To bi-amp or not to bi-amp? - AVS Forum
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post #1 of 54 Old 10-04-2006, 04:24 PM - Thread Starter
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I just recently purchased the following Speakers, which are all set up to be bi-amped (if I choose to):

B&W 805S Fronts
B&W HTM4S Center
B&W SCMS Rears

I am considering a few different choices, including a used Linn amp from a friend. However, I am also seriously considering going with a Rotel RMB-1095 OR 2 rotel RMB-1075s and biamping all 5 speakers.

If price were equal between both Rotel setups (which it basically is) would I be better off going with the single larger 200x5 amp or biamping the speakers with 2 5x120 amps?

Opinions?

-Ethan
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post #2 of 54 Old 10-04-2006, 04:31 PM
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IMHO, biamping is not worth the bother/complexity if you do not use an external line-level crossover and remove the inbuilt filters. However, you can find MANY threads and comments on this issue here on AVS and on every other audio-related forum on the web. IMHO, again, all opinions have been expressed already.

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post #3 of 54 Old 10-04-2006, 04:45 PM
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single larger amp is better.

Mike C
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post #4 of 54 Old 10-05-2006, 07:43 AM - Thread Starter
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Thanks for the responses. Would purchasing one of the new digital 7 channel amps and using the extra 2 channels to biamp the front L/R channels be worth the hassle, or would this also be more hassle than it is worth in your opinions?

-Ethan
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post #5 of 54 Old 10-05-2006, 08:19 AM
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One good thing about bi-amping is that if you're driving the amp to nearly maximum capacity, you'll have the highs distortion-free at all times because they require less power than the low frequencies. Though, it's still not a good idea to be driving the amp so hard that you're frequently clipping on the lows for extended periods of time.
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post #6 of 54 Old 10-05-2006, 10:09 AM
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Quote:
Originally Posted by Ebanks
Thanks for the responses. Would purchasing one of the new digital 7 channel amps and using the extra 2 channels to biamp the front L/R channels be worth the hassle, or would this also be more hassle than it is worth in your opinions?

-Ethan
I actually have this dilema myself... I mean, I _HAVE_ the extra channels, so I could take advantage. I even have the appropriate Y-cables and speaker wire lying around, so it's no added cost... (Just picked up a Rotel 1077)

However, at some point I'll want to move from 5.1 to 7.1, so if I get used to the sound from bi-amping, maybe I'll be less happy with single amping.

Of course, my dealer may be happy as I might feel the need to get a stereo amp (;

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post #7 of 54 Old 10-05-2006, 10:16 AM
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Quote:
Originally Posted by Clepto
I actually have this dilema myself... I mean, I _HAVE_ the extra channels, so I could take advantage.

Ahhhh.... but you have already assumed there will be an advantage, so you are predisposed to hear one. ;)

Just because there is a knob doesn't mean you should turn it.
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post #8 of 54 Old 10-05-2006, 10:22 AM
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Originally Posted by whoaru99
Ahhhh.... but you have already assumed there will be an advantage, so you are predisposed to hear one. ;)
I more meant take advantage of the fact I have extra channels lying around, in actuality I'm pretty skeptical of any audible advantage, especially at my listening levels.

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post #9 of 54 Old 10-05-2006, 10:33 AM
 
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To reword what Kal said, but int more authoritative terms, it is a fact that there is no benefit to merely hooking up two amps to the passive crossover of a speaker. The amp still has to swing the full voltage load, its just that the peak current the amp sees (and thus the peak power) it sees will be less. This only solves the problem of having a power amp with a weak power supply. Getting a superior amp would solve this problem with considerably more simplicity.

The benefits to biamping primarily come from filtering the signal in active components at line level so you can have predictably flat response. Also, eliminating passive components between the amps and the drivers allows the amps to have better control over the drivers.

Most of these hifi companies only add two sets of terminals for marketing value, not an actual engineering reason to do it. The sales they could potentially lose from ignorant audiophiles demanding to (incorrectly) biamp, or even worse, biwire the speakers far exceeds the value of the terminal itself. It may be good business practice to add two pairs of terminals, but its not good audio practice to attempt to make use of them.
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post #10 of 54 Old 10-05-2006, 12:32 PM
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Besides power supply, what else inhibits the amount of power an amp can supply?

