Question about warps and tracking at outer edge of LP

Discussion in 'Audio Hardware' started by VU Master, Aug 3, 2022 at 4:13 PM.

  1. VU Master

    VU Master Senior Member Thread Starter

    I recently installed an Audio Technica VM540ML on my friend’s Empire turntable, which has a Grace 700 tone arm.

    Install went smoothly, anti-skating and tracking force are good. Everything sounds great, but he’s noticed an odd problem. On certain pressings the setup seems very sensitive to record warp at the outer edge of an LP. If I shine a bright light and look closely I can see that on each revolution the cartridge jumps up and down slightly and briefly. We hear a little distortion (in the left channel only) and of course a slight thump. This happened on 2 out of 10 records that we tried. It only lasts for about the first 30 seconds of the track.

    I took two of the “problem” records back to my place and played them on my SL-1200, which has a Share V15 (with a brush in front of it) and a different model Grace arm. Not only was there no thump or distortion, I couldn’t even see the cartridge lift up at all in the places where it did on his TT. It was as if I was playing a totally different pressing. Amazing to me.

    I discussed this with by buddy Kevin Gray, and he thought it might be a tonearm resonance issue. I am also wondering if this might be a downside of the AT cartridge having better LF response. I mean, clearly there is a warp in the record, which should be passed on the speaker cones, at least to some extent. Maybe it has to do with the cart’s compliance? I don’t think there are any flaws on his turntable platter that could cause this. And I wonder if the brush on my cartridge helps it "ride" the warps somehow.

    I should mention that his Grace arm has had some kind of makeshift repair near the base, and may have some kind of mechanical issue.

    Not sure what to make of this. Has anyone else run into this? Any thoughts? It’s not a huge problem but maybe it can be improved in some way. Thanks in advance.
     
  2. Davey

    Davey NP: Carmen Villain ~ Only Love From Now On (2022)

    Location:
    SF Bay Area, USA
    Do you know which Grace tonearm he has, there are a few in that 700 series, is it one of the lightweight series, maybe the G-707? I guess they are all pretty light, though, so shouldn't be any problem with the VM540ML compliance and warps. Without knowing more bout the damage, hard to say anything about that. The brush on the Shure helps damp the tonearm in the subsonic region, so will help it track warps better without causing extra movement due to the tonearm-cartridge resonant frequency ... maybe one of you could record a short video on your phone and post it ...

    Top to bottom: G-704, G-707, G-714, G-727, G-747...

    [​IMG]
     
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  3. VU Master

    VU Master Senior Member Thread Starter

    Davey, thanks a lot for your reply. His tone arm looks like your 4th or 5th photo. I can't spot the difference between those two, but I'm pretty sure it's one of those two. I'll ask my friend if he can make a short video showing the issue. Thanks again!
     
  4. patient_ot

    patient_ot Senior Member

    Location:
    USA
    If you can take a measurement of the resonance, that would help. I doubt there is a mismatch issue though.

    My 1200 arm has the KAB damper and with my AT cartridges usually doesn't have much trouble with these types of warps. The cartridge can vibrate a little on them, but doesn't lose groove contact or shake violently.

    If you have doubts about the mechanics of the particular arm, I would get it checked out. "Makeshift repair" doesn't sound good. I would guess this arm is a few decades old and may need checking over or servicing.
     
  5. doctor fuse

    doctor fuse Forum Resident

    Is the bottom of the cartridge housing “bottoming out” on the warps, and making contact with the vinyl? I have a cartridge which does this.

    A good, almost free solution is to make a makeshift DIY fluid dampening system. Just use shampoo or glycerin, if you don’t have access to silicone oil, for now. It just takes trial and error to scavenge parts that fit your table and arm. Even the crudest versions I have made have dropped all woofer pumping.

    Here is a good basic primer:

    Tonearm Damping Article By Bob Graham The BAS Speaker Volume 3 No. 4, January 1995
     
  6. VU Master

    VU Master Senior Member Thread Starter

    Thanks for all the replies. I'm now suspecting that this is actually a feedback problem. This evening I asked my friend to do some tests using headphones with the power amp off. It seemed that the problem went away if the left speaker (which is closest to the TT, and on a bare floor) was disconnected, and also that the problem grossly worsened or vanished, depending on the volume setting.

    I'm thinking that at the volume setting we used, maybe the system was on the verge of feedback but not quite running away. In that state, I think a small warp on a record could have an exaggerated effect, if the thump it produced was close to the resonant frequency of the feedback from the speaker. (Sort of like the way a vocal mic on the verge of feedback will begin to sound very shrill, due to positive feedback in the midrange frequencies.) I'm going to go back there this weekend and will follow up when I know more.

    (doctor fuse, the cartridge was not bottoming out when this happened.)
     
    Last edited: Aug 4, 2022 at 1:20 AM
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  7. VU Master

    VU Master Senior Member Thread Starter

    I went out there again with a bunch of test gear and finally figured out what was going on. I’ve been an audio tech/engineer for 45 years, and this was one of the oddest things I’ve ever seen.

    My previous suspicions about this were completely wrong, and to explain what happened it’ll be easiest to start over from the beginning.

    The original problem we noticed was that at the beginning of certain disks, a brief once-around thump was heard on the left channel. Besides the audible thump, audio would partly cut out on the left. Looking at the cartridge, I could see the stylus making a distinct vertical jump at the moment this happened. The problem didn’t happen at all when I played those albums on my own home system.

    We happened to notice that if we set the preamp so that the left side of the TT was fed to both speakers, the thump and drop out were only heard on the left; not on the right at all. This surprising observation told us that the problem couldn’t be in the turntable. By reversing left and right at the preamp inputs and then outputs, we determined that both of the preamp outputs were problem free. So this just left the power amp and speakers. Thinking it unlikely to be the speaker, I pulled the power amp out, and noticed that one of the 6550 tubes on the left channel was warm, but not nearly as hot as the other 3. We plugged in a spare and voila, the problem vanished.

    One might ask: how could a bad tube produce a rotational thump from the turntable? Here’s how. That very low frequency thump put a big power demand on the amp. But because of the bad tube the left channel had very little headroom and went right into clipping every time the stylus hit that small warp on the disk. The thump itself was really too low in frequency to be audible (though I could see it on a ‘scope) but when the amp went into clipping it added a lot of harmonics (overtones) to the thump, which WERE audible. And because the left channel of the amp was going into clipping, whatever music was on the left channel also became distorted during the thump.

    I think the fact that his stylus jumps at that point on the disk only means that his cart has better very low frequency response than mine does. But once the amp was fixed the ability of the cart to reproduce the small warp didn’t really matter, because the thump was relatively low level and when amplified properly, wasn’t audible at all. (Maybe if he had a good sub it would be more apparent.)

    Those darn electrons can really do crazy things sometimes. Anyway, with the new tube, everything works great and sound quality is perfect.

    Thanks again to those who replied here.
     
    Last edited: Aug 5, 2022 at 10:06 PM
  8. Leonthepro

    Leonthepro Skeptically Optimistic

    Location:
    Sweden
    I think the Shures brush is helping a lot, kinda damping the tonearm.

    Edit:
    Cool you found the solution, would not have guessed that but it makes sense.
     

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