I'll jump in here for Bob and he can add / change as he sees fit. Others are always welcome to do the same as i am not above making mistakes. I would rather be corrected and get "spanked" in public than to spread "disinformation".
When you bridge an amp, it effectively sees HALF of what the actual loudspeaker load is. In other words, what appears as an 8 ohm load to an "unbridged" amp now looks like 4 ohms to a bridged amp. If you doubt this, take a look at what an amp is rated at per channel into four ohms and combine the output of the two channels together. You will then have what the amp is rated at in bridged mode ( in MOST cases ) with an 8 ohm load.
A common figure for a pretty decent sized amp would be 250 X 2 @ 8 ohms / 400 X 2 @ 4 ohms. Bridging the two channels together would typically result in a rating of 800 X 1 @ 8 ohms with 4 ohm loads not recommended by most manufacturers. That is because a REAL 4 ohm speaker would EFFECTIVELY be seen as a 2 ohm load by the bridged amp. Only BEEFY amps can run bridged at 4 ohms. If an amp can run bridged with a 2 ohm load, you KNOW that it is built BETTER than a tank.
The reason that the damping factor is reduced has to due with the reduction in effective impedance that the bridged amp sees. If you look at the spec sheets, damping factor is based on a specific load impedance and frequency. Most amps have a lower damping factor at higher frequencies. As such, a common place to rate them is at 1 KHz and, even if not specified, typically at 8 ohms.
Damping factor is based on the difference between the output impedance of the amp and the impedance of the load. The greater the variance between the two results in a higher damping factor. Damping factor is typically thought of as how tight of a grip the amplifier has on the speaker i.e. commonly referred to as "control" of the drivers. Since going from an 8 ohm load to a 4 ohm load effectively halves the difference in impedance, the damping factor is now cut in half and "control" is reduced by appr 50%. Whether this is noticeable or not has to do with how high the damping factor was to begin with.
The opposite occurs as you go up in load impedance. Using a speaker rated at 4 ohms and then switching to an 8 ohm load effectively doubles the damping factor. Going from a 4 ohm load to a 16 ohm load QUADRUPLES the damping factor. This typically results in notably tighter bass reproduction.
All things being equal, a lower impedance speaker is tougher to control for ANY amp. This is true even though the amp may be able to generate more power due to the lower resistance. In this specific case, more power does NOT necessarily mean more control over the drivers and you might actually end up with "looser" or "sloppier" low frequency output.
As a general rule, solid state ( SS ) amps have a MUCH lower output impedance ( typically way below 1 ohm ) than tube amps ( sometimes as high as 1 or 2 ohms ). This is one of the reasons why many folks prefer to run higher impedance speakers with tube amps. Not only does it pull less current from the tubes ( which are more at home producing high voltage rather than high current ), the damping factor is increased. In a comparative analysis, a SS amp with an output impedance of .5 ohms and a speaker load of 4 ohms is equivalent in damping factor to a tube amp with an output impedance of 2 ohms and a speaker load of 16 ohms. This is why many tube amps are said to suffer from "mushy" or "round" bass when running low impedance speakers. The damping factor or "control" over the drivers is drastically reduced.
Given the specifics of the speakers in question ( ML hybrids ), i can see quite a dilemma. Whereas the mid and high frequency section is electrostatic ( prefers high voltage and can be quite reactive ) and the woofers are dynamic ( prefers high current ). You really need a top notch amp(s) for speakers like this to perform at their potential. While tubes typically are thought of as being "voltage amplifiers" and seem well suited to the drive characteristics of an E-stat, they typically do not like the low impedance drop that ALL E-stat's exhibit at high frequencies. Obviously, poorly built and designed tube amps need not apply for this job. On the other hand, the woofers need a lot of current making them more suitable for use with a beefy SS amp. Only problem is that they typically don't generate the higher voltages that really make E-stat's sing.
In order to achieve optimum results with ONE amplifier driving this type of load, you would need something that offered very high voltage capabilities along with something that was of a high current design. Sounds like the best of both worlds, right ??? Now you know why some people running "hybrid" speakers run tubes on top and SS on the bottom end. Hybrid speakers with a hybrid amplifier arrangement. Do i hear Detlof's name coming up ??? : )
The only alternatives that i know of using a similar approach would be amplifiers designed around some type of high current / high voltage switching power supplies. The Bel Canto MIGHT fall into this category. I don't know enough about it to say one way or the other. Some others that would APPEAR to be well suited to this type of application might be the aforementioned Innersound amp and the Sunfire amps. Both of these two amps run high rail voltages and are quite stable into low impedances. This is evidenced by the "doubling down" of power output as impedance is halved. It is quite possible that the larger Bel Canto's also fall into a similar category.
I know that Perreaux's have also been known to drive VERY reactive loads ( i.e Apogee's ), even in bridged configuration. While they are also of a high rail voltage / high current design, they make use of a more standard power supply arrangement. This means that they run WAY hotter, are WAY heavier and WAY less efficient in terms of power consumption. Some Krell's might also be suitable candidates but also fall into the "big clunker" category like the Perreaux's. Whether any of these compliment the tonal balance of the rest of the system or your personal taste is anybody's guess though.
With all of that in mind, i would suggest following the "K.I.S.S." principle. Instead of running four amps and the associated cables, i would look for one or two amps of equivalent or higher power and quality that could do the job. While doubling up on your amps would obviously give you more of what you already have, you might be able to achieve even greater results AND keep it simpler for about the same amount of money. Otherwise, i can see you moving into an active crossover and doing major mods to the internals of the speakers ( i.e. bypassing the crossovers ) to achieve what you are looking for.
