Help Eliminating the "thump" you get on powered speakers

Discussion in 'General Electronics Chat' started by Butterworth, Nov 18, 2011.

  1. Butterworth

    Thread Starter Active Member

    May 6, 2009
    135
    3
    Hey everyone, I tried searching for the thread that had a similar question to this, but it did not come up... Anyways, I have a custom subwoofer amplifier circuit that was used in my rebuild of an older AR-8" powered subwoofer.

    Now when I turn off my receiver/amp, I get that dreaded thump as the circuit shuts down.

    How can I remove that 'thump' on the subwoofer?

    Thanks.
     
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Most amps use a relay that keeps the speaker disconnected for a while on turn on and then instantly opens at turn off to avoid transients. That's what I always used.
     
  3. Butterworth

    Thread Starter Active Member

    May 6, 2009
    135
    3
    Well I know this amp doesn't have a relay, that thump can be shorted out somehow via a capacitor near the input? I seen a post about it in the past but have yet to find it again. I want to get rid of this thump, there is no relay in this amp, so how can this be done?
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    bountyhunter has given some valid advice. I have built power amps and have used a relay to eliminate the ON-OFF thumps in the speaker, which is not good for the speaker, btw. There is no relay in your amp. You have to add this yourself. Simply add an RC circuit to delay turning on the relay for about 2 secs. Add a diode so that the RC circuit discharges as soon as the power is turned off.
     
  5. Butterworth

    Thread Starter Active Member

    May 6, 2009
    135
    3
    The only problem I see is, where to put this rc circuit and relay. I have attached a schematic for your reference. If you can advise where to put the add-on circuit it would be greatly appreciated. :)
     
    Last edited: Nov 21, 2011
  6. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    The contacts of the relay will go between the output of the amp at SK8 and the speaker connection LS+. The coil of the relay wil be powered from the +40V line through the RC and diode circuit. I will draw a circuit when I have the time, or someone else may be able to help.

    While you're at it I would add a fuse to the speaker. Your amp is DC coupled to the speaker. If something terribly goes wrong you will blow your speaker. A fuse can be a fast blow fuse, a thermal resettable solid-state fuse or a small incandescent light bulb. We would have to know the power rating of the amp and your speaker to make recommendations.
     
    Last edited: Nov 21, 2011
  7. Butterworth

    Thread Starter Active Member

    May 6, 2009
    135
    3
    The original project with some pictures is located here.

    Based on what I know, the woofer is an older model Acoustic Research 8" powered subwoofer. I believe it was originally 80W RMS @ 8Ω.

    I am currently using that Velleman K8060 discrete amp, it has an RMS of 70W, max 100W into 8Ω, or 200W into 4Ω.

    I have a slow blow fuse inline with the power supply and the amp board, if something goes wrong on the amp, it should still blow. Would adding this other fuse be redundant?

    The RC circuit can be a simple resistor, capacitor & diode? I may have some larger diodes laying around, but I know for sure I have some 1N4148's kicking around, or are those no good for this voltage?
     
  8. kam-22

    New Member

    Nov 21, 2011
    19
    0
    Good day to all in this thread..

    After a quick look at the amp circuit it's a standard type. there are many ways to add a simple speaker protection circuit. ways like a small aux supply using 12v relay circuit. or even better one with ac loss that cut's the relay with in a 100ms and turns on within say 2 seconds. velleman kit's do a speaker protection unit. or try Rod Elliott's audio pages he has a speaker protection that will cater for your needs.

    adding a fuse in the o/p speaker was done back in the 70's/80's but is best to leave this out and add a relay as the above comments in this tread..
     
  9. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    Here is my speaker relay. I cannot guarantee that this will work for you. Maybe others can speak out as well.

    [​IMG]

    R1 should be 1/2W. C1 should be aluminum electrolytic rated to handle at least 50V.

    Here is an example of a 5V 5ma relay, Part No. 9007-05-40
    The contacts are rated for only 1A. This might be good enough or you can look for 2A to 3A rating.

    http://www.cotorelay.com/9000_Spartan.pdf
     
  10. Butterworth

    Thread Starter Active Member

    May 6, 2009
    135
    3
    Thank you for this Mr.Chips, I have one question though, SK8 is followed by this relay circuit, then hooks up to the LS+ on my speaker? The amp schematic shows SK8 & LS+ together, I am assuming that means SK8 leads to LS+ on the speaker terminal?
     
    steveaac likes this.
  11. kam-22

    New Member

    Nov 21, 2011
    19
    0
    the above circuit is a simple timer, the fuse feed is not needed in this case.. this circuit will not act if there is large dc offset on the speaker o/p of the amp... it's best to add a speaker protection unit that will soft start plus detect large offsets and shut off the speaker from the amp saving your speaker from being burnt!
     
  12. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    Yes, I have drawn it incorrectly. SK8 is probably the socket or terminal at the back of the amp. You would disconnect the wire that leads to this terminal. The relay contacts will go in series with this disconnected wire and the SK8 terminal.

    So what I have labelled as LS+ should really be SK8.
     
  13. kam-22

    New Member

    Nov 21, 2011
    19
    0
    the above circuit is just a timer on/off the fuse is not needed..plus there is no dc large offset detector to protect the speaker! it is best to add a full protection unit that does the job. timer on/off dc protection.
     
  14. kam-22

    New Member

    Nov 21, 2011
    19
    0
    try Elliot sound products for speaker protection circuit.
     
    Last edited: Nov 21, 2011
  15. Butterworth

    Thread Starter Active Member

    May 6, 2009
    135
    3
    Ok, this makes more sense to me now. Can I use any 5V relay? I have a couple laying around but I don't know if they will work.

    This is what I found...
    [​IMG]

    It is more complicated, I need 3 transistors and 6 diodes... How would this even hook up to my amp?
     
  16. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Here's a working circuit I used to eliminate the turn-on/turn-off bumps in my headphone amplifier. You need a higher current relay for a big power amp application.
     
    steveaac likes this.
  17. kam-22

    New Member

    Nov 21, 2011
    19
    0
    Hello Butterworth have you checked all of the information on the eps site regarding the set up? how it works, part of the circuit connects to the ac secondary side on the power amp pcb..look at the four large diodes and where your wires from the power transformer connect to it that's where the ac sense are soldered, next connect the second part of the circuit that needs dc from the + rail of the power supply rod states use 24 volt relay with 10amp contacts..this is a straight forward circuit to build and works very well indeed I've used it in my own power amp. follow all the information on this unit. also because it's meant for 2 channel use just use one side for your amp. and leave one side not connected.
     
  18. kam-22

    New Member

    Nov 21, 2011
    19
    0
    if you need a ready built one try velleman kits
     
  19. kam-22

    New Member

    Nov 21, 2011
    19
    0
    butterworth.. did you see the whole circuit on esp site.. there's more useful stuff on there.
     
  20. kam-22

    New Member

    Nov 21, 2011
    19
    0

    remove c1 r2 d1 and d2 for one channel use...
     
Loading...