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Old 14th November 2003, 04:04 PM   #1
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Default Balanced line questions

I'm making an amplifier and seperate active crossover with an input selector. My first problem is whether or not to include the preamp with the amplifier or the crossover. Including it in the crossover would give me added noise rejection because the amplifier would not be as sensitive to noise in the input. The disadvantage here is that I would need to link the grounds of the crossover and amp. I plan to use no capacitors between these two. The amplifier has no caps and amplifies DC.

Now, my understanding is that a balanced line system uses three wires, ground, + and -. The reason regular wiring picks up noise is because the ground line (if it is earth ground) is 100% immune to noise but the signal line is not, therefore the noise causes a voltage difference across the wire and this is audible. Now if I want to make a balanced line system and just use UTP wire, say CAT5, can I use the transformer ground if it won't be connected to the chassis or earth ground? I'm counting on the fact that both the crossover's transformer ground and signal line will be affected by noise equally, therefore eliminating it. I'm planning not to run any ground between the amplifier and crossover. The only thing the amplifier will see is the voltage difference between the two wires. Is this "groundless" balanced line possible?
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Old 14th November 2003, 04:18 PM   #2
tiroth is offline tiroth  United States
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http://sound.westhost.com/balance.htm
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Old 14th November 2003, 04:39 PM   #3
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Quote:
Example: The differential output resistance in his project is 2 x 220 Ohm = 440 Ohm. If you apply a common mode voltage U at both outputs, a current of 2 x U / 220 Ohm will flow, i.e., the common mode output resistance is 110 Ohm (1/4 of the differential output resistance).
Is adding equal resistors to the lines necessary as to subject the lines to an equal amount of noise?
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Old 14th November 2003, 04:51 PM   #4
pooge is offline pooge  United States
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Balance is all about relative impedances. Think of the source and load as a Wheatstone bridge circuit. Balancing impedances will cancel out noise common to the + and - lines. Not easy to do, especially at all frequencies, since capacitances come into play at high frequencies. See white papers at:

Jensen Transformers
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Old 14th November 2003, 04:55 PM   #5
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Interesting...

So If my crossover/preamp has an output impedence of 330 ohms and it's - line is a direct tap the the transformer ground, the signal line will be more vulnerable to noise and this will unbalance the line, correct? To balance the line I would have to put a 330 ohm resistor between the - line and transformer ground?
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Old 17th November 2003, 04:54 AM   #6
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Okay, what I'm saying is, can I turn an unbalanced line into a balanced line by adding equal value resistors on the output connectors?
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Old 17th November 2003, 05:03 AM   #7
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Quote:
Originally posted by Solid Snake
Okay, what I'm saying is, can I turn an unbalanced line into a balanced line by adding equal value resistors on the output connectors?
Effectively, yes. A lot of semi-pro audio gear uses that technique.

But if your line is going to drive an input transformer, I wouldn't bother. An input transformer will give you nearly as much common-mode rejection fed from an unbalanced source as a balanced source.

For example, Jensen's JT-11P-1 gives you about 100dB of common-mode rejection from an unbalanced source and 107dB from a balanced source.

se
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Old 17th November 2003, 05:41 AM   #8
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No, I'm driving the base of a 2N2222 and I'm disappointed to find that my amp makes a very good AM radio. I think this is because the GND input has a 0 ohm impedence and the signal has a 4.7K ohm (minimum) impedence. I will try adding a 4.7K to ground.

By the way, what is common-mode?
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Old 17th November 2003, 05:56 AM   #9
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Quote:
Originally posted by Solid Snake
No, I'm driving the base of a 2N2222 and I'm disappointed to find that my amp makes a very good AM radio.
You're using a 2N2222?

Quote:
I think this is because the GND input has a 0 ohm impedence and the signal has a 4.7K ohm (minimum) impedence. I will try adding a 4.7K to ground.
You mean your source has an output impedance of 4.7k or do you have a literal 4.7k resistor in series with the base? What's the schematic you're working off of?

Quote:
By the way, what is common-mode?
Common-mode means a signal (or more typically noise) which exists equally in magnitude and polarity on both lines, as opposed to differential (or normal) mode where it exists equally in magnitude but of opposite polarity on each line.

se
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Old 17th November 2003, 07:39 AM   #10
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Quote:
You're using a 2N2222?
Yes it drives my low current pre-amp with good results.

I'm using a laptop to generate sound. For protection reasons I added a 4.7K resistor on the signal wire but not on ground.
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