Thoughts on this little oddity, anyone ...?

[Arfa Daily]

Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has two
completely separate and identical power amps, one of which was running very
hot very quickly. No schematics, but problem was quickly traced to an open
circuit resistor, by comparison ohms checks between the two amps. In order
to replace the resistor, it was necessary to remove the offending ("B")
power amp to get to the board underside. The power rails and ribbon from the
mixer desk section were on removable connectors, but the 4 pole speakon
connector was bolted to the rear panel and the wires were soldered directly
to the board. When I came to unsolder these wires from the socket, I noticed
that the "B" channel one was wired the opposite way round to the "A" channel
one. I noted this down to check on later.

When the amp was tested ok, I bolted it back in, and reconnected the speakon
socket the same as it was when I unsoldered it. The wires lay exactly where
they originally were, and looked absolutely factory-original, even down to
the one which went to the uppermost connector tag, having a rubber sleeve on
it, exactly the same as on the "A" channel. Except the two channels were
definitely wired arse-about-face to one another. On both channels' speakons,
the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel
"A" the ground wire was connected to the "+" pair, and on the "B" channel,
the ground was on the "-" pair.

I fired it up again, and put a sine wave in, then hooked up two channels of
my 'scope, one to the "A" output and one to the "B", both with the same
polarity, and was surprised to find that the two signals were completely
anti-phase, which meant that with the reversed connections to the speakon
sockets, they would be back in phase again to the outside world.

I then went back to the power amps' front ends near where the ribbon
connector was, and found the same thing. At the same point on each amp, the
signals were in opposite phase to one another. I then spotted three little 3
pin header blocks, with one pin pair on each designated "B", and the other
pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to the
power of' position. On the B amp, these three jumpers were set to "B", and
on the A amp they were set to "A", so clearly, these jumpers reverse the
phase of the signal coming in from the desk section. The two amps then
process the signals in antiphase to one another right to the final output,
where the 'correct' phase relationship is again restored, by wiring the
speakon connector 'wrongly'.

I have puzzled over this, and can think of no good reason for doing it that
way. I did wonder at one point if it was anything to do with being able to
make the amps bridgeable, but I downloaded a copy of the user manual, and
there is a dire warning paragraph towards the end, specifically saying that
under no circumstances should any attempt be made to operate the amps in
bridge. It also warns against making any connections between the two amps or
any speakers connected to them, although in actual fact, the 'ground' side
of each output is truly ground, and is common to both amps, the whole (dual)
power supply, and all metalwork.

What am I missing here ? Not of any real consequence, as the unit is mended
and back in its original condition, but for academic interest, it would be
good to understand the designer's thinking.

Arfa

 

[Michael A. Terrell]

Arfa Daily wrote:

Had an Allen ? Heath PA20-CP 2 x 500 watt powered mixer in today. Has two
completely separate and identical power amps, one of which was running very
hot very quickly. No schematics, but problem was quickly traced to an open
circuit resistor, by comparison ohms checks between the two amps. In order
to replace the resistor, it was necessary to remove the offending ("B")
power amp to get to the board underside. The power rails and ribbon from the
mixer desk section were on removable connectors, but the 4 pole speakon
connector was bolted to the rear panel and the wires were soldered directly
to the board. When I came to unsolder these wires from the socket, I noticed
that the "B" channel one was wired the opposite way round to the "A" channel
one. I noted this down to check on later.

When the amp was tested ok, I bolted it back in, and reconnected the speakon
socket the same as it was when I unsoldered it. The wires lay exactly where
they originally were, and looked absolutely factory-original, even down to
the one which went to the uppermost connector tag, having a rubber sleeve on
it, exactly the same as on the "A" channel. Except the two channels were
definitely wired arse-about-face to one another. On both channels' speakons,
the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel
"A" the ground wire was connected to the "+" pair, and on the "B" channel,
the ground was on the "-" pair.

I fired it up again, and put a sine wave in, then hooked up two channels of
my 'scope, one to the "A" output and one to the "B", both with the same
polarity, and was surprised to find that the two signals were completely
anti-phase, which meant that with the reversed connections to the speakon
sockets, they would be back in phase again to the outside world.

I then went back to the power amps' front ends near where the ribbon
connector was, and found the same thing. At the same point on each amp, the
signals were in opposite phase to one another. I then spotted three little 3
pin header blocks, with one pin pair on each designated "B", and the other
pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to the
power of' position. On the B amp, these three jumpers were set to "B", and
on the A amp they were set to "A", so clearly, these jumpers reverse the
phase of the signal coming in from the desk section. The two amps then
process the signals in antiphase to one another right to the final output,
where the 'correct' phase relationship is again restored, by wiring the
speakon connector 'wrongly'.

I have puzzled over this, and can think of no good reason for doing it that
way. I did wonder at one point if it was anything to do with being able to
make the amps bridgeable, but I downloaded a copy of the user manual, and
there is a dire warning paragraph towards the end, specifically saying that
under no circumstances should any attempt be made to operate the amps in
bridge. It also warns against making any connections between the two amps or
any speakers connected to them, although in actual fact, the 'ground' side
of each output is truly ground, and is common to both amps, the whole (dual)
power supply, and all metalwork.

What am I missing here ? Not of any real consequence, as the unit is mended
and back in its original condition, but for academic interest, it would be
good to understand the designer's thinking.

If both amps are fed the same signal, it reduces the maximum current
requirements for both the + and - rails. That also lowers the current
in the system ground, reducing crosstalk & possible ground loops. This
is done in some RF circuits, as well.


