total harmonic distortion. The APA range of amplifiers are delivered with
the anticlip system adjusted at a threshold of 1% THD (hard limiting). However, this value can
be switched to a softer limiting (5% THD) via an internal jumper (see figure 2).
The great convenience of such a circuit in any kind of installation has to be remarked:
The clear advantage of a limiting system in front of conventional compressors is that the former
does practically not alter the dynamic range, acting only when the distortion threshold is
reached.
CAUTION: Changes on the ANTICLIP configuration have to be performed by a qualified
technician.
3.6. Output connections
The OUTPUT section on the rear panel features Speak-on connectors (12).
Whenever you need to operate the amplifier in BRIDGED mode (mono or bridged), you must set
the “STEREO BRIDGED" Switch (16) to BRIDGED. The input signal is taken from the INPUT
connector on channel I (20) and the signal output to the loudspeaker is on pins 2+ and 2- of
the Speak-on connector “CH1”. The same contacts (2+ and 2-) of the Speak-On connector of
CH2 also carry the bridged signal in order to facilitate the parallel connection of loudspeakers.
ATTENTION: Terminals 1+, 1- must only be used in STEREO operation and terminals
2+, 2- in BRIDGED operation. Other combinations would lead to poor output quality.
When operating in BRIDGED mode, always make sure that the resulting load impedance of the
installation is never below 4 Ohm.
The connection cable that joins the amplifiers outputs and the loudspeakers must be of
good quality, sufficient section and as short as possible. This is most important when the
distances to cover are long ones i.e. up to 10 meters it is recommended to use a section not
inferior to 2.5mm
2
and for superior distances 4mm
2
.
