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PE Subwoofer Pre-Amp - Exposed

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  • PE Subwoofer Pre-Amp - Exposed

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

    Introduction:

    Tom Z started a thread where he identified this product. That it seemed ideal for small active, 2.1 systems.

    Gordy had previously bought two of these units. He perceived it difficult to dial in a specific LP XO point:” The pot that controls the freq. does not seem to do much like it is adjusting nothing. Almost like it is too sensitive and only controlling a portion of the freq. scale it is supposed to.”

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

    Summary:

    The PE Subwoofer amp suffered from two major issues. First, the component values chosen for the LP filter created a variable cut off frequency (F3) between 9 Hz and 107 Hz. Second, as the cut-off frequency was lowered, the module’s overall gain would be drastically reduced.

    Taken together, the frequency adjustment pot had a extremely narrow adjustment range.

    Several components were replaced to correct these issues. The modified unit has an F3 adjustment range of 43 to 228 Hz. Gain reduction has been eliminated over that range. And the entire module performs near an ideal subwoofer LP pre-amp.

    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.jpg



    In stock form, the values chosen for the filter’s two capacitors and two resistors, in conjunction with the variable resistance of the potentiometer adjustment range, vary the frequency from a high F3 of 107 Hz down to 9 Hz. If we assume the desired adjustment range is between 60 Hz and 107 Hz, then only ~25% of the pot's rotation is useful in the desired range.

    The remainder of potentiometer's rotation, ~75%, adjusts the F3 frequency from 60 Hz to 9 Hz, fairly useless especially for smaller systems where this low cost sub pre-amp would likely be used.

    Graph 1.jpg

    Initial Testing:

    Measurements confirmed the adjustment range limitation. Testing also uncovered a peculiar problem. As expected, increasing the LP potentiometer’s CCW rotation lowers the F3 cutoff frequency. But in addition, the overall system gain was also significantly reduced

    Graph 2.jpg

    Further analysis revealed that the filter’s bias resistor impedance was too low @ 10K ohms. The increasing impedance of the 50K pot was allowing a significant portion of the AC signal to bleed through the filter's 10K bias resistor. With the two gangs of the 50K pot in series and in series with the two fixed 4.7K resistors, almost 90% of the signal was bleeding through the 10K bias resistor to ˝ Vcc, nominal ac ground.

    In 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). This modification expands the F3 adjustment range up to 228 Hz.

    Next, the 50K LP frequency potentiometer was replaced with a 20K “A” taper potentiometer. The reduction in overall resistance limits the minimum F3 frequency to 43 Hz. In addition the total adjustment range from 43 Hz to 228 Hz becomes more linear over the pots full 300 degree rotation.

    The solution for the gain issue was to remove the filter’s bias resistor (R7). The overall gain is set by the gain potentiometer and is now independent of the LP potentiometer resistance.

    Normally, a much higher value for bias resistors are chosen (e.g., 100K ohms) to avoid this issue. However, the op-amp gain stage has a bias resistor raising the ac coupled input signal to ˝ Vcc. Since the op-amp gain stage is DC coupled to the filter op-amp stage, there was no need to add bias at the filter op-amp. The signal entering the filter stage was already biased to ˝ Vcc by the gain stage.


    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

  • #2
    Awesome Mike! Well done!

    Dan
    _____________________________
    Tall Boys
    NRNP Computer Sub
    The Boxers
    The Hurricanes
    The Baronettes
    Conneccentric
    UX3

    Comment


    • #3
      Very nice!
      Electronics engineer, woofer enthusiast, and musician.
      Wogg Music

      Comment


      • #4
        The mods you made would have added to the piece cost minimally if implemented by the manufacturer, and they didn't just make the device better - they were the difference in being useful in the intended application or not. So I have to ask the question... why would a manufacturer make a device that doesn't work when they could have so easily made one that does?

        Dan
        _____________________________
        Tall Boys
        NRNP Computer Sub
        The Boxers
        The Hurricanes
        The Baronettes
        Conneccentric
        UX3

        Comment


        • #5
          This is an awesome set of mods sir. Thank you for taking the time to do this. I plan on giving this a try soon.
          There are some majorly smart dudes on this board!
          TomZ
          *Veneering curves, seams, using heat-lock iron on method *Trimming veneer & tips *Curved Sides glue-up video
          *Part 2 *Gluing multiple curved laminations of HDF *Cello's Speaker Project Page

          *Building the "Micro-B 2.1 Plate Amplifier -- Part 1 * Part 2 * Part 3 * Part 4 * * Part 5 'Review' * -- Assembly Instructions PDF

          Comment


          • #6
            Originally posted by DanP View Post
            The mods you made would have added to the piece cost minimally if implemented by the manufacturer, and they didn't just make the device better - they were the difference in being useful in the intended application or not. So I have to ask the question... why would a manufacturer make a device that doesn't work when they could have so easily made one that does?

            Dan
            ​Possibly engineered by someone that wouldn't use it as intended. The solution doesn't change cost to manufacturer at all, like for like part swaps +- pennies. Perhaps this will lead to a revision. Perhaps someone at PE could pass this along to the supplier?
            Electronics engineer, woofer enthusiast, and musician.
            Wogg Music

            Comment


            • #7
              Okay- A and B are what taper? A = audio/log? B = linear? or vice versa?