--
You can't have a sense of humor, if you have no sense.

 

[Cydrome Leader]

Arfa Daily <arfa.daily@ntlworld.com> wrote:

Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has two
completely separate and identical power amps, one of which was running very
hot very quickly. No schematics, but problem was quickly traced to an open
circuit resistor, by comparison ohms checks between the two amps. In order
to replace the resistor, it was necessary to remove the offending ("B")
power amp to get to the board underside. The power rails and ribbon from the
mixer desk section were on removable connectors, but the 4 pole speakon
connector was bolted to the rear panel and the wires were soldered directly
to the board. When I came to unsolder these wires from the socket, I noticed
that the "B" channel one was wired the opposite way round to the "A" channel
one. I noted this down to check on later.

When the amp was tested ok, I bolted it back in, and reconnected the speakon
socket the same as it was when I unsoldered it. The wires lay exactly where
they originally were, and looked absolutely factory-original, even down to
the one which went to the uppermost connector tag, having a rubber sleeve on
it, exactly the same as on the "A" channel. Except the two channels were
definitely wired arse-about-face to one another. On both channels' speakons,
the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel
"A" the ground wire was connected to the "+" pair, and on the "B" channel,
the ground was on the "-" pair.

I fired it up again, and put a sine wave in, then hooked up two channels of
my 'scope, one to the "A" output and one to the "B", both with the same
polarity, and was surprised to find that the two signals were completely
anti-phase, which meant that with the reversed connections to the speakon
sockets, they would be back in phase again to the outside world.

I then went back to the power amps' front ends near where the ribbon
connector was, and found the same thing. At the same point on each amp, the
signals were in opposite phase to one another. I then spotted three little 3
pin header blocks, with one pin pair on each designated "B", and the other
pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to the
power of' position. On the B amp, these three jumpers were set to "B", and
on the A amp they were set to "A", so clearly, these jumpers reverse the
phase of the signal coming in from the desk section. The two amps then
process the signals in antiphase to one another right to the final output,
where the 'correct' phase relationship is again restored, by wiring the
speakon connector 'wrongly'.

I have puzzled over this, and can think of no good reason for doing it that
way. I did wonder at one point if it was anything to do with being able to
make the amps bridgeable, but I downloaded a copy of the user manual, and
there is a dire warning paragraph towards the end, specifically saying that
under no circumstances should any attempt be made to operate the amps in
bridge. It also warns against making any connections between the two amps or
any speakers connected to them, although in actual fact, the 'ground' side
of each output is truly ground, and is common to both amps, the whole (dual)
power supply, and all metalwork.

What am I missing here ? Not of any real consequence, as the unit is mended
and back in its original condition, but for academic interest, it would be
good to understand the designer's thinking.

Arfa

are these amps run from one power supply?

 

[N_Cook]

Arfa Daily <arfa.daily@ntlworld.com> wrote in message
news:Xc0uq.25576$gy7.110@newsfe28.ams2...

Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has two
completely separate and identical power amps, one of which was running
very
hot very quickly. No schematics, but problem was quickly traced to an open
circuit resistor, by comparison ohms checks between the two amps. In order
to replace the resistor, it was necessary to remove the offending ("B")
power amp to get to the board underside. The power rails and ribbon from
the
mixer desk section were on removable connectors, but the 4 pole speakon
connector was bolted to the rear panel and the wires were soldered
directly
to the board. When I came to unsolder these wires from the socket, I
noticed
that the "B" channel one was wired the opposite way round to the "A"
channel
one. I noted this down to check on later.

When the amp was tested ok, I bolted it back in, and reconnected the
speakon
socket the same as it was when I unsoldered it. The wires lay exactly
where
they originally were, and looked absolutely factory-original, even down to
the one which went to the uppermost connector tag, having a rubber sleeve
on
it, exactly the same as on the "A" channel. Except the two channels were
definitely wired arse-about-face to one another. On both channels'
speakons,
the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel
"A" the ground wire was connected to the "+" pair, and on the "B" channel,
the ground was on the "-" pair.

I fired it up again, and put a sine wave in, then hooked up two channels
of
my 'scope, one to the "A" output and one to the "B", both with the same
polarity, and was surprised to find that the two signals were completely
anti-phase, which meant that with the reversed connections to the speakon
sockets, they would be back in phase again to the outside world.

I then went back to the power amps' front ends near where the ribbon
connector was, and found the same thing. At the same point on each amp,
the
signals were in opposite phase to one another. I then spotted three little
3
pin header blocks, with one pin pair on each designated "B", and the other
pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to the
power of' position. On the B amp, these three jumpers were set to "B", and
on the A amp they were set to "A", so clearly, these jumpers reverse the
phase of the signal coming in from the desk section. The two amps then
process the signals in antiphase to one another right to the final output,
where the 'correct' phase relationship is again restored, by wiring the
speakon connector 'wrongly'.

I have puzzled over this, and can think of no good reason for doing it
that
way. I did wonder at one point if it was anything to do with being able to
make the amps bridgeable, but I downloaded a copy of the user manual, and
there is a dire warning paragraph towards the end, specifically saying
that
under no circumstances should any attempt be made to operate the amps in
bridge. It also warns against making any connections between the two amps
or
any speakers connected to them, although in actual fact, the 'ground' side
of each output is truly ground, and is common to both amps, the whole
(dual)
power supply, and all metalwork.

What am I missing here ? Not of any real consequence, as the unit is
mended
and back in its original condition, but for academic interest, it would be
good to understand the designer's thinking.