I'm not saying that you're wrong Cowclops, but if you could explain the reasoning behind this statement and why it is or is not correct, it would help the rest of us understand...
http://www.anthemav.com/NewSitev2.0/...echsupp.html#5
Quote:
With the jumpers removed on a biampable speaker, the impedance of each section is not the usual 4 or 8 ohms, but several hundred if not more at the frequencies that the amp is "not supposed to be amplifying". Higher impedance means less current draw. No meaningful amount of current, no wasted power.
...
Using the passive crossover in the speaker is indeed the correct way to biamp.
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post #11 of 54 Old 10-05-2006, 12:39 PM
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Wasted power is not a real issue for monoamping but it is for biamping since the HF amp has a relatively easy job compared to the LF amp. You could use a lower-power amp for the HF but then you face issues of matching sensitivity and timbre. Conversely, with one amp doing both, the power burden, compared to that for an LF-only amp, is only marginally greater. So, imho, it simply comes down to whether the (mono)amp is adequate. If not, you can either get a bigger one or add another one.

EDIT: My comments above were about passive biamping.

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post #12 of 54 Old 10-05-2006, 12:50 PM
 
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Its voltage rails. Even with NOTHING connected to the speaker terminals (i.e. no current drawn, an open circuit) a power amp can not swing a voltage greater than its rails. If your amp has 35V rails, it can't put out a signal beyond (or even near that, really), without significant distortion. Regardless of the current draw, the voltage fed to the input times the voltage gain of the amp will result in the voltage it wants to swing to. If it can't reach it, you've got clipping (or at least some sort of nasty distortion). If you take one amp that can swing a max of about 25V (80 watts into 8 ohms assuming unlimited current supply) and then passively biamp with another identical amp, your headroom is still only 25V at instantaneous point in time. Say you had a signal that was a 100hz sinewave and a 10khz sine wave summed together. Each sine wave has a peak of 15V before summing. When you add the signals together, the new amplitude of the signal is 30V. If you fed this into two "25V" power amps separately, they both see a 30V signal even if they may be supplying no current at all. The signal will be clipped and you will have distortion. This is why passive biamping doesn't work.

The reason active biamping DOES work is because the signal is filtered before it gets to the power amp. Take a 100hz 15V sine wave and a 10khz 15V sine wave, add them together, separate them again in a crossover, and one amp will get a 100hz 15V sine wave and a greatly attenuated 10khz sinewave (you can calculate exactly how much based on the rolloff of the crossver). So individually, each amp might only see a 15.1V peak instead of a 30V peak. They will also have, just like in the passive biamping situation, more current output by way of TWO transformers rather than only one.

Active biamping A) increases the amps control over the driver because there are no longer any passive components in there, B) reduces intermodulation distortion (the narrower the bandwidth, the less intermodulation distortion there is), and C) also increases the total amount of potential power.

Passive biamping doesn't accomplish A) because you still have passive components in the signal path. It doesn't accomplish B) because both amps still have to amplify a full range signal. It doesn't "really" accomplish C) because you're still limited to the voltage rails of each amp. In short, passive biamping does nothing.
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post #13 of 54 Old 10-05-2006, 12:56 PM
 
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Oops, and while what I said does explain the the fallacy on Anthem's website, I didn't specifically address it.

There is no wasted "power" because the amps aren't supplying power when they see the high impedance of the stuff the crossover isn't supposed to let through to the driver. There IS, however, wasted VOLTAGE headroom. And to prevent distortion, you have to be able to supply both the necessary voltage and the necessary current simultaneously. 1V at 500amps won't help, and 50V at 1mA won't help either. You have to have both. Passive biamping, aside from the other problems it doesn't solve, doesn't solve the voltage headroom issue.
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post #14 of 54 Old 10-05-2006, 01:15 PM
 
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And at the possibility of looking like a doofus posting three times (I don't like editing posts unless its a mistake I catch immediately after posting it), I'll ALSO say that Anthem's overall conclusion is pretty much the truth.