Hope this helps and keep us posted with what you decide to do. Sean
>
When you bridge an amp, it effectively sees HALF of what the actual loudspeaker load is. In other words, what appears as an 8 ohm load to an "unbridged" amp now looks like 4 ohms to a bridged amp. If you doubt this, take a look at what an amp is rated at per channel into four ohms and combine the output of the two channels together. You will then have what the amp is rated at in bridged mode ( in MOST cases ) with an 8 ohm load.
A common figure for a pretty decent sized amp would be 250 X 2 @ 8 ohms / 400 X 2 @ 4 ohms. Bridging the two channels together would typically result in a rating of 800 X 1 @ 8 ohms with 4 ohm loads not recommended by most manufacturers. That is because a REAL 4 ohm speaker would EFFECTIVELY be seen as a 2 ohm load by the bridged amp. Only BEEFY amps can run bridged at 4 ohms. If an amp can run bridged with a 2 ohm load, you KNOW that it is built BETTER than a tank.
The reason that the damping factor is reduced has to due with the reduction in effective impedance that the bridged amp sees. If you look at the spec sheets, damping factor is based on a specific load impedance and frequency. Most amps have a lower damping factor at higher frequencies. As such, a common place to rate them is at 1 KHz and, even if not specified, typically at 8 ohms.
Damping factor is based on the difference between the output impedance of the amp and the impedance of the load. The greater the variance between the two results in a higher damping factor. Damping factor is typically thought of as how tight of a grip the amplifier has on the speaker i.e. commonly referred to as "control" of the drivers. Since going from an 8 ohm load to a 4 ohm load effectively halves the difference in impedance, the damping factor is now cut in half and "control" is reduced by appr 50%. Whether this is noticeable or not has to do with how high the damping factor was to begin with.
The opposite occurs as you go up in load impedance. Using a speaker rated at 4 ohms and then switching to an 8 ohm load effectively doubles the damping factor. Going from a 4 ohm load to a 16 ohm load QUADRUPLES the damping factor. This typically results in notably tighter bass reproduction.
All things being equal, a lower impedance speaker is tougher to control for ANY amp. This is true even though the amp may be able to generate more power due to the lower resistance. In this specific case, more power does NOT necessarily mean more control over the drivers and you might actually end up with "looser" or "sloppier" low frequency output.
As a general rule, solid state ( SS ) amps have a MUCH lower output impedance ( typically way below 1 ohm ) than tube amps ( sometimes as high as 1 or 2 ohms ). This is one of the reasons why many folks prefer to run higher impedance speakers with tube amps. Not only does it pull less current from the tubes ( which are more at home producing high voltage rather than high current ), the damping factor is increased. In a comparative analysis, a SS amp with an output impedance of .5 ohms and a speaker load of 4 ohms is equivalent in damping factor to a tube amp with an output impedance of 2 ohms and a speaker load of 16 ohms. This is why many tube amps are said to suffer from "mushy" or "round" bass when running low impedance speakers. The damping factor or "control" over the drivers is drastically reduced.
Given the specifics of the speakers in question ( ML hybrids ), i can see quite a dilemma. Whereas the mid and high frequency section is electrostatic ( prefers high voltage and can be quite reactive ) and the woofers are dynamic ( prefers high current ). You really need a top notch amp(s) for speakers like this to perform at their potential. While tubes typically are thought of as being "voltage amplifiers" and seem well suited to the drive characteristics of an E-stat, they typically do not like the low impedance drop that ALL E-stat's exhibit at high frequencies. Obviously, poorly built and designed tube amps need not apply for this job. On the other hand, the woofers need a lot of current making them more suitable for use with a beefy SS amp. Only problem is that they typically don't generate the higher voltages that really make E-stat's sing.
In order to achieve optimum results with ONE amplifier driving this type of load, you would need something that offered very high voltage capabilities along with something that was of a high current design. Sounds like the best of both worlds, right ??? Now you know why some people running "hybrid" speakers run tubes on top and SS on the bottom end. Hybrid speakers with a hybrid amplifier arrangement. Do i hear Detlof's name coming up ??? : )
The only alternatives that i know of using a similar approach would be amplifiers designed around some type of high current / high voltage switching power supplies. The Bel Canto MIGHT fall into this category. I don't know enough about it to say one way or the other. Some others that would APPEAR to be well suited to this type of application might be the aforementioned Innersound amp and the Sunfire amps. Both of these two amps run high rail voltages and are quite stable into low impedances. This is evidenced by the "doubling down" of power output as impedance is halved. It is quite possible that the larger Bel Canto's also fall into a similar category.
I know that Perreaux's have also been known to drive VERY reactive loads ( i.e Apogee's ), even in bridged configuration. While they are also of a high rail voltage / high current design, they make use of a more standard power supply arrangement. This means that they run WAY hotter, are WAY heavier and WAY less efficient in terms of power consumption. Some Krell's might also be suitable candidates but also fall into the "big clunker" category like the Perreaux's. Whether any of these compliment the tonal balance of the rest of the system or your personal taste is anybody's guess though.
With all of that in mind, i would suggest following the "K.I.S.S." principle. Instead of running four amps and the associated cables, i would look for one or two amps of equivalent or higher power and quality that could do the job. While doubling up on your amps would obviously give you more of what you already have, you might be able to achieve even greater results AND keep it simpler for about the same amount of money. Otherwise, i can see you moving into an active crossover and doing major mods to the internals of the speakers ( i.e. bypassing the crossovers ) to achieve what you are looking for.
Hope this helps and keep us posted with what you decide to do. Sean
>