Problem with preamp on audio amplifier

Discussion in 'General Electronics Chat' started by bluebrakes, Jan 29, 2014.

Thread Status:
Not open for further replies.
  1. bluebrakes

    Thread Starter Active Member

    Oct 17, 2009
    Hi everybody,

    Skip to the bottom for the questions or read through for the fault description

    Since I play electronic drums, I've been meaning to get hold of an active floor monitor (speaker with built in amp) for a while for gigs and stuff.

    I found this faulty one brand new from a shop on ebay, so I decided to buy it as a little project.

    There are a choice of two inputs into the amplifier, one is a microphone and the other is a line level input.

    The issue with it is that the line-in input hisses when the level control is turned up.

    The microphone input is perfect and there isn't a single whiff of ground noise when it's cranked to max.

    If the input levels are set to zero and the master volume control is set to max there is no hissing either.

    This leads me to believe that the preamp of the line-in input is at fault. However, I should point out that if I play an MP3 player or whatever through the line levels, it does produce the sound as it should, except there is hissing accompanying it. The hissing gets louder or quieter as the line-in level potentiometer is adjusted on the control panel.

    So today I took the amplifier apart and it all looks fairly straight forward.

    The back of the control panel of the cover (with the EQ controls and input sockets) has a PCB on the back with a single 4 pin connector going to the PCB from the rest of the circuitry of the amplifier (see picture of PCB removed).

    This PCB seems to be the preamp for amplifier.


    Back of the PCB (which I photoshopped to show the various duties of the PCB tracks for reference purposes)...

    The board appears to use 4580 IC, which is a duel operational amplifier and uses four of them in total on the PCB. The 16 pin IC is an LM13700 (Dual Operational Transconductance Amplifiers with Linearizing Diodes and Buffers)

    The far left 4580 appears to be responsible for the Line level output.
    Top left 4580 i think might be something to do with EQ
    Top right 4580 I think deals with EQ and combining the two inputs (mic and line in)

    Bottom right 4580 duel opamp seems to be amplifying the mic input on one side and the line level on the other side before going to either input level adjustment potentiometers (shown with orange tracks).

    Am I right in thinking that the 4580 IC in the bottom right hand corner (which uses an opamp for each input) at fault?

    What is the best way to test to see if it's a faulty IC?
  2. MrChips


    Oct 2, 2009
Thread Status:
Not open for further replies.