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Grounding

Jas0_0

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Hi everyone,

Thanks in no small part to advice here, I am now set up with digital RIAA. The rig is sounding excellent and far quieter than with analogue RIAA, but I’m wondering if it could be better. I am noticing a pronounced hum if I turn up the volume with no music playing. An analyser tells me this is at around 50Hz so I’m guessing it’s a mains ground loop. My experiments suggest it’s possible to remove, and I’m hoping someone here will have a practical way to do this.

My setup is:

Turntable + MC cart
AEA TRP mic pre
RME ADI-2 Pro
Mac mini with digital RIAA

I’ve traced the hum to the mic pre. To do this I disconnected the turntable leads from the mic pre. I muted one channel. On the other channel I plugged in a spare XLR cable with no plug on the other end and twisted the bare signal cables together. This revealed the same noise profile, with the 50Hz hum, as when the turntable was connected. What was odd was that when I accidentally touched the twisted ends on the mic pre’s metal case, the hum almost entirely disappeared.

What does this mean? Is there a way I could achieve this same effect with the turntable connected?

Any suggestions gratefully received.

Thanks,

James
 

pma

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Thanks in no small part to advice here, I am now set up with digital RIAA. The rig is sounding excellent and far quieter than with analogue RIAA, but I’m wondering if it could be better. I am noticing a pronounced hum if I turn up the volume with no music playing.

You would need to provide detailed drawing of your audio chain with all cables described.

A proper design and built of analog phono gives NO hum even at full volume, "with no music playing".

phonofet_50R_PSD.png


phonofet_2m_PSD.png
 

DonH56

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Hi everyone,

Thanks in no small part to advice here, I am now set up with digital RIAA. The rig is sounding excellent and far quieter than with analogue RIAA, but I’m wondering if it could be better. I am noticing a pronounced hum if I turn up the volume with no music playing. An analyser tells me this is at around 50Hz so I’m guessing it’s a mains ground loop. My experiments suggest it’s possible to remove, and I’m hoping someone here will have a practical way to do this.

My setup is:

Turntable + MC cart
AEA TRP mic pre
RME ADI-2 Pro
Mac mini with digital RIAA

I’ve traced the hum to the mic pre. To do this I disconnected the turntable leads from the mic pre. I muted one channel. On the other channel I plugged in a spare XLR cable with no plug on the other end and twisted the bare signal cables together. This revealed the same noise profile, with the 50Hz hum, as when the turntable was connected. What was odd was that when I accidentally touched the twisted ends on the mic pre’s metal case, the hum almost entirely disappeared.

What does this mean? Is there a way I could achieve this same effect with the turntable connected?

Any suggestions gratefully received.

Thanks,

James

The AEA TRP mic pre is a very high gain preamp for ribbon microphones. It is not a turntable preamp. I am not sure where or how you are doing RIAA equalization? The preamp also has very high input impedance so you probably need a low-impedance termination/load for your MC cartridge. Unless the TT has a line output? Off-hand that preamp does not seem the right tool for the job.

That aside, cartridge preamps typically have a ground connection for the turntable's ground wire. You may be able to make this work by connecting the ground wire connection from your TT to the preamp, assuming you can find a good place to do that. Either a ground or chassis screw, or the ground shield on one of the XLR connectors, may do it.
 

mhardy6647

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I can tell you how I approach* ground loop 'issues'.
A piece of wire and a couple (or a bag full) of clip leads.
Find a "hard ground" on component X (e.g., a screw head that is connected to the chassis) and connect it (with clip leads -- and a piece of wire if needed to reach) to component Y. Does the hum cease? Finished.
Still humming? Repeat all combinations and permutations of components X, Y, Z, W (etc.).

1615899027210.png


One other thought (not knowing where you live) -- does your turntable have a polarized mains plug? If not, and if it is permissible/possible with your mains wiring and outlet, reverse the polarity of the tt's mains plug in the wall socket. This is a classic way to minimize hum/noise from the dawn of hifi (in the US, where we had nonpolarized, 2-prong mains plugs for many, many years).

________________
* NO WARRANTY EXPRESS(ED) OR IMPLIED! :)
 
OP
Jas0_0

Jas0_0

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Hi everyone, thanks for your suggestions and apologies for the slow response.

I've been playing today and I think the mic pre might have a fault. One channel seems to have consistently higher noise at the same gain. I've tried swapping the inputs and outputs around to see if it was a fault in the cart/TT cable or AD converter. In all iterations, channel 1 (left)
of the mic pre always seems to produce more noise.

Will get in touch with the manufacturer - unless there's anything else you suggest?

Thanks again,

J

AEA TRP channel comparison.png
 
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