Bridge mode

[Waddycall]

Right, thanks folks - that wraps it up for me. Also had a look at a few more class d heads. Fender rumbles are also bridged at least the 150, 350 and 800 are.

If I’m looking for a power amp again in the future (which I may be) I’d be tempted to go small and put it in bridge mode. No point in paying for and carrying twice as much amp as you need.

 

[Downunderwonder]

8 hours 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?

Power is voltage x current. If you supply more voltage while the load is the same you get current going up by the same factor. That means power increases by the square of the voltage.

 

Handily summed up as

Power = volts²/resistance when using Watts Volts and Ohms.

 

That's DC maths for resisistors but if you work with the RMS of an A/C voltage it also applies to speaker impedances.

 

[Downunderwonder]

1 hour ago, Waddycall said:

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.

When the question is asked it is always a two channel amp with significant power available from both channels. In most cases there's a cab rated to just over the single channel output. A bad plan brews, they know it's a bad plan, but they ask the question anyway.

[BassmanPaul]

A thing to remember is that if you want to be twice as loud you'll need to increase power by a factor of ten.

 

800W is 800W whether derived from a bridged power amp or from a single channel of a larger power amp. The risk of damage depends on the capabilities of the connected speaker system. 800W into a 300W speaker system will eventually cause damage no matter how careful you are.

[Phil Starr]

Oh, what the hell.

 

First of all don't worry about an amp which is designed to work in bridged mode only.  There's nothing inherently wrong with that, nobody is asking you to redesign the amp and you can't undo or alter the way the designer/manufacturer has gone about making the amp. Just play bass and don't worry about it.

 

If you have a stereo amp which is capable of operating in bridged mode then you do need to think twice about how you use it. Power goes up with the square of the voltage so theoretically bridging which doubles the voltage gives you four times more power through the same speaker. That's theoretically, but in practice the amps power supply and maybe other components won't give or be able to handle the current. Probably the speakers won't either. Not all amps or speakers are created equal so at this point you need to go to the manual and hope the information is available, correct and in language you can understand. If you understand and are confident in what you are doing then all will probably be fine but if you aren't a technical person why go down this route? 

 

If you are trying to understand this genuinely then it isn't complex but you need to know a few things.

 

Electrical Power is Voltage x Current

 

Ohm's Law is that Current is Voltage/Resistance (for speakers resistance is more accurately impedance which is partly frequency dependant but ignore that for now, assume an 8ohm speaker is 8ohms)

 

These two formulae can be joined so that  Power = Voltage Squared/Resistance or Current Squared x Resistance

 

In bridged amps the two amps are wired so that as one amp goes negative the other goes positive so the voltage the speaker sees is double the power supply voltage.

 

Most amps are limited in the amount of current they can supply and run hotter at high currents. This is where the manual might help but probably won't. It get's more complex than this when you go into detail but this is the basics  https://en.wikipedia.org/wiki/Electric_power

Edited March 9, 2022 by Phil Starr

[Waddycall]

Thanks Phil,

it all makes sense. I understand ohms law but some posts have led me to believe that its more complicated than that. I was also unsure if there was additional risks from bridging caused by some kind of phase shifting or something else I might make up but if volts are volts bridged or not then its all good with me.

i enjoy tinkering with gear, experimenting with tone and like to understand the technical aspects hence the original post👍

[agedhorse]

Actually, voltage, current, impedance and power are directly related and can not be separated. Double the voltage into the same nominal impedance and the power quadruples, that's non-negotiable. Power factor, or the phase angle between the voltage and current doesn't really matter because it applies equally to both situations.

 

Power = Voltage squared divided by the nominal impedance. The reason the power quadruples when the voltage doubles is due to the squaring of the voltage. 

 

Bridging carries the risk of one of the two speaker conductors being accidentally connected to ground. Because the output of the amp is not ground referenced, grounding either terminal can damage the amp.

 

Some amps are designed from the ground up to be bridged, these are generally fairly tolerant of real world loads. Some amps bridge as an afterthought, these tend to be more vulnerable.

 

Many class D amps are designed using bridged topology, in part to improve the power supply efficiency when driving inductive loads. This is an important part of some of these amps.

[Phil Starr]

On 09/03/2022 at 18:50, Waddycall said:

Thanks Phil,

it all makes sense. I understand ohms law but some posts have led me to believe that its more complicated than that. I was also unsure if there was additional risks from bridging caused by some kind of phase shifting or something else I might make up but if volts are volts bridged or not then its all good with me.

i enjoy tinkering with gear, experimenting with tone and like to understand the technical aspects hence the original post👍

I have a vague worry about how hard it is to learn this stuff nowadays. Like most of my generation I leaned most of what I know from practically tinkering with stuff and building my own gear from projects in magazines like Practical Wireless and Wireless World. There were also loads of books popularising electronics and DIY was a real hobby with loads of support. All of this stuff is now on the internet but so is a lot of poorly written and incorrect information. To be honest there is way more known with human knowledge doubling every few years. When I started it was all valves! At least BassChat is full of people trying to be helpful