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Please Help Ambient Bass Resonance

I have a problem I just noticed last night. I believe it's mechanical. Perhaps the vinyl folks can explain what I am hearing and suggest a solution.

First my equipment: Lyra Delos Cartridge; VPI Classic TT; ARC PH-8 Phono Pre; ARC Ref 5 SE linestage; and ARC CD-8 CDP.

The Delos output is .6mV. The PH-8 fixed gain is 58db and its S/N spec is 75db. The Classic is grounded to the PH-8. I also use a Delrin clamp and periheral ring.

Because the Delos output is about as low as ARC recommends for the PH-8, I have to turn the Ref 5's gain up to about 70-75%. Normally, noise and hiss is acceptable.

Last night I was playing a record and I turned the gain up to 75+% and I heard a low freq bass ringing that started to feedback on itself and got louder as I left the gain up. It would stop when I turned the gain down to 65 or 70%.

I believe its mechanical because I did some experimenting with the rest of my gear. Step 1: I clicked the Ref 5 to the CDP and turned the gain up to over 95%. Dead silence. Step 2: I turned off the TT and put the stylus on the record. I heard the bass ringing. Step 3: I disconnected the lemco connector from the junction box -- that is I disconnected the cartridge from the PH-8. Turned the gain up. Dead silience. Step 4: I reconnected the cartridge and kept it suspended off the record, motor on or off, turned up the gain. Silence.

I believe that what's happenening is that the record is acting as a low freq. sound collector and the vibrations are being picked up by the cartridge and fed back on itself. Hard to be sure about the Hz level but it may be in the 60 Hz neighborhood. I surmise this because I get the feedback with or without my sub, which cuts off at 50 Hz and the him seems to come out of the bass woofers.

As I said, the problem becomes apparent when I turn the gain up past 75 or 80% with the TT on or off, when the stylus is on the record.

Any explanations and suggestions? Did I find Fishing's missing bass? Check his OP. LOL ;>')

Seriously ... what is going on?

P.S. Just tried something else. I checked the hum situation without the peripheral ring. Still have bass ringing feedback, but I had to turn the gain up even higher for it to kick in. Again, it only happens when the stylus is on the record -- regardless if the motor is on or off.

Thanks guys
bifwynne
buconero117
2,311 posts
 
Acoustic energy is one possible reason, assuming the condition only exists when music is actually being played. Find a way to acoustically shield the equipment as a test.
sounds_real_audio
677 posts
 
When you say you turned up the gain to 75% just what do you mean. Is that 25% below the point where your amplifiers would clip, or just 75% of where the dial would turn?
bifwynne OP
2,069 posts
 
Thanks for all of the suggestions. I think the idea that had the most chance of working was moving the TT further away from the speakers. It didn't really make a difference.

The best thing I've done so far is to use an extra rubber washer with the Delrin clamp. For those who own a VPI table, you'll know that the table comes with a single rubber washer which is put on first, then the record, then the clamp. I had an extra washer, so I put the extra washer on top of the record, directly underneath the clamp. That seemed to raise the feedback threshold well beyond the level I would ever listen to the music.

Btw, I've used MM carties before, Can't say I recall having the problem. Also, I used to use the Soundsmith Zephyr, a MI cartridge, and the Dynavector, a MC cartridge. Again, I don't recall experiencing the problem.

Just a thought -- I have a 3 gram headshell weight. I wonder if using it to increase the effective mass of the arm would make a difference??

Just thinking out loud here -- where the heck is the Delos picking up the ambient sound? Yes, there are motors throught the house and laundry room, but could that be the source? Or is it possible there is electrical inductance between the tone arm cables and maybe a power line that in some back is feeding a 60 Hz current back into the moving coil parts of the cartridge, thereby causing vibration to feedback through the system as Al described?? Al what do you think??

Thanks again.
almarg's avatar
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almarg
8,673 posts
 
Hi Bruce,

I doubt that ambient sounds or 60 Hz coupling have anything to do with the problem, although experimenting with the turntable ground, as was suggested above, would seem to be worthwhile. Perhaps even trying having no ground connection to the preamp. Obviously, have the system turned off while you are making any of those changes, and have the volume control turned down when you initially power up.

Regarding ambient sounds, as I indicated in my first post in the thread I linked to earlier:
A high gain feedback loop that is unstable at some frequency or frequencies does not require much if any input stimulus for oscillations to occur.... So what is probably happening is that some extremely tiny (and inevitable) low frequency vibration causes the cartridge to output a tiny signal, which is amplified by a high gain factor, resulting in an output from the nearby speaker that causes further vibration to be mechanically transmitted to the cartridge, which is further amplified by that high gain factor, etc., etc. The root cause of the problem is that the overall loop is unstable at some low frequency or frequencies, when the overall gain through the loop is above a certain amount.
The only suggestion that occurs to me at this point, besides the grounding experiment, is to continue to experiment with turntable placement, and the related suggestions the others provided above.

Best,
-- Al
bifwynne OP
2,069 posts
 
Thanks Al, I tried the grounding idea already. No change.

Not a lot of options about moving gear around. My wife finished the basement, carpet furniture, etc and there's simply not much I can do. So far, the double rubber washer trick seems to help a bit. At least the feedback oscillation doesn't kick in unless I turn the linestage gain up to ridiculous levels, which I never would use. There's no discernable feedback at "normal" listening levels.

Curiously, the peripheral ring seems aggravate the problem.

The delos is a nice sounding cartridge. As long as the problem doesn't get worse, I can live with it.

Thanks for the help.
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