Turntable humm confusion

capecodder

So, I noticed a few months ago that I was getting that typical annoying background humm when playing in my phono mode. More noticeable as volume goes up, again which is typical. Anyway, I tried to rectify by reaching around back of the equipment a few weeks ago (equipment located in built-in shelf unit, and can't see what I'm doing without removal) and "wiggling" this and that. After a few wiggles humm vanished. Cool.

This past weekend I switched cartridges (moved from a MM to a MC) in my Linn TT so I disconnected everything and realized that by "wiggling" wires i had DISCONNECTED the TT ground wire to the preamp ground post. Oops. This was during period of no humm mind you.

After setting up the cartridge and hooking things back up i reconnected the ground. Humm returns. Disconnect, humm stops. Uhhh, isn't that the opposite of what should happen? Why am I getting the feedback humm when not grounded to the preamp? This is a fairly new phenomenon I think.

FWIW, Linn LP12 connected to vintage Sony TA-2000 preamp (low and high output capable), NAD 218 main amp and HSU amp for sub among other stuff..

Bottom line is that when ground not connected things seem fine, so I am leaving things this way. I assume, I hope, that by not having it attached, it is not posing a potential problem for the equipment.

billbillw

You are getting some kind of ground loop when the TT ground is connected. I find this happens when you have mixed equipment, some with 3-prong power (grounded) and some with 2 prong power (ungrounded). I have ended up "lifting" the ground on all my 3-prong powered equipment to get rid of such hums. I don't like having my TT ungrounded for various reasons.

SCompRacer

Hums with turntables can be due to lack of shielding (picked up from external sources) or a ground loop. Check to see that the cartridge is connected properly and the head shell leads are not touching one another or the tone arm. (If you were to check for proper polarity of the tone arm leads from the RCA connectors, always disconnect the cartridge before testing with an Ohm meter). Before testing, disconnect the TT from the pre amp and from the mains. Leave the tone arm cable plugged into the bottom of the tone arm.

Depending on manufacture date, your Linn will either have a two or three prong power cord. If it is three prong, check for continuity of the ground lug to the sub chassis. There should also be a ground wire from the tone arm, to the sub chassis, to one of the nuts that secures the stainless top plate to the plinth.

If your Linn has the Valhalla PS, with it still unplugged from the mains, check for continuity from the tone arm ground spade to the four motor wires where they attach to the PS board. (Blue, Red, Grey, Grey). Check for continuity from the tone arm ground spade to the two power wires. (Black and White). If you have continuity, you may have a shorted motor or PS board.

capecodder

Thanks guys. Lots of info there. Need to digest. Is there a real negative to having the TT not grounded to the preamp?

mhardy6647

Nope, operate in the configuration that gives you minimum hum.

If the tt's P/S plug isn't polarized, you could try reversing it in the AC mains socket and see if the hum's minimized with the ground line connected to the tt from the amp/preamp (whatever).

capecodder

mhardy6647 wrote: »

Nope, operate in the configuration that gives you minimum hum.

Okey, dokey.

hearingimpared

Riding everyone's coattails here I can add one more piece of advise and that is to use a 3:2 ratio cheater plug throughout your rig. I can't believe that audio manufacturers even used the three prong plug; that "neutral" causes more ground loop hums and makes the chasis of gear active RF antenae then take into consideration the ground potential capacitance and you've got a whole world of **** to deal with.

capecodder

hearingimpared wrote: »

Riding everyone's coattails here I can add one more piece of advise and that is to use a 3:2 ratio cheater plug throughout your rig.

So... I should add 3 to 2 adapters to all the 3-pronged plugs hooked into the common power strip? How about if a 3 prong (my main amp for instance) is connected to a separate wall socket independent of the power strip?

hearingimpared

It can't hurt to try it as you may even find it is an inexpensive tweak as the noise floor on your rig overall is lowered.

I read somewhere where Frank Van Alstine wrote a paper or something on the evils of three prong connections. All his gear comes with two prong connections.

I haven't had the hum problem in my rig so I could not even tell you that I've tried it. I know that when I was advised as to how to strap my two mono block that the additional chasis ground strap between the two blocks would eliminate any chance of ground loop hum.

SCompRacer

When I ran into gear that caused ground loop problems, I only lifted the ground on it, not everything. They are a pain to chase. Just be methodical in your search for the problem.

capecodder

Jeez, shoulda taken an electrical engineering course. I never have understood this stuff. I readily admit to high level of ignorance to electrical circuitry and terminology. I'm pretty good at looking at something and taking it apart and putting it back together but I rarely know what the f i'm doing...

SCompRacer

Does your LP12 have a grouned plug (three prong)? Early ones had two prong IIRC. Mine had a grounded plug.

capecodder

I just unplugged it this weekend, you'd think i could remember. I think it is 3 prong but I really don't know. It is a 1988 model, I think, if that helps. Will look tonight.

madmax

Just remember that the ground you are lifting is called a "safety ground" and is called that for a reason. Once it is lifted the only ground your rig has is the non-guaranteed grounds from other equipment through the interconnects. If a problem arises you are not as safe as you were. Something to weigh in with as you make your decisions.
madmax