Is +3db using a Y-cable on sub input a myth?

[cnjvh]

Sure, you've doubled the input by populating both L+R inputs which should give you a 3db gain, but haven't you reduced the input by 3db since you've split the signal (halved the energy?) Should be a wash, shouldn't it?

[PolkThug]

Its not a myth, the volume is raised on those types of amps.

[michael_w]

I believe it's actually a 6dB gain not 3.

Just get an spl meter and test it out for yourself if you're skeptical.

[jdhdiggs]

Or you could just balance your sub in either case and it wouldn't matter.

[John K.]

CN, it's good to be skeptical about these things and there're plenty of audio myths involving players, receiver/amplifiers and even pieces of wire that should be shot down when they rear their ugly heads. This, however, isn't one of them. What you propose seems initially to make sense: first the voltage is divided and then recombined, so the net effect should be no change. However, as we become better-informed about audio we learn(at least hopefully)that many things that seem to make sense or that seemed to be apparent from mere listening aren't actually true.

Here the reality is that the Y doesn't actually create a voltage divider. It's a parallel circuit and the voltage on the two branches of the Y remains essentially the same as the input voltage. A discussion and proof of this can be studied here (http://www.tpub.com/neets/book1/chapter3/1-24.htm).

Next, the two unchanged voltages from the Y are fed into the two inputs of the sub amp, which typically has a voltage summing circuit at that point, such as the one illustrated and discussed here (http://www.play-hookey.com/analog/experiments/summing_amplifier.html). As shown, the summing of the two voltages results in a doubing of the voltage, and since the formula for dB voltage ratios is 20xlog(V2/V1)dB, the log of 2 being about 0.3 results in an increase of about 6dB.

So here the "myth" has been proven true. Note that this ordinarily isn't a major benefit since turning up the receiver's sub output voltage a bit accomplishes the same thing and most receivers have more than enough low distortion sub output voltage to keep sub amps "awake" .

[dorokusai]

You don't really need one at all.

[cnjvh]

Thanks for this detailed explanations everyone. This was merely a curiosity for me because as others have stated it makes no difference in the long run - just calibrate everything properly and be done with it. It's just that whenever I read a post about the 3db gain there was this thought in my mind of "wait a minute - that shouldn't work, should it?" so I had to ask someone.

Anyway, I have an SVS PB10 with a single line in so its a non-issue for me ;-)

[wingnut4772]

I called SVS once and asked them this and they told me no.

[jdhdiggs]

It's a "no" because if you level set after hookup, it will be the same volume regardless of what you did before the level set. The myth here is that it is a "free" 6dB without putting any more strain on the amp, which is not true.

Gotta be the first time Doro and JohnK agreed so it must be true.

[Demiurge]

Club Polk Myth Busters.