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Martin Logan's cross-over (not sure which speakers you have) with passive bi-amping allows each (hi / lo) connections to only draw power in that bandwidth. While active bi-amping usually yields superior sonic results, this passive bi-amping is very effective and less expensive than having to also buy an active cross-over. You do get significant benefits of passive bi-amping in this configuration over higher powered amps with bi-wire. The reason is the electrostatic panels can go to relatively low impedance. Depending on the music there may at the same time be a large bass spike (kick drum or something equally demanding). This asks for a large amount of power to the woofer, while the impedance is very low. This is an extremely difficult demand to the amplifier. By bi-amping (passive) these two demands are separated, so one amp can be handling the electrostatic (low impedance) demands and the other can be handling the bass (high power/impact).
Not so fast. With passive bi-amping, both amps still see the full range of the input signal. Yeah, you get twice the power if you use one more amp like you had before but you don't get the benefits of limiting the bandwidth of the INPUT signal to each amp. This is what active bi-amping is all about and the benefits in that case are considerable.
Now having said that, passive bi-amping can be attractive with some speakers. If the high & low pass sections on the ML's are isolated from each other when configured for bi-wiring, then, during passive bi-amping, each amp will only see 1/2 of the x-over instead of the whole thing. As such, your amps will probably deliver more current to the individual drivers than a single amp would of the same combined wattage looking into the entire x-over. This also means you will have slightly better control over each driver as well.
Also, as is usually the case for electrostatic panels, as the frequency goes down impedance goes up, sometimes considerably. If this is so with the ML's, then the amp would be asked to supply more voltage than current at lower frequencies. This may reduce the stress on the amp running the panels and may be audible. Similarly, as the frequency rises, there is probably an increase in the impedance of the low frequency section. It won't be as dramatic as the rise of the panel, but it too may help to reduce the stress on that amp and may also be audible.
When I passively bi-amped my ML Quests by using another Forte 4a (yes, that amp did a great job with the Quests), I immediately noticed a step up in quality, with all the usual suspects (slam, soundstage, transparency...)going up a notch. The basic charater of the combined amps was the same as a single amp, just noticably more refined and dynamic. The point here being, if you have an amp that you like with your ML's but want more power, just add another amp.
If you do decide to passively bi-amp, one thing you need to pay attention to is to use a HIGH QUALITY splitter (assumming your pre-amp does not have an extra output set)and use identical, high quality interconnects all around.
1953 makes a good point which may have not been clear in my first post. The input to the amp is the full bandwidth (as 1953 correctly stated), but the load is only the hi/low that it is going to, so the demands on the amplifier as I stated in my first post are limited by the passive crossover. It will, as 1953 mentioned, make a substantial improvement.
Like other things audio, it's controversial, but like mono-blocs, bi-amping allows for use of (potentially) very short speaker cables. Many claim significant improvements with the use of short speaker cables.
My amps will be back from SMc Audio (for upgrading & matching) next week, and I'm going to use them for passive bi-amping (Vandersteen speakers cannot be active bi-amped without screwing up the internal X-over up) as recommended by Vandersteen and SMc. I'll post a short report when I've gotten some time on the system. Cheers. Craig.
craig, i tink yule like yer bi-amped set-up, but as yuve also had yer amps seriously upgraded, i imagine lotsa improvement wood be evident even if using only *one* amp in a bi-wire mode.
supakit, whether or not 4 channel for passive bi-amping is better than bridging & having 2-channel w/bi-wiring, depends, in part on the power-requirements of your speakers. if, for example, yer bass driver/s really need 200w to come alive, & yer 4-channel amp is, say, 100wpc, then bridging the amps to two-channel, w/anywhere from 200-400wpc, will prolly be better. but, if each channel of yer four-channel amp is more than enuff power for the load it'll see in a bi-amped configuration, it will prolly sound better when run bi-amped.
hope this helps, doug s.
ryllau, there are many pre's that have more than one set of main output connectors. but, i woodn't necesarily use this as the criteria for choosing a preamp. there are quality adaptors that allow ewe to connect a pair of i/c's to one output. even tho my pre has *three* outputs, i still have to use adaptors, because i use an active x-over which has only one set of hi-pass outputs. i run a pair of amps from my hi-pass outs to wertically bi-amp my monitors.
but, when bi-amping, ewe *do* have to be even more aware of impedance mismatches - adding a second amp to yer pre effectively lowers the input impedence the pre is seeing. for example, if ewe are using two amps rated at 20k-ohms input impedence, the preamp will *see* a 10 k-ohm impedence. and, longer i/c's, which can be attractive in a wertically bi-amped set-up, to reduce the length of the speaker cable, can further exacerbate any impedance mismatches. basic rule of thumb is that yer amps' input impedance needs to be *at least* ten times greater than the output impedance of the preamp, & in some instances, even this amount may not be enuff.
hope this helps, doug s.
I have a pair of Legacy Classics I am driving with a Sunfire Cinema. I have tried driving them with single channels then biwiring and then passive bi-amping and there was a steady improvement in the sound. More highs and lows with each change and better clarity and definition. A rather interesting thing, which I can't explain is that the speakers response got much flatter when I went to vertical biamping (as confirmed by my SPL meter). I had a +3 to +5 dB boost over much of the bass range and after the vertical biamp, it's now +-1.5 dB down to 40 Hz. Now I'm just waiting for the funds to go with Active crossovers...
(Someone with an EE can say this better than I, but bear with me.) When you passively biamp you are removing the crossover elements of the high and low pass sections of the speaker from each other. Because there is no series or parallel interaction between these elements, the impedance variations with frequency the amp sees are gentler (and the lower amp is not forced to put high current into the high frequencies.) The frequency response improvement is probably due to the smaller impedance slope,phase angle, and possible higher overall impedance from the bass to the mid range and from mid range to highs when you biamp compared to it run as a single speaker. This slope/phase angle causes frequency response/tonal interaction with the amps. As the slope approaches zero (ala a classic 8 ohm resistor)there is less frequency reponse interaction, all other things being equal, (eg see the Stereophile tests of many tube amps to see such a plot.) Bravo on your disciplined investigation and sharing of information that truly benefits this discussion.
Thank you for that explanation. I have done quite a bit of reading up on the web on passive biamping and most of the discussions of improvements seemed to focus on the benefits to the higher frequencies due to lessened intermodulation distortion with the lower frequencies, so I really wasn't expecting the result to be apparent in the bottom end. (Although I'm very, very happy with the response I got).