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About make an audio power amplifier.

Discussion in 'General Electronics Chat' started by Nikolai Petrenko, Jan 30, 2016.

  1. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    ".Use 5A L and Plastic C instead of huge e-caps.". referred to my suggestion to use cheaper, better SMPS with lower ripple using inexpensive small LC.


    In your case, you need, huge e-cap, depending on your tolerance to muddy distorted bass with 100 Hz hum at high power. Without the added complexity of cascode current sources, your supply ripple rejection will be poor in PA at high power.

    What C value?
     
  2. spec

    spec Well-Known Member Most Helpful Member

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    The 1K resistor has little effect on the voltage gain of the opamp stage in the audio band, it just gives isolation of the input from stray capacitances etc at high frequencies. Remember that the OPA 2134 has a higher frequency response than normal opamps. The 100 Ohms on the output of the opamps is to isolate the output of the OPA2134.

    spec
     
  3. spec

    spec Well-Known Member Most Helpful Member

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    Here is an outline circuit of the Elliot speaker protector with AC monitoring and relay drive. This basically has the functionality of your circuit, as discussed, but simplified and rationalised. There is always a danger with monitoring circuits that they will not turn on. I have not investigated this aspect. The time constants R12/C10 and R13/C9 will give around one second before AC monitoring starts.

    spec
    2016_06_19_iss01_ETO_SPEAKER_PROTECTOR.png
    ERRATA
    (1) C9 should read 22uF

    NOTES

    (1) R10 is not strictly necessary but is just good practice.
     
    Last edited: Jun 20, 2016
    • Like Like x 1
  4. dave

    Dave New Member

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  5. Nikolai Petrenko

    Nikolai Petrenko Member

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    I already have 10000uF old Epcos cap (extract from old amplifier).
     
  6. Nikolai Petrenko

    Nikolai Petrenko Member

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    My active crossover, certain has many error:
    Active crossover.png
     
  7. spec

    spec Well-Known Member Most Helpful Member

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    Impressive and well executed.:cool: A good schematic layout helps a lot to understand, check, analyze, modify, and fault-find a circuit.

    Suggest these minor amendments to reduce cross-talk and possibly improve PCB layout:
    IC2A change to IC1B
    IC3A change to IC2B
    IC2B change to IC3A

    IC1A and IC2B need a 47 Ohm resistor between their outputs and the high pass filter inputs.

    Are you planning on combining the filter function and preamp all in one to reduce the number of gain stages that the audio signal goes through?

    One thing I did notice is that you have two low pass filters where one could do the job. This can be implemented by combining the left and right audio channels, for the bass channel, before the low pass filter. On reflection I am not sure if it is better as the signal overhead may be less with a single low pass filter- something to think about.

    In an amplifier chain there is always a trade off between distortion, dynamic range, and noise and active filters are notorious headroom thieves so this is an area to be investigated.

    Why have you suddenly changed from an OPA2134 to an NE5534 for the bass sub output?

    spec
     
    Last edited: Jun 20, 2016
  8. Nikolai Petrenko

    Nikolai Petrenko Member

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    Thank, I have fixed the error, I have wrong at two low-pass filter, gain=2 is replace by unity gain
    Active crossover.png
     
  9. spec

    spec Well-Known Member Most Helpful Member

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    Hy Nicolai,

    I have had a more detailed look at the filter and preamp designs.

    This is what I suggest:
    (1) Combine the preamp (mine) with your filter circuit.
    (2) Replace the two filter input opamps with the two output opamps from the preamp (left and right channels) in the preamp.
    (2) Delete IC4
    (3) Delete IC3B
    (4) Connect R15 top to IC1A output
    (5) Connect R16 lower to IC1B output
    (6) The output of the IC3A is then the output to the sub amplifier

    That will leave one spare OPA2134 opamp (I think).

    spec
     
    Last edited: Jun 20, 2016
  10. spec

    spec Well-Known Member Most Helpful Member

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    About earthing and star points: While it is good that you are attending to the earths and hopefully supply lines, it is a little more complicated than just connecting every earth line to one point. The important thing is to visualize what the opamp sees at its non inverting and inverting inputs and also practical implications. Tony Stewart mentioned similar.

    As a result, you may have a few earth star points. Don't let this put you off though. The earthing can easily be sorted. The main thing is to get the topology and functionality correct. The next stage will be fine-tuning and optimization.

