AI-1 and bridged amps, why is it bad?

On3s&Z3r0s

I know this has been discussed tangentially in other threads, but I figured rather than attempt a threadjack I'd start one to discuss just this topic.

I have some SDA 2B's, which will be TL'd eventually. Anyway, I have a big ol heap of NAD gear including a reciever and matching standalone amp (both are 55wpc in stereo mode and bridge to 150wpc). I was thinking about building an AI-1 and putting these in bridged mode and using the receiver to drive one speaker and the amp to drive the other. But, I've seen folks on the board (H9, not to name names) say that this is a bad idea.

So, my question is why exactly is this a bad idea? Is it just a matter of this not being likely to improve the sound, or is it that this is not how the AI-1 was intended to work and it would end up cooking something? I know the AI-1 is primarily intended to allow the use of non-common ground amps, but it sounds like people are using real monoblocks with them as well. So do stereo amps that are bridged to mono operation behave differently enough from true monoblocks to create a problem?

Sorry about my audio electronics naivete... someday I hope to be able to read an amp schematic a little, but until then I'm scared of making my wife angry by, say, burning down our house. :redface:

Schurkey

The AI-1 or equivalent will allow the separate amplifiers.

Powering the stereo pair of speakers with similar design amplifiers (one stand-alone, one integrated into a receiver) and both running in bridged mode may not be the best sonic solution. But aside from sound quality issues, you're unlikely to do damage to the speakers.

Bridged amps tend to be picky about being asked to service low-impedance loads; and the SDA speakers in general tend to be on the low side. I don't remember what the SDA 2B is rated for impedance. You'll want to assure that the amps you bridge will be able to grunt out enough amperes to satisfy your speakers at the volume level you listen at.

There are those that feel a bridged amp is poor sonically; and they're not all wrong. Some forms of distortion cancel when you bridge an amp, and others get worse. Your ears will tell you the truth.

On3s&Z3r0s

Schurkey wrote: »

...
Bridged amps tend to be picky about being asked to service low-impedance loads; and the SDA speakers in general tend to be on the low side. I don't remember what the SDA 2B is rated for impedance. You'll want to assure that the amps you bridge will be able to grunt out enough amperes to satisfy your speakers at the volume level you listen at...

Does using the AI-1 present a lower impedance than not using it? Both the amp and receiver have a switch for 8ohms (high) and < 8ohms (normal). I leave them set to 4ohms/normal. Their specs show them increasing wattage into 2ohm loads, but I doubt they're what you'd consider a high-current amp. At least it doesn't seem like the 2B's (in their stock incarnation anyway) are very hard to drive based on the different amps/receivers that have been able to handle them.

If the low impedance is a problem, what am I risking? Just tripping the protection circuitry of the amp?

Schurkey

On3s&Z3r0s wrote: »

Does using the AI-1 present a lower impedance than not using it?

If there's any effect on impedance, I think it would be minimal. What I'm concerned about is the amp in bridged mode may be less tolerant of a low-impedance load.

On3s&Z3r0s wrote: »

Both the amp and receiver have a switch for 8ohms (high) and < 8ohms (normal). I leave them set to 4ohms/normal. Their specs show them increasing wattage into 2ohm loads, but I doubt they're what you'd consider a high-current amp.

What is the watts/impedance specification of the amp in bridged mode? Some amps may be rated into 4- or 2-ohm loads in stereo mode; but only rated for an 8-ohm load when bridged. THAT would be a problem.

On3s&Z3r0s wrote: »

At least it doesn't seem like the 2B's (in their stock incarnation anyway) are very hard to drive based on the different amps/receivers that have been able to handle them.

Far as I know, they're a reasonably easy load with regard to capacitance/inductance, but the resistance/impedance is lower than the "standard 8 ohm" load. Volume level is also a consideration; if you don't listen in a huge room or with the volume at stadium levels, things are a lot easier on the amp.

On3s&Z3r0s wrote: »

If the low impedance is a problem, what am I risking? Just tripping the protection circuitry of the amp?

If the amp clips, you might be tripping the polyswitches and/or risking damage to the tweeters. Again--how loud do you listen?

On3s&amp;Z3r0s

Schurkey wrote: »

If there's any effect on impedance, I think it would be minimal. What I'm concerned about is the amp in bridged mode may be less tolerant of a low-impedance load.


What is the watts/impedance specification of the amp in bridged mode? Some amps may be rated into 4- or 2-ohm loads in stereo mode; but only rated for an 8-ohm load when bridged. THAT would be a problem.

Actually, that's something I hadn't noticed before... the specs go down into 2ohms in stereo mode, but only down to 4ohms in bridged mode. The way NAD gives its wpc specs has always been a little goofy. In both stereo and bridged, they only give a continuous average power output number into 8ohms (this is the 150W bridged). Then they give an IHF Dynamic Power number, which bridged is 250W/8, 330W/4. But then there's legalese about some countries requiring measurements in bridged mode to be done at the 4ohm setting, which reduces the RMS... blah, blah.

