QSC Audio SF-1 User Manual

SF-1
Subwoofer
Filter

Owner’s Manual

9

FPO

Rev. A

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SF-1 SUBWOOFER FILTER

INTRODUCTION

The SF-1 Subwoofer Filter is an upper slot accessory for QSC MXa Series amplifiers, which feature Level
One Open Input Architecture. The accessory mounts in the back of the amplifier above the standard input
panel.

Ideal for adding subwoofer amplification to an existing full-range system, the SF-1 offers adjustable and
selectable low-pass filtering for both channels of the amplifier, in addition to adjustable high-pass subsonic
filtering. The filter slopes are 12 dB per octave, and the filter frequencies are programmable via provided
SIP resistor networks.

A slide switch, SW1, allows you to configure the SF-1 with a low-end “boost” which provides a suitable
equalization for use with B6 speaker enclosures. A 10-position DIP switch, SW2, affords you the versatility
of setting the accessory in many different configurations, such as mono summing and single channel
bypass. The configuration process is detailed below.

The use of the SF-1 for cinema subwoofer installations can reduce the chance of speaker damage by digital
soundtracks. Low frequency information, below the box tuning frequency, can destroy speakers at power
levels far below their rated capacity. The SF-1 can remove these damaging frequencies and also provide
the needed equalization to flatten and extend the response of popular cinema subwoofers. These include:
Electro-Voice models TL880D, TL550D, TL3512, TL440 and JBL 4645 and 4645B.

Existing cinema surround systems typically have limited low frequency response. New digital surround
formats now have full-range surround channels that extend frequency response much lower than previous
optical surround formats. The SF-1 may be used to protect surround speakers from damage, due to
excessive low frequency operation. It is desirable, however, to eventually upgrade the capability of the
surround systems to handle the increased power and low frequency response requirements of digital
soundtracks.

25

Figure 1

Figure 1 shows where the pertinent switches, resistor networks, and connectors are located on the SF-1
board.

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CONFIGURATION

SETTING THE SUBSONIC FILTER FREQUENCY

The subsonic (highpass) filter on Channel 1 cannot be bypassed, and Channel 2 should only be bypassed
in the bridged mono mode. Plug-in SIP resistor network RN3 sets the high-pass frequency for both channels
1 and 2. Table 2 shows the resistor value to be used for the frequency you desire. Typical subsonic
frequencies available with the provided SIP resistor networks are 20, 25, 32, 35, 40, 50, 63, and 80 Hz.
Consult your subwoofer manufacturer if you are unsure of what frequency to use.

If you desire a 6-dB boost at the low end of the subwoofer frequency range, set the slide switch SW1 to “ON”;
otherwise set it to “OFF.” Such a boost is popular with users of Electro-Voice B6 subwoofer enclosures.

TABLE 2

Frequency Resistor Network Value (RN3 or RN4)

20 82K Factory set for RN3
25 68K
32 56K
35 47K
40 39K
50 33K
63 27K

80 20K
100 18K
135 12K
160 10K
200 8.2K
250 6.8K Factory set for RN4

SETTING THE SUBWOOFER UPPER FREQUENCY LIMIT

The low-pass filter sets the upper limit of the subwoofer frequency range. Plug-in SIP resistor network RN4
sets the low-pass frequency for both channels 1 and 2. Table 2 shows the resistor value to be used for the
frequency you desire. Typical frequencies available with the provided SIP resistor networks are 100, 135,
160, 200, and 250 Hz. Consult your subwoofer manufacturer if you are unsure of what frequency to use.
Whatever frequency you choose should be higher than that of the subsonic filter.

SETTING DIP SWITCH SW2

Determine the configuration you desire and set the DIP switches as described below.

Positions 1 through 4 and 9 and 10

First, decide whether you need “Mono Sum” or “Stereo” operation. This will determine the settings of
positions 1 through 4 and 9 and 10 of DIP switch SW2.

• Mono Sum

Because it is difficult for listeners to sense directionality of low audio frequencies, many subwoofer
systems are set up for monaural operation, with both channels summed together. For mono
summed operation, set the amplifier mode switch to “STEREO” and SW2 as follows:

Position 1 OFF
Position 2 ON
Position 3 ON
Position 4 OFF
Position 9 OFF
Position 10 ON

Continue to “Positions 5 through 8” and set those switches to engage both channels’ low-pass
filtering.

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