Impedance matching, preamp to power amp.

Discussion in 'General Electronics Chat' started by John Douglas, Sep 22, 2019.

  1. John Douglas

    Thread Starter New Member

    Aug 7, 2016
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    I built a stereo power amp, using two kitset 20W amps. For the front end I was using a mixer with an attenuator. This worked well. I decided to install a kitset (AliExpress) preamp. Now the output from the power amps has low volume and is distorted. I suspect this is due to an impedance mismatch. The trouble is I don't have a clue how to fix it. What sort of circuit do I need and do I put between the amps?
     
  2. Audioguru

    Expert

    Dec 20, 2007
    11,249
    1,349
    You forgot to tell us which AliExpress amplifiers and preamp you have. I looked at AliExpress and see that they have many different ones.
    Then we need to see the schematics which they might not show until you buy them. Can you scan the schematics and post them here please?
     
  3. John Douglas

    Thread Starter New Member

    Aug 7, 2016
    8
    0
    Thanks for your reply. I have attached two PDF files. 20W Amp contains the circuit diagram for the 20W power amps. I have asked AliExpress for a circuit diagram for the Pre-Amp, but it seems that the only thing they have is the component layout, I have included that in case it's of any use. Further details on the Pre-Amp are:

    CIRMCH NE5532 OP-AMP HIFI Amplifier Preamplifier Volume Tone EQ Control Board DIY KIT.
    Brand Name: CIRMECH
    Maximum Power Per Channel: 10W
    Model Number: WM-025
    Channels: 2 (2.0)

    I hope this information us of use.
     
  4. John Douglas

    Thread Starter New Member

    Aug 7, 2016
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    Further to my query, I hjave managed to source a circuit diagram.
     
  5. dendad

    Distinguished Member

    Feb 20, 2016
    2,921
    810
    For a start, have you checked the pre-amp is actually working ok?
    Do you have an oscilloscope?
     
  6. Ylli

    Active Member

    Nov 13, 2015
    747
    218
    You are connecting the output of the preamp (volume control wiper) the the + input of the power amp, right? Just leave the - input to the amplifier floating.
     
  7. John Douglas

    Thread Starter New Member

    Aug 7, 2016
    8
    0
    No. Unfortunately not. I'm just an amateur at this, I've got some hand tools and a multimeter.

    I have seen a video on YouTube with a guy testing one of these pre-amps and he put the problem down to the supply at the NE5532's being too low, causing clipping.
     
  8. John Douglas

    Thread Starter New Member

    Aug 7, 2016
    8
    0
    Yes it is wired correctly.
     
  9. John Douglas

    Thread Starter New Member

    Aug 7, 2016
    8
    0
    I have solved the problem. I fell into a trap for young players. There are two regulators on the board, one for +12V and the other for -12V. Sods law says that, without adequate instructions, I would put them in the wrong positions.

    Thanks to all for your help.
     
  10. TeeKay6

    Member

    Apr 20, 2019
    337
    83
    @John Douglas
    Thank you for being kind enough to let us know of your success!
     
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