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Thread: Is this thump normal?

  1. #1
    Senior Member pyonc's Avatar
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    Is this thump normal?

    Hi friends,

    Please look at the initial couple of seconds of this video clip.
    When the owner turns on the power switch of JBL SG520 preamp driven by JBL SE408S power amp,
    you see the woofers moving out with rather a big audible thump:

    https://www.youtube.com/watch?v=StnL...ature=youtu.be


    Okay, I heard the same audible thump from JBL SE400S, basically the same model as SE408S, all produced in late 1960s.
    Some say this particular model has that thump due to its T-circuit electrical board, which I'm not sure of.
    Do you think this is the normal thump? Does it cause any damage to the woofers over the long run?
    On googling, I see something like this:
    "Pre-amps can pass DC and some amps do not have a built in protection circuit which can be damaged by DC
    and which can be passed on to the speakers which can also be damaged."


    Is this the case with this vintage SE408S or SE400S? I heard the same thump from JBL SA640 power amp.

    Thanks for your feedback and advice, as always.

    PS: I don't hear any such thump when I use SG520 with a Crown amp.

  2. #2
    Super Moderator yggdrasil's Avatar
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    Some amplifier design's has this turn-on thump.

    A few years back I had a Norwegian amplifier with turn-on thump. The designer offered an upgrade, and when I called to make the appointment, I had the pleasure of discussing this thump with the designer. He ensured me that any normal, working woofer, would not be damaged by the turn-on thump in his amplifiers.

    Later I have built quite a few different amplifier designs, and the only times I have built a turn-on thump are with single ended designs. None of the push pull-designs (that I built) have turn-on thump.

    "Pre-amps can pass DC and some amps do not have a built in protection circuit which can be damaged by DC
    and which can be passed on to the speakers which can also be damaged."
    Some (working) pre-amps kan pass DC, some not. This is a design choice.

    Power-amps can implement protection against DC in several ways, depending on the design goal of the power-amp. Simplest protection comes from a series capacitor at input. More complicated designs have circuits monitoring input and / or output for DC.

    Looking at commercial offerings my guess would be that:
    * professional amps have built-in protection
    * high-power amps have built-in protection
    * low-power, high-end amps probably won't have built-in protection
    * older amps - you'll have to check
    Johnny Haugen Sørgård

  3. #3
    Senior Member pyonc's Avatar
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    Quote Originally Posted by yggdrasil View Post
    Some amplifier design's has this turn-on thump.

    A few years back I had a Norwegian amplifier with turn-on thump. The designer offered an upgrade, and when I called to make the appointment, I had the pleasure of discussing this thump with the designer. He ensured me that any normal, working woofer, would not be damaged by the turn-on thump in his amplifiers.

    Later I have built quite a few different amplifier designs, and the only times I have built a turn-on thump are with single ended designs. None of the push pull-designs (that I built) have turn-on thump.


    Some (working) pre-amps kan pass DC, some not. This is a design choice.

    Power-amps can implement protection against DC in several ways, depending on the design goal of the power-amp. Simplest protection comes from a series capacitor at input. More complicated designs have circuits monitoring input and / or output for DC.

    Looking at commercial offerings my guess would be that:
    * professional amps have built-in protection
    * high-power amps have built-in protection
    * low-power, high-end amps probably won't have built-in protection
    * older amps - you'll have to check
    Thanks a lot for your kind and detailed reply. So, if the designer is right, I don't have to worry about this woofer movement as a result of the thump. Looks like vintage JBL power amps including JBL SA640 produced in 1980 have all that typical thump when turned on.
    I've got SA640 and SE408S at the moment. Some recommend using a speaker selector to avoid the thump, but I'm afraid that will affect the sound quality.

