It could be the fabled Grado hum others have mentioned. If you search the Web extensively, you'll see somebody who sells an after-market kit for shielding the motor on Rega tables. Which will be of doubtful use, but I merely mention it for edification purposes.
The ground is certainly supposed to go to the amplifier. Which you don't have. Perhaps with a different phono stage...
Here's the main thing: are you sure you have shielded phono cables? This is vital.
1. Remove turntable IC's from Li'l Rat:
a. hum persists
b. touch ground contacts on RCA ins: hum decreases slightly
c. touch metal body of Li'l Rat: hum decreases more
2. Re-insert IC's
a. touch tonearm: hum decreases a bit
b. play around with phono > Li'l Rat RCA's = major loud hummage...sounds like a short in the IC's from the turntable. Sometimes an entire channel blinks out with only huge hum.
HOWEVER, why is it that the same basic hum still happens when just the Li'l Rat is plugged into the JMT with no turntable IC's stuck in it?
Alright, freaky evidence: I was experiencing a hum with my EarMax Pro. This was pretty much exactly the same pitch as one that I hear around me, from the garage (could be lights, the garage door opener) or the walls or something. I have no headphones on and I hear that pitch.
Anyhow, that was the pitch of the EMP's hum and it is roughly the pitch of the turntable's hum. The turntable has it way more.
Could this just be dirty power?
I mean, I am touching metal things on the system left and right, changing the mix of hums and intensity of overall hum, so wouldn't it be a ground problem? Could it be both?
Someone was talking about a power line ground screw-up when I had posted about the EMP hum...maybe that is the same problem, only seeming like a player problem?
I really don't get any of how this stuff works (e.g. why I can influence it at so many different parts of the chain)...any ideas?
Another thought: did you check to see if a hum is produced when the turntable is not connected to the phono stage at all? You should certainly try that, to eliminate the stage itself as the problem.
...will I be able to listen to the phonograph player without the preamp? Do you mean by sticking it's RCA's directly into the amp? I have a JMT, so that's unfortunately not feasable (don't have the right adapters).
60 cycle hum...YES! Alright, I produced a test tone at 60Hz and this is the same pitch...actually, an octave above, but that might be funky overtones, since a pure 60 is low-ish and getting down somewhat into subaudio (or at least attenuated audio).
Can you tell me exactly what a "ground loop" means (so I know what to look for) and what I can do about it in practical terms?
Despite people's dumping on the Grado cartridge, I don't think that is the problem. It sounds like a grounding issue. Its hard to predict whats going on with grounds, but more experimenting should help. One thing that is uncommon to me, and I'm not sure how to address, is that the li'l rat, being battery powered, has no earth ground. What I've always done in the past is ground the turntable to the chassis ground of the preamp. I use an old Technics SL-210, with a Grado Black cartridge (i've experienced no hum issues, despite the naysayers!)
How will I be able to listen to the phonograph player without the preamp? Do you mean by sticking it's RCA's directly into the amp? I have a JMT, so that's unfortunately not feasable (don't have the right adapters).
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Sorry. I meant: connect preamp to JMT and see if you get a hum when the turntable is not connected to the preamp. --But I hadn't realized the rat preamp is battery-powered. At which point I'm sure you won't get the hum when connected that way.
(And no, you wouldn't be able to listen to the TT
)
I am of the camp that thinks the Grado may not be the problem. I think you have to the get the whole rig properly grounded. You might try grounding the TT to a water pipe (cold), and if that fails, grounding the JMT to the water pipe and the TT to the JMT. At least you won't get a.m radio that way.
It has been said that Grado cartridges can hum when used with VPI or Rega turntables due to their unshielded motors. I have never run into this problem using Grados on my old Rega Planer 2 turntable and I didn’t experience it this last time. Losing the hum is, however, the reason Grado made the low output version of their Reference line. While I haven’t had any hum problems, your mileage may vary. (I had a hum problem with my Grado Cartridge. Accent on Music in Mt. Kisco, NY helped me solve it. I moved the turntable further away from my other components. As always, choose your dealers wisely.–DH)
From Grado's own site:
quote:
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It has been said that Grado cartridges can hum when used with VPI or Rega turntables due to their unshielded motors. I have never run into this problem using Grados on my old Rega Planer 2 turntable and I didn’t experience it this last time. Losing the hum is, however, the reason Grado made the low output version of their Reference line. While I haven’t had any hum problems, your mileage may vary. (I had a hum problem with my Grado Cartridge. Accent on Music in Mt. Kisco, NY helped me solve it. I moved the turntable further away from my other components. As always, choose your dealers wisely.–DH)
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There is plenty more on other websites.
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Yes -- precisely. The problem is reported with TTs that have unshielded motors. By far the majority of reports are with Regas. Hence, not only the Rega version of some Grado cartridges, but also the shielding kit for Regas, an aftermarket product I mentioned above.
I don't know for sure that the Dual in question has a shielded motor, but I would certainly think it does. I believe that is the much more standard design, and Dual is a maker of mass-market TTs. I would expect at one time there were hundred of thousands of Dual TTs running Grado cartridges; without shielding kits.
But of course this is guesswork and the Grado *could* be the problem. OTOH, I think there's hardly any experience base for running an AC-powered TT into two battery-powered devices, and I suspect the problem lies in getting a proper ground for said rig.
I am not an expert on ground loop. Ground loop happens when there is potential difference between electrical ground. The return current on the ground caused the hum. The only solution is to isolate where the ground loop occur. I think you are doing the right thing,
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"1. Remove turntable IC's from Li'l Rat:
a. hum persists
b. touch ground contacts on RCA ins: hum decreases slightly
c. touch metal body of Li'l Rat: hum decreases more"
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This tells me the TT is not the casuse rather it's between the Li'l Rat and the amp.
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"2. Re-insert IC's
a. touch tonearm: hum decreases a bit
b. play around with phono > Li'l Rat RCA's = major loud hummage...sounds like a short in the IC's from the turntable. Sometimes an entire channel blinks out with only huge hum".
_______________________________
This is normal. Your body can introduce hum or reduc hum..
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"HOWEVER, why is it that the same basic hum still happens when just the Li'l Rat is plugged into the JMT with no turntable IC's stuck in it?"
_____________________________
Now this is interesting, there should be no ground loop if one of boxes is batterry powered. My next guess is you have a bad interconnect between JMT and Lil'rat.
This could be either the connector or the IC itself. Try different IC to eliminate IC problem. If not the IC, then I think you might have a defective Lil rat
...has reminded me that I haven't said anything about the power sources: I am using wallwart 9V battery eliminators. I have been using the JMT this way and have used the Grado RA-1 this way with much sucess, but could this be it?
BTW; radio shack also sells a ground loop breaker. If indeed, ground loop is to blamed (we have not established that fact yet), that could be a solution.
...from NeedleDoctor.com just told me that I ought to try cleaning all the contacts (makes sense, because some of them look a bit tarnished or something), then regrounding the unit, then trying it again. He said if that doesn't work, I might probably want to look into a different cartridge.
It wouldn't take much residual AC on a wall-wart to make a hell of a hum through a phono stage type preamp. Output on that Grado cart is something under 5 mv. That's millivolts, as in thousanths of volts. If you've got even 1/2 millivolt ripple in the PSU, you're probably going to get some hum.
Try battery power for the phono stage and the JMT. Then if the hum is still there, you've got a problem with grounds somewhere in the table system.
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