Re: Sorta on topic, shop stereo and using the phono input

On 2008-10-04, Jon Anderson <janders1957@xxxxxxxxxxx> wrote:

[ ... ]

working. I only have phono left for input, but wasn't sure
it would work, so using an 1/8 to RCA adapter, I plugged in
my MP3 player. Still running through the Optimus speakers
and probably a good thing. There was a hell of a loud noise
that only slightly resembled the Robin Trower song playing,
then the speakers went dead. They are both deader than
doornails. I know the amp will output far more than they
will handle, but I had the volume way down.

Not possible to have it down enough. Phono inputs are for
signal levels around perhaps 15 mV, while the line level inputs (same
connector) are for levels around 1.0 V or so -- sometimes more. That is
a gain of 50X or so. I don't think that your volume control has a
position lowe enough to make this work -- even neglecting the distortion
which would come from overdriving the input stage that much. (The input
stage is before the volume control, so it can't protect that part.)

To add to the problems, the signal to the line level inputs is
presumed to be fairly flat in frequency response. However, the output
of a good phono cartridge has a serious frequency variation, so the gain
varies with frequency to adjust the level properly. At one time (late
1950s), there were quite a few different equalization curves, now we are
down to a single one for all brands of records -- I think.

What you *really* need is a box which will switch a selection of
inputs into a single input of your amplifier. So -- you set the
amplifier/preamp to "AUX" or "AUX1" or whatever you have available, and
then operate the switch on the separate box. It would probably be
beneficial to have volume controls for each input so you don't get a
major jump in volume as you switch from input to input. If you can get
away without the individual volume controls, all you need is a metal
box (aluminium is fine), two phono jacks for each input, and a pair for
the output, plus a two pole rotary switch with enough positions to
handle the number of input you need -- plus one or two spares for when
you find other things which you need.

But stay clear of the phono inputs with anything but the
connectors from a turntable with a phono cartridge. (If you have the
schematics of your amplifier and a reasonable knowledge of electronics,
you could bypass the phono preamp stages and go directly to the switch
inside there -- but I would not do this to a nice amplifier/preamp (or
receiver).

Anyway, since a sound card will output to speakers or
headphones equally well,

A lot of computer speakers have small amplifiers inside them, so
you are not really directly driving the speakers from the sound card.
If the speakers have a wall wart which needs to be plugged in to make
them work, they have the amplifier.

I assumed the test would validate
hooking up the computer before running wires across the
shop. I'd still like to do this, but it sure seems there's
some mismatch between what a sound card/MP3 player puts out
and what a phono input wants to see.

A major mismatch, as described above. :-)

Is there a quick and
easy way to match up the signals?

If you are capable of designing an attenuator circuit with the
inverse of the equalization curve in the phone preamp -- you could do
this -- but ...

Or should I just make up a
switch box and switch the the input from the computer with
one of the other inputs I'm already using.

That is the way to go.

Good Luck,
DoN.

--
Email: <dnichols@xxxxxxxxxxx> | Voice (all times): (703) 938-4564
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