Pete Alcock Posted December 29, 2011 Share Posted December 29, 2011 I have a PA12CP (Allen and Heath desk with a 500+500W amp built in) and some time ago discovered a bizarre feature that the speaker outputs are out of phase with one another (OK, polarity reversed for the pedants). I've always carried a speakon adaptor to correct this whenever I've used it, and vowed to fix it whenever I had time. I've not had it from new, and the desk has obviously been repaired in the past (some screws misisng etc.). My first thought was that whoevere'd mended it had put the spade connectors onto the speakons the wrong way round. Curiously, this isn't the case, and the "+" from each amp, is correctly wired to the output sockets. The signal from the mixer to the power amps underneath comes down a ribbon cable, so there's no opportunity for things to get mixed up there. The only thing is on each power amp module there is a row of 3 links, each with jumpers. These appear to allow each power amp to pick off the signal it needs (A or B) from the ribbon. These could be balanced, and if so, could be a way the poarity may have got reversed. So the question is, does anyone know what the designation of these links is, and how they should be set? Just don't have a service manual you see... Thanks as always, Pete. Link to comment Share on other sites More sharing options...
laolu Posted December 30, 2011 Share Posted December 30, 2011 Is the desk purely stereo or can it be bridged? The blockdiagram (http://www.allen-heath.com/uk/products/pages/productdetails.aspx?CatId=PASeries&ProductId=PA12CP) does not suggest so. Just curious. If it were me, I would keep using the "phase plug" instead of fiddling around with jumpers inside. I'm afraid, I might do something worse. But hat's just lazy me.... :P Happy New Year! Norbert Link to comment Share on other sites More sharing options...
StevenW2 Posted December 30, 2011 Share Posted December 30, 2011 It might seem obvious, but have you tried taking it to your nearest A&H approved service centre? They will have access to the service data on the A&H web site and would be able to swap the jumpers if that is what is needed. That is why they are there! (Note of intetest - we are an Approved Service Centre) Link to comment Share on other sites More sharing options...
Pete Alcock Posted December 30, 2011 Author Share Posted December 30, 2011 Thanks Steven for the offer, Most things analogue, I mend myself, and I did suss out what the jumpers are for, and the problem is not with them. In the end, I ran out of time, and for expediency's sake, just swapped the wires on the speakon output socket. Case closed, but a mystery I shall have to get to the bottom of one day... Pete. Link to comment Share on other sites More sharing options...
Mixermend Posted December 30, 2011 Share Posted December 30, 2011 At some point in the past somebody has clearly taken out the intended reversal to one of the Speakon connectors on Pete's mixer (which does look 'wrong' to be fair) - and actually introduced an incorrect polarity reversal. After the power amplifier volume control in the mixer section - there are a pair of op amps U5A and U5B - which feed the L + R signals onto the ribbon to the power amplifiers - with a polarity reversal. So this reversal has to be taken out on the wiring to the Speakon connector. Some other commercial power amplifier designs use this technique - which is supposed to reduce the load on the power supply, but I would like to prove that this idea does actually have real benefits...... Link to comment Share on other sites More sharing options...
rdelaplaza Posted February 11, 2018 Share Posted February 11, 2018 As Feb 2018... this is a very OLD topic but the Allen & Heat PA12-CP still existing and I have one of them which goes to work 4 or 5 time a month in public venues. I have a concern... in this powered console the power amplifier can be "bridged" to have a mono amplifier doubling the power of the single amps... Will this polarity reversal affect the operation of the amps in "Bridged mono mode" ? Depending on how this is whole unit is factory wired internally this reversal may be actually be "short circuiting" both power amplifiers and making them to work against each other in terms of polarity, resulting in a big cloud of black smoke and a needless death of pair of amps... A good and deep look at the schematics and a low power testing with a multi-meter or a oscilloscope in hand is in order to reveal the truth about this mystery. Am I Wrong or hallucinating ? Robert Link to comment Share on other sites More sharing options...
DrV Posted February 11, 2018 Share Posted February 11, 2018 A few points - first , bridging two amps doubles the output voltage therefore potentially quadrupling the power available into an identical load but usually amps are specified into a higher impedance when bridged so reducing the output back to a safe level. Second, if you inadvertently remove a polarity reversal you are not going to harm anything. The two amps will be driving the speaker terminals in phase with each other so you will simply get no output. The way this type of amp is wired is as follows: The stereo speaker sockets are indeed wired opposite polarity to each other so on one channel the groundy end of the amp goes to -ve and the output end goes to +ve. In the other channel these are reversed. This means that the input to one of the amps has to be driven with reverse polarity to compensate. Why bother? The reason is you then have a separate "Bridged Mono" speaker connector which takes the output terminal of both amps (which are being driven with opposite polarity, remember?). Now you don't need to "switch" the amp into bridge mode, you just need to use the appropriate speaker connector and make sure you are feeding both channels with an identical mono signal. As this type of powered mixer usually has a simple means of driving both channels with the same signal this is straightforward. And it removes the need for a bridge mode switch - marginal decrease in cost, and in complexity for the user at the expense of confusing someone working on the innards for the first time! Clear as mud? Dave Link to comment Share on other sites More sharing options...
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