How do you bridge an amp?

[highfigh]

I did elaborate in post#8. No issue if words like your "effectively...", or Ryan's "the speaker impedance the amp sees is halved. Though it would be better if people just keep it simply by saying the amp's output voltage is doubled, for the same input signal, that would factual, impedance being halved is not factual, but again it is okay if "effectively, the amp sees... etc. are thrown in there.

The rms power thing is just another example of the power of the internet, that can turn something wrong to become acceptable. Someone as knowledgeable as you are surely know there is (or more like was now..) no such thing as rms watt or W rms like there are Vrms and Irms.

I was looking for something in the form of an explanation that described why it may be misleading or feels wrong.

The amplifier's output normally "sees" the negative end of the circuit through the load, but it's not normally referenced to the other channel's output. While it may not be half, it IS different- if you want, maybe you can explain the actual difference, to clarify it for the OP and others and do the same for RMS, because it is such a cloudy area.

I don't know how long you have been involved in audio, but RMS Watts reared its head in the early-'70s (probably around the time when the FTC said it wanted better power ratings). When I was 15 (around that same time), I bought my first stereo and when I saw the power rating expressed in RMS Watts, I asked what RMS was. Rather than define it, the guy just said 'Root Mean Squared' and left it at that. I knew what Root, Mean and Square meant, from my math classes, but it left a lot to interpretation without the rest of the info.

That answer showed the typical level of knowledge about the technical details and that continued relatively unchanged for years, partly because the manufacturers' technical training meetings for dealers were often held by sales reps, who often knew less than the salespeople did. Unfortunately, that remains true now- trying to get factual info from manufacturers is like asking a rock how it came to be. Some of us looked for answers and info, while the "better" salespeople closed more sales as we informed people when they asked questions.

When I ask questions like that, I'm doing it as a way to get clarification because I don't always see definitions and details that could explain something more fully for others.

 

[Good4it]

Just don't consider it a bluepeint for all amps. My take was it seemed a better idea until the more knowledgeable pointed out the foibles....

Basically what you are saying is “ there’s more than one way to bridge an amp “.

 

[lovinthehd]

Basically what you are saying is “ there’s more than one way to bridge an amp “.

More saying there are different amps out there....

 

[PENG]

I was looking for something in the form of an explanation that described why it may be misleading or feels wrong.

The amplifier's output normally "sees" the negative end of the circuit through the load, but it's not normally referenced to the other channel's output. While it may not be half, it IS different- if you want, maybe you can explain the actual difference, to clarify it for the OP and others and do the same for RMS, because it is such a cloudy area.

I don't know how long you have been involved in audio, but RMS Watts reared its head in the early-'70s (probably around the time when the FTC said it wanted better power ratings). When I was 15 (around that same time),I bought my first stereo and when I saw the power rating expressed in RMS Watts, I asked what RMS was. Rather than define it, the guy just said 'Root Mean Squared' and left it at that. I knew what Root, Mean and Square meant, from my math classes, but it left a lot to interpretation without the rest of the info.

That answer showed the typical level of knowledge about the technical details and that continued relatively unchanged for years, partly because the manufacturers' technical training meetings for dealers were often held by sales reps, who often knew less than the salespeople did. Unfortunately, that remains true now- trying to get factual info from manufacturers is like asking a rock how it came to be. Some of us looked for answers and info, while the "better" salespeople closed more sales as we informed people when they asked questions.

When I ask questions like that, I'm doing it as a way to get clarification because I don't always see definitions and details that could explain something more fully for others.

Then I guess I misunderstood your question. Have to go now but will return with maybe a numerical example.
Regarding the rms power term, for one thing, you will not find such term in any physics or EE text books, it is not taught, nor should it be, in colleges and universities. Your reference "guy" is obviously right about RMS being Root Mean Square and you can follow the definition to come up with a real Root Mean Square Power, but it won't be in Watts and will likely have no practical use, therefore meaning.

Simply put, the power formula is (for resistor load) Power in Watt = RMS Volt X RMS Ampere, that's it.

A quick google found someone who seems to hate the term (not me, I am okay with it, though as an EE I just feel wrong about such things).

https://www.hifi-writer.com/he/misc/rmspower.htm

 

[PENG]

I was looking for something in the form of an explanation that described why it may be misleading or feels wrong.

As I mentioned in post#8, it is statements like the following that I thought could be misleading and I also said the way Ryan stated it was fine.

"........it is worth noting that bridging cuts the resistance load or ohms in half; this can quickly cause your amplifier to overheat and shutdown. "

Reason:

Stating it that way may mislead less technically inclined people to believe the impedance of a load, whether it be a test resistor or a real loudspeaker, would somehow be changed by a bridged amplifiers. That would be false. The impedance characteristics of a loudspeakers does not change just because the amp is changed, or bridged with an identical amp.

Bridging two identical amps results in the output voltage being doubled, that is why the output power increases, not because the load impedance is halved (because it isn't magically halved). In fact, doubling the voltage would theoretically result in 4X (not in practical term) the power developed into the same 8 ohm load, where as halving the impedance of the load to the same amp without being bridge with a second one, with the same input signal, the amp (single, un-bridged) would deliver 2X the power developed into a 4 ohm load, i.e. half of 8 ohm. Of course in the real world, very few amps can "double down", and no bridged amp(s) can quadruple the output. The Benchmark amp apparently came close, rated 100 WPC, and 380 W bridged. It obviously has the power supply and output devices designed to take full advantage of the doubled output voltage due to bridging, and probably have very low output impedance. In either case, halving the impedance from 8 to 4 ohms without bridging, and bridging with the same 8 ohm load would result in roughly the same total output power. The bridged amp(s) can do better with higher impedance than 8 ohms but not with impedance below 4 ohms.

