Clarify this for me. Whether one input or two via a Y lead, the voltage applied by the LFE output across the inputs will remain the same. Connecting to two inputs in parallel, will halve the impedance seen by the amp which would double the current required. I also thought doubling a signal results in a 3dB increase, with 6dB requiring a quadrupling?
I'm quite happy to be wrong, I just need this clearing up.
I know the OP doesn't address this point, but what are your thoughts on high/speaker level inputs as an alternative for the stereo boys? I can't remember seeing them on any of the M&Ks I've had dealings with.
Russell
Woohoo! Let's start ALL over.
First before we go any further is the LFE issue: THE SUBWOOFER OUTPUT IS NOT LFE !!!!!!!!!!!!!
Again: THE SUBWOOFER OUTPUT IS NOT LFE!!!
To absolutely clarify:
LFE is an
effects channel used in the optical track in movie theaters because there is not enough dynamic range in the optical channels for Low Frequency Effects. Movie theaters usually have 5 channels plus a sub channel PLUS an LFE channel. A theater can get away with multiple bass sources because the room is so big - typically much larger than the wavelength of the lowest frequency of interest, i.e. 55 feet at 20Hz.
The actual LFE channel itself may go down to almost DC and up to 120 Hz (it's -3db point) and in the beginning (that would be 1996 or so) there was consternation about what happens to the "lost" area between 80 and 120 Hz when this stuff is summed to the subwoofer out jack, and it is that level of audio nitpicking which busy's people's brains and allows such things as republicans to slip into the white house.
You might enjoy my freq to wavelength chart here:
http://www.soundoctor.com/freq.htm
On DVD's released for home use, most
MOVIE soundtracks have 5 full freq channels of surround info (L C R Ls Rs) and one channel called the "Boom" track; this is the LFE in a movie theater.
The bass management circuit in every (Dolby / THX) receiver strips everything below 80 Hz off of each of the 5 channels, sums it into mono, adds the LFE channel (if any) and
THAT is what comes out the
subwoofer jack.
[[ I have a WHOLE other tirade about 5,6,7,n.1 misinformation, but I'm still working on that for my website & book]]
As far as the levels go, These are not lightbulbs drawing current. An audio SOURCE is low impedance driving a very much HIGHER impedance load, or if your'e using a Y connector, loads.
At the input to a sub each input is buffered an then summed into mono. They are buffered so that the actual RCA connectors are not shorted to mono in case you are simply parallelling the inputs to your stereo power amp... otherwise everything in the room would be summed into mono.
If you take ONE RCA cable and plug it in and measure the output you get a 0dB reference. If you take a Y cord and plug in the 2nd input you get 6dB more. That's VOLTAGE. (that also is 4 x the power)
When you change ANY signal 3dB, (you are adjusting a volume control or gain control) that doubles or halves the power... going from 1 watt to 2 watts is 3dB; going from 10,000 watts to 20,000 watts is 3dB.
In the old instances of broadcast devices, they were balanced 600 Ohms on BOTH the inputs and outputs. When you terminate any given source impedance / resistance with the equivalent load impedance/resistance the voltage on that line will halve, i.e become 6dB less.
Nowadays, the source impedance of nearly any output is low; ideally as low as possible (0 ohms) but of course this is not possible so a buildout resistor is usually used to present a source Z of 50 to 200 ohms to the outside world. The input Z of the next stage is typically 10k, 20k, 25k, 50 k ohms for solid state equipment and often higher, 100k - 500k for tube equipment. Therefore, practically speaking, the input stage does not "load" the previous output stage.
In the case of line level equipment all we are really doing is generating a VOLTAGE and then MEASURING that voltage at the next succeeding stage or stages.
In the case of SPEAKER level equipment, we are mostly transferring POWER from the amp to a load (the speaker) so there ARE higher currents that we are dealing with and the impedances are somewhat more important.
Again, in the days of tube amps, you had an output transformer with taps to match the load impedance so that all the available POWER was transferred fom the amp to the load.
Nowadays with solid state amps the source impedance is very low (oh, perhaps from .1 ohm down to .001 ohm) and when this source impedance is divided by the load impedance (typically 8 ohms) that gives you the damping factor.
When the amplifier pushes the speaker cone in a direction then the elasticity of the mechanism attempts to move the c one back to the center position. During this time the speaker acts as a generator. The voltage it is generating has to go somewhere... where it goes is back into the power amp (we could hope back into the storage caps) and the entire output circuitry of the power amp acts as an "active load" But not to digress too far...
Since the human perception of hearing roughly follows a log pressure relationship, a +6dB change is twice the voltage, twice the air pressure, and to most trained ears is "about" twice as loud, although strictly speaking there IS NO SUCH THING as twice as loud, just as there is no such thing as "twice as beautiful". Once you study audio levels with a calibrated attenuator you can just about discern a 1dB change; you can "just"
easily discern a 1-1/2 dB change; and a 6dB change "seems" like twice, although to most people not familiar with audio, they think that a 10 dB change is "twice" or "half". It is a very interesting phenomena, also dependent upon the range of frequencies present while the attenuator change is taking place. I have discerned that the wider the frequency range present, the more level change it takes to affect the human perception, and this is oppositt to what logic or a "mental" experiment dictates! Try it!
As a suggestion, get yourself a 27 or 31 band graphic eq and learn each frequency (and level change).
It's like playing a piano or a saxophone - at first it seems impossibly hard and then it just pops into place in some alloted slot in your brain, and you simply "feel" it.
Barry
