Why balance input for phono amp?

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#4
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.
 
#7
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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#10
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)
 
#12
Our full balanced (actually differential) preamp, the RTP3D, has three paralled input sockets, 1/ XLR, 2/ REDEL (a superior balanced connector from LEMO) and 3/ RCA.

The RCA socket is simply wired cross the two active balanced inputs, as in 99% of the time, neither side of the tonearm cable is grounded,and this acts as an effectively balanced input.

Only DECCA cartridges confuse the situation a little, with ther
commoned earth output (only three pins on the cartridge).

Regards, Allen
 
#13
Joachim Gerhard said:
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)
Click to expand...

I have found tapping the termination network to preamp ground and connecting the shield at one end to it works well. Have you observed asymmetry in motor drive? I have used a Grado with 23K and 120pF to ground on each input this way, and have observed nearly perfect balance on the two sides of the first stage (diff-in/diff-out).
 
#14
i use the rca all the time (wbt next gen or eichmann) in pseudo balaced. i have a wheaton arm that does not ground the tonearm cable. my balanced stage needs no earth connection so it looks we have a topological similarity here although the way impedace is handled is very particular with my design. this circuit comes from the medical industry to detect brainwaves. i do not publish it for diy but certified and qualified dealers may be provided with a service manual. the circuit should be reparable but i think it whoud litterally work for ever at least in the forseable future. it´s a bomb proove design of great simplicity and sofistication.
 
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