  4. #4
    Senior Member DavidF's Avatar
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    Quote Originally Posted by pyonc View Post
    Thanks a lot for your kind and detailed reply. So, if the designer is right, I don't have to worry about this woofer movement as a result of the thump. Looks like vintage JBL power amps including JBL SA640 produced in 1980 have all that typical thump when turned on.
    I've got SA640 and SE408S at the moment. Some recommend using a speaker selector to avoid the thump, but I'm afraid that will affect the sound quality.
    I have used Adcom, an old Hafler, and a couple of B&K amps that had various degrees of a thump, cone motion, or both. I think it was the Adcom that actually had cone motion when turned off, as well, as the power supply caps discharged.

    On the Adcom I changed some of the low level driver transistors using matched pairs that diminished, but did not completely eliminate, the turn on thumps.

    Right or wrong I attributed the degree of noise and motion to the age of components and the difference in timing that they came up to temp. I always checked the DC at the speaker terminals to ensure things settled down as the amp came up from a cold start before use with speakers. With one the B&K amps you could see spikes in DC of several volts at turn on before it immediately dropped to millivolts as expected.
    David F
    San Jose

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    Senior Member SEAWOLF97's Avatar
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    Quote Originally Posted by DavidF View Post
    I have used Adcom, an old Hafler, and a couple of B&K amps that had various degrees of a thump, cone motion, or both. I think it was the Adcom that actually had cone motion when turned off, as well, as the power supply caps discharged..
    My Adcom sends a little "chirp" about 10 seconds after powering OFF, but no turn ON thump. Have always used the practice of turning GAIN to zero before powering ON with all amps
    (it's just old skool SOP)
    “If you think that’s the dumbest thing you’ve ever heard,
    just wait a couple minutes!”

  6. #6
    Senior Member Baron030's Avatar
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    Hi

    I did a quick google search and I could not find the JBL SA640 schematic. But, I did turn up the service manual for the SA600 and SA660 amplifiers, which should be very similar to the SA640. The service manual does suggest that up to 0.1 volt of DC current is normal. And with high efficiency speakers that would create an audible thump. If you any concerns than you may want to test the outputs with a volt meter. The service manual does describe a “zero adjustment” potentiometer, which you could re-adjust to reduce the DC current to a lower value. And I would think that the SA640 would also have a “zero adjustment” potentiometers mounted somewhere on the main circuit board, with one potentiometer for each channel.

    Here is the link to the service manual:
    http://www.jblproservice.com/pdf/Vin...A600~SA660.pdf

    Baron030
    Last edited by Baron030; 06-23-2015 at 08:15 AM. Reason: to fix a typo

  7. #7
    Senior Member pyonc's Avatar
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    Quote Originally Posted by Baron030 View Post
    Hi

    I did a quick google search and I could not find the JBL SA640 schematic. But, I did turn up the service manual for the SA600 and SA660 amplifiers, which should be very similar to the SA640. The service manual does suggest that up to 0.1 volt of DC current is normal. And with high efficiency speakers that would create an audible thump. If you any concerns than you may want to test the outputs with a volt meter. The service manual does describe a “zero adjustment” potentiometer, which you could re-adjust to reduce the DC current to a lower value. And I would think that the SA640 would also have a “zero adjustment” potentiometers ounted somewhere on the main circuit board, with one potentiometer for each channel.

    Here is the link to the service manual:
    http://www.jblproservice.com/pdf/Vin...A600~SA660.pdf

    Baron030
    Thanks for your feedback. Does this thump have anything to do with the so-called T-Circuit?
    According to the manual, it says T-Circuit is "an analog computer-type operational DC amplifier. All stages are direct coupled, including the output stage so that accurate control of the loudspeaker is maintained to DC. In the JBL T-Circuit, there are no audio transformers of any kind, no coupling capacitors, no reactive components to affect the response of the stability of the circuit in any way."

  8. #8
    Senior Member pyonc's Avatar
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    Quote Originally Posted by DavidF View Post
    I have used Adcom, an old Hafler, and a couple of B&K amps that had various degrees of a thump, cone motion, or both. I think it was the Adcom that actually had cone motion when turned off, as well, as the power supply caps discharged.

