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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

Showing 3 responses by dougdeacon

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.
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.
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.
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