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Thread: Crown IC 150 Preamp popping noise - source of problem?

  1. #31
    Join Date: Oct 2016

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    Quote Originally Posted by jimkarl View Post
    Measurement taken relative to the chassis.
    I would not rely on the chassis as a good reference point. Use a known 0V point such as the end of R20 or the end of R17. 0V and earth are not necessarily the same thing. In fact, "earth" is a very ambiguous word.

  2. #32
    Join Date: Apr 2017

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    Quote Originally Posted by RothwellAudio View Post
    I would not rely on the chassis as a good reference point. Use a known 0V point such as the end of R20 or the end of R17. 0V and earth are not necessarily the same thing. In fact, "earth" is a very ambiguous word.
    I tried using the end of R20 - same readings from that point as well.

    I also took that 4.7uf tant out of circuit and confirmed about -4V on the plus side were C12 had been, NO voltage on the other side on R13. Circuit is definitely broken with it out so that confirms no board trace leak. Put (another) fresh 4.7uf back in, just in the one in a million chance the new one was bad somehow, same readings once back in. About -2V on the R13 side. How is that possible?? It just doesn't make any sense. With R13 no going anywhere (disconnected from the selector switch) the ONLY other is through R20 to ground. the 4.7uf removal established there is absolutely no voltage present on that side. Perhaps I should replace R20 anyway, even though it measures fine? But I thought the Cap did all the DC blocking anyway?

  3. #33
    Join Date: Apr 2017

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    Quote Originally Posted by Light Dependant Resistor View Post
    So what measurement do you get when measuring R13 upper and lower when switched to AC ?
    It could be there is a very low frequency oscillation from the phono board and the duty cycle nature of
    multimeters is translating an average DC voltage like a RMS ( Root Mean Square )
    because it is averaging then resident within a much larger AC voltage.

    The typical cause of low frequency oscillation is positive feedback
    As I see it Q4 causes emitter degeneration at Q3 which couples to the base of Q1 and then Q2 giving amplification
    R16 and R11 are feedback and signal related with a lot of reliance on Q4 collector to present opposite phase, as the emitter of
    Q4 is also a feedback path having emitter degeneration. So its not great at keeping positive feedback from starting.
    At C14 we see ability for Q4 to not be steady as its emitter is AC coupled. And emitter of Q4 taking shorter path
    through C14 and C16 for delivering amplified audio. Q6 is a cap multiplier

    I would check R29 variable as there is a lot of reliance on establishing a form of frequency summing point via C16
    In preference using a mosfet with a tiny bit of resistance prior to ground establishes a much better ability for Q4 to
    be predictable.

    I really need to breadboard the phono circuit to see what its characteristics are.

    Cheers / Chris
    On AC reading, that Phono board R13 yields about 0.06vac coming off it. About the same on either side of it.
    Check R29 on the main board. 5.6K as it should. Broke it trying to take out to confirm so now there's a brand new one there ugh...)

  4. #34
    Join Date: Sep 2013

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    So it looks like there is just some residue DC voltage contained in the capacitor
    Using a parallel 1k resistor across resistor R20 which is 100k, dissipate that voltage
    from C 12 it should take 30 seconds and not return once the 1k resistor is removed.

    You could load the phono board output slightly by changing R20 for 22k, the effect will
    be to achieve lower noise

    Did you replace the capacitors at the high gain op amp - which I advised earlier ?.

    Cheers / Chris

  5. #35
    Join Date: Apr 2017

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    Quote Originally Posted by Light Dependant Resistor View Post
    So it looks like there is just some residue DC voltage contained in the capacitor
    Using a parallel 1k resistor across resistor R20 which is 100k, dissipate that voltage
    from C 12 it should take 30 seconds and not return once the 1k resistor is removed.

    You could load the phono board output slightly by changing R20 for 22k, the effect will
    be to achieve lower noise

    Did you replace the capacitors at the high gain op amp - which I advised earlier ?.

    Cheers / Chris
    i don't understand how there could be residue voltage across (now) 3 C12 capacitors that were put there. The original, and now two new ones. How can there be residue voltage on a brand new capacitor? Also - why isn't that same level of residue voltage present on the other channel?

    I'm uncomfortable with changing values, even if to improve performance, because it's not yet clear to me that the source of the problem. My goal is to first and formost get it back to operating - as spec'd. then to talk about possible improvements.

    As for the 1uf tants on the main board, they came in Sat but I have not put them in yet since I have been chasing down this phono board voltage problem. Are you saying that they are in any way responsible with the voltage problem we are experiencing on the phono board? If so i don't see how. Or are you just saying that because of the voltage problem on the phono board, those main board caps aren't handling things well. If so that's a symptom, good to fix, but shouldn't the priority be to fix the source of the problem and not just mask it? Maybe I'm just not understand your intent clearly.

