Bridge mode

[Waddycall]

Recent threads including one about my new power amp have got me curious about bridge mode and it’s perils. I can understand the potential risk of having too much power but get an impression that it’s that power specifically delivered in bridge mode that can cause problems. Is this the case or is it as simple as too much power is bad, bridge mode or otherwise?

Also my old Ashdown RM800 evo II operated permanently in bridge mode with no indication of this other than in the manual (I think). Is it common for class d bass heads to run permanently in bridge mode (I think all the rm evos do)  If so are cabs connected to these amps at more risk than those connected to other amps of similar power?

my new power amp puts 830w into 4ohm. My old RM head  puts 800w into 4 ohms. My 8 ohm cab has a continuous power rating of 1200w. Other than the extra 30w is the power amp more likely to damage my cab than the RM?

id appreciate any comments in thicko speak.

Edited March 8, 2022 by Waddycall

[Dad3353]

There are thousands of PA systems, world-wide, running in bridge mode, many on a 24/7 basis. There's nothing particular about this mode per se that makes it, for any adequate speaker system, different. To a cab, it's just applied tension across its terminals. As for power ratings, the 'real world' audible difference between 800 and 830 (or even 1200...) is negligeable. If there's any risk of damaging cabs, in this, or any other, situation, I would question the matching, or even the necessity, of such wattage. Any decently-efficient cabs would be crushing concrete at those acoustic levels anywhere other than a wide-open space such as an open-air concert. However, if one's music relies on this to be heard, so be it, and accept that damage may occur, either to the rig, or one's ears, or the building, or all of the above. If it's a concern, turn it down or double up on the cabs. 

[Downunderwonder]

Too much available voltage is the problem. With inexperienced users it's like giving a toddler the leash on a rottweiler. Sure the dog is well trained and only does what it's told, but it just isn't a good idea.

 

The environment in which you are 'needing' a metric buttload of power isn't conducive to hearing the cries of dying speakers if you overdo it.

 

Getting all available headroom into play is highly overrated.

 

The last 3dB of volume achievable is only going to be realised with twice as much power as before. 3dB isn't much extra loudness.

 

If it isnt loud enough you need more speakers, or a quieter guitarist.

 

I was reading specs on BB2 this morning. Max output 137dB iirc. With half the max power it's still 134 freaking dB.

[stevie]

For a 12" speaker to produce 137dB, it would have to be able to handle 5,000 watts and have an xmax of 12 inches. Just sayin' 😃

[Waddycall]

Thanks, the bridge mode or not makes no difference makes  sense.

Something else  I can’t get my head round-

power amp manufacturers seem to recommend choosing an amp that’s twice the rated power of the cabs. I guess that makes sense for a fixed venue type installation but if I followed that advice I’d be connecting a 2.4kw amp to my 210!

apparently I also need clean headroom - how much is that?  I need to avoid clipping the amp as that will damage the cab but don’t want so much power I risk overpowering the cab. Where’s the balance?

by the way I’m not having any issues - just curious. My old Hartke Ha350 (250w into my cab) is plenty loud enough - my power amp is bridged to compensate for low input voltage. I’ve no intention of blowing anything up or deafening myself.

 

 

[Bill Fitzmaurice]

This question comes up often enough on my forum that I added an FAQ reply:

When should I bridge? The answer is almost never. Forget about the silly power ratings that manufacturers post for bridged output, that's just advertising piffle aimed at the unwashed masses. Bridging isn't about power, it's about voltage swing. You use it when your amp doesn't have enough voltage swing to drive the speaker to its displacement limit. Nine times out of ten that's because the speaker has a high impedance, say 16 ohms. The tenth time is when your amp is rated at less than a quarter the power output that your speaker is.
If you do bridge when you don't need to the doubled voltage swing quadruples your chances of blowing drivers. Bridging into multiple cabs also can cause the amp to overheat, as bridging typically doubles the amp minimum load impedance, while using multiple cabs lowers the load impedance.

