i may be mistaken here, but an amp that is not designed to accept balanced signals won't really make any discernible music, just a lot of noise, if a balanced signal is used...

the difference between regular rca and balanced signal, is not that one is high level and one is low level, but rather how the signals are delivered. (one wire carrying signal, and the other carrying ground; or both wires carrying signals but opposite of each other( + -))

so in your statement " If your amp only has single-ended inputs, then it won't benefit from common mode rejection like the differential input amp has"... you really shouldn't be saying that you are missing out on a benefit and that it will still work, but rather that if you did this, you would be sorely disappointed at the result because it won't give you anything other than noise.

 

Firstly, lets clear up exactly what balanced input means. Balanced input really means nothing in regards to the amplifier. Its where the input cabling feeding the amplifier has identical impedances on both signal wires. Unless you are running a balanced cable (i.e. twisted pair) directly from the HU, you are not running a BALANCED input into the amplifier. Even if you splice twisted pair into the amplifier from the stock Bose AMP harness with IXOS twisted pair RCA cables, you still aren't doing so from the Bose AMP harness to the HU so there is an impedance mismatch.

That said, you basically are left with two types of inputs on the amplifier - Single-Ended and Differential

Single-Ended takes whatever +/- signals you feed it and the + feeds into your audio pre-amp circuitry while the - is referenced to the amps ground.

Differential takes your +/- signals you feed it and then feeds it into a differential amplifier where the audio signal is extracting by subtracting the two signals.

The benefit of the latter is that any common mode noise coupled into the signal wires will be canceled out. However, this only works perfectly when you have a balanced cable feeding the amplifier input - in other words, a cable where both the +/- signals have identical impedances and noise couples into them equally. Without balanced cabling, you'll have a bit of mismatch and when you subtract the +/- you will still be left with some residual common mode noise component.

But that said, when using a differential input, you will still get some benefit with the differential noise reduction as opposed to using an amplifier that has a single-ended input scheme.

And since the stock BOSE HU puts out extremely low level signals (less than 1V), any noise reduction scheme you can add to your system is a benefit - hence why differential inputs are recommended - as they provide you with good common mode rejection.

 

now i'm no professional, but i have a few questions about what you said.:

" Unless you are running a balanced cable (i.e. twisted pair) directly from the HU, you are not running a BALANCED input into the amplifier." -- i'm not sure how twisted pair wires have anything to do with whether or not the signal is balanced. if the signal output is a balanced signal (like the outputs from the bose HU), this is very different from a regular standard rca output. wiring has nothing to do with whether the signal is balanced or not. the difference between a balanced signal and a standard rca signal is how the signal is delivered. the rca type, only the positive wire is amplified, sending the signal to the amp, and the - wire acts as a common ground with the amp. with balanced outputs, both the + and - wires are amplified, but out of phase of each other to reject noise.

so i don't understand why you say that if you splice wires from the amp wiring harness into rca's, that you aren't running a balanced signal. The balanced signal starts at the HU outputs, and if you take the + and - and splice that into rca's, why would the signal change because of that? It doesn't, right?

 

A balanced signal basically means the cable running the signal is . .. well . . balanced. In otherwords, both the + and - have the EXACT same impedance (not resistance) from source to load. Since the magnitude of noise induced on a wire is directly related to the impedance of the wire, if both + and - have the same impedance, the same magnitude of noise will be induced on each one.

The benefit of this is that the balanced signal then is fed into a difference amplifier where the + and - are subtracted from one another. This eliminates the common mode noise induce on the wire in the following fashion:

Output = ((Positive Wire Signal) + Noise) - ((Negative Wire Signal) + Noise)

Output = Positive Wire Signal - Negative Wire Signal

So the noise cancels out.

However, although the signals may be sent in a differential fashion from the BOSE HU, its not really a balanced connection, especially if you have splicing occuring and/or attaching typical RCA cables. Both splicing and the addition of RCA cables will cause impedance mismatching. Whereas a twisted pair will have the two signal wires with the same impedance (think geometry as thats really what makes up the characteristics of a transmission line), an RCA cable has a center conductor surrounding by an external braided shield which both have vastly different impedances.

Also, with a differential input, both + and - are not amplified. The "difference" of those signals is what is ultimately amplified.

Of course, I agree this is all technobabble nitpicking and in practice, as long as you do the best you can, you'll be okay. Use an amplifier with differential inputs (such as JL audio), and make those connections between HU and amplifier as clean and neat as possible.

 

but for the sake of anyone else reading this... if you try to use the balanced input on an amp that doesn't have that circuitry it will offer much more noise than regular, correct?

 

U gotta go with the 12" alpine sub and amp. It kicks and will give u what ur looking 4!

 

Balanced Lines / Differential Inputs go together. Without one or the other, you wouldn't get the full benefit of balanced lines.

When an amplifier says it accepts balanced inputs, it basically means it has a differential amplifier at the input. However, it doesn't require balanced inputs to work.

 

you may have misunderstood me... i was talking more about someone using the stock bose HU with an amp that doesn't have differential balanced circuitry..(just accepts regular rca inputs)... this would cause more noise, right?

 

It wouldn't necessarily cause more noise, but it wouldn't reject *as much* noise, so your SNR would be lower. Does it make a difference enough to be audible? Don't know. But seems like everyone experience here is to go with the balanced differential input amp.

 

Thanks,
I've done several simple installs, but don't know alot. Like I have no idea what balanced inputs are. Is it easy to find that out on amps specs? Like a JL 500.1 for example. I don't see anything on it about balanced inputs.

Does the stock amp only run the rear speakers?

 

jl's have balanced input curcuitry...so that should be good to go there. the stock amp runs all the speakers...what i was suggesting was that if you wanted "fronts only" type sound (i am a fan, never really cared for rear speakers) then you could use the rear output on the HU to provide the signal to your sub. you can then use the fader (fade front and rear) to control the volume of the sub channel (used to be the rear speakers)