Using an amplifier in bridged mode causes it to "see" a load impedance equal to half of the speaker’s actual impedance. The Motion 20’s nominal impedance of 4 ohms would be seen by a bridged amp as 2 ohms. I doubt that your amp could handle a 2 ohm load with good sonic results. And I wouldn’t be surprised if the amp’s self-protection mechanisms were triggered if it was operated in that condition, causing the amp to shut down.
It might be a different story with the IQ5, which has an 8 ohm nominal impedance. But I suspect that even in that case you would get better sonic results, albeit probably a bit less power, via biamping.
Regards,
-- Al
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P.S: Although it probably won’t be an issue, especially if your preamp (or whatever component is driving the amp) is solid state rather than tube-based, a point to be aware of regarding biamping is that since the preamp will be driving two amp channels for each speaker it will see the amp’s input impedance divided by two. Unless, that is, it provides four or more channels, or it is one of the rare two-channel preamp designs which provide two pairs of output jacks for each channel ***and drives each of them with a separate output circuit,*** rather than the much more common approach of simply connecting them together and driving them with the same output circuit. (Asterisks added for emphasis).
Regards,
-- Al
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Good stuff for sure Al. Thanks for the insight. My pre/pro is the Marantz AV-7005. It's handling the bi-amping of the Kef's at the moment, without any issues I'm aware of. I have yet to place the ML's into the mix. So, should I be using the Front High or Front Wide for the second channel for the tweets?
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Follow up: I know that the Surround Back is commonly used, but I have surround back speakers so that option is off the table.
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... should I be using the Front High or Front Wide for the second channel for the tweets? I have no experience with av pre/pros, and perhaps someone who does will chime in and address this question. But as I see it what matters, assuming that the goal is accurate behavior of the amp/speaker combination, is simply that the signals provided to the two sections of the speaker (and therefore to the two corresponding channels of the amp) should be identical. Best of luck with the new speakers! Regards, -- Al |
When you bridge you need to be careful of the amp manufacturer's specifications.
Usually they provide a minimum speaker impedance when bridging, which is 2x or more the minimum when un-bridged. It's all math, but as mentioned before when you bridge:
2x Max Voltage, therefore:
4x Power (at 8 Ohms) (since P proportional to V squared).
The increase in power is limited in large part by the thermal dissipation of the amplifier.
Will you gain much? Meh. It takes 10x the power output to double the apparent volume (10dB). You'll only gain about 6 dB this way.
Bi-amping (without a crossover) is also kind of limited, but with weak power supplies can bring out a lot. Since the current for bass notes is now separate, the power supplies are stiffer, and you may experience more dynamic, faster and less muddy amplifier this way. However this implies you have a separate power supply. With a multi-channel amp, this benefit is less clear.
Bi-amping WITH a crossover is where the magic, and complications lie. :) Best left to experts and active speaker makers.
Assuming your amplifier is spec'd for bridging with your speakers, I'd encourage you to try both and see if you find one you like best.
Best,
E
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Gah, I meant to say:
The increase in the minimum impedance requirement when bridged is related to the maximum safe thermal dissipation of the amplifier, in watts.
This is the same reason why most power amps won't support 2 Ohms. The power dissipation needed would cause a thermal meltdown. Hopefully your amp will shut off before this happens!
Best,
E
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I appreciate the input from everyone. It sounds like I will stick to the bi-amping, especially with the 4 ohm ML's. I have plenty of amp channels available, it should just be a matter of getting the right signals from the pre to the amp(s) from what I can gather. What would the cons be for splitting the front L/R outputs at the preamp to provide the signal into the amp? Is this also an impedance consideration?
