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Old 29th November 2009, 10:31 AM   #1
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Default Why balance input for phono amp?

All phono cartridges have only 2 pins, why there are some expensive phono preamp has a balance input?
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Old 29th November 2009, 10:35 AM   #2
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Eeh, because you don't need three pins for balanced operation.
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Old 30th November 2009, 10:20 AM   #3
Calvin is offline Calvin  Germany
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Hi,

beeing a coil, the pickup is a floating balanced source. So its quite intuitive to use a balanced input stage, don´t You think?

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Old 30th November 2009, 10:29 AM   #4
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real balanced is when the cardridge had 6 pins. one in the midlle of two coils per side.you are refering to pseudo balanced that works in praxis very well to supress common mode signals like hum and radiation. i never needed an earth cabel for my balanced design. it is very imun to outside influences although it has a theoretican noise disadvantage (johnson noise) of 3dB compared to an unbalanced phonostage of half complexity.
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Old 30th November 2009, 10:40 AM   #5
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do not look further than that at this pdf of an "obsolete" part. it shows what i mean but
that wisdom has all but disappeared.
http://www.diyfactory.com/data/SSM2017.pdf
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Old 30th November 2009, 10:46 AM   #6
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You can buy similar IC's today.
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Old 30th November 2009, 11:09 AM   #7
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The transistor "data amplifier" or instrumentation amplifier was described as early as 1968 (proably earlier). The noise penalty will depend on the real part of the source impedance. For say a 400 Ohm DCR coil it would be easy to make a differential amp with a dB or less penalty. Even easier for a MM input.

A problem lies with partitioning gain and the RIAA time constants. So for phono stages there might be a conflict. For MC, where a fixed differential first gain stage might make more sense you have the noise issue.

EDIT - I noticed one of our add banner folks "Seventh Circle Audio" has some good schematics that cover current IC's and discretes in differential preamps.
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Last edited by scott wurcer; 30th November 2009 at 11:18 AM.
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Old 30th November 2009, 11:09 AM   #8
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is the third pin really needed for balanced operation?
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Old 30th November 2009, 11:13 AM   #9
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per anders, there is a missunderstanding. i did not put forward my apreciation of that chip but found the definition of several types of ballanced (they say diferential) in the form of drawings lower down on the pdf
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Old 30th November 2009, 11:19 AM   #10
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for true balanced (two diffential) you have to cut the signal in half, one half with positive the other with nagative polarity and as long as the signal chain is separated and similar external noise intrusion (the commen mode signal) can be canceled anytime in a final differencial stage that cancels out unwanted noise (substrakt) and amplfies and conducts the signal (add)
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