Re: Ipod to stereo amplifer: getting volume

Jitt (tser827@xxxxxxxxx) writes:
In article <475009b0.1246905@xxxxxxxxxxxxxxxxxx>,
NoSpam@xxxxxxxxxxx says...
On Thu, 29 Nov 2007 08:55:27 -0800, Jitt <tser827@xxxxxxxxx> wrote:


My kids had Creative mp3 players that would not produce
sufficient sound volume when connected to "aux" inputs
directly. I made an adapter cable that loaded each headphone
output with a 33 ohm resistor; this gave useable volume.
A later Samsung unit did not have the problem.

OK, I just have to ask: How does loading with a 33 ohm resistor
*increase* the volume? Normally we would expect a load to only
*decrease* volume, since it becomes the bottom leg of a voltage
divider with the output impedance of the source the top leg.

Or is there some sort of automagical sensing in the output circuit,
that increases the level when it sees a low load impedance?
Seems possible, and maybe even logical, so the same connector
can serve as headphone and line out. But I haven't encountered
it before... it this common?

Best regards,


Bob Masta

DAQARTA v3.50
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!

I was thinking the current-oriented output was unable to
drive the relatively high impedance input; the 33 ohm load
allowed the output circuit to function, developing enough
signal voltage across the resistor to give useable volume.

That's a viable explanation, depending on the actual output
circuit. I haven't a clue what they use in those things, but
certainly if they had something that used the headphones to
to supply the current to the output stage, it certainly would
provide a terribly weak signal unless there was a suitably low
load there.

I thought, though, that they'd generally use something like
an op-amp, where there's current to the output device whether
or not there's a load. I do note that out of three portable
CD players, one portable cassette player and one MP3 player
that I have, none require a low impedance DC load to provide
suitable output level. Their outputs vary in level, and they
all seem to be lower level than my component CD player (though
my component DVD player seems to suffer from a lower output
level), but not a major difference in level.

Michael

.

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