Arfa

Antiphase amps discussed here a year or so ago re Yamaha Stagepas and/or
another amp not A&H, I forget the make. I think ther concensus view was to
balance up power draw on ps + and - rails for deep near impulse audio output

 

[Phil Allison]

"Arfa Daily"

What am I missing here ? Not of any real consequence, as the unit is
mended and back in its original condition, but for academic interest, it
would be good to understand the designer's thinking.

** The idea is sometimes known as the "OOPS" system - for Out Of Phase
Stereo.

It balances the load on the common PSU at any point in time, reducing
voltage drop and electro cap ripple voltage.

Bob Carver used the idea in his famous M400a " Carver Cube" amp, et alia.

http://www.oaktreevintage.com/web_photos/Stereo_Amps_Pre/Carver_M-400A_Stereo_Amp_web.jpg

http://www.doebbe.com/hifi/pics/Carver_M400_ret_3s.jpg


Fancy A&H picking up on it .....




.... Phil

 

[N_Cook]

Yamaha EMX5016 mixer amp and Stagepas range and Lab Gruppen amps use
antiphase/ out of phase arangement

 

[Mark Zacharias]

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:Xc0uq.25576$gy7.110@newsfe28.ams2...

Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has two
completely separate and identical power amps, one of which was running
very hot very quickly. No schematics, but problem was quickly traced to an
open circuit resistor, by comparison ohms checks between the two amps. In
order to replace the resistor, it was necessary to remove the offending
("B") power amp to get to the board underside. The power rails and ribbon
from the mixer desk section were on removable connectors, but the 4 pole
speakon connector was bolted to the rear panel and the wires were soldered
directly to the board. When I came to unsolder these wires from the
socket, I noticed that the "B" channel one was wired the opposite way
round to the "A" channel one. I noted this down to check on later.

When the amp was tested ok, I bolted it back in, and reconnected the
speakon socket the same as it was when I unsoldered it. The wires lay
exactly where they originally were, and looked absolutely
factory-original, even down to the one which went to the uppermost
connector tag, having a rubber sleeve on it, exactly the same as on the
"A" channel. Except the two channels were definitely wired arse-about-face
to one another. On both channels' speakons, the "1-" and "2-" pins were
linked, as were "1+" and "2+". But on channel "A" the ground wire was
connected to the "+" pair, and on the "B" channel, the ground was on the
"-" pair.

I fired it up again, and put a sine wave in, then hooked up two channels
of my 'scope, one to the "A" output and one to the "B", both with the same
polarity, and was surprised to find that the two signals were completely
anti-phase, which meant that with the reversed connections to the speakon
sockets, they would be back in phase again to the outside world.

I then went back to the power amps' front ends near where the ribbon
connector was, and found the same thing. At the same point on each amp,
the signals were in opposite phase to one another. I then spotted three
little 3 pin header blocks, with one pin pair on each designated "B", and
the other pair "A" and a little 1 or maybe an I next to the "A" in a sort
of 'to the power of' position. On the B amp, these three jumpers were set
to "B", and on the A amp they were set to "A", so clearly, these jumpers
reverse the phase of the signal coming in from the desk section. The two
amps then process the signals in antiphase to one another right to the
final output, where the 'correct' phase relationship is again restored, by
wiring the speakon connector 'wrongly'.

I have puzzled over this, and can think of no good reason for doing it
that way. I did wonder at one point if it was anything to do with being
able to make the amps bridgeable, but I downloaded a copy of the user
manual, and there is a dire warning paragraph towards the end,
specifically saying that under no circumstances should any attempt be made
to operate the amps in bridge. It also warns against making any
connections between the two amps or any speakers connected to them,
although in actual fact, the 'ground' side of each output is truly ground,
and is common to both amps, the whole (dual) power supply, and all
metalwork.

What am I missing here ? Not of any real consequence, as the unit is
mended and back in its original condition, but for academic interest, it
would be good to understand the designer's thinking.

Arfa

Carver has also done this on some models. Not so good when someone tries to
demonstrate through a common grounded speaker switcher...

Mark Z.

 

[N_Cook]

Mark Zacharias <mark_zacharias@labolgcbs.net> wrote in message
news:4eb918cc$0$28491$a8266bb1@newsreader.readnews.com...

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:Xc0uq.25576$gy7.110@newsfe28.ams2...
Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has
two
completely separate and identical power amps, one of which was running
very hot very quickly. No schematics, but problem was quickly traced to
an
open circuit resistor, by comparison ohms checks between the two amps.
In
order to replace the resistor, it was necessary to remove the offending
("B") power amp to get to the board underside. The power rails and
ribbon
from the mixer desk section were on removable connectors, but the 4 pole
speakon connector was bolted to the rear panel and the wires were
soldered
directly to the board. When I came to unsolder these wires from the
socket, I noticed that the "B" channel one was wired the opposite way
round to the "A" channel one. I noted this down to check on later.

When the amp was tested ok, I bolted it back in, and reconnected the
speakon socket the same as it was when I unsoldered it. The wires lay
exactly where they originally were, and looked absolutely
factory-original, even down to the one which went to the uppermost
connector tag, having a rubber sleeve on it, exactly the same as on the
"A" channel. Except the two channels were definitely wired
arse-about-face
to one another. On both channels' speakons, the "1-" and "2-" pins were
linked, as were "1+" and "2+". But on channel "A" the ground wire was
connected to the "+" pair, and on the "B" channel, the ground was on the
"-" pair.

