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Design bass amplifier

Discussion in 'Audio' started by josejos, Mar 6, 2018.

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  1. Audioguru

    Audioguru

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    Sep 24, 2016
    Jose says "the bass seems to influence the offset" so I think the bass is one polarity pulses that offsets the output until the input and output capacitors charge or discharge to the average voltages of the one-sided pulses. It doesn't matter unless the levels are so high that the output clips the peaks of the pulses.
     
  2. WHONOES

    WHONOES

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    May 20, 2017
    Is that on a single supply. If so then he needs an output capacitor to remove the DC content from the speaker. He will indeed see a fairly large offset voltage if that is the case.
     
  3. Hopup

    Hopup

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    Jul 5, 2015
    Are you making amplifier for subwoofer or just speakers with bass/mid and tweeter?
     
  4. Audioguru

    Audioguru

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    Sep 24, 2016
    Bose calls their dinky little 4" woofer a sub-woofer. It does not produce sounds below 100Hz so it ain't a sub-woofer.

    I have a pretty good sounding Sylvania 2.1 sound system in my TV/computer room that cost $25.00 CAN on sale. It has two satellite mid/high speakers each with two 1.5" long throw tweeters and a port, and one 5" woofer in a ported enclosure. It is advertised to produce 150 Watts total output (!) but its power transformer is marked 9VAC/1.1A which is only 9.9W. About 4W makes heat so the three amplifiers produce a total of about 5.9W.
     

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  5. WHONOES

    WHONOES

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    May 20, 2017
    If you wanna make big bass then you need correspondingly big speaker with amps to match.
    Josejos:::: If you only want to make a sub bass unit, give us some idea of what you want and with what you hope to achieve it.
     
  6. josejos

    josejos

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    Mar 6, 2018
    Hi, thanks for the explication Audioguru and WHONOES, it helps a lot :) .

    I think it's handy to give more info over my project.
    I am working to build a stereo system with a audio spectrum analyzer and a FM receiver and other stuff.
    The prototype off the audio spectrum analyzer is finished and now i wanted to start with the amplifier section off the stereo. As good speakers cost a lot, i wanted to reuse the speakers of a home cinema a have laying around.
    The link of the manual with extra information of the speakers of the home cinema:
    https://www.manualslib.com/manual/247160/Panasonic-Sc-Ht56.html#product-SB-HWX50

    For the stereo i wanted to make a specific amplifier board to drive the two tweeters and the subwoofer.

    I have already designed the filter for the bass amplifier but when i measured the frequency response the response is not ideal also after some tweaking, for the convenience i used the online calculator from analog devices. The schematics off this design with the stereo to mono converter is given below.

    upload_2018-3-11_20-15-20.png
    The bodeplots of this design seems a lot better, but the output signal at Bass_signal_amp seems drifting a frequency of 50Hz. I first tought it was the capacitors C2 and C3 that charges and discharges a little bit with the low tones of the bass. I measured the voltage over C2 and that seems to confirm my theory.
    Noise at output filter:
    upload_2018-3-11_20-39-54.png
    Math voltage over C2
    upload_2018-3-11_20-39-21.png
    Then i decided to rebuild the circuit with a symmetric power supply to eliminate this problems.
    upload_2018-3-11_20-44-3.png
    The circuit works the same but has also the same noise as the asymmetric powersupply version.
    After extra investigation and a FFT analysis i have some extra info over the noise.
    FFT at the input of the filter (mono):
    upload_2018-3-11_20-46-58.png
    FFT after the first filter stage:
    upload_2018-3-11_20-48-2.png
    FFT at the output
    upload_2018-3-11_20-48-59.png
    So the noise is a 50Hz (seems like some noise of the power line) and increase after a filter stage, i used a other power supply and decoupled the power supply with 100µF but that doesn't changed a lot. I also tried the TL082 as Opamp but that also doesn't changed a lot.
     

    Attached Files:

  7. Audioguru

    Audioguru

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    Sep 24, 2016
    Didn't you know that a lousy old LM358 is terrible for audio?
    1) It produces horrible crossover distortion.
    2) It is noisy (lots of hissss).
    3) It has trouble producing high levels above only 2kHz.
    Use a modern opamp designed for audio instead. An OPA134 single, OPA2134 dual and OPA4134 quad audio opamp is good.

    Why do you need a 4th-order lowpass filter? It is designed wrong and is oscillating at about 50Hz and 280Hz.
    Your tiny Japanese speakers cannot produce low frequencies or high frequencies anyway.
     

    Attached Files:

  8. Audioguru

    Audioguru

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    Sep 24, 2016
    I simulated your odd filter:
     

    Attached Files:

  9. josejos

    josejos

    18
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    Mar 6, 2018
    Ok, thanks for the effort to read my explination.
    No i didn't know that the lm358 is so bad audio opamp, i already have ordered some OPA2134 :).

