balanced preamp question

[bonsai171]

What exactly is a balanced input/output, and what is the advantage of having them? This seems to be placed in conjunction with references to xlr connectors, but sometimes 1/4" too. Recently i was investigating professional active crossovers like the Dbx 223 (Stereo 2 way, 3 way mono)

http://www.zzounds.com/item--DBX223

In the description, they say that there are "TRS balanced ins and outs." Does that mean that the only output signal it can send is balanced? One of the amps this would run to is an interm, which has 1/4" and xlr inputs. Also, for the preamp, would it have to be balanced too? I have heard that there won't be any difference in the sound you get if your sources (cd, dvd, tuner, etc) aren't balanced.. Does that mean they would still work well with a balanced setup?? Ok, maybe this could be worded a little bit easier.

here's what i'd like to do:
A friend of mine recently hooked up my speakers (Advent 2002-1980's 2 ways with a set of replacement tweeters-Dayton Silk domes) in active mode (complete elimination of passive components) with an electrovoice active crossover, two interm amps, and a b&k preamp. The sound blew me away! There was an unbelievable amount of clarity coming out of the tweeter, and the mid and bass were impressive too. Overall, it seemed like a huge veil was lifted off the speakers. The best frequency was 3000Hz i think.. I would like to reproduce this same type of sound, and am wondering what components to buy. I have one interm amp (170W X 2) which should be good for pushing the woofers. That leaves an active 2 way crossover, an amp for the tweeters, and a preamp. Many of the active crossovers have balanced inputs/outputs, so is it necessary to have a preamp with balanced inputs/outputs too, and will that work with a rotel cd player, toshiba dvd player, and a realistic tuner?? The preamp is an obvious first step, but of course it needs to be the right one before starting all this Also, any thoughts on whether to use an active pro crossover like the dbx 223 or to custom build one for the speakers?? This seems like enough writing for now.. maybe too much Thanks!

Dave

[jwb]

Deep breath...

Balanced or differential signalling is very well explained on this forum, on the Internet, and in literature (try Horowitz & Hill). So I won't cover it.

Advantages of differential signalling: potentially less noise picked up on cables between components, and probably the signal level will be greater. Some amplifier designs will have lower distortion for a given power level when using the differential input, as compared to the single-ended input,

Is it advantageous to have some differential runs even if some are still single-ended? Absolutely.

Is it necessary to use the differential output of equipment with both kinds? Not at all.

TRS == tip-ring-sleeve.

If some equipment has a differential output, it also has a single-ended output. Just ignore the inverted pin of the differential output.

In my view the best system would be all differential. If you are building your own equipment there's no reason to omit differential signalling.

Cheers,
jwb

[slowhands]

Originally posted by bonsai171
What exactly is a balanced input/output, and what is the advantage of having them? This seems to be placed in conjunction with references to xlr connectors, but sometimes 1/4" too. ...... Dave

Balanced cables have two so-called "balanced" (equal but opposite polarity) signals flowing through them. The benefit is that if hum or noise is induced in the cable on long runs, it is common to both signals ("common mode") and will be ignored by a balanced receiver circuit at the input of the next device. Balanced signals are essential in pro audio installations, where cable runs are long and electrical noise abounds.

There are two common standard connectors for balanced signals: XLR and TRS. TRS means Tip, Ring, Sleeve, for the three sections on a 1/4 inch plug (exactly like on your stereo headphones). Tip goes to the +signal, Ring to the - signal, Sleeve to shield/ground. Similarly, an XLR connector has the +, -, shield lines. XLR connectors are larger and more expensive but have a locking feature. Many pro devices have both TRS and XLR inputs, because then they are compatible with most existing pro sound system cables.

There are few if any mass market consumer audio devices with balanced inputs, so there is little need for balanced inputs in small home systems. However, there are some situations in which they are handy.

One case is using pro gear in home systems, with suitable adapters. A "mono" 1/4 inch plug will also work on TRS inputs (for short runs) bringing the tip and sleeve signals in and connecting the ring to sleeve. Adapters from 1/4 inch mono plugs to RCA jacks (available at Radio Shack and many other suppliers) make it very easy to connect traditional RCA cables found on most home audio gear to pro gear. Of course you lose the benefits of the balanced inputs, but that is seldom a major concern in compact home installations. This scheme allows you to easily use some of the excellent pro gear with balanced inputs, such as active crossovers, equalizers, and spectrum analyzers. These pro devices generally outperform their consumer audio counterparts.

There are other cases too. Balanced cables work well in home installations for connecting between a system and remote amps, which are near the speakers for whatever reason. You may have far lower losses of high frequencies and less noise using balanced cables. I'm sure there are many other unusual situations where they are handy, but for the majority of home systems they are not so useful.

[maylar]

Originally posted by slowhands
Balanced cables have two so-called "balanced" (equal but opposite polarity) signals flowing through them.

Actually, "Balanced" has nothing to do with polarity. A transmission line is balanced when both signal lines have equal impedance to ground. Any ground referenced noise picked up in the cable will therefore be identical on both wires.

The benefit is that if hum or noise is induced in the cable on long runs, it is common to both signals ("common mode") and will be ignored by a balanced receiver circuit at the input of the next device.

Correct. There is also a tendancy to refer to differential signals within equipment as "balanced", even though the equal impedance requirement (the definition of balanced) may not be met. Differential signals have the ability to reject hum and noise that's possible within the grounding system. Differential inputs are easy, since opamps are inherently differential by their nature. Differential or balanced outputs however require an additional amplifier stage which increases complexity, size and cost.

[Steve Eddy]

Originally posted by maylar
Differential or balanced outputs however require an additional amplifier stage which increases complexity, size and cost.

You can balance an output without any additional amplifier stages. You can use a transformer of course. Or you can mirror the output network on the existing output stage which may be as simple as a single resistor.

se

[maylar]

You can balance an output without any additional amplifier stages. You can use a transformer of course.

Absolutely. The phone company has been doing that for 150 years. But hi-fi balanced transformers... cha-ching $$.

Or you can mirror the output network on the existing output stage which may be as simple as a single resistor.

Got any schematics? I'm very curious. Thanks.

[Steve Eddy]

Originally posted by maylar
Absolutely. The phone company has been doing that for 150 years. But hi-fi balanced transformers... cha-ching $$.

Yeah, they can get expensive. Sowter has a rather inexpensive output transformer, their 4652 which sells for about 30 pounds.

Got any schematics? I'm very curious. Thanks.

Yeah, but I'm on another computer at the moment so I'll have to post that later.

But just imagine you've got a simple opamp output with a 1k ohm buildout resistor in series with the opamp's output. Just take another 1k ohm resistor with one end connected to the "non-inverting" output and the other end connected to ground.

Voila. You've got a balanced output with each line having the same impedance with respect to ground.

se

[bonsai171]

it sounds like balanced might be a good option for me.. Since active systems require a lot of amplifiers and an electronic crossover, i would like to maybe eventually rackmount all that stuff and store it out of the way somewhere like in a closet (ventilated! ), and then have a furniture piece of some kind to hold the preamp and sources. Any suggestions on a good 2 way active pro crossover? Thanks

Dave

[fcel]

Since we're on the subject of balanced pre-amp ....

I have seen very expensive pre-amp with only one pair of balanced output. My BOSOZ only has 1 pair of balanced output too (so far). Can I wire up another pair of balanced output in parallel to the existing balanced output? I don't see any harm to it (I think) but would it reduce the output level of each pair of balanced output? And reduced by how much (db?)?

I ask because I want to biamp my speakers.