Vinyl - PSW404 Low Frequency 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?

bigbro1206

Thanks to those Polk Audio Club members who were able to point me in the right direction!

Its true. 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!

Dr. Spec

A set of Vibra-Pods will work better than your carpet, but you are on the way! Good deal.

bigbro1206

Thanks for getting back to me, Dr. Spec. What are Vibra-Pods? Never heard of them before. How do they work and where do I get them? Are they expensive?

tjmmdennis

You can get vibrapods in the polk store at polks website.

bigbro1206

All right! I'll check it out. Thank you.