I fired it up again, and put a sine wave in, then hooked up two channels
of my 'scope, one to the "A" output and one to the "B", both with the
same
polarity, and was surprised to find that the two signals were completely
anti-phase, which meant that with the reversed connections to the
speakon
sockets, they would be back in phase again to the outside world.

I then went back to the power amps' front ends near where the ribbon
connector was, and found the same thing. At the same point on each amp,
the signals were in opposite phase to one another. I then spotted three
little 3 pin header blocks, with one pin pair on each designated "B",
and
the other pair "A" and a little 1 or maybe an I next to the "A" in a
sort
of 'to the power of' position. On the B amp, these three jumpers were
set
to "B", and on the A amp they were set to "A", so clearly, these jumpers
reverse the phase of the signal coming in from the desk section. The two
amps then process the signals in antiphase to one another right to the
final output, where the 'correct' phase relationship is again restored,
by
wiring the speakon connector 'wrongly'.

I have puzzled over this, and can think of no good reason for doing it
that way. I did wonder at one point if it was anything to do with being
able to make the amps bridgeable, but I downloaded a copy of the user
manual, and there is a dire warning paragraph towards the end,
specifically saying that under no circumstances should any attempt be
made
to operate the amps in bridge. It also warns against making any
connections between the two amps or any speakers connected to them,
although in actual fact, the 'ground' side of each output is truly
ground,
and is common to both amps, the whole (dual) power supply, and all
metalwork.

What am I missing here ? Not of any real consequence, as the unit is
mended and back in its original condition, but for academic interest, it
would be good to understand the designer's thinking.

Arfa

Carver has also done this on some models. Not so good when someone tries
to
demonstrate through a common grounded speaker switcher...

Mark Z.

Someone last week asked my advice on bridging high power amps. My advise was
don't - bridge inputs if you must, thinking that any fault tends to lead to
spectacular failure all round, I'd forgotton about this ,antiphase, reason
for not bridging

 

[Baron]

Hi Arfa,

Arfa Daily Inscribed thus:

Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has
two completely separate and identical power amps, one of which was
running very hot very quickly. No schematics, but problem was quickly
traced to an open circuit resistor, by comparison ohms checks between
the two amps. In order to replace the resistor, it was necessary to
remove the offending ("B") power amp to get to the board underside.
The power rails and ribbon from the mixer desk section were on
removable connectors, but the 4 pole speakon connector was bolted to
the rear panel and the wires were soldered directly to the board. When
I came to unsolder these wires from the socket, I noticed that the "B"
channel one was wired the opposite way round to the "A" channel one. I
noted this down to check on later.

When the amp was tested ok, I bolted it back in, and reconnected the
speakon socket the same as it was when I unsoldered it. The wires lay
exactly where they originally were, and looked absolutely
factory-original, even down to the one which went to the uppermost
connector tag, having a rubber sleeve on it, exactly the same as on
the "A" channel. Except the two channels were definitely wired
arse-about-face to one another. On both channels' speakons, the "1-"
and "2-" pins were linked, as were "1+" and "2+". But on channel "A"
the ground wire was connected to the "+" pair, and on the "B" channel,
the ground was on the "-" pair.

I fired it up again, and put a sine wave in, then hooked up two
channels of my 'scope, one to the "A" output and one to the "B", both
with the same polarity, and was surprised to find that the two signals
were completely anti-phase, which meant that with the reversed
connections to the speakon sockets, they would be back in phase again
to the outside world.

I then went back to the power amps' front ends near where the ribbon
connector was, and found the same thing. At the same point on each
amp, the signals were in opposite phase to one another. I then spotted
three little 3 pin header blocks, with one pin pair on each designated
"B", and the other pair "A" and a little 1 or maybe an I next to the
"A" in a sort of 'to the power of' position. On the B amp, these three
jumpers were set to "B", and on the A amp they were set to "A", so
clearly, these jumpers reverse the phase of the signal coming in from
the desk section. The two amps then process the signals in antiphase
to one another right to the final output, where the 'correct' phase
relationship is again restored, by wiring the speakon connector
'wrongly'.

I have puzzled over this, and can think of no good reason for doing it
that way. I did wonder at one point if it was anything to do with
being able to make the amps bridgeable, but I downloaded a copy of the
user manual, and there is a dire warning paragraph towards the end,
specifically saying that under no circumstances should any attempt be
made to operate the amps in bridge. It also warns against making any
connections between the two amps or any speakers connected to them,
although in actual fact, the 'ground' side of each output is truly
ground, and is common to both amps, the whole (dual) power supply, and
all metalwork.

What am I missing here ? Not of any real consequence, as the unit is
mended and back in its original condition, but for academic interest,
it would be good to understand the designer's thinking.

Arfa

I'll bet that your first suspicion is right and the amps have or are
being used in bridge configuration in some other gear.

--
Best Regards:
Baron.

 

[Dave Platt]

In article <j9c8ni$qfq$1@dont-email.me>, Baron <baron@linuxmaniac.net> wrote:

What am I missing here ? Not of any real consequence, as the unit is
mended and back in its original condition, but for academic interest,
it would be good to understand the designer's thinking.

Arfa

I'll bet that your first suspicion is right and the amps have or are
being used in bridge configuration in some other gear.

There's another possibility, which I saw mentioned a few years ago.

By configuring the amp so that the two channels are operating in
opposition to one another, you may increase the amp's ability to
handle short periods of very high output (brief peaks) slightly.

