JBL LSR28P Technical & Service Manual page 30

The curves in the figure below plot the frequency response from a +4dBu input signal at the
outputs of U2 pin 1, U5 pin 7, U6 pin 7 and U7 pin 1.
U2A acts as a second order low pass filter as shown by the top curve. U5A and B form a bell
equalization cut centered on about 600Hz. U6 A and B form a second bell equalization cut
centered at about 1KHz. U7 A and B along with U8A form a notch filter at around 2.6KHz.
The response at any point along the signal path relies on an accurate frequency response
characteristic from preceding stages. The curves in the figure allow ready identification of a
stage which is causing frequency response errors. An inaccuracy in a single section can result in
gross inaccuracies in the overall response result.
AUDIO PRECISION LSR28PL1
10.000
5.0000
0.0
-5.000
-10.00
-15.00
-20.00
-25.00
-30.00
10
• The curve with a drooping response below 80Hz is the result of selecting the -2dB bass cut
switch SW1E.
• The curve with extended bass response is the result of selecting the Bypass switch SW1D.
• The curve with flat response down to about 30Hz with a well damped roll-off below 30Hz is
the standard response with all switches down.
The action of the bass response DIP switches SW1 D, E and F can be checked by examining the
frequency response at U2 pin 7. The four possible response curves are plotted on the figure
below.
Low Frequency Signal Path Gain
LEVEL(dBu) vs FREQ(Hz)
100
30
Low Frequency Section
1k 10k