Re: Active Electronics
- From: Benj <bjacoby@xxxxxxxxxxx>
- Date: Thu, 28 Feb 2008 22:22:14 -0800 (PST)
On Feb 29, 12:23 am, bass...@xxxxxxxxx wrote:
$69.95, but the question is can one be built with an 18 volt power
supply without interference or hum?
I've been considering building something like this only powering the
bass through a stereo cable using the power from inside my amp(s).
Here's the deal in a nutshell. In active basses (and some other
things) you have a stereo jack. The "tip" of the jack is the signal
output from the bass. The "ring" contact of the jack which is
designed to contact the second wire in a stereo cable, instead
contacts the body of the mono cable plug and shorts that terminal to
"ground" which is the shield of the mono cable. Typically the 9v (or
18v) NEGATIVE of the battery goes to that terminal on the bass jack.
So dig. When the bass has no cable plugged in the jack terminal floats
and the battery is disconnected. When you plug in a mono cable it
shorts the "ring" terminal (battery negative) to the body of the plug
(ground) and it thus turns on the battery by putting the negative side
to ground. THAT is how an active bass turns on and off by inserting
the cable.
OK. So what if you insert a STEREO cable? Well the "ring" terminal
that formerly was grounded now goes to the OTHER wire in the cable.
And that cable then goes to whatever. So the bottom line is that the
NEGATIVE of the internal battery goes to that wire. So NOW if you take
an old 9v battery apart and take the terminals off the top and solder
a wire between them you now have made a shorting plug for where the
battery goes. So if you take the battery OUT of the bass and install
the short instead, with a stereo wire you now have the "RING" on the
cable going to the POSITIVE wire of what used to be the battery. In
other words your extra wire in the stereo cable is now a POWER line
direct to the instruments electronics! Hence to power the instrument
remotely all you need to do is connect a 9v or 18v battery at the FAR
end of the stereo cable between ring and ground (body of the jack).
"OK, Mr. Bongo, if you are so smart, then why haven't you done this
yet?" Here come the "problems". One is that if you use a battery for
power, the battery is REALLY isolated from all powerlines and hum and
noise. This is what makes battery power so cool. If you try to use a
"wall wart" there will be some coupling or even ground loops or even
perhaps some line voltage between that power supply and the one in
your amp. Expect HUGE hum and noise! This was why my idea was to
actually use the power in the AMP to provide the "phantom" power to
the bass. That way both amp preamp and the bass active circuits use
the SAME power and it avoids all the problems. A good start. Of
course you may have to 'adjust" the amp power (most likely 12 or 15
volts) if your bass can't take that voltage. Getting 18v may be even
more tricky.
But wait there is more! Notice in the description of that PMT battery
box they talk a lot about short protection and "fault warnings". This
has to do with the fact when you plug a stereo jack into your bass
that is supplying power on the "ring' terminal, that terminal will
SHORT for an instant as you jam the plug into the jack hole! The
power supply MUST, repeat MUST be able to take that short without
damage. This means that if you suck power from the amp, there must be
a "protection circuit" between the amp power supply voltage and the
line to bass. The protection fault circuit just opens up the
connection when there is a short. As you can see this is where things
start to get a bit more complicated!
Of course you COULD just take a vow to ALWAYS have the power off when
plugging and unplugging bass cords... Yeah SURE! How long to you think
it would be until you forget (right before the most important gig of
your life, of course)? Bottom line: I"m still using batteries. But
the general concept is certainly doable and I'd love to set up my amps
this way! Any engineers out there with a nice short protection
circuit...Please dump it in here!
That's the story!
Benj
.
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