 

Quote

power amp manufacturers seem to recommend choosing an amp that’s twice the rated power of the cabs. I guess that makes sense for a fixed venue type installation but if I followed that advice I’d be connecting a 2.4kw amp to my 210

Not likely. There may be tens with a thermal rating of 600 watts, but I'm not aware of any that will take more than half that before reaching their mechanical limit. Also, power amps are intended for PA use, where plenty of headroom is desirable. That's not necessarily the case with instrument amps.

Edited March 8, 2022 by Bill Fitzmaurice

[Waddycall]

Cool, so this is getting interesting! What’s the difference between power and voltage swing? Is it due to the fact that it’s not a simple dc voltage at a steady current?

[Bill Fitzmaurice]

Voltage swing is what determines cone excursion, which determines volume. Power is what you advertise to convince people to buy your product.

[Dad3353]

9 minutes ago, Waddycall said:

Cool, so this is getting interesting! What’s the difference between power and voltage swing? Is it due to the fact that it’s not a simple dc voltage at a steady current?

 

Just as a simple way of illustrating, take an amp with a voltage to its power stage of 40v. Whatever the currrent, the amp's PSU cannot deliver more volts. To get the watts up, the current must increase, and the o/p transistors may not be capable. In comes 'bridge' mode, where one amp, with the same 40v limit, is bridged with an identical, but phase-inversed, '-40v' amp. The cab 'sees' an 80v swing. That's what bridging achieves; 'double' the power for the same current. OK, it's a simple view, but that's the gist of it. 

Edited March 8, 2022 by Dad3353

[Waddycall]

Ace. So it’s basically the combined push pull on the driver? 

And there is a difference between 800w bridged and 800w un-bridged.

[Dad3353]

6 minutes ago, Waddycall said:

Ace. So it’s basically the combined push pull on the driver? 

And there is a difference between 800w bridged and 800w un-bridged.

 800w bridged and 800w per channel unbridged..? Seems odd to me.

[Waddycall]

1 minute ago, Dad3353 said:

 800w bridged and 800w per channel unbridged..? Seems odd to me.

Sorry what I should have said is that it sounds like there is a difference between using one channel of an amp that gives 800 W per channel and using both channels of a smaller amp bridged at 800 W. 

 

 

[Waddycall]

And what does that mean to users of class d amps like the Ashdown RM that are bridged as standard? Are they more likely to blow speakers than users of single channel analogue amps of the same power rating?

[Dad3353]

44 minutes ago, Waddycall said:

Sorry what I should have said is that it sounds like there is a difference between using one channel of an amp that gives 800 W per channel and using both channels of a smaller amp bridged at 800 W. 

 

A cab connected to one or the other would not react any differently. Tha amps with such a specification would have a different price on them, though, if of the same quality. 

[Waddycall]

That would suggest that if I wanted a an amp at say 300w (to move away slightly from the power concern issues) theres no difference between buying a single channel amp rated at 300w or buying a small two channel amp and bridging it to get the equivalent 300w which seems contradictory to the posts regarding voltage swing or am I missing the point? That would also suggest that anyone using one channel of a two channel power amp would have been better off getting a low power one and bridging it to get the same output.

[Bill Fitzmaurice]

2 hours ago, Dad3353 said:

the same 40v limit, is bridged with an identical, but phase-inversed, '-40v' amp. The cab 'sees' an 80v swing. That's what bridging achieves; 'double' the power for the same current. OK, it's a simple view, but that's the gist of it. 

When you double the voltage swing into a speaker the cone excursion increase is the same as what occurs with four times the power. That's what leads to mechanical damage.

Quote

am I missing the point?

Yes. You're not considering impedance. Engineers seldom look at power, we look at voltage and current, the former with respect to the capability of speakers, the latter with respect to the capability of amps. Power is an attempt to simplify the equation, but in truth it just further muddies things. 

Edited March 8, 2022 by Bill Fitzmaurice

[Waddycall]

8 minutes ago, Bill Fitzmaurice said:

Quote

 

Yes. You're not considering impedance.

