Thoughts on this little oddity, anyone ...?

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

Reply to
Arfa Daily
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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.
Reply to
Michael A. Terrell

are these amps run from one power supply?

Reply to
Cydrome Leader

very

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directly

noticed

channel

speakon

where

on

speakons,

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the

3

that

that

or

(dual)

mended

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

Reply to
N_Cook

"Arfa Daily"

** 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.

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

... Phil

Reply to
Phil Allison

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

Reply to
N_Cook

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

Mark Z.

Reply to
Mark Zacharias

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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

Reply to
N_Cook

Hi Arfa,

Arfa Daily Inscribed thus:

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.
Reply to
Baron

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                                    AE6EO
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
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Reply to
Dave Platt

Dave Platt Inscribed thus:

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

--
Best Regards:
                          Baron.
Reply to
Baron

"Arfa Daily"

** 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

Reply to
Phil Allison

Thanks Michael. Good thoughts.

Arfa

Reply to
Arfa Daily

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

Reply to
Arfa Daily

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

Arfa

Reply to
Arfa Daily

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

Arfa

Reply to
Arfa Daily

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

Reply to
Arfa Daily

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

Reply to
Arfa Daily

Such amps are naturally adaptable for bridging.

Mark Z.

Reply to
Mark Zacharias

"Arfa Daily"

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

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

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

** 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

Reply to
Phil Allison

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