mm@mvuxd.att.com (Martin H Meyers) (01/03/91)
I am interested in bi-amping by B&W 801s (currently bi-wired with Tara Return). I have a lot of questions: 1. How much power is required for the tops (mid-range and tweeter) where the cross-over is, I believe, around 300 Hz? I'd like to use a low to medium power tube amp (less than 100 watts per side). How much is necessary? 2. Is an active cross-over required or can I simply feed the passive crossover already used for bi-wiring? I have a dealer who says I need an active crossover if the top and bottom amps are not identical. Could someone give me a technical explanation in more detail. 3. I am currently playing around with modifying an ST-70 (Curcio mods ala Glass Audio). What I do not have is any original documentation or schematics for the ST-70. Could someone please send me a copy? I'll glady reimburse for any copying charges, etc. Anyone with experience with the Curcio mod or other updated ST-70s? 4. The ST-70 has a mono switch which I assume (no schematic) doubles the input feed and parallels the outputs to double the output power to 70 Watts mono power. How does this sound? Any compromises involved in bridging to mono? Can bridging to mono be done with the Curcio mod as well? 5. The Dynaco is currently driving my bedroom speakers. Would anyone hazard an opinion on using them on the top of the 801s???? Thanks in advance, Marty Meyers
hull%janus.Berkeley.EDU@ucbvax.Berkeley.EDU (Christopher Hull) (01/08/91)
In article <8661@uwm.edu> you write: > > I am interested in bi-amping by B&W 801s (currently bi-wired with >Tara Return). I have a lot of questions: > >1. How much power is required for the tops (mid-range and tweeter) where >the cross-over is, I believe, around 300 Hz? I'd like to use a low to medium >power tube amp (less than 100 watts per side). How much is necessary? > >2. Is an active cross-over required or can I simply feed the passive crossover >already used for bi-wiring? I have a dealer who says I need an active >crossover if the top and bottom amps are not identical. Could someone give me >a technical explanation in more detail. > You can use a passive crossover, however that will negate most of the advantages of bi-amping since both amps will have to deal with the whole spectrum. In that case both amps will need to be high powerd, especially the one for >300hz, since its distortion will be most audible. If you use a low level crossover (either electronic or passive, but before the amps!) than each amp can be about 1/2 of the power you would normally require. If you are going to equalize your subwoffer than you would need more on the bass amp. I recommend that the high pass part of the crossover be either passive or tube, since you will introduce audible distortion with a solid-state x-over for frequencies > 300hz. Unfortunately, the B&W were designed for steep x-overs I believe. If you want to use slopes > 6db/octive than you will need to use an active x-over for sure. In that case I recommend tube x-overs or hybrids. Chris Hull hull@janus.berkeley.edu
strong@tc.fluke.COM (Norm Strong) (01/21/91)
In article <8754@uwm.edu> hull%janus.Berkeley.EDU@ucbvax.Berkeley.EDU (Christopher Hull) writes: }In article <8661@uwm.edu> you write: }> }> I am interested in bi-amping by B&W 801s (currently bi-wired with }>Tara Return). I have a lot of questions: }> }>1. How much power is required for the tops (mid-range and tweeter) where }>the cross-over is, I believe, around 300 Hz? I'd like to use a low to medium }>power tube amp (less than 100 watts per side). How much is necessary? }> }>2. Is an active cross-over required or can I simply feed the passive crossover }>already used for bi-wiring? I have a dealer who says I need an active }>crossover if the top and bottom amps are not identical. Could someone give me }>a technical explanation in more detail. }> } } }You can use a passive crossover, however that will negate most of the }advantages of bi-amping since both amps will have to deal with the whole }spectrum. In that case both amps will need to be high powerd, especially }the one for >300hz, since its distortion will be most audible. } }If you use a low level crossover (either electronic or passive, but before }the amps!) than each amp can be about 1/2 of the power you would normally }require. If you are going to equalize your subwoffer than you would need }more on the bass amp. I recommend that the high pass part of the crossover . . . I must disagree with the statement that each amp can be 1/2 the power you would ordinarily need. Unless you spend your time listening to white noise, both amps will have to be full power--especially the bass one. This arises because you don't know what note is going to tax the amp to the limit. In my experience, it's usually in the bass end. -- Norm Strong (strong@tc.fluke.com) 2528 31st S. Seattle WA 98144
peter@rex.cs.tulane.edu (Peter Tapscott ) (01/23/91)
[Many lines of reference deleted. -tjk] I was trying to not write about this, since it seems like an interseting topic that could require a lot of time to discuss thoroughly. Oh, well, here goes: I see you having three options: (1) Simple Bi-amp with two stereo amps (or 4 mono amps) (2) Low-level crossover, probably an active crossover (3) A hybrid of 2 & 3, probably a passive low-level crossover. To explain: (1) This means dedicating a stereo amp to each speaker. The crossovers in the speaker are used to isolate the drivers from the parts of the frequency spectrum that are not needed, as they do now. The advantage here is somewhat similar to bi-wiring if you only consider the output impedance of the amp. What I see being of more importance is the greatly reduced current requirements of the unused frequency portions. The amp still has to produce the VOLTAGE it did before bi-amping (as Norm points