Went to a home where the client has a Technics phono (SL-1200 I believe) through a Kenwood stereo receiver KR A5050. Medium sized subwoofer.

When the stylus is placed on a record, regardless of whether the dial on the Technics is in the On or Off position, a loud thumping kind of builds up. It strikes me as a type of feedback or reverberation of some sort, but I'm not sure.

Everything appears to be grounded.

It's an architecturally unique house, where the walls are glass and they kind of slope down in a dome shape, kind of like a UFO. Currently all of the equipment, including the subwoofer, is in a sloped down corner, so wondering/suspecting that might have something to with the sound reverberations.

Unfortunately I didn't capture any audio or video at the visit.

I know it's hard to pin down from the description above, but figured I'd give it a shot to post here.

Sorry, but what's the name of that Technics dial? See [Link: kabusa.com] and tell us, please. I guess maybe it's POWER? Why wouldn't you say "POWER" in that case?

You say "A thumping kind of builds up." Do you mean you can hear a slight thump, then it gets louder and louder and louder until the amp grenades? Thumping to me means a repeated low frequency sound of some sort. How often does it repeat? Can you keep time to it? Is it dead silent between thumps?


Thumps are not 60 Hz or 120 Hz signals, so, yeah, okay, there's no hum.

I've heard thumping in a few cases, one of which is at the beginning of a Pink Floyd album, so... what record was on the turntable and what other records did you try?

Get a 45 RPM record, or take an LP you don't care about and play it at 45. Does the thumping behave the same? If its speed of repetition is about 4/3 the speed of the thump when you play at 33, then it's related to the rotation of the turntable.

I've heard thumping when bearings don't work quite right and when the stylus pressure is too high and the cartridge is smacking the record. I also used to live 100 feet from the railroad tracks off of San Fernando Road in Glendale and on Sunday nights, when the freight trains took off northward from LA, the stylus would thump right out of the groove until I improved the turntable's isolation.

I don't see reverberations being an issue unless the place is hundreds of feet in at least one direction.

Was there ever a time when the thumping noise did not occur?

If you unplug the turntable power, does the thumping occur?

What is the turntable sitting on? Is it dead still? Is it thick? If you rap on it when the stylus is down and the turntable power is umplugged, do you hear that rapping through the speakers?

Try out anything else you can think of to pinpoint what's making this happen.

Turn the subwoofer off and see if that fixes it. If it does, try and isolate the turntable so that the vibration caused by the sub doesn't work it's way back through the turntable.
Having said this, it would be surprising for that particular turntable to have this issue.
Easy to test though.

Certainly the SL-1200 is one of the better Technics turntables, but in my opinion they all are an acoustic feedback risk.

You can quickly test the acoustic feedback hypothesis by slowly advancing the Volume control with the stylus placed on a record sitting on the stationary platter. If the thumping noise suddenly starts as the Volume control rotates to a certain setting, you have acoustic feedback. If the thumping tracks the Volume control, it is not acoustic feedback -- there is probably a mechanical noise associated with the building or the cabinet.

If you have acoustic feedback, the subwoofer is likely to be in the mix. Try simoneales suggestion of turning OFF the subwoofer and repeat my Volume control test. If you can advance the control farther without the subwoofer, then the subwoofer is in contributing to the feedback, but may not be the sole contributor. Other candidates are speakers sharing the same shelf with the turntable. Ultimately, you'll need to isolate the turntable from the speakers, but an interesting trick would be to invert the absolute phase of the speakers and the relative phase of the subwoofer. Your criteria for improvement is how far you can rotate the Volume control before feedback strikes. This is a 50/50 shot at improving things, but it is cheap.

Keep in mind that energy from the speakers and subwoofer is both conducted through the structure and radiated through the air. Ultimately, you need to prevent this energy from reaching the record surface. The turntable's suspension should provide this isolation, but you could count the turntables with effective suspension without running out of fingers and the 1200 is not on this list. A compliant pad under the subwoofer could help attenuate conducted energy. Another scheme is to add a heavy plate under the turntable and compliant pads under the plate. There is some science to this and there are isolation mounts that can be purchased that are tuned to reject frequencies of interest. (one needs to know the mass of things and the frequencies) One can concoct a decent isolation platform with a slab of slate or marble and some tennis balls. Cut the balls in half and place them under each corner of the slab. Experiment with cutting tuning slits in the balls. If you cut too far, chuck that set and start over. Your success criteria will be how far you can advance the Volume control.

Look at the album (I'm using the singular because nothing was mentioned about trying another one)- is it warped, or completely flat? Even if it has a little blip, a subwoofer makes a mountain out of a mole hill and this would have been rare in the '70s or '80s because A) subwoofers in their current form didn't exist, B) most speakers didn't do 20Hz and C) most, if not all receivers had a subsonic filter that was meant to remove the frequencies that could cause this kind of problem. Look at the subwoofer, if it's direct-firing and the grill is removable- does the cone vibrate when it's not thumping?

If the turntable needs isolation, a more attractive thing to use is from Rockler Woodworking stores, called Bench Cookies. These are made for mounting or placing on a work bench while staining/finishing, using a router or doing other operations that require keeping it off of the bench top. They're cheap enough, too.

[Link: rockler.com]

Thanks guys. Ernie, yes you are right WAY too vague for definitive troubleshooting, but these are good tips.

I've included a line item in the proposal "Troubleshoot phono issues", hourly. So if he goes for it I'll give your guys' suggestions a try.

Oh, hell, I forgot this was at a client's house. I got into the mindset that this was your personal horror.

Follow only the simplest of the tests I suggest. You might even try having someone hold the turntable in the air, power unplugged, stylus on the record, and seeing if the thumping occurs. That should only take a minute or so to do and could be a big help.

I can hear someone in sales yelling "Sell him a better turntable!"