A challenge—connecting a pre-amp with power amp

Discussion in 'General Electronics Chat' started by ranatungawk, Feb 16, 2016.

  1. ranatungawk

    Thread Starter Senior Member

    Oct 30, 2008
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    This device was originally built as two parts—MP3/Radio player + mini power amplifier. Due to malfunction of the mp3/radio unit, I need to replace it with a new MP3/Radio unit. So I bought a new module from Ebay. However the circuit of the new module is little different from the old one.

    From the old Mp3/radio modules, only 3 lines come as inputs to the power amp (Left-in; Rght-in & Common ground [-]). Please see the old pic. However on the new module, two small amp ICs (8002A) are used to amplify left and right channels separately before going to the power amp (please see new pic). Further there is no a common ground pin, but all 4 lines come out from four IC pings (instead of 3 ping out , there is a 4 ping out). So if I connect the New Mp3 module as I have illustrate in the “New pic”, 8002A ICs of the radio/mp3 module will burn out because only one negative (-) line is goes through the two circuit (mp3/radio module is powered up through a 5V regulator from the Power supply of the power amp). So please explain me how can I connect this new module with the power amp ?:(

    Herewith, I have attached the data sheet of the 8002A IC for your information.

     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    Pin 7 of the 8002A IC is ground. Can you check with an ohmmeter to see which output wires are connected to pin 7? Most likely there are two ground return wires on the output and you can connect them both to the input ground for the power amplifier and also use them as the ground return for the power supply..
     
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  3. wayneh

    Expert

    Sep 9, 2010
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    I'm not so sure after reading the datasheet. See Rf. The problem is using a power amp as input to another power amp. Both expect line-level inputs. You may need a different coupling.
     
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  4. AnalogKid

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    Aug 1, 2013
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    The 8002 is called a BTL amp - Bridge-Tied Load. It is a way to almost double the voltage across a speaker to get more power out of a low-voltage system. The tradeoff is that neither speaker lead is grounded, but that is not a serious problem for you. For each channel, each + and - output has the full audio signal voltage, but sitting on a DC offset equal to Vcc/2. Pin 8 for each channel is the non-inverting output, which preserves signal phase. In your new circuit schematic, disconnect the two - outputs and leave them floating, leave the MP3 module GND and amp GND connected as shown, AND add coupling capacitors between the MP3 + outputs and the amp inputs. If the amp has input coupling capacitors, then you do not need to add external ones.

    ak
     
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  5. ranatungawk

    Thread Starter Senior Member

    Oct 30, 2008
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    Dear AnalogKid;

    Thanks a lot for your advanced reply !! according to your comments i modified the diagram and attached herewith--Please verify it and let me know whether i have understand your advice correctly . meanwhile please tell me what type of caps should i put there ? are they electrolytic ? values of them ? (there is no any inbuilt coupling capacitors) does this change the sound quality of the system ?

    Thanks again :)
     
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  6. AnalogKid

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    Probably electrolytic, yes they are on your schematic correctly, and no they won't affect sound quality more than the 8002.

    The best way to determine the coupling capacitor size is the know the input characteristics of the amp. Is the amp an IC or an assembled module? Either way, part number or link?

    ak
     
  7. ranatungawk

    Thread Starter Senior Member

    Oct 30, 2008
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    This is a module and unfortunately i don't have the circuit diagram of it. however considering the data sheet of the IC (8002A) can you please suggest a cap size . data sheet is attached to the main post of this thread
     
  8. AnalogKid

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    The transfer of audio from one device to another places three impedances in series - the output impedance of the source, the impedance of the coupling capacitor (which varies with frequency), and the input impedance of the destination. Usually the output impedance is so much less than the input impedance that it can be ignored. So the coupling capacitor is in series with the input impedance, and these form a high-pass filter. This is why I asked about the amp. We don't know the actual amp input impedance, but we can make an educated guess. 10 K seems a bit low for a general-purpose device, but if we divide that by 10 for extra margin, and want a minimum corner frequency of 20 Hz, then

    f = 1 / (2 x pi x R x C)
    or
    C = 1 / (2 x pi x f x R)
    = 1 / (6.28 x 20000) = 7.96 uF
    Therefore, the coupling capacitors are 10 uF.

    ak
     
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  9. wayneh

    Expert

    Sep 9, 2010
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    You can also just experiment with different values, if you care to cut the size or cost of the caps. There's a good chance that a 1uF would sound just the same, and a 1uF non-electrolytic is affordable.
     
  10. ranatungawk

    Thread Starter Senior Member

    Oct 30, 2008
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    Thanks Wayneh! it's a god point! the size and the cost wont be a matter, but can we put an non-electrolytic cap here ? shouldn't it be a electrolytic one ?
     
  11. ranatungawk

    Thread Starter Senior Member

    Oct 30, 2008
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    Thanks genius ; I'll do this and let know the result. thank you so much for your time and the support!
    only one thing need to be asked : would this cap work as a hi-pass filter and reduce the bass -quality of the amp ?
     
  12. AnalogKid

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    As explained above, thecoupling capacitor does form a high pass filter. You can calculate the corner frequecy to be whatever you want.

    ak
     
  13. wayneh

    Expert

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    That would be the more cost-effective way to get plenty of capacitance for cheap. But otherwise, a capacitor is a capacitor.
     
  14. AnalogKid

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    Beware the wrath of Walter...
     
  15. wayneh

    Expert

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    I thought about adding some caveats but decided brief was better. ;)
     
  16. AnalogKid

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    Walter is not happy...
     
  17. ranatungawk

    Thread Starter Senior Member

    Oct 30, 2008
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    Good Day!
    I tested your solutions and found they work good. I noted:

    1.With the common ground, speakers out a little less sound than the (-) pin from the IC; but it's O.K
    2. When a 10uF coupling capacitor is put, sound quality goes down a little. but without a coupling capacitor it work fine (no hum).

    i saw on the data sheet of the IC:

    "It does not require output coupling capacitors or bootstrap capacitors."

    If so, can i use this without coupling capacitors when one speaker pin is connected to the common ground? or is it good to put a cap anyway ?
     
    Last edited: Feb 20, 2016
  18. AnalogKid

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    You need a coupling capacitor somewhere. That comment on the datasheet is for when the chip is being used as intended, as a speaker driver, and you are not.

    ak
     
  19. ranatungawk

    Thread Starter Senior Member

    Oct 30, 2008
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    Edited.jpg
    Thanks for the information. if it works fine without a coupling capacitor when even it is used a pre-amp for the discussed power amp, is it still necessary a coupling capacitor?--pls see the attachment

    what happen if it's used a non-polarized 10uf cap instead of a polarized one ?
     
  20. AnalogKid

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    Since you don't know what is on the input stage of the amplifier, use the capacitor. Non-polarized is better, but usually more expensive or hard to find.

    ak
     
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