    It is quite handy that you have posted the filter design because it will be useful for me on another project. I hope you don't mind me stealing it?:)

    spec
     
    Last edited: Jun 20, 2016
  11. spec

    spec Well-Known Member Most Helpful Member

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    If it is not too much hassle, operate the OPA2134 at 17.5V. They sound better with high voltage supply lines and also you get more overhead.

    spec
     
  12. spec

    spec Well-Known Member Most Helpful Member

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    This is what I was thinking about for the combined preamp and filters:


    2016_06_20_iss01_ETO_AUDIO_PRE-AMP_ver2.png
    ERRATA
    (1) Change R16 to 10K
    (2) Change R24 to 10K
    (3) Change R25 to 10K
    (4) Change pin identity from, 'LEFT MID OUTPUT' to 'LEFT MID/HIGH OUTPUT'
    (5) Change R10 to 47R
    (6) Change R1 to 47R
    (7) Change R19 to 47R
    (8) Change R22 to 47R

    DATA SHEETS
    (1) OPA2134
    http://www.ti.com.cn/cn/lit/ds/symlink/opa2134.pdf
     
    Last edited: Jun 21, 2016
  13. Nikolai Petrenko

    Nikolai Petrenko Member

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    EDITED
    How many schematic and design I have and will steal from you? :D:D:D
    As you know, NE5532 are my choice now.
    Why call output of high-pass filter is "mid"? I think should be "mid+treb.", may I am too stupid fastidious? :D
    And only need one opamp in low-pass filter section, so go for NE5534?
     
    Last edited: Jun 20, 2016
  14. Nikolai Petrenko

    Nikolai Petrenko Member

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    Oh, your design not only help me to save money, it also reduce the duty on power supply, also save more PCB and space too. One dual opamp has been kicked out :D. What will happen if 12K5 arenot available in the store? Any answer if it happen?
    With your lastest schematic and plugged NE5532, is it neccessary to use coupling caps for the power amp input?
    The Elliot amp has voltage gain=23 (was I wrong), and Elliot also said DC offset< 100mV, so maxium DC in the input without capacitor will be <4mV. <4mV DC offset, is it possible? Or any wrong in my think?
     
  15. Nikolai Petrenko

    Nikolai Petrenko Member

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    And each star point will be connected to main star point? :D
     
  16. spec

    spec Well-Known Member Most Helpful Member

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    :joyful:

    Not possible without decoupling capacitors, so they would lead to more expensive overall.

    Correct- fastidious is good.:cool:

    Got it

    spec
     
  17. spec

    spec Well-Known Member Most Helpful Member

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    Those are only notional resistor values at the moment and not worth worrying about. You can make any resistor you like by parallel and series standard resistors.

    The formula is, (R16/2) + R25= 15K

    Yes- but other complications arise because of the high NE5532 input current and offset voltage.

    Not wrong. The Gain of the Elliot amp is 1+ R5/R4= 1+22K/1K = 23
    http://sound.westhost.com/project3a.htm. But due to C3 the gain is 1 at DC.

    You are on the right track but detail not right. At a first approximation though, yes you can DC couple throughout, providing you use the OPA2134.

    spec
     
    Last edited: Jun 20, 2016
  18. spec

    spec Well-Known Member Most Helpful Member

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    Just for reference, Rod Elliot audio power amplifier, Nikolai version:

    (1) OUTPUT TRANSISTORS (Q7/Q8)
    ON, NJW3281/NJW1302 http://www.onsemi.com/pub_link/Collateral/MJW0281A DATASHEET.PDF
    (2) DRIVER TRANSISTORS (Q5/Q6)
    ON, MJE15028/MJE15029 http://www.onsemi.com/pub_link/Collateral/MJE15028-D.PDF
    (3) VAS TRANSISTOR (Q4)
    ON, (MJE243G)/MJE253G http://www.mouser.com/ds/2/308/MJE243-D-113102.pdf
    (4) LONG TAILED PAIR TRANSISTORS (Q1/Q2)
    xxx/xxx
    (5) LED (D1)
    xxx
    (6) TAIL CURRENT (Q3)
    xxx
    (7) VBE MULTIPLIER (Q9)
    xxx

    VOLTAGE GAIN: 23
    SENSITIVITY: 1.414V peak (1V RMS)

    [​IMG]
     
    Last edited: Jun 21, 2016
  19. Nikolai Petrenko

    Nikolai Petrenko Member

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    Well, I forgot the C3 :p, so input and output DC will be almost the same.
     
  20. spec

    spec Well-Known Member Most Helpful Member

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    The voltage difference between the amplifier output and zero volts (which is what counts) will be dependent on.
    (1) The difference between the VBEs of the two long tail pair (LTP) transistors
    (2) The difference between the LTP hFEs
    (3) The difference between the LTP transistor collector currents, which are not necessarily equal in the Elliot amplifier.

    spec
     
    Last edited: Jun 21, 2016
  21. Nikolai Petrenko

    Nikolai Petrenko Member

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    What about (10k/2) + 10k =15k??? :D
     

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