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PE Subwoofer Pre-Amp - Exposed

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  • wogg
    Very nice!

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  • DanP
    Awesome Mike! Well done!


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  • Millstonemike
    started a topic PE Subwoofer Pre-Amp - Exposed

    PE Subwoofer Pre-Amp - Exposed

    Note: To view pictures in full size, right click the picture and select "open in new window".


    Then he shipped his last unit to me for analysis, thanks Gordy.


    The subwoofer pre-amp performs three functions: (1) It sums a L-R stereo signal to provide a combined subwoofer output signal; (2) It provides an adjustable gain for the subwoofer output; and (3) It supports an adjustable LP frequency.

    One of two potentiometers controls gain that can be varied between zero gain (i.e., no signal out) and ~10.5x gain. This potentiometer works as expected. An audio frequency, 1st order bandpass filter is integrated into this circuit. The HP F3 is fixed at 3 Hz. The LP F3 decreases with increasing gain but never gets below ~10 KHz @ maximum gain. So it doesn't affect the subwoofer out signal.

    The second potentiometer controls the sub's LP F3 frequency and this is where some problems exist.


    Original Module: 9 Hz to 107 Hz Adjustment Range

    The sub's LP filter is a 2nd order Sallen-Key topology as are most active, analog 2nd order filters. The original filter's Q factor is 0.73; very close to .707, aka Butterworth dampening. After modifications, the Q factor is virtually unchanged at 0.74.

    Schematic Stock.jpgGraph 1.jpg

    Initial Testing:Graph 2.jpgIn a single supply op-amp circuit, signal bias resistors are needed to shift the 0 value of an ac coupled signal to half the supply voltage. This allows the op amp to reproduce the positive and negative portions of the signal (i.e., instead of V- to V+, the signal has a DC offset and spans 0V to 2V.


    First, the capacitors in the Sallen-Key filter were reduced to allow a higher F3 frequency. The 0.22 uf capacitor, C5 (EDIT: not C6), was replaced with a 0.1 uf capacitor. The 0.47 uf capacitor, C6 (EDIT: not C5), was replaced with a .22 uf capacitor (i.e., EDIT move C5 to C6 position and insert a 0.1 uf cap into C5 postion)

    Combined Component Layout Modified Schematic.jpg

    Final Testing:

    Post modification, the measured results coincide with the expected performance of the modified module.

    Graph 3.jpg

    The 20K "A Taper potentiometer can be procured from Digi-Key (TT Electronics # P092NQC15AR20K).The part cost ~$2.40 and the shipping cost was reasonable at ~ $3.50. Most any film / polyester capacitor can be used for the 0.1 uf replacement.

    Side Note:

    The measurement graphs show the effect of the 1st order HP at the input stage (F3 = 3 hz). If a higher HP point is needed to limit sub Xmax, replacing the two input signal decoupling capacitors can achieve a higher HP F3 (aka PLXXO). With the existing 4.7uf capacitors, the 1st order HP F3 is 3 Hz. Replacing those two capacitors with lower values yields the following: (1) 1.0 uf, F3 = 16 Hz; (2) 0.68 uf, F3 23 Hz; (3) 0.47 uf, F3 = 34 Hz and (4) 0.22 uf, F3 = 48 Hz. The effect of the HP stage on subwoofer performance can readily be modeled in WinISD.
    Last edited by Millstonemike; 03-21-2017, 07:25 PM. Reason: The wording in the "Solution" paragraph had C6 and C5 reversed. This has been corrected in the paragraph in bold. The original red markups on the schematic are correct and have not changed