The reasoning is this: most of the time, high-amplitude signals are
going to be in phase between the two channels. If the two channels
are operating with the same polarity, they're both going to be drawing
on the same power-supply rail during these peaks (i.e. both will be +
or both will - at any given moment). This sudden load will tend to
draw down the power-supply bypass cap for that rail, and/or will
result in a large amount of current flowing through one side of the
power supply rectifier chain. The other rail's supply capacitor won't
have any load on it at that moment, and its rectifier won't be
carrying much if any current.

On the other hand, if you invert the audio signal coming into one
channel, and then wire up the output of that channel to the speakers
"backwards", you end up with a situation in which the two channels are
usually drawing power from _opposite_ power-supply rails. This will
decrease the worst-case sag in the rail voltages, and also tend to
reduce and even out the current flow in the rectifiers (and reduce
voltage drop and heating slightly).

As a result, an amp which was designed with a "not-so-stiff" power
supply system, may be able to deliver a slightly higher amount of peak
power before it begins to clip and distort.

--
Dave Platt <dplatt@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

 

[Baron]

Dave Platt Inscribed thus:

In article <j9c8ni$qfq$1@dont-email.me>, Baron
baron@linuxmaniac.net> wrote:

What am I missing here ? Not of any real consequence, as the unit is
mended and back in its original condition, but for academic
interest, it would be good to understand the designer's thinking.

Arfa

I'll bet that your first suspicion is right and the amps have or are
being used in bridge configuration in some other gear.

There's another possibility, which I saw mentioned a few years ago.

By configuring the amp so that the two channels are operating in
opposition to one another, you may increase the amp's ability to
handle short periods of very high output (brief peaks) slightly.

The reasoning is this: most of the time, high-amplitude signals are
going to be in phase between the two channels. If the two channels
are operating with the same polarity, they're both going to be drawing
on the same power-supply rail during these peaks (i.e. both will be +
or both will - at any given moment). This sudden load will tend to
draw down the power-supply bypass cap for that rail, and/or will
result in a large amount of current flowing through one side of the
power supply rectifier chain. The other rail's supply capacitor won't
have any load on it at that moment, and its rectifier won't be
carrying much if any current.

On the other hand, if you invert the audio signal coming into one
channel, and then wire up the output of that channel to the speakers
"backwards", you end up with a situation in which the two channels are
usually drawing power from _opposite_ power-supply rails. This will
decrease the worst-case sag in the rail voltages, and also tend to
reduce and even out the current flow in the rectifiers (and reduce
voltage drop and heating slightly).

As a result, an amp which was designed with a "not-so-stiff" power
supply system, may be able to deliver a slightly higher amount of peak
power before it begins to clip and distort.

Interesting analysis ! Though I thought Arfa said "Dual" power
supplies. Unless he meant dual rails from a common transformer.

--
Best Regards:
Baron.

 

[Phil Allison]

"Arfa Daily"

I fired it up again, and put a sine wave in, then hooked up two channels
of my 'scope, one to the "A" output and one to the "B", both with the same
polarity, and was surprised to find that the two signals were completely
anti-phase,

** When servicing audio, knowing the polarity of the test signal and if
there is significant phase shift is important. A scope triggering off the
incoming wave will it not reveal this in single channel mode - so AF used
both channels.

A much better way is to use the External Synch input on your scope and link
it to a fixed output on your bench audio oscillator ( create on if you have
to). This way, you see instantly if the signal's phase has reversed or has
a large phase shift.

Also, triggering will be rock steady with nearly any kind of distorted,
noisy or contaminated signal on the scope screen.

Try it out for a day or two - you will never go back.


.... Phil

 

[Arfa Daily]

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:GaCdnTdypPvqMSXTnZ2dnUVZ_o6dnZ2d@earthlink.com...

Arfa Daily wrote:

Had an Allen ? Heath PA20-CP 2 x 500 watt powered mixer in today. Has two
completely separate and identical power amps, one of which was running
very
hot very quickly. No schematics, but problem was quickly traced to an
open
circuit resistor, by comparison ohms checks between the two amps. In
order
to replace the resistor, it was necessary to remove the offending ("B")
power amp to get to the board underside. The power rails and ribbon from
the
mixer desk section were on removable connectors, but the 4 pole speakon
connector was bolted to the rear panel and the wires were soldered
directly
to the board. When I came to unsolder these wires from the socket, I
noticed
that the "B" channel one was wired the opposite way round to the "A"
channel
one. I noted this down to check on later.

When the amp was tested ok, I bolted it back in, and reconnected the
speakon
socket the same as it was when I unsoldered it. The wires lay exactly
where
they originally were, and looked absolutely factory-original, even down
to
the one which went to the uppermost connector tag, having a rubber sleeve
on
it, exactly the same as on the "A" channel. Except the two channels were
definitely wired arse-about-face to one another. On both channels'
speakons,
the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel
"A" the ground wire was connected to the "+" pair, and on the "B"
channel,
the ground was on the "-" pair.

I fired it up again, and put a sine wave in, then hooked up two channels
of
my 'scope, one to the "A" output and one to the "B", both with the same
polarity, and was surprised to find that the two signals were completely
anti-phase, which meant that with the reversed connections to the speakon
sockets, they would be back in phase again to the outside world.

I then went back to the power amps' front ends near where the ribbon
connector was, and found the same thing. At the same point on each amp,
the
signals were in opposite phase to one another. I then spotted three
little 3
pin header blocks, with one pin pair on each designated "B", and the
other
pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to
the
power of' position. On the B amp, these three jumpers were set to "B",
and
on the A amp they were set to "A", so clearly, these jumpers reverse the
phase of the signal coming in from the desk section. The two amps then
process the signals in antiphase to one another right to the final
output,
where the 'correct' phase relationship is again restored, by wiring the
speakon connector 'wrongly'.

