loud thud from speakers when powering up

[hjames]

Try to power up SE 400s without preamplifier connected.
If it is gone, use Adcom ACE 515 or the like. It starts first preamplifier and after approx. 30 seconds, the power amplifier.

Adcom ACE-515

I have 2 Adcom ACE-515s connected to the switched outlet on my Integra 9.8DTC preamp.
They control the 2 poweramps in that system, so its painless for my partner to use the system.

I started using one of them 6 or so years ago with the Biamped JBL 4341 Monitors, then got another more recently
They've been great for the years I've been using them. Flawless.

One controls power to a B&K ST202-Plus amp that runs the 2 main speakers (200w/ch x2)
One controls power to a B&K AV5000 Series II amp that runs Center, sides and rear speakers (125w/ch x5)

Basically, then have a sensor plug that goes into the switched outlet of your preamp.
So when you turn the preamp on, it triggers the ACE units - they have a timed delay to apply power
to the Poweramps AFTER the preamp is on and running.
They also manage a nice power-down sequence for the gear as well.

[pyonc]

Try to power up SE 400s without preamplifier connected. Is it gone, use Adcom ACE 515 or the like. It starts first preamplifier and after approx. 30 seconds, the power amplifier.

Adcom ACE-515

Well, SE400S has no power switch, so I plug its power cord into the switched outlet of SG520 preamp.
Of course, I tried plugging it in separate power strip, but the problem got worse.
When I plugged it, I heard not only a sharp electrical bang from the unit, but loud thump from the speakers....

[Odd]

Well, SE400S has no power switch, so I plug its power cord into the switched outlet of SG520 preamp.
Of course, I tried plugging it in separate power strip, but the problem got worse.
When I plugged it, I heard not only a sharp electrical bang from the unit, but loud thump from the speakers....

Was preamplifier connected?

[DavidF]

Was preamplifier connected?

Same question, but also to ask have you tried power up with nothing connected to the power amp imputs?

[NickH]

You've definitely got some faulty component then. You should by no mean hear a pop from the amp. Stop using it and take it to a tech before you damage something in it that can't be replaced.


I use an audio research amp to power 4 altec 416-8b woofers. There's no thump at turn on. But there's a good one at power down. The high sensitivity drivers make it worse. Amps that have output transformers are more likely to do this do to the inductance of the transformer.

[pyonc]

Was preamplifier connected?

Yes, it was. Otherwise, I would not have heard this thump through the speakers (woofers, more exactly)...

[pyonc]

Same question, but also to ask have you tried power up with nothing connected to the power amp imputs?

Just hooking this amp into its matching preamp SG520, with no amp inputs connected, I see no noise from the unit.

[pyonc]

You've definitely got some faulty component then. You should by no mean hear a pop from the amp. Stop using it and take it to a tech before you damage something in it that can't be replaced.


I use an audio research amp to power 4 altec 416-8b woofers. There's no thump at turn on. But there's a good one at power down. The high sensitivity drivers make it worse. Amps that have output transformers are more likely to do this do to the inductance of the transformer.

Does this 'high sensitivity drivers' apply to JBL 4343, too? This SE400S use what they call T-circuit output stage.

[hjames]

Yes, it was. Otherwise, I would not have heard this thump through the speakers (woofers, more exactly)...

I think that was the whole point they were trying to establish -
troubleshooting the system by UNPLUGGING the poweramp, powering up the PREamp, and then plugging in the power amp to see if you get a thump.

If there is NO THUMP, its quite possible using something like the Adcom ACE-515
to power up the amp AFTER the preamp is on would avoid the problem.

But if you don't test that theory, you'd never know.

Looks like 3 of them on ebay in the $60-80 range - plus some much higher, of course ...

[pyonc]

I think that was the whole point they were trying to establish -
troubleshooting the system by UNPLUGGING the poweramp, powering up the PREamp, and then plugging in the power amp to see if you get a thump.

If there is NO THUMP, its quite possible using something like the Adcom ACE-515
to power up the amp AFTER the preamp is on would avoid the problem.

But if you don't test that theory, you'd never know.

Looks like 3 of them on ebay in the $60-80 range - plus some much higher, of course ...

Thanks much for your help and advice, as always.
Actually I finally have got to talk with the technician who did go through this unit.
He first asked me to unplug the power cord from the pre-amp in power-on state to see if I hear any thump. My answer was No.
Then he wanted me to open the unit, and take out the pilot light inside which's connected to amplifier power.
(The light comes on when the amp is powered up). When I took it out, and still heard loud thump through the speakers,
he concluded this unit had some problem and wanted to have it sent back to him for repair.
Now he finally found what's the problem there. I mentioned rusty capacitors, and he mentioned something like 'power discharge'...

[pyonc]

Thanks so much to you all who tried to help me out!

Actually, the original technician who went through this unit confided this unit had some problem.

I'll report back to you as soon as I have his repairs report.

[NickH]

I would say all jbl speakers are relatively high efficiency.

[SEAWOLF97]

Adcom 555. Heavy thump at turn on and noticeable cone motion. A little “tick” after turn off as the Caps discharged. DC offset high at turn on but quickly stabilized and DC offset at the terminals was well within spec after the initial turn on. I changed some driver board transistors that lessened the thump but it never went away. Not sure I matched the transistors well enough. Changing the power supply caps made no difference in the thump.

My 555 (series 1) has NO turn-on THUMP at all , but does have what we call a "fairy fart"* as the caps are draining , about 10 seconds after turn-off.

* like a weird chirp