P.I.P.–AMCb
Page 7
Low-Frequency EQ
After the crossover, low frequencies
are equalized by a second-order
high pass filter with a Q of 2. Such a
filter is specified by Thiele
2
and in a
more recent paper by Keele
3
where
it is used to form a sixth-order
Butterworth high pass response with
a vented box low-frequency system.
The filter uses the Sallen-Key non-in-
verting second-order high pass to-
pology. The 9-segment “BOX EQ”
DIP switch (Figure 2.1) sets the +6
dB boost frequency as shown in the
table in Figure 3.4. There are fifteen
settings (including FLAT). Figure 3.5
shows typical response curves for
these settings.
2
A.N. Thiele, “Loudspeakers in Vented Boxes,” JAES, Vol 19 Part 1, May 1971, Part 2, June 1971.
3
D.B. Keele Jr., “A New Set of Sixth-Order Vented Box Loudspeaker System Alignments,” JAES, June
1975.
FREQUENCY
1 K 2 K200
–45˚
–90˚
–135˚
CHANNEL 1
CHANNEL 2
0˚
–180˚
–225˚
–270˚
–315˚
300 400 500
Fig. 3.2 Crossover Frequency
Response (800 Hz)
Fig. 3.3 Crossover Phase
Response (800 Hz)
LOW-FREQUENCY (BOX) EQUALIZATION
Switch Settings
+6 dB
Frequency (Hz)
OFF
OFF
OFF
OFF
OFF
OFF
---
20
24
28
30
36
38
42
44
56
65
68
102
120
180
FLAT
ON
ON
ON
ON
ON
ON
ON
ON
1
OFF
OFF
OFF
OFF
OFF
OFF
---
ON
ON
ON
ON
ON
ON
ON
ON
2
OFF
OFF
OFF
OFF
OFF
OFF
---
ON
ON
ON
ON
ON
ON
ON
ON
34
OFF
OFF
OFF
OFF
OFF
OFF
---
ON
ON
ON
ON
ON
ON
ON
ON
56
OFF
OFF
OFF
OFF
OFF
OFF
OFF
OFF
OFF
OFF
OFF
OFF
OFF
OFF
ON
7
OFF
OFF
OFF
OFF
OFF
OFF
OFF
---
ON
ON
ON
ON
ON
ON
ON
89
OFF
OFF
OFF
OFF
OFF
OFF
---
ON
ON
ON
ON
ON
ON
ON
ON
OFF
OFF
OFF
OFF
OFF
OFF
---
ON
ON
ON
ON
ON
ON
ON
ON
OFF
OFF
OFF
OFF
OFF
OFF
OFF
---
ON
ON
ON
ON
ON
ON
ON
Fig. 3.4 Low-Frequency EQ Settings
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