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B4 pre-amp

Discussion in 'reference' started by PigletsDad, Oct 21, 2011.

  1. PigletsDad

    PigletsDad pfm Member

    Neil Jadman's B4 Schematic, BOM and a few build notes can be found here.


    Here are a few diagnostic notes:

    If you have a DVM, here are some useful places to look, with the rough voltages expected.

    The part numbers come from Neil's schematic to linked to above.

    I will only give top channel numbers - obviously repeat them for the other channel.

    Do the measurements with power applied, but the power amp turned off, and no signal input. All measurements are with respect to ground.

    1) Input terminal - junction of R5 and C5. Should be close to 0V (say within 20mV).
    2) Output terminal - junction of R1, R2, R6, LED2 and LED3. Agains should be close to 0V (within say 100mV)
    3) +ve rail - junction of Q1, Q2, Q3 and D1. Round about +4V - anything from say 3.5 to 4.5 is fine.
    4) -ve rail - junction of Q4, Q5, Q6 and D3. Round about -4V.

    If all those are fine, circuit has basic DC voltages OK. Next is to check internal currents, by measuring voltage across R1, then across R2. Both should read about 0.6V. If this is OK, then problem is almost certainly in the signal path, rather than the active circuit.

    To diagnose the signal path you need a signal source - a generator is ideal, but you can download an ISO of a test CD and burn that and put it in your CD player. Failing that, any music CD with a fairly constant level.

    Keep the power amp off, but connect your test signal source. Put your DVM on AC volts, and measure the voltage at the input socket. Now trace it through the circuit; first after your volume control, then at the input terminals, the input side of C3, output side of C3,
    R6, output side of C6, output terminals on board, output socket.

    Hopefully, these will give you some clues as to where the problem is.
  2. Jo Sharp

    Jo Sharp Pulls on doors marked push

  3. Jo Sharp

    Jo Sharp Pulls on doors marked push

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