Replacing a passive crossover with an active one requires more knowledge than most end users have. The "correct" way to biamp is to design a proper circuit, actively, to fix the shortcomings of the driver. This is not possible with an off the shelf black box. I converted my speakers to active biamping, but I also had the schematic for the factory-biamped version of the speaker to go off of. While it is not in good business sense to recommend against a course of action that would sell more amps (i.e. "Don't biamp at all if you can't do it right"), the reality is that unhooking the passive crossover and hooking up the wrong active crossover is worse than just leaving it as is. Passive biamping will sell more amps, but is pretty much pointless for the end user.
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post #15 of 54 Old 10-05-2006, 01:26 PM
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Ethan -

In my experience bi-amping DOES work. I ran my Aerials off 2 channels of the 5125 and it did fine... then I bi-amped them with 4 channels off the 5125 and the improvement was immense.

I would however recommend that you buy a higher current capable amp than relying on bi-amping with lesser amps. For example - I then replaced my 4 channels of 5125 power with 2 channels from a Levinson 336. You want to talk about night and day....

I guess what I'm trying to say is buy the best amp that you can... more channels of amplification isn't always better - but if you bi-amp with the same power then it will most certainly be better than not.

Sorry if that's confusing. BTW... I'm going to keep the 5125 (to bi-amp my center channel and drive a pair of surrounds). :)

What's your budget - just curious? You can score some great deals on used gear.

Jim
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post #16 of 54 Old 10-05-2006, 01:33 PM
 
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Anecdotal evidence makes baby jesus cry.
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post #17 of 54 Old 10-05-2006, 01:41 PM
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Quote:
Originally Posted by Kal Rubinson
IMHO, biamping is not worth the bother/complexity if you do not use an external line-level crossover and remove the inbuilt filters. However, you can find MANY threads and comments on this issue here on AVS and on every other audio-related forum on the web. IMHO, again, all opinions have been expressed already.

Kal
Assuming this is true, then you need to consider that the complexity of active biamping is an order of magnitude greater. Essentially, active biamping requires you to redesign one of the most difficult, mathematically complex subsystems in your speakers, and do it better than a skilled designer with sophisticated modeling software, experience and an electronics lab at his disposal. So even if in passive networks are inferior to active crossovers, it doesn't mean that you can do it better.
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Exactly, swampfox. Though as long as "stick with the passive crossover" is a given, "don't bother with biamping" should be a given too. The benefits you get from active crossovers (which, as we agreed, aren't trivial to implement) aren't achieved with passive crossovers. So, just get the biggest single amp you can afford.
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post #19 of 54 Old 10-05-2006, 01:49 PM
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Quote:
Originally Posted by Swampfox
Assuming this is true, then you need to consider that the complexity of active biamping is an order of magnitude greater. Essentially, active biamping requires you to redesign one of the most difficult, mathematically complex subsystems in your speakers, and do it better than a skilled designer with sophisticated modeling software, experience and an electronics lab at his disposal. So even if in passive networks are inferior to active crossovers, it doesn't mean that you can do it better.
I agree with you in general but I never said that "passive networks are inferior to active crossovers." I said that in order for biamping to be anything more than just more-amping, line-level crossovers are required. I never said that it was simple or that it was practical for the average DIY guy.

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post #20 of 54 Old 10-05-2006, 01:56 PM
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Quote:
Originally Posted by Kal Rubinson
I agree with you in general but I never said that "passive networks are inferior to active crossovers." .
I know. I didn't mean to "put words in your mouth".

SM
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post #21 of 54 Old 10-05-2006, 02:02 PM
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Quote:
Originally Posted by Cowclops
Anecdotal evidence makes baby jesus cry.
So do stupid comments. :rolleyes:
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post #22 of 54 Old 10-05-2006, 02:04 PM
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If you take one amp that can swing a max of about 25V (80 watts into 8 ohms assuming unlimited current supply) and then passively biamp with another identical amp, your headroom is still only 25V at instantaneous point in time.
But this 25V of headroom is dependant on your volume level; on the fact that you are asking for that voltage swing at 80 watts. If that same source signal was delivered to your amp when it was only outputting 1 watt, then the voltage swing is only 2.8 by my calcs. So I don't really buy into your analogy of having limitations even with no power draw, but I can see what you're saying.

