Posted by Dave Rat on January 06, 2004 at 20:35:59:
In Reply to: Fighting for Power posted by Kevin Reimer on January 05, 2004 at 13:00:21:
: Dave, I know a few discussions have come up from the article you wrote a few years back, but my question is this. If you reverse the leads coming out of the board, will that work. That way you only have one wire that is switched and the rest can remain untouched. Will that fry other equipment or not. I wouldn't think it would, but obviously you have quite a bit of experience in this so I was wondering how you pull this off. Thanks in advance.
I am not really sure I understand what you mean by "reverse the leads coming out of the board" but I can try and address that and the "will that fry equipment" part below:
There is a surprisingly vast amount of fear and misunderstanding regarding amps in bridged mode. So let me try and clarify.
1) Typically power amps come in two channel to chassis or commonly known as 'stereo'
2) Typically each channel output has a red or + terminal that has the AC-audio voltage present and a black or - terminal that is grounded. I am not saying that there are not exceptions, I am merely stating that the vast majority and nearly all stereo pro audio amps are configured this way.
3) Typically, when you apply a positive going voltage to the 'in phase' pin of the input connector of channel 1 of an amp, you should see a positive going voltage on the channel 1 output of the amp. The same goes for channel 2 of the amp, positive on the 'in phase pin' = positive on the red output terminal.
4) Since the black output terminals are typically grounded together, they just are zero volts. If you are in doubt about whether you particular amp has grounded black terminals you can either open the amp and look and see if they are shorted together and/or measure the resistance between them and assure that there is a dead short from black to black terminal.
5) Mono-Bridged mode is no magic secret complicated voodoo thing. Typically all that amps do to achieve bridged mono mode is simply to 'send channel 1's input signal, reversed in polarity, to the input of channel 2'.
Now if you remember point #3 above, a positive signal on to the input of channel 1 creates a positive signal on the red output terminal of channel 1. In bridged mode, that stays the same but the amp sends a reversed signal to channel 2. So in bridged mode a positive signal to channel 1 creates a positive signal on channel 1 output and a negative signal on channel 2 output.
Now here is the cool part of bridged mode. The two red terminals both have voltage on them that are exactly opposite. So when the channel 1 output is producing a positive voltage, channel 2 is producing a negative voltage. When you hook your speakers across the two red terminals they see 'twice the voltage' and more power.
The formula to calculate how much more power is Voltage squared, divided by load.
So if your amp puts out 50 volts rms into 8 ohms in stereo mode that would be 50x50=2500 divided by 8 = 312.5 watts for one channel.
If that amp was able to maintain that same voltage output bridged into 8 ohms it would put out 100 volts rms between the red terminals and would put out 100x100=10000 divided by 8 = 1250 watts bridged mono.
And although the #s I list above are in an ideal perfect amp, you can see how much more power and amp tries to produce in bridged mode. This big boost and the lack of understanding of this concept is why people are 'scared' of bridged. They try it without understanding it and they blow up there speakers.
And now the most important aspect regarding the 'Fighting for Power' article. None of the above scary stuff has anything to do with the article.
In that article, I never recommend using an amp in bridged mono mode. I never recommend hooking speakers between the two red terminals. Bridged mono is awesome if used properly but I repeat, 'Fighting for Power' does not describe using an amp in bridged mode!
Fighting for Power describes a method of gaining extra watts out of an amp by wiring chanel 2 in revese polarity and describes a way 'you can save on making a reverse phase adapter by using the bridged switch to reverse polarity on channel two'.
Basically, if you dont feel comfortable using the bridged switch to perform the polarity reversal of channel 2, a simple "y" cable will work just the same. Be sure to do a pin 3-2 swap on the XLR going to channel 2 of the amp.
6) I know of no amp that will sustain damage by puting it in bridged mono mode while a normal load is connected between the red and black terminals, if there is any valid info to the contrary, I would love to see it.
Kevin, I hope that helps, I will gladly explain more and do my best to answer any questions. I am pleased at the fact that this has become such a hotly debated topic and enjoy especially when it sparks the passions in people.
In the posts I have read in that past I have seen many people express many very biased opinions but unfortunately surprisingly few (possibly none) actually go out and take the time to test and prove whether their opinion or criticism has merit.
Happy New Year!
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