out above), but the current requirements are virtually zero for frequencies outside the band allowed by the speakers crossover. That is, the bass amp only produces current for 300hz & below, the mid-tweeter amp only produces current for 300hz and up. Each amp still has to produce volatge for the entire spectrum. Back when I used to make a lot of prototype amps, it was really easy to make an amp that measured perfectly (as read by my crude instruments) with no current requirements. Add a real load, though, and all sorts of distortion showed up. This was particularly true for crossover distortion. (2) With an active crossover, you can tweak a lot of parameters, such as crossover frequency, slope, (perhaps) Q, type (Butterworth, etc.) The internal crossovers in the speakers are disconnected. Your power requirements will decrease with frequency, since the amps now have to only produce the voltage and current for their specific frequency ranges. A modest sized tube amp for the tweeter section might sound great. (3) This is a hybrid I thought of from your description. This method still uses the crossovers in the speakers. Assuming that your preamp has a substantially lower output impedance than the power amp input, you could use a simple passive filter of one or two poles, using high quality capacitors and resistors. I would place the frequencies of these filters so that the crossovers in the speakers completely control the drive to the drivers, but so the amps don't have to produce voltage for the entire audio spectrum. For example, the bass amp could have a frequency response of 500hz, so the 300hz crossover in the speaker is providing the rolloff, not the crossover I describe. ADVANTAGES AND DISADVANTAGES: (1) AD: Simple Biamp: any one can make it. You need a good Y connector if your preamp has only one output, but this could be made easily. (You can buy low-fi Ys at Radio Shack.) This method is recommended by Vandersteen for the 2Ci, BTW. The NAD integrated amp manual I saw that suggests upgrading by Bridge-amping the internal power amp to one speaker, and buying a second power amp to bridge to the second speaker, I would be interested if better sound doesn't result from simple bi-amping as described here. (Perhaps a reader has this setup and could run the experiment?) DIS: Requires four identical amps. The mid-tweeter amp will just be idling most of the time, in terms of power output. (2) AD: Active Crossover: Really a nice way to match drivers that have different characteristics. you don't have to use identical amps: amps can be tailored to the drivers. DIS: The B&W 801s have very carefully designed crossovers. They might even have some form of passive frequency compenstation built in (I don't know). These crossovers have been proven with instrumentation far beyond what most audio experimenters have, so I would use caution in trying to redevelop the crossovers. Also, a high quality avtive crossover is expensive. This is an expense you could put into buying better amps, or even records & CDs. (3) AD: Hybrid technique: as with the active crossover method, identical amps are not required. If you want a transistor behemoth on the bottom end and a tube gem for the high end, fine. With a low crossover, like 300hz, though, I would not make the mid-tweeter amp underpowered. This contains most of the information you listen to. (For example, telephones have about 300-3Khz response, and we can clearly understand the spoken word even though the fundamental frequencies for many voices are missing.) You must depend on a well designed crossover, but that is not a problem with the B&W 801s. If you were talking about <$500, it may come into play, but not for the multi kilo-buck speakers. DIS: you have to make the crossover. This is easy enough for me: a couple resistors and capacitors, but if you aren't an electrical engineer, and a pretty competant technician, this could be a problem. Of course, if you need help, I bet you could be able to hire some consulting help from the net (not me). BTW, I use a slightly different version of #3 for my subwoofer system. The crossover took about 20 minutes to design and about 20 minutes to build. Scrouging parts was probably another 20 minutes. But I never said that it is a work of art. (2-pole passive filter.) Will the original poster please post the results of his experiments? Thanks! -- |----------------------------------------------------------------------| | peter@versatc.VERSATEC.COM -OR- {ames|apple|sun}!versatc!peter| | Peter Tapscott - Xerox Engineering Systems, Versatec Products | | 2805 Bowers Avenue, Santa Clara, Calif (408)982-4235 |
jj@alice.att.com (jj, like it or not) (01/25/91)
Anyone seriously considering "real" bi-amping
(which means a low-level crossover in front of the amps)
should probably look into various asymetric designs for
crossovers.
Why? Because you wind up with (for instance)
a low pass of and a high pass of
2s+1 s*s
________ _________
s*s+2s+1 s*s+2s+1
Which, you note, adds up to 1, with no phase-problems,
and no amplitude problems.
This does have a problem, in general you wind up with slower
crossover points somewhere, usually at low freq's.
I have done this with a machine-optimized 5th order
transfer function, where the highpass looked like third order
and the low pass like second order. The q of all the
sections was quite low, because of the design, so I think
you COULD even build it with valves for that IM distortion
warmth, if you'd insist.
You just have to do the math, and then figure out the
parts values for ONE of the filters.
You get the other one by direct subtraction, thus taking care
of any significant parameter problems around the crossover
point.
-----
You can make this an article if you want. I don't have
time.
jj
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