(I apologize in advance, if this is starting to look like "battle of the
In article <znr928845466k@TRAD>, Mike Rivers <mriv...
>In article <7jieda$...
>> IOW, you need something to isolate one channel's output from the other (a
>> "summing node", as it's called), so that one output can't attempt to
>> "drive" the other! This needs to be at least an order of magnitude
>> higher impedence than the output itself, to avoid either possibly frying
>> or otherwise distorting the outputs!
>Geez, you guys are a bunch of wimps. You can only blow stuff up when
>there's power behind it, and there's no significant power behind the
>output of a consumer CD player. (and, no, I didn't ask which brand)
>People have been connecting these things together with Y cables ever
>since there's been stereo. True, you can get a bit of distortion,
>maybe, but nothing blows up. For $1.29, it's certainly worth a try.
I haven't seen many damage problems with home gear, but I remember seeing a
number of car audio components (electronic crossovers, etc) get blown
output preamp circuits from just Y-connecting things together, so I'm a
little more cautious than most in this regard... call it hard-learned
lessons, if you will...
Also, I've seen a couple of home audio components (for example, my old
Sanyo D3 cassette deck, may it R.I.P. after 10 years of good service),
that simply would shut down with the outputs paralleled... now, THAT'S an
inconvenient thing to have happen, in the middle of a gig, no?
>The problem with putting resistors in series with the outputs is that
>if the value is high enough to provide good isolation, you're building a
>voltage divider which comes close to dropping the voltage in half, which
>means you'll need more gain to record the mono signal, which will
>increase the noise. But you can experiment if you wish.
Actually, you'll have just as much of a voltage divider, if you connect
the outputs in parallel!
You've STILL got an identical impedence per channel, just less of it
(100 ohms vs. 1100 ohms)... which just means the channels will have to
sink more current in an out-of-phase or uncorrelated signal condition,
than with the resistors I suggested...
Actually, to get nit-picky, that really should be:
INSIDE CD PLAYER | OUTSIDE OF CD PLAYER
voltage sources |
(preamp out driver |
| CD |
| output |
|\ V impedence |
| \ Ro1 | R1
|L hot \-----/\/\/\-*|*-------/\/\/\/\-----*-----*
| \ 100 | 1K-ohm | |
| (Eo left) \ ohms |RCA plug | |
| / | | |
|L ground /---------*|*---* | *---* output mono hot
| / | | |
| / | *----------------------*---* output mono ground
|/ CD | | |
output | | |
|\ impedence | | |
| \ Ro2 | R2 | |
|R hot \ --/\/\/\-*|*-----/\/\/\/\-------* |
| \ 100 | 1K-ohm |
| (Eo right) \ ohms | |
| / |RCA plug |
|R ground /---------*|*--------------------------*
| / |
| / |
Note that the Kirchoff impedence loop of the new circuit is the same as in
your circuit... but it's much more effective a learning tool to consider
the effect of the voltage source in the circuit, as well (as it's what
we're trying to protect!)
>Look at the path from the L hot. The voltage goes through R1, R2,
>and RR in series on its way back to ground. Since RR is almost
>negligable when compared to the sum or R1 and R2, ignoring it means that
>the voltage at the junction of R1 and R2 (where the output is taken)
>will be divided in half.
Only if the signal is ZERO on one channel!
Look at the "revised", more precise circuit representation I did: If the
same signal is present on both channels, then there will be NO
attenuation, since the voltage at 'L hot' and 'R hot' will be identical.
However, if there is NO signal on one channel, but something on the
other, then YES, it will be attenuated by 6 dB, no matter whether you use
the buffer resistors or not! And this is EXACTLY a proper mono sum, I
submit... anything that's JUST on the left or right channel SHOULD be 6
dB down from the "center" image stuff, when folded to mono... there's
HALF as much energy there in the original stereo case, because of ONLY ONE
CHANNEL providing the information!
In the extreme case, if you have two completely out-of-polarity signals
on the channels, you will have ZERO output, as one voltage will cancel
the other (the resistors will be sinking ALL the signal, as one voltage
will be the inverse of the other, with a virtual ground at the output).
However, with any recording where the engineer has "done his job" and
insured PROPER MONO COMPATABILITY to his mix on whatever album is in the
CD player, you won't ever have an outright cancellation situation like
>In the grand scheme of things, this is probably insignificant, but then
>most likely so is tying the two outputs directly together.
Well, if you want to take the chance, so be it... I personally rather be
safe than sorry, especially when the gig is important...
GALAXY convention --------- Anime Weekend Atlanta 5- October 8-10,1999
/| || //| // /| ,, //~// //~// //~// ----- Marriott Gwinnett Hotel
//|| ||//||// //|| ./ //_// //_// //_// --- http://www.anime.net/~awa
//~~|| |/ |/ //~~|| / ,,_// ,,_// ,,_// Gordon Waters-gwat...@crl.com