I have puzzled over this, and can think of no good reason for doing it
that
way. I did wonder at one point if it was anything to do with being able
to
make the amps bridgeable, but I downloaded a copy of the user manual, and
there is a dire warning paragraph towards the end, specifically saying
that
under no circumstances should any attempt be made to operate the amps in
bridge. It also warns against making any connections between the two amps
or
any speakers connected to them, although in actual fact, the 'ground'
side
of each output is truly ground, and is common to both amps, the whole
(dual)
power supply, and all metalwork.

What am I missing here ? Not of any real consequence, as the unit is
mended
and back in its original condition, but for academic interest, it would
be
good to understand the designer's thinking.


If both amps are fed the same signal, it reduces the maximum current
requirements for both the + and - rails. That also lowers the current
in the system ground, reducing crosstalk & possible ground loops. This
is done in some RF circuits, as well.


--
You can't have a sense of humor, if you have no sense.

Thanks Michael. Good thoughts.

Arfa

 

[Arfa Daily]

"Cydrome Leader" <presence@MUNGEpanix.com> wrote in message
news:j9aahv$k3m$1@reader1.panix.com...

Arfa Daily <arfa.daily@ntlworld.com> wrote:
Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has two
completely separate and identical power amps, one of which was running
very
hot very quickly. No schematics, but problem was quickly traced to an
open
circuit resistor, by comparison ohms checks between the two amps. In
order
to replace the resistor, it was necessary to remove the offending ("B")
power amp to get to the board underside. The power rails and ribbon from
the
mixer desk section were on removable connectors, but the 4 pole speakon
connector was bolted to the rear panel and the wires were soldered
directly
to the board. When I came to unsolder these wires from the socket, I
noticed
that the "B" channel one was wired the opposite way round to the "A"
channel
one. I noted this down to check on later.

When the amp was tested ok, I bolted it back in, and reconnected the
speakon
socket the same as it was when I unsoldered it. The wires lay exactly
where
they originally were, and looked absolutely factory-original, even down
to
the one which went to the uppermost connector tag, having a rubber sleeve
on
it, exactly the same as on the "A" channel. Except the two channels were
definitely wired arse-about-face to one another. On both channels'
speakons,
the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel
"A" the ground wire was connected to the "+" pair, and on the "B"
channel,
the ground was on the "-" pair.

I fired it up again, and put a sine wave in, then hooked up two channels
of
my 'scope, one to the "A" output and one to the "B", both with the same
polarity, and was surprised to find that the two signals were completely
anti-phase, which meant that with the reversed connections to the speakon
sockets, they would be back in phase again to the outside world.

I then went back to the power amps' front ends near where the ribbon
connector was, and found the same thing. At the same point on each amp,
the
signals were in opposite phase to one another. I then spotted three
little 3
pin header blocks, with one pin pair on each designated "B", and the
other
pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to
the
power of' position. On the B amp, these three jumpers were set to "B",
and
on the A amp they were set to "A", so clearly, these jumpers reverse the
phase of the signal coming in from the desk section. The two amps then
process the signals in antiphase to one another right to the final
output,
where the 'correct' phase relationship is again restored, by wiring the
speakon connector 'wrongly'.

I have puzzled over this, and can think of no good reason for doing it
that
way. I did wonder at one point if it was anything to do with being able
to
make the amps bridgeable, but I downloaded a copy of the user manual, and
there is a dire warning paragraph towards the end, specifically saying
that
under no circumstances should any attempt be made to operate the amps in
bridge. It also warns against making any connections between the two amps
or
any speakers connected to them, although in actual fact, the 'ground'
side
of each output is truly ground, and is common to both amps, the whole
(dual)
power supply, and all metalwork.

What am I missing here ? Not of any real consequence, as the unit is
mended
and back in its original condition, but for academic interest, it would
be
good to understand the designer's thinking.

Arfa


are these amps run from one power supply?

Sort of. It's one bloody great torroidal tranny, for sure, but it looked as
though there was enough filter caps on the board for it to be otherwise two
independant supplies. Certainly, two sets of + / - / gnd cables came out of
the psu for the two amps, but I must admit that I did not look at its design
closely enough to see if there was two sets of secondary windings, and two
rectifiers. The ground was certainly common to both amps, and one leg of the
output on each amp was that same ground.

Arfa

 

[Arfa Daily]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:9hse0iFjllU1@mid.individual.net...

"Arfa Daily"

What am I missing here ? Not of any real consequence, as the unit is
mended and back in its original condition, but for academic interest, it
would be good to understand the designer's thinking.


** The idea is sometimes known as the "OOPS" system - for Out Of Phase
Stereo.

It balances the load on the common PSU at any point in time, reducing
voltage drop and electro cap ripple voltage.

Bob Carver used the idea in his famous M400a " Carver Cube" amp, et alia.

http://www.oaktreevintage.com/web_photos/Stereo_Amps_Pre/Carver_M-400A_Stereo_Amp_web.jpg

http://www.doebbe.com/hifi/pics/Carver_M400_ret_3s.jpg


Fancy A&H picking up on it .....




... Phil

Thanks Phil. That seems to be the concensus that's beginning to form here.

Arfa

 

[Arfa Daily]

"Mark Zacharias" <mark_zacharias@labolgcbs.net> wrote in message
news:4eb918cc$0$28491$a8266bb1@newsreader.readnews.com...

