I'm almost ready to order the 970, can't afford the 990. My question, I don't understand balanced or unbalanced on the amp output. I'd appreciate any help, thankyou Phil

An unbalanced connection basically has the signal line (think the tip of an RCA connector) and a shield that completes the circuit, and helps absorb radio frequency interference before it gets into the signal line.

A balanced connection uses 2 signal lines, one with the signal's polarity reversed (so when the signal goes up, on the second line it goes down instead). A balanced line also has a shield to absorb the RFI before it gets to the signal. When a balanced signal gets to the next piece of equipment it goes through either a differential amplifier, or a transformer, which takes that second signal line and flips it back into polarity with the first. The two signals are then added together.

When the polarity is flipped at the end, any noise that was picked up by both signal lines is now reversed polarity between the two lines and is cancelled out. You also get a +6dB boost in signal when they are added together. It is also possible to disconnect the shield from the circuit in order to break ground loop issues as you have a complete circuit between the hot (+) and cold (-) lines.

For the most part balanced connections are desirable for runs over 20' long. Some equipment may have better sounding circuitry on the balanced side, but it can be a gamble as differential amplifiers and transformers all have distinct character which may or may not be helpful to the aesthetics of the system.

My personal feeling on the matter for hifi equipment is that if you don't have long runs, or special equipment that requires balanced, don't worry about it.

You're almost right with regard to balanced and unbalanced signals, except that unbalanced signals often also have a shield that's not part of the signal circuit - look at the Outlaw audio interconnects for a great example.

But the OP seems to be asking about balanced vs unbalanced ampifier output stage designs. I don't feel comfortable enough with that subject to try and explain it to someone else.

EDIT:

I think the 7700 is a balanced output (and input) design, the older amps were not.

That shield still must be connected at one end or the other in order to actually stop RFI from getting into the signal. It will connect with the 'ground' wire and as such still is considered part of the signal circuit.

If the shield is not connected on either end it can help somewhat, but the effects are extremely minimal if it is not grounded to something.

You are correct though, I misinterpreted his request, he was curious about balanced output amplifiers.

I haven't looked into them much, but from a brief overview it looks like they are very similar to a bridged amplifier, where you have two amplifiers working in tandem, one with reverse the polarity of the other, to drive the speakers. I would imagine this would have much of the same effects as the balanced line level does.

I don't generally consider something connected at one end and nailed to common part of a signal circuit, but I suppose it depends on your world view. In any case, the shield shouldn't carry any signal in either topology, the noise rejection balanced systems exhibit is from the signals both floating with respect to ground, allowing them to both have similar input impedance and thus both pick up a similar amount of noise, which can then be subtracted out.

I wonder why Outlaw went to a balanced output final section? I can't see the relatively big signal and low resistance of that circuit needing noise immunity.

But in an unbalanced circuit the common is the reference used for the signal. If there is noise added, no matter how, it can have an effect. Of course in short runs this pretty academic and should have no perceivable effect unless you have major RFI issues. That bad of RFI issues may cause problems with balanced lines also though (I once saw a system that had horrible hum, and I discovered they put their amp racks on top of a 5,000watt power drop cable. Even with all balanced cables, the equipment itself is unbalanced internally for most equipment and the amps were picking up the noise).

As far as balanced output goes, I personally don't see the need as well, although I'm curious if it might allow for higher power due to what basically is two amps working together. Certainly noise rejection shouldn't be an issue.

Absolutely, however the signal current is carried by a conductor that is connected to chassis ground; the shield wire should also (at one end) be connected to ground but it's no more a signal conductor than the grounding conductor in your home is a current carrying part of the circuit.

The reason balanced connections have better noise immunity is twofold:

• They usually have lower input impedence compared to the non-common connected signal side of a single ended input. Lower impedance means lower induced voltage, all else equal.
• In both systems the signal is the differnce bewteen the two wires, however, in a single ended configuration the impedance to common between the two is vastly different by design, whereas in a balanced system they should be closely matched; thus when the single ended system amplifies the difference in the two signal wires, the noise is not significantly cancelled because the much lower impedance to common of the "shell" wire prevented any appreciable noise from being induced on it, therefore there is nothing useful to subtract out. Essentially the asymetrical impedances caused asymetrical noise to be induced.

And just as in your home wiring, the neutral and ground tie together at the electrical box, so noise generated on the ground can appear on the neutral and thus affect the 'signal' on the hot leg.