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KB9ALN > TECH 22.07.03 08:51l 100 Lines 4278 Bytes #999 (0) @ WW
BID : 3814_KB9BYQ
Read: DB0FHN GUEST OE7FMI
Subj: Re: KB2VXA > Dual-Phase AF Amp
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0AR<WB0TAX<N5VDA<N8DA<KB9BYQ
Sent: 030722/0625z @:KB9BYQ.#APL.WI.USA.NOAM [Appleton, Wi] $:3814_KB9BYQ
Hello Warren and other readers,
Warren, I have to disagree with some of your assertions concerning what
you call "push-pull push-pull" amplifiers, more commonly called
"bridged" power amplifiers.
I have edited out some portions of your message, only for brevity. In the
second paragraph, you state:
"The typical juke box amplifier uses a rather unique method of
delivering that thundering bass without undue distortion. Simply
stated it is a push pull push pull wiring arrangement. Each channel
is a standard push pull output, but here's the twist."
One input is wired backward putting that channel 180 degrees out
of phase with the other. The midrange and tweeter are likewise wired
backward reinverting the phase so the sound output is in phase, normal.
The resulting push pull output is fed to the woofer, doubling it's
output."
I haven't worked on all juke-boxes, but I have to say I haven't seen a
bridged power amplifier as you describe. I have seen multiple amplifiers,
in some cases Bi-amped. In addition, some juke-boxes include an additional
mono amplifier to power a 70-volt public-address style speaker system
The old Rock-Ola and Seeburg juke-boxes that I worked on simply had
powerful conventional push-pull amplifiers and a LOT of high-end roll-off
to emphasize the bass. The Rock-Ola 440 often sounded quite muddy as a
result. The other reason for doing this was to filter out the scratches
and tracking distortion present in well-worn 45's they used to play.
One is far more likely to find bridged power amplifiers in auto stereos.
There's an obvious advantage there - increased power output without
investing money and space into a switched-mode boost power supply to get
higher voltages for a more powerful amplifier.
In another paragraph you state:
"Many audiophiles make a big mistake when strapping the channels
of a stereo amp and using two of them for stereo sound. They simply
strap the inputs together and also the outputs putting the channels
in parallel which accomplishes nothing. What needs to be done is to
invert the phase of one channel at the input and connect the
loudspeaher between the two "hot" output terminals which will result
in the out of phase signals being applied to the load. Again you have
a push pull push pull amplifier."
In most transistor amplifiers, paralleling the outputs will make them blow
up, accomplishing smoke! The exceptions are certain pro audio amplifiers
(like Crest and QSC, for example) that can be operated in either bridged
or parallel mode. They have a 3-way switch on the rear panel for this.
Tube-type (valve) amplifiers with power transformers can't be operated in
Bridged mode as they won't be properly loaded. They can be safely
paralleled, I've done this. However, a better solution is to bi-amp the
amplifiers. It greatly reduces intermodulation distortion and gives much
more effective wattage.
One more correction:
"Oh, one more thing. This is a common internal arrangement for
medium and high power output transistors and modules. Some may be
surprised to learn that it's called a Darlington amplifier (device).
73 de Warren, KB2VXA@WT3V.#CNJ.NJ.USA.NOAM"
A darlington amplifier is not a bridged amplifier. A bridged pair of
amplifiers may utilize darlington devices, but many single push-pull
amplifiers use them as well. These are not the same thing.
While most power amplifier modules, such as Sanyo's STK series use
darlington pairs as output transistors, there are precious few that are
internally wired for bridged operation. Bridged amplifier outputs need to
be isolated from the chassis ground (earth), and from each other. This
would not provide the design flexibility that most engineers need.
It might also interest you to know that most high-power transistor
amplifiers use paralleled output transistors. The arrangement is similar
to that found in linear power supplies - equalizing resistors are used in
the emitter circuits to keep the most efficient transistor from "current
hogging" and overheating.
Hope this clears up some misconceptions. 73,
--
Andy Nemec
Internet E-Mail: kb9aln@gbonline.com
Amateur Network Addresses:
amprnet - kb9aln@kb9aln-5.ampr.org
AX.25 - KB9ALN@KB9ALN.#GRB.WI.USA.NOAM
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