DIYbanter - View Single Post - "Floating Ground"

Michael A. Terrell

wrote:
On Sat, 22 Apr 2017 18:56:51 -0700, Jeff Liebermann
wrote:

On Sat, 22 Apr 2017 08:21:01 -0500, Foxs Mercantile
wrote:

On 4/22/2017 1:54 AM, Phil Allison wrote:
The amp uses bridge mode output - right ?
So both speaker terminals are amp outputs.

I'm going to go with this as the correct answer.
The audio output of a Motorola Spectra is like this, and
there are warnings repeatedly through the service manual
NOT to ground either side of the audio.

This has nothing to do with "hot chassis" or floating
grounds.

I just hate to agree with you, but y're right. Nobody builds AC-DC
transformerless radios and audio amps these daze.

It's probably a bridge amp:
https://en.wikipedia.org/wiki/Bridged_and_paralleled_amplifiers
That looks like this:
https://en.wikipedia.org/wiki/Bridged_and_paralleled_amplifiers#/media/File:Bridge_amp.GIF
Notice that neither speaker wire is grounded.

If you want to look at the output of a bridge amp with a scope, you
need to have a scope with an A-B (that's channel A minus channel B)
input. Connect one probe each from each scope channel to each speaker
lead. Connect both scope ground leads to ground.

You're right. First off, it has a power transformer.
Secondly it is an amp made to be bridged. After reading the user manual
a second time, I determined that this floating ground is only needed in
Bridge mode.

I have never had an amp that was bridgable (that I know of anyhow). I
dont think this is possible with the old tube amps, but I could be
wrong. This amp is a solid state, with power transistors (not chips for
the output). 16 power transistors to be exact.

Although I like tube amps, I could not pass this amp up for the price he
wanted. It's rated at 300W per channel using 4ohm speaker load, or 200W
per channel using 8 ohm speakers. Or it's brigable to be a 600W mono
amp. I'm gonna have fun with this thing....

It's a Altec Lansing 9444A power amp. It appears it was made around 1988
to 1990.
And Altec Lansing is top of the line equipment.

It was popular for industrial and commercial sound systems. It was a
US company, and I've seen their equipment for decades. I've been out of
the business for years, so I don't know if they are still around.

Some stadiums used their equipment, with thousands of watts of
amplification, with each amp driving a single zone. Some had N+1
installations that would automatically switch a spare amp into service
if needed.

This is a commercial amp, with balanced XLR inputs, so I will have to
buy or make some adapters so I can connect 1/4" or RCA jacks to it for
home use. But I know that my speakers wont handle that full power
either, but I suppose I don't have to run it wide open either....

Here is the manual, which shows how to connect an unbalanced input.

https://www.manualslib.com/download/365773/Altec-Lansing-9444a-Power-Amplifier.html

--
Never **** off an Engineer!

They don't get mad.

They don't get even.

They go for over unity! ;-)