These Modifications are supplied by customer Grant Spangler and Apex Jr assumes no responsibility for accuracy. The following is part of an e-mail Grant sent us. Thanks Grant!
Date: Monday, December 11, 2000 9:54 PM
Here is a preliminary set of numbers. The "Bass Boost" is actually the 1 dB highpass filter consisting
of U3B/C14/C15/R26/R27. By changing the transfer function, the "Boost" goes away, and the subsonic filter
is left. Please note that changing C5 & C6 to 47pf will enhance the performance of the highpass output. It
will set the 3db knee at 100 KHz rolling at 6dB per octave. Also the value of C18 should be changed to .015 ufd.
This will accommodate one more 6 dB rolloff pole at 320 Hz. The ultimate bass lowpass rolloff will become 18 dB
per octave (versus12) in the midrange. COG or PPN is recommended for all frequency-determining caps. The closest
5% resistor value for changing R26 to a Butterworth hipass
(per Doug Goldberg) is actually 91K. This is not an extremely common value, so a 100K should work if that's what
you have on hand. For a Bessel filter, the closest 5% value is 130K, with a 1% resistor it would be 137K .
Best Regards,
Grant Spangler
Butterworth and Bessel Transfer Functions
Resistor Values for 12 dB Highpass Filters
Butterworth 12dB Highpass Filters Bessel 12 dB Highpass Filters
| Frequency | R18/R19/R26 | R17/R20/R27 | R18/R19/R26 | R17/R20/R27 | |
| 5 Hz | 225079 (226K) | 450158 (453K) | 351077 (348K) | 468103 (464K) | |
| 6.3 Hz | 178638 (178K) | 357268 (357K) | 278634 (280K) | 371506 (374K) | |
| 8 Hz | 140674 (140K) | 281349 (280K) | 219432 (221K) | 292564 (294K) | |
| 10 Hz | 112540 (113K) | 225079 (226K) | 175538 (174K) | 234052 (232K) | |
| 12.5 Hz | 90032 (90.9K) | 180063 (182K) | 140430 (140K) | 187241 (187K) | |
| 16 Hz | 70337 (69.8K) | 140674 (140K) | 109711 (110K) | 146282 (147K) | |
| 20 Hz | 56270 (56.2K) | 112540 (113K) | 87769 (88.7K) | 117026 (118K) | |
| 25 Hz | 45016 (45.3K) | 90032 (90.9K) | 70215 (69.8K) | 93621 (93.6K) | |
| 32 Hz | 35169 (34.8K) | 70337 (69.8K) | 54856 (54.9K) | 73141 (73.2K) | |
| 40 Hz | 28135 (28.0K) | 56270 (56.2K) | 43885 (44.2K) | 58513 (59.0K) | |
| 50 Hz | 22508 (22.6K) | 45016 (45.3K) | 35108 (34.8K) | 46810 (46.4K) | |
| 63 Hz | 17863 (17.8K) | 35727 (35.7K) | 27863 (28.0K) | 37151 (37.4K) | |
| 80 Hz | 14067 (14.0K) | 28135 (28.0K) | 21942 (22.1K) | 29256 (29.4K) | |
| 100 Hz | 11254 (11.3K) | 22508 (22.6K) | 17554 (17.4K) | 23405 (23.2K) | |
| 125 Hz | 9003 (9.09K) | 18006 (18.2K) | 14043 (14.0K) | 18724 (18.7K) | |
| 160 Hz | 7034 (6.98K) | 14067 (14.0K) | 10971 (11.0K) | 14628 (14.7K) | |
| 200 Hz | 5627 (5.62K) | 11254 (11.3K) | 8770 (8.87K) | 11703 (11.8K) | |
| 320 Hz | 3517 (3.48K) | 7034 (6.98K) | 5486 (5.49K) | 7314 (7.32K) | |
| 400 Hz | 2813 (2.80K) | 5627 (5.62K) | 4388 (4.42K) | 5851 (5.90K) |
Butterworth and Bessel Transfer Functions
Resistor Values for 12 dB Highpass Filters
Boost Removal
Doug Goldberg another Apex Senior owner has successfiuly removed the bass boost. According to Doug all that needs to be done is to change R26 from the stock 18k to a value of 100k. This removes the boost and sets the corner frequency of the subsonic filter to 12.5Hz.
I had someone model the circuit for me and come up with a couple other
boost points so to speak. To get the boost at 20Hz you woul need to change
C15 to a .22uF cap. This gives about 5dB boost at 20Hz.
To get a 25Hz boost you would need to change C15 to .22uf, and also change
R26 from 18K to 12K. This would give about 6dB boost at 25Hz.
John E. Janowitz