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By changing two (2) resistor values, you can adjust the pre-amp filter Q to obtain the desired level and frequency of bass
boost for your particular subwoofer design.
Note: Soldering and basic electronic skills are required. Only qualified electronic technicians should perform this
modification. Any damage to the amp due to improper modifications, is the sole responsibility of the user. We
strongly recommend that the user thoroughly test the performance of the amplifier prior to making any
modifications.
1. Removing the pre-amp board: Take care not to scratch the amp faceplate while removing the pre-amp board.
A. Remove the knobs from the Phase, Crossover and Level controls.
Note: Knobs are pressed-on (no set screws) and at times may be difficult to remove.
B. Using needle nose pliers, remove nut and washers holding Phase, Crossover and Level
controls.
C. Remove screws holding binding posts, RCA connectors and on/off switch.
Note: Do not remove AC jack and voltage selector screws.
D. Unplug the wiring harness from main board and remove the pre-amp board.
Note: You will need to cut the wire tie holding the wire bundle.
2. Changing the Resistors
A. Locate R26 and R27 on the pre-amp board. The default values for these resistors are
R26=56K, R27=120K (no boost).
B. Using a low wattage soldering iron and desolding braid, remove the two (2) resistors.
Be careful not to apply too much heat to the p.c. board traces.
C. Using the charts below, determine the proper resistor values to achieve the desired boost.
Note: Resistors are 1/4 watt and values listed should be multiplied by 1,000.
Example: 56 = 56,000 ohms or 56K ohms. The Fc indicates the "corner or cut-off" frequency (Hz).
This is the frequency where the amp starts to roll off on the low end. The higher the level and
frequency of the boost, the higher the Fc (corner frequency). The Fc is also an indication of
the frequency where the "rumble" filter starts to take effect.
D. Install the resistors making sure that you put the correct resistor (R26 or R27 ) in the proper
location on the p.c. board. Tip: If you plan on experimenting with different boost levels,
mount the resistors on the back side of the p.c. board so they can be easily changed
without removing the pre-amp board.
3. Reinstall pre-amp board
A. Reinstall the pre-amp board in reverse to the removal procedure listed above.
B. Make sure that you remember to reconnect the wiring harness to the main amp board.
C. When reinstalling the control knobs, first turn all controls fully counter clock wise to help align
the white indicator markings on the knobs to their proper positions.
1dB of Bass Boost
Boost Freq. R26
R27
(Hz)
(k Ohms)
(k Ohms)
20-24 Hz
56
220
25-30 Hz
47
180
31-35 Hz
39
150
36-40 Hz
27
120
Boost Freq. R26
(Hz)
20-23 Hz
24-30 Hz
31-35 Hz
36-40 Hz
Adjusting EQ "Bass Boost"
Instructions for Modifying EQ
Filter Fc
Filter Q
Boost Freq. R26
(Hz)
(Hz)
14
1
20-24 Hz
17
1
25-30 Hz
21
1
31-35 Hz
28
1.1
36-40 Hz
4dB of Bass Boost
R27
Filter Fc
(k Ohms)
(k Ohms)
(Hz)
27
270
19
22
220
23
18
160
29
15
150
34
2dB of Bass Boost
R27
Filter Fc
(k Ohms)
(k Ohms)
(Hz)
47
220
16
33
180
21
27
150
25
22
120
31
5dB of Bass Boost
Filter Q
Boost Freq. R26
(Hz)
(k Ohms)
1.6
20-23 Hz
22
1.6
24-30 Hz
18
1.5
31-35 Hz
15
1.6
36-40 Hz
12
3dB of Bass Boost
Filter Q
Boost Freq. R26
(Hz)
(k Ohms)
1.1
20-23 Hz
33
1.2
24-30 Hz
27
1.2
31-35 Hz
22
1.2
36-40 Hz
18
R27
Filter Fc
Filter Q
(k Ohms)
(Hz)
330
19
1.9
220
23
1.7
180
31
1.7
150
35
1.8
R27
Filter Fc
Filter Q
(k Ohms)
(Hz)
220
19
1.3
180
23
1.3
150
28
1.3
120
34
1.3
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