Emubitter Posted November 28, 2019 Share Posted November 28, 2019 Hi, I have used Onken boxes with a 10 inch woofer for duty below 160 Hz with my front loaded horns for many years . Recently the plate amp in one of the boxes died and I purchased a pair of Dayton SPA 250 to replace both amps. Having installed both amps... one works perfectly but the behaviour of the other is frightening. I had noticed excessive cone excursion when the unit first powered up but that seemed to settle down. Playing moderate level program everything appeared to be working.normally.. Turning up the volume somewhat, I think it was FLeetwood Mac - Tusk, suddenly the woofer was flapping wildly, creating and awful racket, clearly bottoming out violently. I cannot automatically assume that the amp is faulty since I installed it myself, is there a silly mistake I could have made? Can you suggest any other root cause, simple test to carry out? Any help or advice, I would be most grateful. Regards, Ken Link to comment Share on other sites More sharing options...
stereo coffee Posted November 28, 2019 Share Posted November 28, 2019 35 minutes ago, Emubitter said: Hi, I have used Onken boxes with a 10 inch woofer for duty below 160 Hz with my front loaded horns for many years . Recently the plate amp in one of the boxes died and I purchased a pair of Dayton SPA 250 to replace both amps. Having installed both amps... one works perfectly but the behaviour of the other is frightening. I had noticed excessive cone excursion when the unit first powered up but that seemed to settle down. Playing moderate level program everything appeared to be working.normally.. Turning up the volume somewhat, I think it was FLeetwood Mac - Tusk, suddenly the woofer was flapping wildly, creating and awful racket, clearly bottoming out violently. I cannot automatically assume that the amp is faulty since I installed it myself, is there a silly mistake I could have made? Can you suggest any other root cause, simple test to carry out? Any help or advice, I would be most grateful. Regards, Ken Hi Ken Are you using a turntable playing a LP. If so most likely is low frequency being fed back from the speaker to the turntable. The cure is to ideally use a wall mounted shelf for your turntable, or even something as simple as some cork mats under the turntable to stop vibration from the turntable coupling back to the speaker. In electronics terms it is positive feedback. Hendrix used it to good effect in many concerts and recordings. Link to comment Share on other sites More sharing options...
Emubitter Posted November 28, 2019 Author Share Posted November 28, 2019 Thanks for the response, no, not using a turntable in this instance.. I understand what you mean, I remember older amps would sometimes have a 'rumble' filter to take out sub sonic frequencies from the tonearm/turntable. I am sure that is not it in this case. Also, only one of the speakers reacts in this way. regards, Ken Link to comment Share on other sites More sharing options...
VanArn Posted November 29, 2019 Share Posted November 29, 2019 The first step would be to interchange the amplifiers and this procedure will reveal if the amplifier or the sub bass speaker is at fault. The usual cause of speaker failure is the corrugated cloth spider splitting or tearing away from the cone apex or the coil former. 1 Link to comment Share on other sites More sharing options...
stereo coffee Posted November 29, 2019 Share Posted November 29, 2019 9 minutes ago, Emubitter said: Thanks for the response, no, not using a turntable in this instance.. I understand what you mean, I remember older amps would sometimes have a 'rumble' filter to take out sub sonic frequencies from the tonearm/turntable. I am sure that is not it in this case. Also, only one of the speakers reacts in this way. regards, Ken Hi Ken Try isolation material then at the speaker cabinet. to see if it stops. With amplifiers, positive feedback manifests itself as what is called motor -boating- and a typical cause is too high a value of coupling capacitor internally but that's not what you have here. Rather your description suggests as acoustic feedback, which usually requires a source component vibrating in sympathy with the amplified version of that source. Carefully check any source component that can couple, perhaps something is switched in that should not be there normally - like a microphone. Also also check the phase of the connection to the speakers. A feature of incorrect phase is for audio to appear as mono. Also check crossover points. In electronics we usually create a lower limit of 20 cycles per second for any audio to commence which usually is sufficient to stop low frequency aberrations. Link to comment Share on other sites More sharing options...