    Is a tl082 also a bad opamp for the application?, i used this opamp and i still got the 50Hz frequency.
    Sure this is not noise from the power supply?

    Ok, is it better to design yourself then to use a online filter wizard. I generated a 3 order butterworth filter, is this a better filter for the application.
    upload_2018-3-12_21-50-55.png
    Sorry i forgot to mention that the 288Hz frequency is from my frequency generator, the 50Hz is the oscillating frequency.

    Strange that the amplitude of the plot rise at frequency's higher then 1KHz.
    I made yesterday a bodeplot with a frequency generator but i have a normal behaviour.
    upload_2018-3-12_21-58-54.png
    Sorry my old FG can't do logarimic sweeps the x axis an y-axis are lineair :( (the total sweep frequency is 2KHz).

    I anyone have good idea's to buy a affordable frequency response analyzer?
     
  10. Audioguru

    Audioguru

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    Sep 24, 2016
    A TL08x is a general purpose opamp with a Jfet input for very high input resistance. It produces frequencies up to 100kHz very well. But a TL07x is for audio because it is a TL08x selected for low noise (a TL08x is probably noisy because all the low noise ones are sold as a TL07x.
    They have a strange behavior if an input voltage becomes within a few volts from the negative supply (or ground if there is a single supply), the output suddenly goes as high as it can producing extreme distortion. it is called Opamp Phase Inversion and it also affects some other older Jfet input opamps.
     

    Attached Files:

  11. Audioguru

    Audioguru

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    Sep 24, 2016
    It looks like you are picking parts randomly instead of designing a 3rd order lowpass filter.
    Here is yours and here is mine:
     

    Attached Files:

  12. josejos

    josejos

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    Mar 6, 2018
    OK, thanks for the info, i definitly use the OPA2134 as opamp :) .
    Ok your filter looks a lot better, i used the online generator to calculate the components.

    How have you designed the your filter, do you have to clue to start with it? (i know the basic off the bodeplots and transferfunctions).
     
  13. Audioguru

    Audioguru

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    Sep 24, 2016
    On the internet there are many tutorials for designing filters but I simply copied the subwoofer filter from an audio site like Elliot Sound Products. There is a Japanese site that calculates parts values for your filter but the schematic of the filter looks like mine and not like yours.
     
  14. WHONOES

    WHONOES

    774
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    May 20, 2017
    I would (and have) used a Sallen-Key equal value component filter. It involves putting a small amount of gain round the amplifier which then controls the Q making it much easier to calculate the RC values required. Have a look in Don Lancaster's Active Filter Cookbook.
    I would also use the OPA604. It is very low noise and distortion and is by far the best sounding opamp I have ever used.
    Having said that I tend to use my own design amplifiers for these purposes these days.
     
  15. josejos

    josejos

    18
    1
    Mar 6, 2018
    OK thanks i have found some info to calculate the filter.
    I have recalculated the circuit and i have about the same values as your filter :).

    I would try to simulate it a lt spice but i got the following error:
    upload_2018-3-14_22-30-46.png
    upload_2018-3-14_22-29-7.png
    Do somebody know how to solve this?
     

    Attached Files:

  16. Audioguru

    Audioguru

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    Sep 24, 2016
    In LTspice, you selected an Opamp2 that needs an additional external spice file that details it. Simply delete it and use "opamp" that is directly ahead in the list of opamps.
     
  17. Sunnysky

    Sunnysky

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    116
    Jul 15, 2016
    If you have a decent 8 Ohms subwoofer (12") then you cnan drive it down to 10Hz with this. for reasonable power.[​IMG]
    The breakpoints for input and output are the intersections of impedance in Red shown on the impedance chart below.
    upload_2018-3-14_22-36-51.png
     
  18. Hopup

    Hopup

    227
    31
    Jul 5, 2015
  19. Sunnysky

    Sunnysky

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    116
    Jul 15, 2016
    affordable log frequency response analyzer ( = Audacity =free on any PC. ) upload_2018-3-15_14-6-17.png

    I just imported some music (Bob James) and selected some portion and >Analyze> Plot frequency Spectrum... You can also generate a Sweep signal = "Chirp" and drive your audio to test it then use Aux input to check response.
     
  20. Audioguru

    Audioguru

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    Sep 24, 2016
    I knew that a TDA2030 was obsolete but today I am surprized to see that the TDA2030A is also obsolete. If you can find one then 12V is its minimum supply where its output to an 8 ohm speaker is only 2W or 3W. The power vs supply voltage graph in its datasheet begins at 24V where its power into 8 ohms is 6W.
     
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