              Oh- the 100uF (C8) polarized cap that is then removed from the circuit by way of removal of R7- Can you just then parallel it then with the 220uF (C3) cap to add to the PSU regulation?

              Thanks, Mike- you really did good here!

              Later,
              Wolf
              "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
              "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
              "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
              "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

              *InDIYana event website*

              Photobucket pages:
              http://photobucket.com/Wolf-Speakers_and_more

              My blog/writeups/thoughts here at PE:
              http://techtalk.parts-express.com/blog.php?u=4102

              Comment


              • #8
                Originally posted by Wolf View Post
                Okay- A and B are what taper? A = audio/log? B = linear? or vice versa?

                Oh- the 100uF (C8) polarized cap that is then removed from the circuit by way of removal of R7- Can you just then parallel it then with the 220uF (C3) cap to add to the PSU regulation?

                Thanks, Mike- you really did good here!

                Later,
                Wolf
                C3 is regulation on Vcc. C8 is regulation on the 1/2 Vcc still connected to the 1/2 Vcc point between R4 & R5 even after R7 is removed.

                A and B are both audio tapers. Unfortunately, even the quality pot mfg.'s (e.g., Bourns) didn't specify the version of the A taper.
                Audio Taper Curves.jpg

                Comment


                • #9
                  So basically, you're removing the .22 cap and reusing it elsewhere in the circuit.
                  The only new components are the .1 cap and the pot?

                  Would you be able to provide a part number suggestion from Digi-Key for the .1 uf cap to get this all in one order?

                  Again, thanks so much for the help making this thing a hugely better product. I have plans for two projects that will use this upcoming.

                  TomZ
                  *Veneering curves, seams, using heat-lock iron on method *Trimming veneer & tips *Curved Sides glue-up video
                  *Part 2 *Gluing multiple curved laminations of HDF *Cello's Speaker Project Page

                  *Building the "Micro-B 2.1 Plate Amplifier -- Part 1 * Part 2 * Part 3 * Part 4 * * Part 5 'Review' * -- Assembly Instructions PDF

                  Comment


                  • #10
                    Maybe DA should hire you as an R&D consultant? It certainly SEEMs so!

                    Comment


                    • #11
                      Originally posted by tomzarbo View Post
                      So basically, you're removing the .22 cap and reusing it elsewhere in the circuit.
                      The only new components are the .1 cap and the pot? Correct

                      Would you be able to provide a part number suggestion from Digi-Key for the .1 uf cap to get this all in one order? Digi-Key doesn't seem to offer a 0.1 uf in a narrow size. Try a KEMET RSBDC3100AA00J, its a box cap w/ correct lead spacing but it's a little larger than the one I used from my parts bin. It should fit. If it doesn't, let me know and I'll mail you some.

                      Again, thanks so much for the help making this thing a hugely better product. I have plans for two projects that will use this upcoming.

                      I'm sending the modified unit back to Gordy next week. Maybe you should wait until he tests it out. I've only had this up on the scope, not a live test.

                      TomZ
                      1

                      Comment


                      • #12
                        Originally posted by Chris Roemer View Post
                        Maybe DA should hire you as an R&D consultant? It certainly SEEMs so!

                        A wise old colleague once said; "Never make the mistake of mixing your avocation with your vocation." Besides, I'd rather help you gents out when I can. Many here certainly helped me.

                        Comment


                        • #13
                          What a great write up! Thanks Mike! My father stills tells me to this day, don't make you hobby your job. Wise indeed.
                          "A dirty shop is an unsafe shop, if you injure yourself in a clean shop you are just stupid" - Coach Kupchinsky

                          The Madeleine
                          The Roxster
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                          • #14
                            Originally posted by Millstonemike View Post

                            C3 is regulation on Vcc. C8 is regulation on the 1/2 Vcc still connected to the 1/2 Vcc point between R4 & R5 even after R7 is removed.

                            A and B are both audio tapers. Unfortunately, even the quality pot mfg.'s (e.g., Bourns) didn't specify the version of the A taper.
                            [ATTACH=CONFIG]n1320160[/ATTACH]
                            I don't see anything else connected to the other end of C8 in your schematic, or is it not all shown? That would leave it out of the circuit. That's why I asked.

                            Later,
                            Wolf
                            "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
                            "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
                            "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
                            "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

                            *InDIYana event website*

                            Photobucket pages:
                            http://photobucket.com/Wolf-Speakers_and_more

                            My blog/writeups/thoughts here at PE:
                            http://techtalk.parts-express.com/blog.php?u=4102

                            Comment


                            • #15
                              Originally posted by Wolf View Post

                              I don't see anything else connected to the other end of C8 in your schematic, or is it not all shown? That would leave it out of the circuit. That's why I asked.

                              Later,
                              Wolf
                              ​It's missing the node dot in the schematic, should be on the same connection where R4 and R5 meet.
                              Electronics engineer, woofer enthusiast, and musician.
                              Wogg Music

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