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Old 30th March 2013, 04:45 AM   #1
BRSHiFi is offline BRSHiFi  United States
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Default Balanced Phono RCA Only

My turntable has only RCA outputs (no XLR support) that cannot be detached from the unit. I am designing a fully balanced phono stage that works both with MM and MC cartridges, with a variable load feature for universal cartridge support.

An RCA cable has a hot and screen. The screen is a heavy duty shield that encircles the hot and connects to ground. If I treat the RCA cables as balanced cable the shield would become hot. XLR connecters have two hots and a shield.

Will hum be picked up since there will be no shield if I use an RCA cable as a balanced transmission line?

It seems like it would not because the hum would have to not be common to both hots. Anyone have experience with this?

Also I am concerned about capacitance since the cartridge I am using is a Shure MM with a 47k impedance.
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Old 30th March 2013, 06:45 AM   #2
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by kingneb View Post
My turntable has only RCA outputs (no XLR support) that cannot be detached from the unit....
does any cartridge/pickup have more than 2 wires for each channel ?
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Old 30th March 2013, 08:14 AM   #3
andyr is offline andyr  Australia
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Quote:
Originally Posted by kingneb View Post
My turntable has only RCA outputs (no XLR support) that cannot be detached from the unit.
The same as many TTs/arms.

Quote:
Originally Posted by kingneb View Post
I am designing a fully balanced phono stage ...
Why are you bothering to do this? What better SQ do you think you will obtain from having a "balanced" phono stage?

Quote:
Originally Posted by kingneb View Post
... with a variable load feature for universal cartridge support.
All you need for this is 2 parallel pairs of input RCAs - 1 for changing load resistance, 1 (for MMs) for changing load capacitance. These RCA sockets take "loaded" plugs.

Regards,

Andy

Last edited by andyr; 30th March 2013 at 08:16 AM.
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Old 30th March 2013, 02:15 PM   #4
MiiB is offline MiiB  Denmark
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You can run the cartridge balanced, the gnd wire is then the shield, and a dual cartridge loading sets the midpoint, but this is rare as true balanced phono-stages are quite rare. some phono-stages even totally float the input and use the output gnd for shield. This can be a very very quiet (hum free) solution, but something I have not seen in any commercial designs. Downside on balanced input is the noise penalty, where it suffers compared to single-ended inputs.
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Old 30th March 2013, 03:04 PM   #5
SY is offline SY  United States
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There's ways to avoid the noise penalty and enjoy the greater noise pickup immunity of balanced. I see that you're in Evanston- if you want to do a bit of hacking and make your TT truly balanced, bring it on up.
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Old 4th April 2013, 05:02 PM   #6
kevinkr is offline kevinkr  United States
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Off topic posts deleted. Guys let's stick to the topic and not crap up other people's threads..
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