Vinyl & Low Frequencey Sub Feedback

bigbro1206

Just brought a set of Rti70 speakers with a PSW404 sub. The system is pretty well matched in terms of power. The amp is a Carver M1.0 (rated at 200 watts RMS per channel). The subwoofer amp in the 404 is rated at 200 watts also. The pre-amp is a Carver C-1 sonic hologram. The RTi70 speakers have a maximum power rating of 250 watts.

The problem I'm have is playing vinyl records. A feedback loop occurs with the 404 sub when I try to turn up the volume when playing vinyl records. Its a serious problem because, not only does the music sound bad, the supersonic low frequency sounds like it could damage the sub as well. It seems the 404 sub is picking up the sound of the cartridge needle rubbing against the record.

The C-1 pre- has amp is equipped with a infrasonic filter designed to eliminate ultra-low-frequency signals associated with phono disk playback when engaged. The filter is working properly because the low frequency feedback is much, much worse when the filter is disengaged.

I can eliminate some of the feedback by setting the phase switch on the 404 to 180 degrees, turning the C-1 sonic hologram on, and leaving the dust cover up on the turn table. But even so, albums can only be played at relatively low volumes compared to CDs. CDs don't require any system adjustments at all to play as loud as I want.

Has anyone else come across this problem? And if so, is there a solution (besides not playing records loud)? Would appreciate your suggestions and ideas. The C-1 can be adjusted to tweak the phono staging (30DB, 35DB, & 40DB). What about replacing the moving-magnet phono cartridge with a moving-coil cartridge? Is that a realistic possibility before I go to the expense of actually doing it?

Any advice?

HBombToo

I have only began researching turntables and have zero experience. I read many docs regarding tuntable hook up and I noticed there is a ground strap from the turtable to the peramp chassis. If this has anything to do with your symptom? i don't know.

HBomb

bigbro1206

Hbomb - Thanks for getting back to me so quickly!

My experience has been that poor grounding problems usually cause a hum in all of the drivers, regardless of the signal source. I believe my system is well grounded, because there is no problem at all when playing CDs or tapes. The low frequency feedback occurs only when playing vinyl records.

If you think of anything else, please let me know. Thanks again for your feedback.

gidrah

To me it sounds more like a connection (cartridge or interconnect) or vibration problem.

You should probably ask this in the 2ch forum as there are some vinyl guys that check/post there more.

Good luck and welcome to the forum!

bigbro1206

Thanks again, Hbomb. How do I get to the 2ch forum?

bigbro1206

I mean Mark. Just thought a little more about your suggestion. Makes sense. My trun table is sitting on a wodden shelf. Tomorrow I'll put some vibration-absorbant material under the turntable and see if that makes a difference. Thank you!

HBombToo

Originally posted by bigbro1206
Thanks again, Hbomb. How do I get to the 2ch forum?

http://clubpolk.polkaudio.com/forum/forumdisplay.php?s=&forumid=22

Dr. Spec

I think your turntable is picking up vibrations and feeding them back through the system.

Isolate it better and the problem should stop.

Tour2ma

Spot on Doc and all hail the mighty gidrah (or whatever he says in his sig these days) for initially ID’ing the real issue here..

A TT's ability to playback is based on vibration. That is what the stylus is doing in the record groove, vibrating back and forth. As such it is not immune to the vibrations in the air, i.e., sound, to the extent that other sources are.

Sound vibrates surfaces, and shelves are among the worst. You have a lot of surface area there to “collect” vibrations and channel them to anything sitting on them, e.g., your TT. Isolation material under the TT will help; finding a more solid surface to sit the TT on will help even more. But you’ve got other problems as well.

The dust cover (DC) is a contributor... you as much as said so when you said opening it “helps”. Here again you have a fair amount of surface area to collect vibrations and transmit them to the TT’s base, then the plinth, then the tone arm pivot, then the tone arm, then the cartridge, then the stylus (...and the knee bone connected to the shin bone…)..

Another neat little destructive effect that can occur here is that some of the vibrations pass thru the DC and get trapped in the volume between the cover and the plinth. Now you have got a little enclosure trapping the waves and they start bouncing around in there, reinforcing, etc., and end up vibrating the record and the stylus via the route described above. So losing the DC during playback is a good idea. Yes the record is exposed directly to the vibes, but it’s less area than it plus the DC.

DC’s are really meant to keep dust away between, not during uses. Many great TT’s don’t have them at all; other DC’s can’t be in place during play, e.g., the AR XA. If dust being attracted to the album during playback is an issue there are little “dust bugs” you can buy and mount to clean the album as it’s playing. (picture below). BTW spinning albums do actually set up a very weak electro-static field that pulls dust to them.

One last thing, and maybe most important of all, your TT may be currently sitting in a bass “hot zone”, i.e., where standing waves are reinforcing. Moving it a foot or two can get you out of this (better) and maybe you can even find a “dead zone” where wave cancellation is occurring (best). If you have a SPL Meter it will greatly help you ID an optimal and hopefully convenient position for your TT. For $35 at Radio Shack it will pay for itself here and in balancing your HT set up. When you’re looking for the best spot don’t just think left or right, also think up and down, 3-D.

You did not say what TT you are running in your rig. Some are better designed than others to handle external vibration. What are you using? "Upgrading should be a last resort, but it's there if you need it.

bigbro1206

Hey! You guys know your stuff. Thank you.

The low frequency feedback was being caused by vibrations from the spinning platter on the turntable. The cartridge was picking up the vibrations producing a very low frequency. The 404 sub amplified it, which intensified the vibration, which was amplified further, which intensified the vibration, etc., etc., creating a low frequency feedback loop.

The turntable sits on wall-mounted shelving that you can get a Lowe’s or Home Depot. The solution turned out to be quite simple. I placed an 18" x 18" piece of left over wall-to-wall carpeting between the turntable and the shelf - pile side down with the turntable siting level on the stiff backing.

Wha-la. We’re cranking this morning listening to Guadrophenia by the Who (vinyl of course). The PSW404 makes Entwistle’s bass sound like he’s somewhere in the room playing live!

Thanks again. Hope I can return the favor sometime.

Tour2ma

Excellent... you knew that piece of scrap carpet would come in handy sooner or later didn't you?

You seem to have things well in hand now, but I also remembered another little trick for your DC. You can buy little felt "dots" at Wal-mart. with adhesive on the backside. Place these at the corners of your TT's base and they should isolate the DC from it. Keep it in mind, just in case...

Glad we could help.

HBombToo

This is really good stuff!

HBomb

Tour2ma

HBomb,
Scoot back over to your Pyle thread. Some thought for you there too.

gidrah

Glad to hear everything worked out. Especially since it was a free fix.