From: email@example.com (Doc)
Subject: How do you determine polarity of a phono amp?
Date: 10 Jan 2003 00:32:01 -0800
NNTP-Posting-Date: 10 Jan 2003 08:32:01 GMT
I have a phono mixer I bought at a pawn shop without a power supply.
This particular model is a Gemini "Scratchmaster" PMX-18. On the
outside the power input only says "15V ac", doesn't specify amps. I
called the manufacturer and after asking several people who didn't
seem to know the answer, someone finally advised me 500 ma as the
rating for the power supply.
I went to a local used electronics supply house and they advised me
that a 12v, supply should be adequate, and they grabbed one rated 12v,
800ma and sure enough, it works. I also found another one that was
rated at 14.5v ac and 1.16 amps, since I thought getting closer to the
specified voltage might be a good idea, and it's my understanding that
it's ok to have ampage that's over the rating, that the unit only
draws what it needs. (yes? no?).
The 14.v unit has no end plug so I have to solder one on, and is of
the 3-prong variety, the 12v one is of the 2 prong variety.
It ocurred to me that I've always heard that correct polarity is
important, and to be honest, I don't know what the polarity
requirements of the unit are, or what the polarity of the 12v 2-prong
power supply is, though it does seem to work. I also find that by
hooking up the Red and White leads from the 14.5v unit to the prongs
of a plug end I bought, it seems to work either way they're hooked up,
and I can't tell any difference in sound or noise floor.
So, some questions
1) How would I determine what the polarity requirement is of this unit
without going back to Gemini? For a couple of reasons - first it was a
pain in the ass just trying to find someone who seemed to know
anything, and also on principle, they seem to be trying to coerce you
into buying THEIR power supply at what I'm sure is a lot more than
simply picking up one locally.
2) Which way should I solder the red/white leads onto the plug end for
which polarity? the longer connector goes to the outer sleeve of the
connector, the shorter one goes to the inner portion. Which way for
pos and which way for neg? I assume the black lead is to the ground on
the 3 prong plug. Should anything be done with that or just leave it
3) Why does this unit seem to work no matter which polarity you use?
I've always heard it's bad for a unit to use the wrong polarity.
Thanks for all input.