    On the Adcom I changed some of the low level driver transistors using matched pairs that diminished, but did not completely eliminate, the turn on thumps.

    Right or wrong I attributed the degree of noise and motion to the age of components and the difference in timing that they came up to temp. I always checked the DC at the speaker terminals to ensure things settled down as the amp came up from a cold start before use with speakers. With one the B&K amps you could see spikes in DC of several volts at turn on before it immediately dropped to millivolts as expected.
    Well, as long as the thump won't cause any damage to woofers or mid/high cones, I'm ready to live with it.

  9. #9
    Senior Member DavidF's Avatar
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    Quote Originally Posted by pyonc View Post
    Well, as long as the thump won't cause any damage to woofers or mid/high cones, I'm ready to live with it.
    Always take consideration of severity. The bump on the you tube video was fairly pronounced but only a short impulse. Wouldn't bother me.

    Most passive xovers will place a capacitor in the path of a mid and the tweeter. If so, these capacitors will block out the DC impulse. If not (or if bi amping) suggest placing a large value cap in series with the driver.
    David F
    San Jose

  10. #10
    Senior Member pyonc's Avatar
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    Quote Originally Posted by DavidF View Post
    Always take consideration of severity. The bump on the you tube video was fairly pronounced but only a short impulse. Wouldn't bother me.

    Most passive xovers will place a capacitor in the path of a mid and the tweeter. If so, these capacitors will block out the DC impulse. If not (or if bi amping) suggest placing a large value cap in series with the driver.
    I see. By the way, I wonder if this bump has anything to do with the so-called T-circuit of this vintage JBL units that include SE400S, SE408S and SA600, all products of the 1960s. According to the manual, it says T-Circuit is "an analog computer-type operational DC amplifier. All stages are direct coupled, including the output stage so that accurate control of the loudspeaker is maintained to DC. In the JBL T-Circuit, there are no audio transformers of any kind, no coupling capacitors, no reactive components to affect the response of the stability of the circuit in any way.


  11. #11
    Senior Member Baron030's Avatar
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    Hi pyonc
    Yes, the thump does have to do with it being a “T-Circuit” amplifier. You can think of the “T-Circuit” as being a discreet component version of a modern integrated circuit op-amp. It is just a lot larger and a lot more powerful than the miniaturized chip version. And as is the case with all op-amps, zero volts at input may not always get zero volts at the output. This called is a DC offset error. So, all of these circuits need to be nulled. In case of the integrated circuit op-amps, manufacturers can trim a resistor on the chip with a laser to minimize this DC off-set error or they provide a chip with an extra pair of leads, which allow for an external means of nulling this DC off-set error. In the case of the “T-Circuit” amp, R9 is adjusted so that the bias currents running though Q1 and Q2 are matched. This causes the DC off-set voltage at the output to be reduced to zero. It should be noted that bias currents in transistors do change with temperature. So, if you are going to adjust R9. It might be best to do it when the amplifier has been run for a while and it has fully warmed up to its normal operating temperature.
    Baron030
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  12. #12
    Senior Member pyonc's Avatar
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    Quote Originally Posted by Baron030 View Post
    Hi pyonc
    Yes, the thump does have to do with it being a “T-Circuit” amplifier. You can think of the “T-Circuit” as being a discreet component version of a modern integrated circuit op-amp. It is just a lot larger and a lot more powerful than the miniaturized chip version. And as is the case with all op-amps, zero volts at input may not always get zero volts at the output. This called is a DC offset error. So, all of these circuits need to be nulled. In case of the integrated circuit op-amps, manufacturers can trim a resistor on the chip with a laser to minimize this DC off-set error or they provide a chip with an extra pair of leads, which allow for an external means of nulling this DC off-set error. In the case of the “T-Circuit” amp, R9 is adjusted so that the bias currents running though Q1 and Q2 are matched. This causes the DC off-set voltage at the output to be reduced to zero. It should be noted that bias currents in transistors do change with temperature. So, if you are going to adjust R9. It might be best to do it when the amplifier has been run for a while and it has fully warmed up to its normal operating temperature.
    Baron030
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    Thanks so much, Baron030! Your explanation clears all my doubts about the relevance of this T-Circuit to thump issues for SE400S, SE408S, etc. If that's the case, audible thump/bump noise from the speakers driven by one of these JBL vintage power amps is inevitable, I feel. I wonder if there is any good way to avoid this kind of "DC off-set error" of vintage JBL unit to reduce the thump noise.

