Bridging amps...???

[oldenough]

I have the opportunity to get another Luxman M-120a power amp locally. These amps are 120 wpc and can be bridged for 300 watts. Question:... what are the pro's and con's of bridging. Thanks.

[artto]

To me, the downside is, if you look closely at the performance specifications (if they provide consistent ones for bridged/non-bridged) is that you will usually find that everything "doubles-up". Not only does the power increase (increase as in combine), but the noise, distortion, etc all also summed along with the power. There may other issues such a phase linearity, etc. which may or may not make a difference to you.

The upside of course is more power. But depending on the amplifier's design you may have load restrictions (ie: it won't do full power into 2 ohms or 4 ohms). Once I figured that out I left my old Crown D60's, D150 and PSA2 in stereo. The PSA2 I would bridge when I used it my bands for sound reinforcement since there we simply needed the power, and noise or increased distortion was not objectionable. Plus you can drive that amp right up against the wall into 2 ohm loads. Pretty much nothing shuts it down except covering with blankets at full output (can't cool itself).

[oldenough]

To me, the downside is, if you look closely at the performance specifications (if they provide consistent ones for bridged/non-bridged) is that you will usually find that everything "doubles-up". Not only does the power increase (increase as in combine), but the noise, distortion, etc all also summed along with the power. There may other issues such a phase linearity, etc. which may or may not make a difference to you.

The upside of course is more power. But depending on the amplifier's design you may have load restrictions (ie: it won't do full power into 2 ohms or 4 ohms). Once I figured that out I left my old Crown D60's, D150 and PSA2 in stereo. The PSA2 I would bridge when I used it my bands for sound reinforcement since there we simply needed the power, and noise or increased distortion was not objectionable. Plus you can drive that amp right up against the wall into 2 ohm loads. Pretty much nothing shuts it down except covering with blankets at full output (can't cool itself).

Thanks Artto, so it would seem there are downsides to this.. though I don't see myself running anywhere near full power into my La Scala's, do you think the distortion or noise would actually be a problem that would be discernible in the real world?

[IB Slammin]

I agree with Art about the specifications......

I bridge two Lux M-117's. The manual states: Stereo 200wpc into 8ohms @ 0.003% THD and BTL Bridged 700wpc into 8ohms @ 0.006% THD

I have gone back and forth, back and forth many times. I can not hear any increase in distortion, let alone double. I have done this for a few forum members and my painfully critical golden-ear wife. (If I change a tube, she asks "what is different"?)

What I do hear is exceptional LF control. The mid's and HF are the same to my ear, and a member owning much more expensive power amps was impressed with the mid's.

So take that for what it's worth. Just my opinion, isolated experience, in my room with my gear.

tc

PS: Artto, I don't recall the D150 as being bridged. ??? DC300 yes.

[artto]

To me, the downside is, if you look closely at the performance specifications (if they provide consistent ones for bridged/non-bridged) is that you will usually find that everything "doubles-up". Not only does the power increase (increase as in combine), but the noise, distortion, etc all also summed along with the power. There may other issues such a phase linearity, etc. which may or may not make a difference to you.

The upside of course is more power. But depending on the amplifier's design you may have load restrictions (ie: it won't do full power into 2 ohms or 4 ohms). Once I figured that out I left my old Crown D60's, D150 and PSA2 in stereo. The PSA2 I would bridge when I used it my bands for sound reinforcement since there we simply needed the power, and noise or increased distortion was not objectionable. Plus you can drive that amp right up against the wall into 2 ohm loads. Pretty much nothing shuts it down except covering with blankets at full output (can't cool itself).

Thanks Artto, so it would seem there are downsides to this.. though I don't see myself running anywhere near full power into my La Scala's, do you think the distortion or noise would actually be a problem that would be discernible in the real world?

That sort of depends on your situation and the equipment. In my room for instance, the noise floor is very low ~ pretty much any noise, if present, is audible. The best bridgable amplifiers out there may not present a noise problem. Less expensive ones might. If you don't really need the power ~ less is more IMO. There are actually some very good arguments for using a simple integrated amplifer, provided no "short cuts" were used in the design, which is unfortunate;y the case with most integrateds. I'm not sure if the Luxman you speak of was made during the years when Luxman was Luxman, or after Apline Electronics took over. If they are pre Alpine you're probably alright. But like I said, I'm just using a non-bridged pair of old Crown D60 for everyday use. At one time I was thinking of bridging them and getting a third for the center channel but after I noticed in the manual that the conversion decreased the S/N ratio I changed my mind.

[artto]

PS: Artto, I don't recall the D150 as being bridged. ??? DC300 yes.

Actually, my D150a has a switch (stereo/mono) on the back for bridging the channels. Maybe the original D150 was different, but being professional amps I don't see why not. I thought pretty much all D series amps are bridgable.

[artto]

And as far as using 300w/LaScala........................

Years ago I remember going to a Phase Linear/Carver demo at the local hi-fi store. Carver was introducing a new "low powered" amp of 100w. They had it hooked up to a pair of LaScala. The rep kept turning the gain up & up & up trying to get the little meter light power indicators to light up but it was already so damn loud that when he cut the sound I felt like I was lifted three feet into the air. Everyone was quite stunned at the silence. The rep laughed and said "I guess these weren't quite the right speakers to choose for this demo". The lowest power level lights were barely lighting up. I think some of the difference we sometimes hear when using higher powered amps stems from the way we set the gain settings on the power amp and preamp. I asked Mark Deneen about this one time because I thought I might just be hearing things but he confirmed that can indeed be the case ~ depends on the amps though ~ some combinations may do this more than others. In other words, if you take your preamp, and power amp, and run the preamp gain higher and turn down the power amps to yield a certain level, doing just the opposite ~ lower preamp gain/higher power amp gain ~ you can essentially change the sound quality, in some ways, almost like a tone control.The higher the power of the power amp, the more this can be exaggerated.