As far as volume goes, like most folks, sometimes I want to see if I can rattle the glassware in the china hutch, but most times I like to listen at saner and more enjoyable volumes.

So, it sounds like there's nothing particularly bad about mixing the AI-1 with bridged amps. Meaning maybe bridged mid-fi equipment with low impedance speakers in general is a bad idea, but there's nothing uniquely bad about doing this with the SDA/AI-1 combo.

It seems weird that so many middle-tier component manufacturers include the bridging option if it's not likely to be useful. Like AVR receiver manufacturers including the option to bi-amp your fronts with your rear surround channels. It seems like everyone agrees that it's a waste of time at best, so why do they bother? And I have to say, as much of a Polk fan as I've become... dual sets of binding posts on a Monitor 30? Really??? It all kinda seems like marketing people gumming up the engineering, and that just makes me really cranky.

tonyb

Why do they bother ??? To sell their products, why else ? Doesn't matter if it's usefull or even logical. Toss in some buzz words and they capture your attention. Marketing 101.

Schurkey

On3s&Z3r0s wrote: »

So, it sounds like there's nothing particularly bad about mixing the AI-1 with bridged amps. Meaning maybe bridged mid-fi equipment with low impedance speakers in general is a bad idea, but there's nothing uniquely bad about doing this with the SDA/AI-1 combo.

Exactly right. The whole purpose of the AI-1 is to allow the compatible SDA speakers to be used with non-common-ground amps, including bridged amps.

heiney9

This subject has been covered so many times. I wasn't going to comment because of that.

When you bridge a stereo amp it see's 1/2 the load because you are summing the 2 channels. For SDA's that means the amps see 3 ohms or less, not a good thing especially for mid-fi components. That's the main reason for not doing it.

Bridging usually adds more noise and distortion as a by product. All you are gaining is theoretical spl and perhaps a tad more headroom in the right set up.

I would not recommend running bridged stereo amps with SDA's as they tend to really stress the amplifiers, regardless of what the spec sheet states and they tend to sound worse and it's not necessary.

What are you trying to accomplish? More spl? Trying to fill a large room?

Anyway, that's my take on it, I wouldn't do it.........but most will do whatever they want.

H9

heiney9

I didn't read your 1st post closely enough. Unless NAD has some really, really funky connections I don't think it's possible to run the receiver and amp at the same time (one to each speaker). That would just be weird anyway. What a hodge podge that would be. Get a proper amp or stick with the seperate amp and sell the receiver and get a pre-amp. I assume this is for a 2ch rig.

H9

On3s&amp;Z3r0s

heiney9 wrote: »

I didn't read your 1st post closely enough. Unless NAD has some really, really funky connections I don't think it's possible to run the receiver and amp at the same time (one to each speaker). That would just be weird anyway. What a hodge podge that would be. Get a proper amp or stick with the seperate amp and sell the receiver and get a pre-amp. I assume this is for a 2ch rig.

H9

NAD seems to purposefully market their mid-fi (and even some higher-end gear) with this scenario in mind. There are lots of cases like this where they designed a standalone amp and then put the identical amp section in a receiver or integrated pre/amp and put bridging switches on everything. Then they allude to doing stuff like this in marketing and manuals without ever being especially clear on how to do it. Anyway, I've done this leaving one channel on the bridged receiver jumpered and sending the other channel to the bridged amp.

I went back to the NAD manual and did find a warning not to bridge into low-impedance speakers even though they do give low-ohm specs. It sounds like mainly you're risking damage to the amp by doing this. They say you're likely to run afoul of protection circuitry or blow fuses.

As far as what I was trying to accomplish... Yes, it's 2-channel and I wasn't going for loud necessarily. I had just seen lots of statements suggesting that more power could give you better detail in the music, but it sounds like that's not the case when bridging if it somehow actually increases distortion. Also, I got the idea maybe there was some magic in having grounds floating with respect to the speakers' circuit path. I guess this has to do with the design of some non-common ground / balanced amps more than just the mere fact of not having the ground present.

Sorry for the redundant thread, but I definitely appreciate you and Schurkey answering. I'm slowly piecing together a better understanding of how this stuff works, and hadn't found some of the specifics in other threads. In my case, it sounds like this isn't even worth experimenting with using my current gear. But it's a good excuse to keep looking for new and different stuff. Thanks again!

heiney9

Hey, do whatever you want. What do those of us in this hobby for 20+ years and having sold some of said product know about anything.

Not being flippant in the above comment. NAD is sold midfi gear, I still wouldn't bridge a stereo amp and then I wouldn't run SDA's with a bridged amp, even if you can because you have the AI-1.

A better amp or other components will give you better detail, etc. Not more power.

Go ahead and try it, then you can decide for yourself

H9