  6. #36
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    Quote Originally Posted by jimkarl View Post
    i don't understand how there could be residue voltage across (now) 3 C12 capacitors that were put there. The original, and now two new ones. How can there be residue voltage on a brand new capacitor? Also - why isn't that same level of residue voltage present on the other channel?

    I'm uncomfortable with changing values, even if to improve performance, because it's not yet clear to me that the source of the problem. My goal is to first and formost get it back to operating - as spec'd. then to talk about possible improvements.

    As for the 1uf tants on the main board, they came in Sat but I have not put them in yet since I have been chasing down this phono board voltage problem. Are you saying that they are in any way responsible with the voltage problem we are experiencing on the phono board? If so i don't see how. Or are you just saying that because of the voltage problem on the phono board, those main board caps aren't handling things well. If so that's a symptom, good to fix, but shouldn't the priority be to fix the source of the problem and not just mask it? Maybe I'm just not understand your intent clearly.
    The original issue was popping noise, now you are chasing DC voltages from coupling capacitors, they are related,
    so let the coupling caps do their job. The Crown has intent to load extremely lightly hence it may be intentional to have
    what are called offset voltages present. Most circuits today do the opposite - they load circuits down to reduce noise.
    A capacitor blocks DC from passing, but in the Crown we see the ground plane is itself capacitively coupled
    so you are unlikely to ever rid the circuit of offsets unless measures are taken to bleed DC voltages continuously

    The best method at lower safe voltages that the Crown uses is to use back to back diodes, these afford connection and also
    a defined ground lift from earth much along the same reasoning why Crown use a capacitor placed to the chassis,

    I would try a parallel diodes back to back around C40 which is a capacitive connection to ground - the capacitor is useless
    on its own ,as it is then frequency dependent passing AC but not DC.

    You can use 1N4007 or UF 4007 types. This should instantly remove the popping noise or any DC on the board
    sitting around latently on coupling capacitors causing the problems.

    You should notice the Crown is then extremely quiet, and will sound much better too.

    So one diode is placed anode ( from the roman word meaning way in ) to the ground plane and cathode ( way out ) to
    the chassis. The other diode is placed anode to the chassis and cathode to the ground plane..... hence back to back
    You could strap these across C40 if there is room.

    This will afford exactly the same isolation that Crown desired, but allow proper DC connection as well
    A measurement will show a semiconductor lift of approximately 0.6v.

    The more complete analysis of this circuit also involves a resistor which sadly will not give the isolation
    Crown wanted, but adding 2 diodes will.

    Hope this helps

    Cheers / Chris
    Last edited by Light Dependant Resistor; 19-04-2017 at 03:58.

  7. #37
    Join Date: Oct 2016

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    Quote Originally Posted by jimkarl View Post
    i don't understand how there could be residue voltage across (now) 3 C12 capacitors that were put there.
    Neither do I.
    Quote Originally Posted by jimkarl View Post
    I'm uncomfortable with changing values, even if to improve performance, because it's not yet clear to me that the source of the problem. My goal is to first and formost get it back to operating - as spec'd. then to talk about possible improvements.
    Seems sensible to me.
    I suggest you measure the voltage on the other side of C12 where it meets R19/R16 and the emitter of the transistor. Then do a little mock-up of that bit of the circuit by having a capacitor and 100k resistor (like C12 and R20) and putting DC on one side of the capacitor, then measure the voltage on the other. This should at least verify that your meter is making sensible readings.

  8. #38
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    Quote Originally Posted by RothwellAudio View Post
    Neither do I..
    Put two capacitors in series such as C12 + C40 and it makes a lot of sense why
    there are residual voltages. Even if one side is chassis grounded. Such is the nature
    of capacitance to store voltage . Back to back diodes will instantly cure.

    Cheers / Chris

  9. #39
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    Quote Originally Posted by Light Dependant Resistor View Post
    Put two capacitors in series such as C12 + C40 and it makes a lot of sense why
    there are residual voltages. Even if one side is chassis grounded. Such is the nature
    of capacitance to store voltage . Back to back diodes will instantly cure.

    Cheers / Chris
    C40 has nothing to do with anything. The OP is measuring voltages with respect to 0V, not the chassis or the mains earth.

  10. #40
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    Quote Originally Posted by RothwellAudio View Post
    C40 has nothing to do with anything. The OP is measuring voltages with respect to 0V, not the chassis or the mains earth.
    How wrong you are, 0v in the Crown iC150 is not zero volts as stated, as 0v is always in series
    with C40. Connect 2 capacitors in series you get voltage at their junction... no mystery to that.

    C40 is on the schematic is a capacitive connection to chassis ground,
    hence a zero volt reference up to the currents involved with a preamp.

    Crown's mistake was to take the floating ground concept a bit too far by relying on capacitors
    alone. Two diodes should retain Crowns concept and allow connection at a semiconductor junction
    at the same time, allowing residual voltages to properly dissipate.

    Cheers / Chris

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