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Old 1st January 2008, 02:52 PM   #1
ungie is offline ungie  Canada
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Default Phono stage common ground hum!!

Hello,

I own a VPI TNT turntable with VPI 12.6 tonearm. I have been using this with my Sonic Frontiers SFP-1 tube phono stage for over a year with a stereo MC cart and have had no noise or hum issues with it. It is totally silent!!
However, I purchased a Shelter 501 MONO cartridge for listening to older mono LPs and I have run into a terrible hum problem. The Shelter has tied grounds: the two ground pins are connected. This is causing the hum with my phono stage. If I simply connect the two grounds together at the input of the phono stage I get hum. Now, is there any simple way for me to isolate these common grounds at the input in some way to get rid of this hum?? Please help!!!!
Thank you!

Andrew
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Old 1st January 2008, 02:56 PM   #2
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What happens if you don't connect one of the grounds from the cart side?
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Old 1st January 2008, 05:05 PM   #3
ungie is offline ungie  Canada
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Quote:
Originally posted by analog_sa
What happens if you don't connect one of the grounds from the cart side?
It still hums.
However, if I only connect one channel to the phono stage input there is NO noise. Now if I had a mono switch on my-preamp I would be set: Just connect one channel to the phono stage and hit the mono switch to sum the channels.
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Old 1st January 2008, 05:37 PM   #4
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Sadly, i don't know anything about mono carts. Assuming it has a single coil which feeds both channels i would think your phono stage does not like short-circuiting the inputs of the two channels, rather than the grounds. In that case i would try including some series resistance between the phono stage input and each of the cart's outputs. This will obviously modify the loading resistance/gain but let's first get rid of the hum.
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Old 1st January 2008, 05:37 PM   #5
tubenut is offline tubenut  South Africa
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connecting a few ohms worth of resistor between input RCA ring and circuit ground may solve it.... It did on my old Radford STA15, (I think 3.6 ohm did the trick effectively separating left and right ground by 7.2 ohms) poweramp though obviously the few ohms there were insignificant, you may need to consider the cart loading as part of all this.

Option 2, make a 1 to two RCA interconnect instead of the pre amp mono switch..... or even just use a nice rca splitter as made and sold by various cable cos with their subwoofer interconnects to connect phonostage to pre amp.
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Old 1st January 2008, 05:45 PM   #6
tubenut is offline tubenut  South Africa
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Have you tried different connections of the arms earth lead? Ie not connecting it or connecting it to the right RCA for eg?
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Old 1st January 2008, 09:04 PM   #7
ungie is offline ungie  Canada
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Quote:
Originally posted by tubenut
Have you tried different connections of the arms earth lead? Ie not connecting it or connecting it to the right RCA for eg?

Yes, I have tried all manner of combinations with the arm earth, and the hum is still present.
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Old 1st January 2008, 09:08 PM   #8
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In any case, the problem lies with the phono pre and how it reacts to commoning of grounds or inputs. You should investigate it separately from the cart/arm/leads. Does it mind shorting the L/R grounds? It shouldn't as those are certainly shorted internally. How does it react to shorting the left and right inputs?
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Old 2nd January 2008, 04:35 PM   #9
Jeb-D. is offline Jeb-D.  United States
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More than likely it's a ground loop. Almost all phono pre's are designed based on the assumption of separated signal sources. So, your circumstance probably wasn't taken into consideration when the designer was doing the grounding scheme.

There are many ways to break the loop, but the easiest and least permanent would probably be to try leaving one of the cartridge ground pins disconnected.
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Old 2nd January 2008, 05:55 PM   #10
ungie is offline ungie  Canada
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Quote:
Originally posted by Jeb-D.
More than likely it's a ground loop. Almost all phono pre's are designed based on the assumption of separated signal sources. So, your circumstance probably wasn't taken into consideration when the designer was doing the grounding scheme.

There are many ways to break the loop, but the easiest and least permanent would probably be to try leaving one of the cartridge ground pins disconnected.
I agree, but I tried that and it still hums.
You now have that long length of wire acting as an antenna.
The only thing I have not tried yet is lifting the ground connection at the phono stage input on one channel. I guess I should give that a go?
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