loud thud from speakers when powering up

[pyonc]

Hi forum friends,I'd like to ask for your help on this "thump" issue of my vintage SE400S amp. As it has no power switch, I plug it into the switched AC outlet of its matching pre-amp SG520, according to the JBL manual. (So, when you turn on/off the preamp, SE400S is turned on/off along with it)The problem is I hear a big "thump" from the speakers (4343) when I turn on the switch of SG520, even with the volume gains of SE400S all the way down. Specifically, the woofers move outward when this thump is heard. I don't hear it when I turn it off, though. Well, I've had any such problems when I used two Crown amps in a bi-amp mode.I hear this "thump" issue has something to do with audio output DC voltage that's separated by capacitors. If that's the case, do you see any problem with the capacitors of this SE400S? Or any other issues related to other problems such as electrolytic capacitors? Please see the pic. (I've purchased it from ebay, and this conscientious seller made sure it has been gone through by a vintage technician with a clean bill of health.)Do I need to worry about any potential damage to the woofers whenever I hear this loud thump with this amp.I look forward to your feedback and opinions. Thanks much in advance.

[gferrell]

Inrush current may or may not damage speakers. Can't tell if caps are bad just looking at them.

[richluvsound]

Depending on your skills and space . I suggest a soft start kit …. or you could just put a CL60 in the + line of the AC . I suggest getting someone to sort it out for you .


Best of luck ,Rich

[pyonc]

Depending on your skills and space . I suggest a soft start kit …. or you could just put a CL60 in the + line of the AC . I suggest getting someone to sort it out for you .


Best of luck ,Rich

Hummm..... I don't get it. Could you explain some more? CL60?

[pyonc]

Inrush current may or may not damage speakers. Can't tell if caps are bad just looking at them.

Thanks. Hope they're just rust, not leakages on the bottom.

[richluvsound]

Hummm..... I don't get it. Could you explain some more? CL60?

http://uk.mouser.com/ProductDetail/G...iEQjXGFRAAAA==


Rich

[Audiobeer]

Your problem probably is in the Caps on the power board as suggested earlier. That's cheap compared to the replacement of drivers. I'd pull the amp and send it to somebody like Terry dewick and have him go through it. Won't cost more than $150 + Shipping. Maybe less!

[pyonc]

http://uk.mouser.com/ProductDetail/G...iEQjXGFRAAAA==


Rich

Thanks. I'm not sure if it helps, because the previous owner told me he didn't experience such a thump at all when he listened to TAD speakers with this amp.
Mine are JBL 4343. So, I'm wondering if this thump might happen, depending on speakers...

[pyonc]

Your problem probably is in the Caps on the power board as suggested earlier. That's cheap compared to the replacement of drivers. I'd pull the amp and send it to somebody like Terry dewick and have him go through it. Won't cost more than $150 + Shipping. Maybe less!

Thanks. Well, I double checked with the previous owner who's an ebay seller (tubejunkie) in LA, specializing in audio. He said his technician did go through it and found no issues at all, including the capacitors...

[yggdrasil]

Thanks. I'm not sure if it helps, because the previous owner told me he didn't experience such a thump at all when he listened to TAD speakers with this amp.
Mine are JBL 4343. So, I'm wondering if this thump might happen, depending on speakers...

No, it is not speaker dependent.

I second Audiobeer's advice: Have somebody look at it.

[pyonc]

No, it is not speaker dependent.

I second Audiobeer's advice: Have somebody look at it.

That's what I guess, too. Thanks.
If you look at the pic, you see some decayed rusts on the bottom of the capacitors, never replaced since this amp came out in 1967.
The thing is except for the thump issue, this vintage amp works flawlessly.

[DavidF]

I have owned a few SS amps that thumped at power-on and power-off. Generally that is normal depending on the circuit design and whether a delay relay is used in the signal path. The issue is not if it thumps, but how much.

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.

B&K 1400. Slight thump and no noticeable cone motion. DC offset within spec. Slight tick as above at discharge.

B&K 442. Moderate thump at turn on and noticeable cone motion. As with the Adcom DC was high at turn on but stabilized and fell within spec quickly. At turn off there was a noticeable thump and each cone was pushed out momentarily as the caps discharged. Again, changing caps made no improvement.

Hafler 200. Heavy thump at turn on. DC offset high in one channel, not used.

All of the above was when a 14 inch woofer was in use. Using a small system with a 6 inch woofer resulted in far less perceived thump sound and less extended cone motion (oddly enough).

At one point I tried an in-line current limiting circuit but this really had a minimal effect on the voltage that was the cause of the thump.

The power switch was removed as a potential source of noise.

I am not skilled enough or patient enough to go through the circuit boards and replace the differential transistors in matched sets. This is what I believed to be the source of the cone displacement and resulting sound. As mentioned, I was able to lessen the effect on the Adcom but when I got careless and blew a side of output transistors, I stopped having fun. I just learned to live with the thump on the B&K and with low low-value quick-blow fuses in line with the woofers.

[pyonc]

I have owned a few SS amps that thumped at power-on and power-off. Generally that is normal depending on the circuit design and whether a delay relay is used in the signal path. The issue is not if it thumps, but how much.

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.

B&K 1400. Slight thump and no noticeable cone motion. DC offset within spec. Slight tick as above at discharge.

B&K 442. Moderate thump at turn on and noticeable cone motion. As with the Adcom DC was high at turn on but stabilized and fell within spec quickly. At turn off there was a noticeable thump and each cone was pushed out momentarily as the caps discharged. Again, changing caps made no improvement.

Hafler 200. Heavy thump at turn on. DC offset high in one channel, not used.

All of the above was when a 14 inch woofer was in use. Using a small system with a 6 inch woofer resulted in far less perceived thump sound and less extended cone motion (oddly enough).

At one point I tried an in-line current limiting circuit but this really had a minimal effect on the voltage that was the cause of the thump.

The power switch was removed as a potential source of noise.

I am not skilled enough or patient enough to go through the circuit boards and replace the differential transistors in matched sets. This is what I believed to be the source of the cone displacement and resulting sound. As mentioned, I was able to lessen the effect on the Adcom but when I got careless and blew a side of output transistors, I stopped having fun. I just learned to live with the thump on the B&K and with low low-value quick-blow fuses in line with the woofers.

Thanks a lot for your kind explanation and sharing your experience.
Like you said, if there is a slight thump, I wouldn't care,
but it is a noticeable big thump with both 15-inch woofers moving outward.
The ebay seller said he didn't hear such thump when he listened to TAD monitors with this SE400S amp.
He had the same set-up, namely SG520 preamp and SE400S, that I had. Only difference is the speakers, mine JBL4343.
So, why doesn't he have any thump on his TAD while I have one on my 4343?
As I'm not so knowledgeable about this circuit and powerboard, I am thinking of returning this unit back to the seller if I can't find the solution.

[DavidF]

Thanks a lot for your kind explanation and sharing your experience.
Like you said, if there is a slight thump, I wouldn't care,
but it is a noticeable big thump with both 15-inch woofers moving outward.
The ebay seller said he didn't hear such thump when he listened to TAD monitors with this SE400S amp.
He had the same set-up, namely SG520 preamp and SE400S, that I had. Only difference is the speakers, mine JBL4343.
So, why doesn't he have any thump on his TAD while I have one on my 4343?
As I'm not so knowledgeable about this circuit and powerboard, I am thinking of returning this unit back to the seller if I can't find the solution.

So, I think I assumed incorrectly. Is this the same combination of preamp and amp that was owned by the seller? Or just the amp? If just the amp, then I'm not sure you eliminated the preamp as a source of the thump?

[Odd]

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