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[dave stojan]

Glad you got a chance to try it first. Something you can do to narrow the difference of before / after is the Dave Rat psuedo-bridge squeeze 'em trick. - Should work a treat on the RMX2450!

Technique review: Set the amplifier for Bridge mode (this sets the two amplfiers within to opposite polarity - i.e. a positive going signal on the input sends one output towards the positive rail while the other output heads for the negative rail). The "trick" is don't run the load (speakers) in bridge mode; plug in the loads almost as you would for stereo operation BUT reverse the polarity on channel 2 (or B, however it's marked). Repeat: hook up across Red to Black on each channel's output NOT across the 2 Reds. Long story short, it lets the big hairy storage capacitors in the power supply go twice as far, typically reducing the "sag" enough to effectively squeeze an extra 20~25% (nearly 1 db!) out of it.

Layman's explanation: Think of the power supply as 2 trampolines (positive and negative) with short legs and the amplifiers themselves as 2 guys bouncing on them. Feeding the same signal to both amplifiers is like having the jumpers joined at the hip, so it's twice the weight landing on a trampoline while the other trampoline has nothing on it. It's easy to imagine that the trampoline skin is going to bottom out as the boys jump higher & higher (remember the short legs I mentioned?). Now if we separate the jumpers and ensure that only 1 is on a trampoline at a time they can both jump higher without bottoming out the trampolines.

Just remember to hook up one of the speakers (customarily channel 2 / B) reverse polarity. Good luck!

[Jess Bruffett]

i gotta clarify that to be sure i am undertanding you because i alway thought i would blow somethingup doing this. turn the amp to bridge mono. then plug one 4 ohm cab into channel A (white wire to red terminal and black to black) and on channel B plug in another 4 ohm cab and this time plug the white wire to the black terminal and the black wire the the red terminal. and operate the amp in all other respects in mono i.e. one input only the channel limiter one channel gain knob and the such?? this wont hurt the amp or the speakers or anything. im not doubting you, just gotta be sure i understand this is the right way before i do this is all. ohh and what will this do to the amount of power the amp is gonna need from the wall?? with all both 2450s in stereo at 4 ohms aside and the 1450's in stereo at 8 ohms a side, i can rune the entire system off one 20 amp circuit with is a nice thought i have to admite, so that is another thing i need to clarify.  also will this work with the channel one and two speakon connectors on the amp?  or do i have to use the binding posts? thanks Dave.

[dave stojan]

Jess Bruffett wrote on Tue, 18 December 2007 12:30

turn the amp to bridge mono.

Good.

Quote:

then plug one 4 ohm cab into channel A (white wire to red terminal and black to black) and

Good.

Quote:

on channel B plug in another 4 ohm cab and this time plug the white wire to the black terminal and the black wire the the red terminal.

Good.

Quote:

and operate the amp in all other respects in mono i.e. one input only the channel limiter one channel gain knob and the such

Righto!

Quote:

this wont hurt the amp or the speakers or anything.

That is correct sir!

Quote:

ohh and what will this do to the amount of power the amp is gonna need from the wall?

Bump it up by a few amperes when yer hittin' hard (i.e. actually using the extra poop - otherwise it's the same). More power out means more power in.

Quote:

also will this work with the channel one and two speakon connectors on the amp?  or do i have to use the binding posts?

As long as you can wire the Speakons to the equivalent binding post connections you're good to go.

[Mike Kivett]

If I recall correctly, didn't Bob Lee say in a post one time that this wouldn't be recommended (or at least would not help) when using QSC amps?  Or switching amps in general?

Ah, found some posts from that thread:

"The Rat method works to squeeze a few extra (and certainly welcome) watts from amps with traditional power supplies. There is no benefit when the method is used with switching power supply amps such as the PL and PLX."

and:

"There is less benefit also if the amp is a "dual mono" type, with a separate power supply for each channel, even if the supplies are conventional, because the reservoirs are separate also.

The reason for the amps with switch-mode supplies not gaining headroom from this techniques is that the primary reservoir is before the power transformer, and it provides energy to both the positive and negative supply rails."

[dave stojan]

Mike Kivett wrote on Tue, 18 December 2007 21:26

If I recall correctly, didn't Bob Lee say in a post one time that this wouldn't be recommended (or at least would not help) when using QSC amps?  Or switching amps in general? Ah, found some posts from that thread: "The Rat method works to squeeze a few extra (and certainly welcome) watts from amps with traditional power supplies. There is no benefit when the method is used with switching power supply amps such as the PL and PLX." and: "There is less benefit also if the amp is a "dual mono" type, with a separate power supply for each channel, even if the supplies are conventional, because the reservoirs are separate also. The reason for the amps with switch-mode supplies not gaining headroom from this techniques is that the primary reservoir is before the power transformer, and it provides energy to both the positive and negative supply rails."

That's correct; the OP is using an RMX2450 - with a conventional style power supply where this technique works.

[Jess Bruffett]

why doesnt everyone with conventional amps do this?? Its awesome, a will admite i dont completely understand how it works because i thought that when in bridge mode the black terminals werent connected to anything so the signal wouldnt ground, but i would have to wrong for this to work and evidentaly it does. i will have a chance today or next week to test this. i am also going to do this with my tops aswell or is it not recommended fro some reason with high/mid cabs(cant imagine why, but im in uncharted territory for me here)

[dave stojan]

Jess Bruffett wrote on Wed, 19 December 2007 12:57

why doesnt everyone with conventional amps do this?? Its awesome, a will admite i dont completely understand how it works because i thought that when in bridge mode the black terminals werent connected to anything so the signal wouldnt ground, but i would have to wrong for this to work and evidentaly it does. i will have a chance today or next week to test this. i am also going to do this with my tops aswell or is it not recommended fro some reason with high/mid cabs(cant imagine why, but im in uncharted territory for me here)

It's a little extra hassle & opens the possibility to brain fart the polarity on a speaker output & many times the little extra just isn't needed - but when you're short on resources it's sure nice to know you can squeeze that silly little 1db more of headroom out of thin air! 1db ain't much but it can make the difference between distortion and high fidelity! I used to do this on a CA9 all the time.

BTW switching modes on an amp doesn't engage or disconnect the black binding posts - or the red ones either for that matter; it simply re-routes the signals going into the power amplifier stages. I see no reason why it wouldn't work for tops as well as subs.