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:Xc0uq.25576$gy7.110@newsfe28.ams2...
Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has two
completely separate and identical power amps, one of which was running
very hot very quickly. No schematics, but problem was quickly traced to
an open circuit resistor, by comparison ohms checks between the two amps.
In order to replace the resistor, it was necessary to remove the
offending ("B") power amp to get to the board underside. The power rails
and ribbon from the mixer desk section were on removable connectors, but
the 4 pole speakon connector was bolted to the rear panel and the wires
were soldered directly to the board. When I came to unsolder these wires
from the socket, I noticed that the "B" channel one was wired the
opposite way round to the "A" channel one. I noted this down to check on
later.

When the amp was tested ok, I bolted it back in, and reconnected the
speakon socket the same as it was when I unsoldered it. The wires lay
exactly where they originally were, and looked absolutely
factory-original, even down to the one which went to the uppermost
connector tag, having a rubber sleeve on it, exactly the same as on the
"A" channel. Except the two channels were definitely wired
arse-about-face to one another. On both channels' speakons, the "1-" and
"2-" pins were linked, as were "1+" and "2+". But on channel "A" the
ground wire was connected to the "+" pair, and on the "B" channel, the
ground was on the "-" pair.

I fired it up again, and put a sine wave in, then hooked up two channels
of my 'scope, one to the "A" output and one to the "B", both with the
same polarity, and was surprised to find that the two signals were
completely anti-phase, which meant that with the reversed connections to
the speakon sockets, they would be back in phase again to the outside
world.

I then went back to the power amps' front ends near where the ribbon
connector was, and found the same thing. At the same point on each amp,
the signals were in opposite phase to one another. I then spotted three
little 3 pin header blocks, with one pin pair on each designated "B", and
the other pair "A" and a little 1 or maybe an I next to the "A" in a sort
of 'to the power of' position. On the B amp, these three jumpers were set
to "B", and on the A amp they were set to "A", so clearly, these jumpers
reverse the phase of the signal coming in from the desk section. The two
amps then process the signals in antiphase to one another right to the
final output, where the 'correct' phase relationship is again restored,
by wiring the speakon connector 'wrongly'.

I have puzzled over this, and can think of no good reason for doing it
that way. I did wonder at one point if it was anything to do with being
able to make the amps bridgeable, but I downloaded a copy of the user
manual, and there is a dire warning paragraph towards the end,
specifically saying that under no circumstances should any attempt be
made to operate the amps in bridge. It also warns against making any
connections between the two amps or any speakers connected to them,
although in actual fact, the 'ground' side of each output is truly
ground, and is common to both amps, the whole (dual) power supply, and
all metalwork.

What am I missing here ? Not of any real consequence, as the unit is
mended and back in its original condition, but for academic interest, it
would be good to understand the designer's thinking.

Arfa

Carver has also done this on some models. Not so good when someone tries
to demonstrate through a common grounded speaker switcher...

Mark Z.

No, I guess it wouldn't be ... :-\

Arfa

 

[Arfa Daily]

"Dave Platt" <dplatt@radagast.org> wrote in message
news:8enoo8-sf1.ln1@radagast.org...

In article <j9c8ni$qfq$1@dont-email.me>, Baron <baron@linuxmaniac.net
wrote:

What am I missing here ? Not of any real consequence, as the unit is
mended and back in its original condition, but for academic interest,
it would be good to understand the designer's thinking.

Arfa

I'll bet that your first suspicion is right and the amps have or are
being used in bridge configuration in some other gear.

There's another possibility, which I saw mentioned a few years ago.

By configuring the amp so that the two channels are operating in
opposition to one another, you may increase the amp's ability to
handle short periods of very high output (brief peaks) slightly.

The reasoning is this: most of the time, high-amplitude signals are
going to be in phase between the two channels. If the two channels
are operating with the same polarity, they're both going to be drawing
on the same power-supply rail during these peaks (i.e. both will be +
or both will - at any given moment). This sudden load will tend to
draw down the power-supply bypass cap for that rail, and/or will
result in a large amount of current flowing through one side of the
power supply rectifier chain. The other rail's supply capacitor won't
have any load on it at that moment, and its rectifier won't be
carrying much if any current.

On the other hand, if you invert the audio signal coming into one
channel, and then wire up the output of that channel to the speakers
"backwards", you end up with a situation in which the two channels are
usually drawing power from _opposite_ power-supply rails. This will
decrease the worst-case sag in the rail voltages, and also tend to
reduce and even out the current flow in the rectifiers (and reduce
voltage drop and heating slightly).

As a result, an amp which was designed with a "not-so-stiff" power
supply system, may be able to deliver a slightly higher amount of peak
power before it begins to clip and distort.

--
Dave Platt

Thanks Dave. This seems to be what everyone is thinking, and makes a lot of
sense when you stand back and think about it

Arfa

 

[Arfa Daily]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:9htrqeFo4sU1@mid.individual.net...

"Arfa Daily"


I fired it up again, and put a sine wave in, then hooked up two channels
of my 'scope, one to the "A" output and one to the "B", both with the
same polarity, and was surprised to find that the two signals were
completely anti-phase,


** When servicing audio, knowing the polarity of the test signal and if
there is significant phase shift is important. A scope triggering off the
incoming wave will it not reveal this in single channel mode - so AF used
both channels.

A much better way is to use the External Synch input on your scope and
link it to a fixed output on your bench audio oscillator ( create on if
you have to). This way, you see instantly if the signal's phase has
reversed or has a large phase shift.

Also, triggering will be rock steady with nearly any kind of distorted,
noisy or contaminated signal on the scope screen.

Try it out for a day or two - you will never go back.