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What would the cons be for splitting the front L/R outputs at the preamp to provide the signal into the amp? Is this also an impedance consideration? I suspect that won’t be a problem in this particular case, although I can’t say that with certainty because the output impedance of the AV-7005 is not specified. As I mentioned earlier, driving two amplifier channels from the same preamp or prepro output stage will result in the preamp or prepro seeing a load impedance equal to the input impedance of the amp divided by two. Your Rotel RKB-650 amp has a specified input impedance of 20K, so two amp channels in parallel would present a load impedance of 10K. That would be a problem for many tube-based preamps, and for at least a few solid state components, but I suspect it would be ok in this case. Also, to minimize the cable capacitance that would be seen by the prepro it would be preferable to split the signal using short y-adapter cables connected to the amp inputs, in conjunction with a single RCA cable for each channel, rather than splitting it at the outputs of the prepro and running two RCA cables for each channel. Although given the short lengths of the interconnects that appear to be required in your setup it probably won’t make much if any difference either way. Regards, -- Al |
I really appreciate you help, Al. I think I am going to experiment with using the Front High outputs to use as the source signal for the tweeters. This assumes the signal will be the same as the Front's. It would then eliminate the potential impedance/loss issues of splitting the signal from just the Front. I'll probably give that go tonight and post the results here for reference.
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Follow up: I tried the Front High pre-outs as the source for the tweeters on the mains, and I was disappointed. They sounded very muted and did not present musically as I had hoped. Now, that being said, I was using the pre in stereo mode, so I presume there was some 'non-processing' going on if you will. When I went to the All Channels selection, they did seem to wake up and became, at least to my non-audiophile ears, much brighter and clearer. Again, I suspect it is a processing matter, so for the time being, I have returned to splitting the front mains into 4 channels for amplification sine I prefer to listen to my music in stereo mode.
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I was just going to chime in here. It really depends on how you are generating the signal for low/high biamp cables. If you use the "special bi-amp" feature in most processors/receivers, they usually have a circuit that analyzes the actual signal that is played by the "low" part of the biamp. It than uses another circuit to subtract the "used" frequencies from the unused. It then outputs this "unused" signal to the "front high pre-outs" or equivalent. The actual result is a significant loss in resolution and impact. This explains why they sounded very muted and dull. It is not recommended and I honestly don't know why these companies are trying to do this "differential subtraction" circuit because it just destroys the sound quality. They really should have just output the normal "full range" signal on both low/high biamp outputs because the speaker would naturally play only what it needs to (based on internal speaker crossover). It's probably best just to use a splitter cable, since you have the Rotel amp with 20k input impedance. Al is correct in stating that splitting this into a 10k + 10k load will be fine for solid state components. Actually, pro audio components usually have 10k impedances a lot of the time. If you want a really good Y-splitter cable, check this one out (I'm assuming you're using RCA): http://audiosensibility.com/blog/products-2/specialty-cables-occ-copper-and-occ-silver/#!/Statement-SE-OCC-Silver-RCA-Splitter-Cable/p/78179109/category=5528439 Or you can look for the Audioquest Y-splitter cable for about $20 or less, or something in between. |
Thanks, aux. I did use the bi-amping feature of the old Onkyo, and it sounded pretty nice, the problem is, I have Surround Backs, and that's the channel the amp uses for the second source. The Marantz does not have a bi-amp setting that I can find in the setup anywhere (probably because it is only a pre/pro), hence the splitting of signals from there to the Rotel. Does anyone know if the Front Height or High outputs are consistent with the Front Mains? Or are they a portion of the signal based on the size/crossover settings?
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More follow-up: The Marantz does have bi-amping capabilities buried in the Amp Assign menu of the setup. You must set it to 'SPKR-C'. Unfortunately for me, anyway, it requires the use of the surround back channels. Oh well. Decisions, decisions. ;-)
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Please excuse my posting frequency, but as you might guess I stick with stuff until the best solution is found. From the FWIW column, the HL and HR are the solution for bi-amping, IF you realize that they are also tied to the Speaker B setting on the front of the pre/pro. I selected A+B and viola`. Thanks to all for your input and help.
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