What if had said 300w into 8ohms? Am I still missing something relating to impedance?

[Bill Fitzmaurice]

Cone excursion. When impedance is halved excursion is doubled for a given voltage swing. When it's doubled excursion is halved for a given voltage swing. One cannot say x watts will result in y decibels, or that x watts will result in y millimeters of excursion. Those equations do not exist. One can say that x volts into z ohms will result in y decibels, or that x volts into z ohms will result in y millimeters of excursion.

[Waddycall]

Thanks Bill. This is good stuff. I appreciate everyone’s comments on this but I’m still a bit confused. Maybe that’s because I’m going round in circles due to thinking about watts/power. 

So a given cab  at a given frequency can handle a certain amount of voltage swing before it suffers mechanical damage?

Is it feasible then that the two theoretical (one bridged one single channel) 300w rated amps  (however iffy the rating may be) could be able to put the same voltage swing onto the same cab? What I think I’m struggling with is the difference between a voltage swing of equal magnitude/amplitude from a single channel amp and a bridged amp. Does that make sense?

i.e, is voltage swing voltage swing or can it have different characteristics dependant on single channel or bridge mode?

im assuming higher power amps can generally put a higher voltage swing into a given load?

Edited March 8, 2022 by Waddycall

[stevie]

I'm  not surprised you're confused.

[Dad3353]

1 hour ago, Waddycall said:

...im assuming higher power amps can generally put a higher voltage swing into a given load?

 

Take your tablets, like that nice doctor fellow said. Chill. Breath. Earl Grey, or camomile tea may help. Take a walk (not, not on the Wild Side..! D'oh..!).
For the purposes of the interogations you're torturing yourself with, there is no difference, none whatsoever, between what comes from one amp channel or two bridged amp channels. Whatever difference there may be is only of interest (and even then, not much...) to a lab technicien studying high-falluting stuff of purely academic use. Now, take those pills, go for a nice long walk and get back to practicing bass playing, there's a good chap. 

 

...

 

 

[Waddycall]

24 minutes ago, Dad3353 said:

 

Take your tablets, like that nice doctor fellow said. Chill. Breath. Earl Grey, or camomile tea may help. Take a walk (not, not on the Wild Side..! D'oh..!).
For the purposes of the interogations you're torturing yourself with, there is no difference, none whatsoever, between what comes from one amp channel or two bridged amp channels. Whatever difference there may be is only of interest (and even then, not much...) to a lab technicien studying high-falluting stuff of purely academic use. Now, take those pills, go for a nice long walk and get back to practicing bass playing, there's a good chap. 

 

...

 

 

Ace - I’ll go with that until someone comes along and appears to contradict it! No mental torture here just genuine interest 🤪

just seems to be an automatic “don’t put it in bridge mode”  response to power amp stuff which I’m curious about especially as it seems some amps are supplied in bridge mode from the factory.

[Dad3353]

1 minute ago, Waddycall said:

...an automatic “don’t put it in bridge mode”  response to power amp stuff ...

 

That's incomplete; it should be 'Don't put it in 'bridge' mode unless you need to, and you know what you're doing.' Some folks burn their fingers on the doubly-live terminals. Others link to earth one or other of the wires. Some use guitar leads to connect to the cabs. (Don't do any of those things..! ) There are many reasons for not bridging, but if it's needed, and done properly, it's fine. As I mentioned right from the outset, there are hundreds of amps working in that mode all over the globe. It's not an Issue. 

[Waddycall]

Yeah but

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

🤣

[Bill Fitzmaurice]

2 hours ago, Waddycall said:

So a given cab  at a given frequency can handle a certain amount of voltage swing before it suffers mechanical damage?

Yes.

Quote

What I think I’m struggling with is the difference between a voltage swing of equal magnitude/amplitude from a single channel amp and a bridged amp.

There is no difference, a volt is a volt. The question is whether or not you need to bridge to realize enough voltage swing to get full output from the speaker. If you don't then there's nothing to be gained with bridging.