... Phil

Interesting. Are you squaring up the generator's sine output, to give a good
sharp transition for syncing the scope ? I was looking at the phase
reversals on this particular unit, by using 2 of the channels on the scope,
and using internal auto trigger derived from CH 1. The sync circuitry on
this Hitachi scope is very good, and will produce a perfectly stable trigger
point from virtually any waveform, no matter how complex or scruffy, but I
can see where it would be useful to be able to have a fixed known trigger
point, and from there, be able to look at relative phasing between amps, and
phase shift within an amp, using a single scope channel.

Arfa

 

[Mark Zacharias]

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:j9b9kd$su9$1@dont-email.me...

Mark Zacharias <mark_zacharias@labolgcbs.net> wrote in message
news:4eb918cc$0$28491$a8266bb1@newsreader.readnews.com...
"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:Xc0uq.25576$gy7.110@newsfe28.ams2...
Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has
two
completely separate and identical power amps, one of which was running
very hot very quickly. No schematics, but problem was quickly traced to
an
open circuit resistor, by comparison ohms checks between the two amps.
In
order to replace the resistor, it was necessary to remove the offending
("B") power amp to get to the board underside. The power rails and
ribbon
from the mixer desk section were on removable connectors, but the 4
pole
speakon connector was bolted to the rear panel and the wires were
soldered
directly to the board. When I came to unsolder these wires from the
socket, I noticed that the "B" channel one was wired the opposite way
round to the "A" channel one. I noted this down to check on later.

When the amp was tested ok, I bolted it back in, and reconnected the
speakon socket the same as it was when I unsoldered it. The wires lay
exactly where they originally were, and looked absolutely
factory-original, even down to the one which went to the uppermost
connector tag, having a rubber sleeve on it, exactly the same as on the
"A" channel. Except the two channels were definitely wired
arse-about-face
to one another. On both channels' speakons, the "1-" and "2-" pins were
linked, as were "1+" and "2+". But on channel "A" the ground wire was
connected to the "+" pair, and on the "B" channel, the ground was on
the
"-" pair.

I fired it up again, and put a sine wave in, then hooked up two
channels
of my 'scope, one to the "A" output and one to the "B", both with the
same
polarity, and was surprised to find that the two signals were
completely
anti-phase, which meant that with the reversed connections to the
speakon
sockets, they would be back in phase again to the outside world.

I then went back to the power amps' front ends near where the ribbon
connector was, and found the same thing. At the same point on each amp,
the signals were in opposite phase to one another. I then spotted three
little 3 pin header blocks, with one pin pair on each designated "B",
and
the other pair "A" and a little 1 or maybe an I next to the "A" in a
sort
of 'to the power of' position. On the B amp, these three jumpers were
set
to "B", and on the A amp they were set to "A", so clearly, these
jumpers
reverse the phase of the signal coming in from the desk section. The
two
amps then process the signals in antiphase to one another right to the
final output, where the 'correct' phase relationship is again restored,
by
wiring the speakon connector 'wrongly'.

I have puzzled over this, and can think of no good reason for doing it
that way. I did wonder at one point if it was anything to do with being
able to make the amps bridgeable, but I downloaded a copy of the user
manual, and there is a dire warning paragraph towards the end,
specifically saying that under no circumstances should any attempt be
made
to operate the amps in bridge. It also warns against making any
connections between the two amps or any speakers connected to them,
although in actual fact, the 'ground' side of each output is truly
ground,
and is common to both amps, the whole (dual) power supply, and all
metalwork.

What am I missing here ? Not of any real consequence, as the unit is
mended and back in its original condition, but for academic interest,
it
would be good to understand the designer's thinking.

Arfa

Carver has also done this on some models. Not so good when someone tries
to
demonstrate through a common grounded speaker switcher...

Mark Z.



Someone last week asked my advice on bridging high power amps. My advise
was
don't - bridge inputs if you must, thinking that any fault tends to lead
to
spectacular failure all round, I'd forgotton about this ,antiphase,
reason
for not bridging

Such amps are naturally adaptable for bridging.

Mark Z.

 

[Phil Allison]

"Arfa Daily"

"Phil Allison"
"Arfa Daily"


I fired it up again, and put a sine wave in, then hooked up two channels
of my 'scope, one to the "A" output and one to the "B", both with the
same polarity, and was surprised to find that the two signals were
completely anti-phase,


** When servicing audio, knowing the polarity of the test signal and if
there is significant phase shift is important. A scope triggering off the
incoming wave will it not reveal this in single channel mode - so AF used
both channels.

A much better way is to use the External Synch input on your scope and
link it to a fixed output on your bench audio oscillator ( create on if
you have to). This way, you see instantly if the signal's phase has
reversed or has a large phase shift.

Also, triggering will be rock steady with nearly any kind of distorted,
noisy or contaminated signal on the scope screen.

Try it out for a day or two - you will never go back.



Interesting. Are you squaring up the generator's sine output, to give a
good sharp transition for syncing the scope ?

** FFS - that processing is inside any decent scope.

Supply it with 0.5volt rms sine wave and you are away.

The sync circuitry on this Hitachi scope is very good, and will produce a
perfectly stable trigger point from virtually any waveform, no matter how
complex or scruffy,

** Stable maybe, phase locked to the audio generator NO.

can see where it would be useful to be able to have a fixed known trigger
point, and from there, be able to look at relative phasing between amps,
and phase shift within an amp, using a single scope channel.

** Most scope have an internal /external switch you can flick anytime.

If you ever deal with tape recorders or tape echos, there is a big
difference with a play back signal.


.... Phil