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How can I bridge a stereo amp with no "bridge" output

Discussion in 'Amps and Cabs [BG]' started by Golem II, Dec 6, 2002.

  1. Golem II

    Golem II

    Jan 4, 2002
    Macon, GA, USA
    I'm using a Yamaha P2201 poweramp (the manual and all the info on are here: http://www.yamaha.com/ycaservice/mp4model/p2201.htm )

    It puts out 200w at 8 ohms per side, and I need a bit more power than that. The manual offers a fairly complicated explanation on bridging the channels: apparently you have to send the signal to one channel, reverse the polarity of it, and send it to the other, then take a speaker wire from one binding post on each side (one positive, one negative) to connect to your speaker. My cabinet doesn't have binding post inputs, just 1/4" and speakon.

    Do any tech guys here know how I can make this process simpler? If I have to buy a splitter box to reverse the polarity, can anyone recommend a particular brand/model that would work well? Also, could I make a "plain speaker wire to 1/4" cable by just removing the 1/4" from the end of a regular 1/4" speaker cable?
  2. Assuming you're interpreting the manual correctly:

    You don't need a special box, just take a regular interconnect cable and reverse the polarity somewhere. You could resolder the connections on one end for example.

    Most bridgeable amps just have a switch to do the phase reversal though, are you sure yours doesn't?

    Yes you can just cut the end off a 1/4" speaker cable. I prefer to solder the bare wires to keep them from fraying.

    Careful when bridging though, misconnected wires can fry amp.
  3. Are you saying to reverse phase on the input and output. Meaning ( Y ) ing the signal to both channels and reversing the phase of one side? Then using the + from one channel and the - from the other to connect the speakers?
  4. Again, I'm going to use the disclaimer that "if that's what the manual says" I don't know why Yamaha, didn't just put a bridging switch, heck even Carvin puts bridge switches on their amps.

    But here's a link to an explanation of a tri-mode bridge for a car amp. This shows an op-amp.


    I dunno if the op-amp would be required for this Yamaha or not, I think it's there because the Elliott example is summing the left and right inputs so you may have to isolate the input that you are inverting.

    Elliott has some more bridging articles in his Projects section.
  5. Golem II

    Golem II

    Jan 4, 2002
    Macon, GA, USA
    If you click the link I posted above, the manual should be available in Adobe Acrobat format. It recommended using something like a passive direct box for the phase-reversal thing. If anyone could click the link and read the manual to see if they can make more sense out of it than I could, that would definitely help.
    Let me clarify that there's no "bridge mode" or even a bridge output for this amp. I guess Yamaha didn't put one there because there was no demand for it at the time it was built (I think it was made sometime in the mid to late 80's.)
  6. Well, normally I wouldn't read through a 59 page manual when it's not my amp...but what the heck, here's how I interpret the manual. They recommend a splitting transformer which will take a single channel signal and split it into two balanced, impedance matched signals. You won't have to do any special wiring tricks to change polarity because they gave you a switch to flip and reverse the polarity on one channel.

    Then you would connect your speaker to the terminals as shown.

    HOWEVER, after reading the manual, UNLESS you have a 16 ohm speaker DO NOT do this!!!

    The amp appears to be only rated for 16 ohm operation in bridged mode. (The amp is only rated for 8 ohm operation in stereo mode):eek:

    So forget about the bridged mode operation! Sorry!!!:(
  7. Golem II

    Golem II

    Jan 4, 2002
    Macon, GA, USA
    I thought this specification was pretty strange. Wouldn't an 8-ohm bridged load just be equivalent to a 4 ohm load on each side? Aside from generating extra heat and placing more strain on the amp, what harm would it cause? I'd assumed that the 16-ohm rating was just a safety guideline, considering that amp makers seem to have published more conservative specifications in the 80's. Now amp specs often contain 2 ohm ratings, though some consider 2 ohms an unsafe impedance load for any amp. Come to think of it, the P2201 manual doesn't even offer a rating for operating one side at less than 8 ohms. Yamaha's "specs" page offers a 4 ohms rating, but I'm assuming this page was created much more recently.

    Just to play it safe, I'll avoid trying to bridge it without some technical assistance, but are there any factors that would make bridging it into 8 ohms dangerous aside from pushing the amp harder than usual?
  8. It pushes the amp very hard, you would be doubling the current draw from each side...Ohm's Law, V=IR, means if you decrease the impedance in half you'll have to double the current.

    You have two limiting factors: the output transistors, and the power supply (the amp's internal transformer). Both will be required to double their output. Also, heat rejection could be a problem, which cuts into the output of all of the components including the aforementioned output transistors and power transformer.

    Many amps will undersize their power transformer because it is very rare--in most applications--to drive both channels equally hard for a long time, so they can save money by downsizing the transformer. Putting the amp in bridge mode puts an equal strain on both channels, taxing the heck out of the transformer. Also, bass guitar amplification is in fact the "worst-case" scenario that an amp would encounter in real life, being very close to a low-frequency sine wave.

  9. If an amp is designed to handle 4 ohms per channel, then in bridge mode it will handle 8 ohms.

    The P2201 is rated 350W into 4 ohms per channel, so I see no problems here.

    Bass guitar is far from worst case. The signal has a very high peak-to-average ratio, loading the amp with an average of around 10% of the peak power.

    So, with this amp, you'd get 700 watts into 8 ohms, bridge mode, and the amp will deliver an average power of maximum 70 watts, and peak at 700 watts.

    My 2 cents.

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