[WMcD]

I think it is un necessary. As Artto describes you've got more than enough power.

- - - -

How this works as far as increasing power 4 times is interesting and puzzled me for a while.

- - - -

Suppose we start with two 4 ohm resistors in series as modeling an 8 ohm speaker. When hooked to one amp (call it left channel amp) the diagram is as following assuming a 1 volt output of the single/single amp:

(1 volt plus from left amp) - - - 4 ohms - - - (center junction with 0.5 volts) - - - 4 ohms - - - (ground or 0 volts)

Important here is that the single amp "sees" 8 ohms between its hot and ground. And there is 0.5 volt drop across each 4 ohm resistor.

When we bridge the pair of amps (left and right) we have to invert the signal to one amp (say the right); and of course we're using the power from both amps. And the speaker is wired between the hot of the left and the hot of the right without the ground connection of the amp.

Now the diagram is:

(1 volt plus from left amp) - - - 4 ohms - - - (center junction with 0.0 volts) - - - 4 ohms - - - (1 volt minus from right amp).

In this second diagram there is a 1 volt drop across each resistor (instead of 0.5). We also recognize that the center junction is what I'll call a "virtual ground" it is always at zero volts. Therefore each amp is now "seeing" 4 ohms as a load. That is because the given amp it is working into a 4 ohm resistor connected between its hot and the virtual ground.

We have to remember the equation that Power = V^2 / R. If we reduce R from 8 to 4 while keeping V constant, we've doubled power.

So by bridging we are doing two things: 1) involving a second amp and 2) halving the impedance seen by each individual amp. so we wind up with 4 x the power. This assumes each individual amp can indeed produce the current necessary to drive 4 ohms without the voltage sagging. Some amps it can not --- and that is why bridged power is not quite 4 x that of an individual amp of the left and right pair.

- - - -

As reported in posts above, the distortion when bridged pretty much doubles. That is because amps in general have higher distortion when more current is demanded by lower impedance -- specifically when going from 8 ohms to 4 ohms. My recall is that the many articles by Nelson Pass regarding his very fine amps bear this out.

- - - -

So what about the LaSalla? I suspect is worse because the LS and K-Horn and Belle all drop below 4 ohms in the bass region (IIRC). Therefore a bridged amp (or rather each of the bridged amps) is going to be "seeing" a 2 ohm load. Therefore I'd expect that the distortion is again doubled. This makes distortion 4 x as high as the rating into 8 ohms. I expect most amps don't like the 2 ohm load in any case.

- - - -

I conclude that you're much better off by not bridging. But you can try it and see what you like.

Wm McD

[oldenough]

I think it is un necessary. As Artto describes you've got more than enough power.

- - - -

How this works as far as increasing power 4 times is interesting and puzzled me for a while.

- - - -

Suppose we start with two 4 ohm resistors in series as modeling an 8 ohm speaker. When hooked to one amp (call it left channel amp) the diagram is as following assuming a 1 volt output of the single/single amp:

(1 volt plus from left amp) - - - 4 ohms - - - (center junction with 0.5 volts) - - - 4 ohms - - - (ground or 0 volts)

Important here is that the single amp "sees" 8 ohms between its hot and ground. And there is 0.5 volt drop across each 4 ohm resistor.

When we bridge the pair of amps (left and right) we have to invert the signal to one amp (say the right); and of course we're using the power from both amps. And the speaker is wired between the hot of the left and the hot of the right without the ground connection of the amp.

Now the diagram is:

(1 volt plus from left amp) - - - 4 ohms - - - (center junction with 0.0 volts) - - - 4 ohms - - - (1 volt minus from right amp).

In this second diagram there is a 1 volt drop across each resistor (instead of 0.5). We also recognize that the center junction is what I'll call a "virtual ground" it is always at zero volts. Therefore each amp is now "seeing" 4 ohms as a load. That is because the given amp it is working into a 4 ohm resistor connected between its hot and the virtual ground.

We have to remember the equation that Power = V^2 / R. If we reduce R from 8 to 4 while keeping V constant, we've doubled power.

So by bridging we are doing two things: 1) involving a second amp and 2) halving the impedance seen by each individual amp. so we wind up with 4 x the power. This assumes each individual amp can indeed produce the current necessary to drive 4 ohms without the voltage sagging. Some amps it can not --- and that is why bridged power is not quite 4 x that of an individual amp of the left and right pair.

- - - -

As reported in posts above, the distortion when bridged pretty much doubles. That is because amps in general have higher distortion when more current is demanded by lower impedance -- specifically when going from 8 ohms to 4 ohms. My recall is that the many articles by Nelson Pass regarding his very fine amps bear this out.

- - - -

So what about the LaSalla? I suspect is worse because the LS and K-Horn and Belle all drop below 4 ohms in the bass region (IIRC). Therefore a bridged amp (or rather each of the bridged amps) is going to be "seeing" a 2 ohm load. Therefore I'd expect that the distortion is again doubled. This makes distortion 4 x as high as the rating into 8 ohms. I expect most amps don't like the 2 ohm load in any case.

- - - -

I conclude that you're much better off by not bridging. But you can try it and see what you like.

Wm McD

Thanks Wm McD....I really ain't into the technicalities of this stuff, but I think the last sentence probably sum's it up. When it comes down to it I think 120 watts is enough. Thanks man...