    By the way, I've found the document on this T-circuit by its original designer B.N. Locanthi:
    http://users.ece.gatech.edu/mleach/papers/tcir/tcir.pdf

  13. #13
    Senior Member Baron030's Avatar
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    After reading the JBL service manual, as long as the DC voltage is below +/- 0.1 volt the amplifier is OK. And a little thump is not a problem. But, if you measure a higher DC voltage than it is time to re-adjust the DC offset potentiometer. With no input signal present, attach a DC volt meter to the output and then slowly turn the DC offset potentiometer to the left or right and you will see the DC voltage change on the meter. Simply adjust it until you read the lowest possible DC voltage. And you will probably want to do this with 8 ohm dummy load resistor attached to the amplifier's output instead of some expensive JBLs.
    Baron030

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    Senior Member pyonc's Avatar
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    Quote Originally Posted by Baron030 View Post
    After reading the JBL service manual, as long as the DC voltage is below +/- 0.1 volt the amplifier is OK. And a little thump is not a problem. But, if you measure a higher DC voltage than it is time to re-adjust the DC offset potentiometer. With no input signal present, attach a DC volt meter to the output and then slowly turn the DC offset potentiometer to the left or right and you will see the DC voltage change on the meter. Simply adjust it until you read the lowest possible DC voltage. And you will probably want to do this with 8 ohm dummy load resistor attached to the amplifier's output instead of some expensive JBLs.
    Baron030
    Sure, will do. Let me report back to you when I'm done. Thanks!

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    Senior Member Baron030's Avatar
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    Now, I just wonder if I can use my Fluke multimeter for this unit. The two GAIN controls you see are sort of volume control for Channel A & B. Any idea on how to check the DC voltage of this unit?:
    Hi pyonc

    Rather than respond to your private email, I thought it might be better to post to the thread, as more people may benefit from it.
    The Fluke multi-meter is the correct tool for the job. What I mean by no input signal present. Is that you short out the inputs or do not connect anything to the inputs and turn the gain controls to their minimum setting. With a dummy load, (aka 8 ohm resistor) attached to the output, you also attach the leads of the Fluke to the output. Depending on polarity of the DC offset voltage, you may need to reverse the leads on the Fluke. And you need to set the Fluke to its most sensitive setting as 0.1 volts = 100 millivolts. If you read a DC voltage that is under 100 millivolts than your amp is good to go. Otherwise, you will need to open the case and re-adjust the zero offset potentiometer. Pictured below is what I think is R9, which is the zero offset “trimmer pot”. You will need a screw driver to turn its ultra-short shaft. And considering that probably not been turned since it left the factory, you may also want to spray it with some de-ox contact cleaner first. Tuning the R9 trimmer pot will change the amount of DC voltage present on the amplifier output. So, try to adjust it to get the lowest DC voltage possible.
    To confirm that the trimmer pot pictured is R9, a capacitor will be attached across both of its outer leads and the center lead be will attached to a very small transistor. I am assuming that channel A is facing up in the picture and channel B is on the circuit board facing down. And these connections will most likely be printed on circuit board. It should not be too hard to trace the circuit on the board to see if it matches the schematic and confirm that the trimmer pot is in fact the DC offset voltage adjuster.

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    Baron030

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