However, if that Anthem statement is correct, and also if the rails were typically the limiting factor, then passive bi-amping would actually drastically reduce the headroom which I'm sure people would hear. I can hear high-frequency distortion pretty easily and I'm sure a lot of people can too. If the resistance of the low end really does hit a few hundred Ohms (say, 300) then that same 25 Volts, 80 watts into 8 Ohms could only deliver 2 watts which represents at least 15 db of headroom taken from the system. I find it very hard to believe that real-world experience wouldn't prove this out if it is indeed the case...
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post #23 of 54 Old 10-05-2006, 03:08 PM
 
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Felgar, I'm not sure how you got "the headroom is reduced" out of what the Anthem website said. The impedance is only "300 ohms or more" at the frequencies the driver isn't supposed to be receiving. This is how crossover works, the reactive components prevent a load that limits power from being delivered. The resistance of a tweeter+capacitor combination might be 8 ohms at 10khz and 308 ohms at 40hz... the power is SUPPOSED to be reduced, its the whole point of a high pass filter. And you'd get inverse numbers for the woofer + inductor combination.

The issue is that the voltage headroom is unchanged with two identical amps. If a swing of [whatever the loudest thing your amp ever has to put out] isn't enough, the voltage swing of two identical amps will be the SAME, not doubled. I.e. if one amp isn't enough, a second identical one isn't enough either. If one amp IS enough power... then two amps would be superfluous. The actual magnitude of power doesn't matter.

You shouldn't have to "buy into" the fact that an amp cannot supply a voltage greater than its open circuit voltage or a current greater than its short circuit current. Its electronics 101.
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post #24 of 54 Old 10-05-2006, 03:20 PM
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Quote:
Originally Posted by Cowclops
Felgar, I'm not sure how you got "the headroom is reduced" out of what the Anthem website said.
I get that headroom is reduced because I don't think this statement is true:
Quote:
Originally Posted by Cowclops
The issue is that the voltage headroom is unchanged with two identical amps. If a swing of [whatever the loudest thing your amp ever has to put out] isn't enough, the voltage swing of two identical amps will be the SAME, not doubled.
Obviously the capabilities of each amp remains unchanged, so total headroom is the same. However in the context of actually using the amps, the amount of required voltage headroom for a given output wattage (volume level) will actually dramatically increase if the resistance increases. Since each amplifier now amplifies with hundreds of Ohms of resistance, the voltage swings will be far greater than if the resistance were kept to around 8 Ohms. I'm basing this off watts x Ohms = volts^2 and using your 80 watts into 8 Ohms with a 25 V swing. If the resistance is 300 Ohms instead of 8, then a 25 V swing is only 2 watts and not 80, and therefore if you're right and the volt swing is the actual bottleneck, you've just reduced the headroom from 80 watts down to 2, or about 15 db.
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post #25 of 54 Old 10-05-2006, 03:32 PM
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The only thing that I can say about passive bi-amping is:

A. You don't gain an increase in voltage output capability.

B. You do gain an increase in current output capability.

C. If your present amplifier can produce the power that your speakers require (voltage and current at speakers lowest impedance), then you gain nothing by bi-amping. Perhaps you take some load off of your receiver's power supply if you bi-amp (reduced current from power supply).

D. If your amplifier is on the weak side (typical receiver 8 ohm amplifier rating), passive bi-amping will work if you have hard to drive speakers. This works in that most speakers have a low impedance dip in the low frequency driver (say 20 to 100hz range). Isolating the low impedance driver with a separate amplifier will result in better current output capabilities.

E. If you want to EQ the low frequency driver, passive bi-amping keep EQ "noise" out of the higher frequency circuit.

F. I use a low pass filter when I bi-amp my speakers (AKA subwoofer amplifier), so my second amplifier is not run "full range". However, I did not have to go out and buy a new amplifier in order to bi-amp. My cost was ZERO.

G. If you have to buy a new amplifier, you should just buy a properly rated amplifier and be done with it.
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post #26 of 54 Old 10-05-2006, 03:37 PM
 
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But I explained why this isn't correct. I don't think you're going to understand it if I reword it a third time, but hell I might as well try.

Their example of "300 ohms" is at a frequency the high pass/low pass filters are BLOCKING. Theres SUPPOSED to be no power drawn at these frequencies. It doesn't change the voltage swing. The drivers are probably between about 3 and 10 ohms impedance WITHIN THEIR BANDWIDTH. All they meant to say is that no power is drawn at frequencies the crossover is supposed to be blocking. Its not burned off resistively, because that is not how inductors or capacitors work.