Note that when bridged the overall resulting load impedance, with the same test load should actually increase, not decrease, but very slightly, due to the increased output impedance of the two amps that are then wired in series.

Now a numerical example:

Consider two identical mono-block amps that are each rated to deliver 100 W into 8 ohms and 150 W into 4 ohms continuously, and are bridgeable to produce twice the output voltage for a load with impedance 8 ohms minimum.

Let's say the amp that has the following specs, similar to that of the Benchmark AHB2:

https://benchmarkmedia.com/products/benchmark-ahb2-power-amplifier
https://www.stereophile.com/content/benchmark-media-systems-ahb2-power-amplifier-measurements

Gain: 23 dB
Rated output: 100 W into 8 ohm load

At 2 V input, the output voltage will be approx. 14.14 V and power into an 8 ohm load is approx. 100 Watts.

Now if we bridge two such amps, the gain will increase to approx 29 dB and:

At the same 2 V input, but inverted for the second amp, the output voltage of the bridged amps that are now effectively wired in series will be doubled to 28.2832 V and power into the same 8 ohm load = 28.2832X28.2832/8 = 400 Watts Again, in real world, very few amps can double down from 8 to 4 ohms, or bridged to double the output into 8 ohms. Some, such as the AHB2 can come very close.

To summarize, "halving the impedance" from say 8 ohms to 4 ohms, all else being equal, have similar effects as bridging two amps/channels in terms of load current and total power output into an 8 ohm load, but that's because of the doubled voltage resulted from two amps/channels wired in series, while the load impedance remains unchanged, not halved. Obviously in the case of bridging, you need 4 amp channels for stereo.


I hope this help clarify my point. If not, maybe wordsmiths like @lovinthehd or @Steve81 wouldn't mind helping me out.

 

[highfigh]

As I mentioned in post#8, it is statements like the following that I thought could be misleading and I also said the way Ryan stated it was fine.

"........it is worth noting that bridging cuts the resistance load or ohms in half; this can quickly cause your amplifier to overheat and shutdown. "

Reason:

Stating it that way may mislead less technically inclined people to believe the impedance of a load, whether it be a test resistor or a real loudspeaker, would somehow be changed by a bridged amplifiers. That would be false. The impedance characteristics of a loudspeakers does not change just because the amp is changed, or bridged with an identical amp.

Bridging two identical amps results in the output voltage being doubled, that is why the output power increases, not because the load impedance is halved (because it isn't magically halved). In fact, doubling the voltage would theoretically result in 4X (not in practical term) the power developed into the same 8 ohm load, where as halving the impedance of the load to the same amp without being bridge with a second one, with the same input signal, the amp (single, un-bridged) would deliver 2X the power developed into a 4 ohm load, i.e. half of 8 ohm. Of course in the real world, very few amps can "double down", and no bridged amp(s) can quadruple the output. The Benchmark amp apparently came close, rated 100 WPC, and 380 W bridged. It obviously has the power supply and output devices designed to take full advantage of the doubled output voltage due to bridging, and probably have very low output impedance. In either case, halving the impedance from 8 to 4 ohms without bridging, and bridging with the same 8 ohm load would result in roughly the same total output power. The bridged amp(s) can do better with higher impedance than 8 ohms but not with impedance below 4 ohms.

Note that when bridged the overall resulting load impedance, with the same test load should actually increase, not decrease, but very slightly, due to the increased output impedance of the two amps that are then wired in series.

Now a numerical example:

Consider two identical mono-block amps that are each rated to deliver 100 W into 8 ohms and 150 W into 4 ohms continuously, and are bridgeable to produce twice the output voltage for a load with impedance 8 ohms minimum.

Let's say the amp that has the following specs, similar to that of the Benchmark AHB2:

https://benchmarkmedia.com/products/benchmark-ahb2-power-amplifier
https://www.stereophile.com/content/benchmark-media-systems-ahb2-power-amplifier-measurements

Gain: 23 dB
Rated output: 100 W into 8 ohm load

At 2 V input, the output voltage will be approx. 14.14 V and power into an 8 ohm load is approx. 100 Watts.

Now if we bridge two such amps, the gain will increase to approx 29 dB and:

At the same 2 V input, but inverted for the second amp, the output voltage of the bridged amps that are now effectively wired in series will be doubled to 28.2832 V and power into the same 8 ohm load = 28.2832X28.2832/8 = 400 Watts Again, in real world, very few amps can double down from 8 to 4 ohms, or bridged to double the output into 8 ohms. Some, such as the AHB2 can come very close.

To summarize, "halving the impedance" from say 8 ohms to 4 ohms, all else being equal, have similar effects as bridging two amps/channels in terms of load current and total power output into an 8 ohm load, but that's because of the doubled voltage resulted from two amps/channels wired in series, while the load impedance remains unchanged, not halved. Obviously in the case of bridging, you need 4 amp channels for stereo.


I hope this help clarify my point. If not, maybe wordsmiths like @lovinthehd or @Steve81 wouldn't mind helping me out.

That's what I was looking for. Even most of the links I found were more like the marketing sheets than technical descriptions. Thanks.

 

[PENG]

Even most of the links I found were more like the marketing sheets than technical descriptions. Thanks.

You got that right!

 

[highfigh]

You got that right!

Getting good info from manufacturers is very similar to my comment about asking a rock how it came to be- it doesn't know. Over time, it's pretty easy to find out who knows and who doesn't, but one thing that I'm sure of- the vast majority of the reps I talk to have never connected/set up a home theater, mixer, adjusted mic/line levels or dealt with a noise problem. One seems to let my questions roll off his back like rain, based on his answers. Makes me wish I worked more with pro audio.