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Seeking help with feedback through audio system

I currently use a high mass, direct coupled turntable and have been experiencing some strange "feedback" type issues - I will explain below.

I'd like to know if anyone else has experienced these type of problems and if there is a potential solution.

Situation:
My table sits on a wall shelf (massive & "home made") which connects/integrates with a load bearing wall. I have it set up this way because I have VERY springy hardwood floors which just don't work with any turntable (suspended or non-suspended).

Everything sounds great but at a certain volume level, I begin to hear what I presume to be feedback. It is a steady upper mid-range resonant tone which does not amplify or get louder, it just remains constant until a few seconds after the overall system volume is lowered. The feedback will continue between songs on a record but eventually gets drowned out by the next song and becomes less audible. I can faintly hear the feedback "within" the next song.

When the volume is lowered, the sound is great - with no feedback issues.

I presume this feedback is caused by the interaction between my room and the cartridge (Shelter 901) - which from what I've read may not be a good match for my unipivot tonearm - further complicating or enhancing the problem. I dunno. I did try moving the table to a different location within the room and the same problem occurred when the system was playing at about the same volume - however I think the frequency or tone-of-the-feedback-tone was a bit lower - perhaps mid-bass feedback.

Again, I'd like to know if any of you have experience anything like this and hopefully you've got suggestions on how to alleviate the problem.
bwhite
12 responses Add your response
dan_ed
2,451 posts
 
Could it be a tube? It's kind of hard to hold it up to the internet so we can have a listen. :) I do also wonder about tonearm chatter with your cart/arm combo, but I don't know if this would be volume related as much as frequency related. You already tried moving the table. This may be totally off the wall but could you somehow dampen the tonearm as an experiment, maybe with a rubberband that you could slide back and forth. Might rule out the unipivot-901 theory.

BTW, very nice system!
bornin50
32 posts
 
Hi

Apologies if this is a stupid question, but if you lift the arm does the feedback go away?

Regards
bwhite OP
807 posts
 
Dan and Bornin - thanks for the replies. Yes - it could potentially be a tube I will give that a try as well as try to dampen the tone arm & report results.

Bornin - yes... when I lift the tonearm, the feedback goes away.

What seems strange to me is that feedback typically amplifies or progressively gets louder. The situation I am experiencing is unique in that the feedback (or resonance or microphonics) remains at a constant volume. Weird.
dougdeacon
3,338 posts
 
Hmm, tough one. This seems like the key behavior to me:

"It ... does not amplify or get louder, it just remains constant until a few seconds after the overall system volume is lowered."

Can you measure/elaborate on this? Exactly what happens when you lower the gain control? Does the behavior change when you cut the gain instantly versus lowering it slowly? Just trying to collect more clues...

On the practical front, if you have or can borrow another cartridge it would be revealing to compare behaviors.
bwhite OP
807 posts
 
Hi Doug, Good questions. This resonant feedback sound tapers off immediately when the volume is lowered and starts immediately when the sound reaches a certain (higher) level. Of course there is a curve in both directions where the sound tapers in or tapers out. Make sense?

The sound does sustain through the part of a record where silence should be (in-between songs) but stops immediately when the cartridge is lifted off the record.

Key here is that at a slightly lower volume level, there is absolutely none of this feedback.

I'll try to borrow another cartridge for test purposes as well as figure out a way to dampen the tonearm.
dougdeacon
3,338 posts
 
Since the gain setting effects whether the sound exists or not, it seems likely the problem's in or after the gain stage. Occam's Razor. Try removing and reinserting the tubes in the Cortese. Or ship the Cortese to me and I'll study it for awhile! I'll loan you my old Phase Linear pre. Maybe the scratchy gain control has cleaned itself up since it was last plugged in (1991).

This just seems more likely than FB in the cartridge/arm. They don't know where gain is set unless its set to atmosphere/room shaking levels. The fact that the sound continues betwen tracks makeas a cart/arm problem even less likely IMO.

Try asking on the Supratek threads if you haven't. Maybe somebody there actually knows something useful.
dan_ed
2,451 posts
 
Really starting to sound like it is down stream from the TT. Is there any difference in the channels? Does it follow one or the other channel?
bwhite OP
807 posts
 
Hi Dan - its tough to tell which channel the feedback comes through. My system is (for lack of a better term) quite holographic - the speakers vanish and origin of any sound is for the most part quite difficult to determine. This resonant feedback sound in particular seems to emanate from somewhere in the room vs. from a particular speaker. I will disconnect one channel at a time and see if the feedback persists. This will certainly help to identify whether or not the problem is a nasty microphonic tube.

Maybe I should try some room dampening too.
dougdeacon
3,338 posts
 
It is really weird. There are some facts that I haven't been able to square with each other:

- it starts/stops with moving the stylus on/off the record
- but it's independent of groove modulation
- it starts/stops with the level of the gain control
- but volume doesn't change with changes in gain control

Armtube resonance? Loose cartridge clip vibrating? Phono stage tube resonating? There are objections to all of them.
bornin50
32 posts
 
Hi again

This should be available on a webcast so we could all hear it and chime in with recommendations.

If your electronics have any tone controls or filters, do they have any effect when the 'sound' is active?

I'm getting a 'subsonic stylus in the groove noise aggravating something in the electronics' type of feeling.

Or maybe I'm just on the wrong medication. :-)
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