Monkeyboi Posted November 29, 2019 Share Posted November 29, 2019 Firstly try swapping channels and see if the problem follows the channel. If it doesn't then I would suspect the amplifier. Whilst a quick thump on turn on is common with some amplifiers, the problem should not persist after the initial switch on. I'd suspect the amplifier. Most plate amps aren't tested before they leave the factory. In the majority of cases quality control is in the domain of the customer. It might just be a faulty bypass capacitor on the board, but it's not IME the Customer's responsibility to diagnose or repair it inside the warranty period. Cheers, Alan R. 1 Link to comment Share on other sites More sharing options...
davewantsmoore Posted November 29, 2019 Share Posted November 29, 2019 (edited) 5 hours ago, Emubitter said: the woofer was flapping wildly, creating and awful racket, clearly bottoming out violently. Your (onken) box does not "load" the woofer at (super) low frequencies. This means that very little power will make the cone move a lot. Your content contains very low frequencies. You need a high pass filter. You could us "EQ" to do this. Vinyl people often call it a "rumble filter"..... or you could use a different box for the driver, one which provides more of a "high pass filter" to the driver (ie. stops it moving so easily at very very low frequencies). Or... as mentioned, your amplifier might be faulty.... However I suspect it is not, and that what I said above is the cause. Test tones (low frequency ones) applied to the speaker boxes will diagnose exactly what is happening. Edited November 29, 2019 by davewantsmoore Link to comment Share on other sites More sharing options...
oohms Posted November 29, 2019 Share Posted November 29, 2019 swap the amps then look closely at the woofers.. could be a faulty amp or the old amp killed a woofer Link to comment Share on other sites More sharing options...
Emubitter Posted December 1, 2019 Author Share Posted December 1, 2019 Update, thanks to all that have put in their two cents worth.. Saturday I swapped the drivers from one box to the other.. problem remained with the same (left) box. Conclusion, probably not the driver at fault. Yesterday, swapped channels at the power amp terminals to see if the issue is coming from upstream.. problem stayed on the left. Conclusion, probably not an upstream problem.. then it gets weird. Played a couple of hours music at moderate volume, no problems then deliberately cranked it loud but could not reproduce the problem any more... reversed the channels to original sides, cranked the volume, still no sign of a flapping woofer.. scratches head, thinks maybe it has fixed itself dubiously... Shut everything off, plate amps left on auto trigger had some dinner, TV on, then suddenly the left amp wakes up and the woofer flaps quite violently for a few seconds! Remember all upstream components are off! flapping dies away quite quickly and the plate amp goes back to sleep. I have no idea what is going on but I cannot help thinking the plate amp is the culprit. Thoughts? Cheers, Ken Link to comment Share on other sites More sharing options...
Monkeyboi Posted December 2, 2019 Share Posted December 2, 2019 (edited) Hi Ken, From what you have just described it looks like the plate amp has a fault. Obviously fixable as the magic smoke hasn't escaped. Perhaps something intermittent or temperature sensitive? Without actually checking it out it is difficult to diagnose by proxy. If it's still under warranty send it in for repair. If you want me to take a look at it, PM me. Cheers, Alan R. Edited December 2, 2019 by Monkeyboi added text Link to comment Share on other sites More sharing options...
davewantsmoore Posted December 2, 2019 Share Posted December 2, 2019 5 hours ago, Emubitter said: then suddenly the left amp wakes up To clarify.... Did the amplifier "auto-wake" unexpectedly..... or was it supposed to (because a signal had been applied to it). Either way, it is likely faulty amplifier based on what described. Given that it still "works", it is likely just a burned component or two.... so try to find a amp tech who will look at it for you, unless you have access to cheap replacement amp. Link to comment Share on other sites More sharing options...
BioBrian Posted December 2, 2019 Share Posted December 2, 2019 7 hours ago, Emubitter said: Shut everything off, plate amps left on auto trigger had some dinner, TV on, then suddenly the left amp wakes up and the woofer flaps quite violently for a few seconds! Remember all upstream components are off! flapping dies away quite quickly and the plate amp goes back to sleep. It's alive! Or one of them might be the new AI model, beta version. Try moderating what you watch on TV while dining (never the news), or offering it some of your delicacies, so it doesn't feel left out and get into a flap. Maybe it would appreciate some counselling? (Feeble gratuitous wink emoji due here). It's not clear what you are feeding them - is it high level speaker wires from the power amp that runs the other drivers? It's not from a preamp, where you could have one channel from the Pre Out and one from Tape Out or something? Plate amps have a rather miserable reputation; it sounds like you have good ammunition for a warranty claim. Have you checked that both plate amps set for 240 V? (I'm scratching too). Link to comment Share on other sites More sharing options...
Emubitter Posted December 3, 2019 Author Share Posted December 3, 2019 @davewantsmore the amp "woke up" with no signal applied.. connected through high level inputs directly to the power amplifier. Power amp off, preamp off etc. I have emailed the supplier since the plate amp is brand new, hopefully a warranty job and all will end well. Cheers, Ken Link to comment Share on other sites More sharing options...
davewantsmoore Posted December 3, 2019 Share Posted December 3, 2019 1 hour ago, Emubitter said: plate amp is brand new In that case, even more likely to be faulty amp. Link to comment Share on other sites More sharing options...
Recommended Posts