You have voltage headroom, and current headroom. If you turn your preamp knob till the amplitude of the signal its outputting is 1V, and feed that to an amp with a voltage gain of twenty, your amp HAS to be able to swing to 20V no matter what you ahve hookd up to the other end. If you hooked up an oscilloscope (a high impedance device) it would still show the clipping. The point is, whether you have one overdriven 25V amp or two overdriven 25V amps, they're BOTH overdriven. If you only need to swing 10V to produce the amount of sound, then you don't need two amps. If you do need to swing more, a second identical amp won't help.

They don't make "300 ohm" speaker drivers. Headphones can have impedances that high but headphones don't usually require huge amounts of power anyway.

Overall, it is quite offensive to be told that something I said is "probably wrong" by somebody who doesn't seem to grasp basic electronics.
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post #27 of 54 Old 10-05-2006, 03:50 PM
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Please don't take offense, just take the time to explain the reasons. Or is it that you care much more about appearing to be right than about helping people with their AV hobby?

I understand what you're saying, but I'm missing why it's true. Are you saying that at the frequencies that the filters are blocking, the wattage is very very low? Because with the same watts and a higher resistance, you get more voltage, right? So basically what you're saying is that the impedance increase is offset by far lower power draw so that at the end of the day the voltage requirements are unchanged? And that's due to the physical way the passive filters work?
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post #28 of 54 Old 10-05-2006, 03:59 PM
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Haha, is that the Ethan with all the turbo'd SC300's? And of course Jim, another 2JZ guy. We need a rotary guy here of course!

There's a lot of factors that will determine if bi-amping produces a noticably benefit or not. Since the drivers on the 805 are so physically small, you might not get a major benefit. Another factor is the design of the amp you're using. If the channels of the amp you're using share a toroid the benefit won't be as high as if each channel has it's own. Another consideration is the crossover design - some higher end speakers have seperate crossover paths for each driver, I don't know if the 805 is like that or not - but if so bi-amping is more beneficial.

Essentially, amplification is cheap ... give it a shot and see. Try it in a controlled manner, with 2 channel stereo. Just use stackable banana plugs and have somebody swap them back and forth on the amp from a mono-amp/bi-amp configuration and see what you think. One thing is almost certain, it can't hurt.

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post #29 of 54 Old 10-05-2006, 04:01 PM
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I'm a rotary guy btw. :D Been meaning to check fc3s but don't even have time to keep up with the forums I'm on now...
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post #30 of 54 Old 10-05-2006, 04:07 PM - Thread Starter
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Quote:
Originally Posted by Jim Swantko
Ethan -

In my experience bi-amping DOES work. I ran my Aerials off 2 channels of the 5125 and it did fine... then I bi-amped them with 4 channels off the 5125 and the improvement was immense.

I would however recommend that you buy a higher current capable amp than relying on bi-amping with lesser amps. For example - I then replaced my 4 channels of 5125 power with 2 channels from a Levinson 336. You want to talk about night and day....

I guess what I'm trying to say is buy the best amp that you can... more channels of amplification isn't always better - but if you bi-amp with the same power then it will most certainly be better than not.

Sorry if that's confusing. BTW... I'm going to keep the 5125 (to bi-amp my center channel and drive a pair of surrounds). :)

What's your budget - just curious? You can score some great deals on used gear.

Jim
I will probably spend $1500-$2000 on the amp, but if the right thing came along would go up to $2500 or so. I feel pretty confident that I can get around 25% off most products MSRP for new product if I shop well, so I guess "MSRP" wise that takes me up to about $3300 or so, but I would like to keep it more in the realm of $2500 MSRP if possible.

I am still undecided on processor at the moment. I am thinking about going to have a look at the Cary Audio stuff at the local dealer as they make a 5x200 amp and the Cinema 11 Processor which I have heard good things about. This would be the upper end of what I would be willing to spend (if my assumptions about the pricing on the cary products is correct), but I have read a number of wonderful things about the Cary products.

Do you have any other recommendations you feel like I should take a look at?

-Ethan

-Ethan
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