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PE Subwoofer Pre-Amp - Exposed

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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".

    Introduction:

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

    Summary:Overview:

    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.

    Analysis:

    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.

    Solution:

    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, 06:25 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

  • Millstonemike
    replied
    Originally posted by bolson2011 View Post

    I haven't modified the original module, I recreated (or at least tried to) it based on your recommendations and the schematic provided, with the changes in place. Essentially, I'm trying to make it work as a variable high pass filter, leaving only ~80-120hz and below pass through that particular filter.

    I guess I should say that I haven't purchased the original module, or the update to it.
    I'm not so sure I can help you with that. Debug a home built board from afar.

    But the need for this type of module arises from time to time. And there isn't an off-the-shelf module that performs.

    When I tested the Sanwu pre-amp, its performance seemed identical to the original PE mode; same two issues. One may not matter. Your target range is close / within the Sanwu's range (and the PE for that matter). The other, decreasing volume with decreasing frequency was a show stopper. But that only requires removing one resistor. The part (ebay / aliexpress) is ~$8 sjipped.. If you want to try that route, let me know. I'll it out and take a look to see if it's feasible.

    If you really need one, I start with the new module and replace parts (both Pots - 50K & 20K - and add back a 10K resistor, IIRC). the parts will cost you ~$10 at DigiKey

    Leave a comment:


  • bolson2011
    replied
    Originally posted by Millstonemike View Post
    I'm confused: "​... make it work as a variable high pass filter, leaving only ~80-120hz and below pass through..."

    Low pass leaving only ~80-120hz and below; or
    High pass leaving only ~80-120hz and above.

    One or the other ...
    My mistake. I want to use it as a subwoofer pre-amp, letting only ~80-120hz and below through.

    Leave a comment:


  • Millstonemike
    replied
    Originally posted by bolson2011 View Post

    I haven't modified the original module, I recreated (or at least tried to) it based on your recommendations and the schematic provided, with the changes in place. Essentially, I'm trying to make it work as a variable high pass filter, leaving only ~80-120hz and below pass through that particular filter.

    I guess I should say that I haven't purchased the original module, or the update to it.
    I'm confused: "​... make it work as a variable high pass filter, leaving only ~80-120hz and below pass through..."

    Low pass leaving only ~80-120hz and below; or
    High pass leaving only ~80-120hz and above.

    One or the other ...

    Leave a comment:


  • bolson2011
    replied
    Originally posted by Millstonemike View Post
    There really wasn't a problem with the input resistors R1 & R2 or a resistor required across RP1.

    Are you trying to modify the module according to my original recommendations? Be aware, circa post #28 there are suggested mods for a totally different application and do not apply to the original mods..
    I haven't modified the original module, I recreated (or at least tried to) it based on your recommendations and the schematic provided, with the changes in place. Essentially, I'm trying to make it work as a variable high pass filter, leaving only ~80-120hz and below pass through that particular filter.

    I guess I should say that I haven't purchased the original module, or the update to it.

    Leave a comment:


  • Millstonemike
    replied
    Originally posted by bolson2011 View Post
    The original part #320-670

    But I recreated the schematic that you linked in the original post in the picture attached.
    There really wasn't a problem with the input resistors R1 & R2 or a resistor required across RP1.

    Are you trying to modify the module according to my original recommendations? Be aware, circa post #28 there are suggested mods for a totally different application and do not apply to the original mods..

    Leave a comment:


  • bolson2011
    replied
    The original part #320-670

    But I recreated the schematic that you linked in the original post in the picture attached.

    Leave a comment:


  • Millstonemike
    replied
    Originally posted by bolson2011 View Post
    I'm using a 12v DC power supply, 2A output.
    That's plenty. Which version of the pre-amp do you have? Please post the exact PE part number.

    Leave a comment:


  • bolson2011
    replied
    I'm using a 12v DC power supply, 2A output.

    Leave a comment:


  • Millstonemike
    replied
    Originally posted by bolson2011 View Post

    I've recreated this schematic, but I seem to have a big problem with significant over-driven signal. The signal is able to pass through the circuit, but is heavily distorted and is still "full-range," if that makes sense. The distortion is so bad that it just sounds like garbage.

    I've tried upping the resistor values for R1 and R2, as well as putting in a resistor in parallel with pins 1&3 of RP1A.

    Any thoughts on how to remedy this issue?
    What is the PS voltage you are using for the module?

    Leave a comment:


  • bolson2011
    replied
    Originally posted by Millstonemike View Post
    Correct. If its not connected, I use a "c" like shape to denote a connection doesn't touch another. Is that really old school? With R7 removed, the schematic looks like this:

    [ATTACH=CONFIG]n1320222[/ATTACH]
    I've recreated this schematic, but I seem to have a big problem with significant over-driven signal. The signal is able to pass through the circuit, but is heavily distorted and is still "full-range," if that makes sense. The distortion is so bad that it just sounds like garbage.

    I've tried upping the resistor values for R1 and R2, as well as putting in a resistor in parallel with pins 1&3 of RP1A.

    Any thoughts on how to remedy this issue?

    Leave a comment:


  • Millstonemike
    replied
    I've tested the Sanwu 2.1 pre-amp module in this thread. It suffers from the same issues of the original version of the PE sub-woofer pre-amp

    Leave a comment:


  • Millstonemike
    replied
    Originally posted by a4eaudio View Post
    Ignoring that the particular implementation of these crossover boards may have problems...assuming there was a board with correct implementation...if I wanted to run two subwoofers, could I run the signal from pre-amp to a single variable crossover board, then split the RCA out with a Y-adapter and run the signal to the two subs separately? That way I know I have the same crossover and gain rather than implementing two separate boards?
    The output impedance of the pre-amp module is 200 ohms. That is low enough to drive the line level inputs on two sub amps (barring some unusually low input impedance).

    But, however, take notice ...

    The original module is no longer available to mod as originally posted. And the new version has more issues than the original - totally useless. I just received a third party module and will be evaluating soon.

    Tune in next week, same Bat time, same Bat channel ...

    Sub Pre Amp.jpg

    Leave a comment:


  • a4eaudio
    replied
    Ignoring that the particular implementation of these crossover boards may have problems...assuming there was a board with correct implementation...if I wanted to run two subwoofers, could I run the signal from pre-amp to a single variable crossover board, then split the RCA out with a Y-adapter and run the signal to the two subs separately? That way I know I have the same crossover and gain rather than implementing two separate boards?

    Leave a comment:


  • Millstonemike
    replied
    Originally posted by thekorvers View Post

    Never cared much for the solder suckers, even the expensive ones. I prefer to use the simple inexpensive copper braid. Try that. It wicks the solder right out:

    https://www.parts-express.com/nte-sw...-5-ft--341-556

    https://www.parts-express.com/nte-sw...-5-ft--341-550
    You know, I never tried copper braid. I'll have to give it a go. I used a $1.79 solder sucker from Fleabay when I modified Gordy's board. though it required some finesse and experience.

    Leave a comment:

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