Food for thought. Picture an amplifer playing full range into a speaker load. By using two different terminals to feed the upper and lower portions of a system we still have an amplifer playing full range into a speaker load. Internally the crossover splits the signal but it doesn't change the amplifer playing full range. Now picture a speaker with a network that is designed to be crossed over externally. We supply an electronic crossover rolling off the amplifer at the desired frequency now creating a more efficient system. If a speaker can be driven without an electronic crossover it is not intended for biampflication. You may be doing more harm than good. Phase,alignment, load, etc. Food for thought. |
Hi folks, I just want to let you know that the problem has been solved. I asked Peter Thompsen from Acoustic Reality and he has been very kind to get into some analysis of the problem. Due to his suggestions it is now clear what the culprit of the problem was: it was probably related to wrong pin configuration of the XLR plugs. Thank you Peter for your suggestions. |
Dazzdax: Now we are getting somewhere. 4K is too low for most preamps and will result in the exact problem that you've described i.e. bloated bass response and trunctuated treble output.
My Dad ran into the same problem in his system several years ago. The dealer had set him up trying to drive a 5K Ohm amp with a preamp that had a higher output impedance. The sonics sounded bad, but much of that was also a speaker based problem. In effect, his whole system sounded bad, so you couldn't really point the finger at any one given thing. In this regards, you might be a lot luckier : )
The simplest solution here is to install series resistors either at the output of the preamp or inside each of the amps. While this will attenuate some of the signal fed into the amps, it should solve much of the tonal balance problems that you're experiencing.
The key here is to find a value that is low enough not to eat up too much signal while being high enough to remove the current drain from the preamp. If you were happy with the tonal balance of the system using one amp per speaker and simply wanted better but equivalent performance, installing 8 K resistors in series with the input of each amp or output of the preamp would put you back appr where you started at.
Personally, i would install the resistors in the amp as that keeps the signal level higher through the interconnects. This is beneficial in more ways than one. Secondly, i would probably use resistors in the 12K region, as this would bring the total load up to appr 10K or so when running both amps in parallel. Even if you chose to run one amp per speaker, the input impedance would be 20K, which is still quite reasonable and easier on the preamp. You should also keep in mind that different brands and types of resistors have different sonic attributes, so you may have to pick and choose your poison there too.
You might want to discuss this with the dealer / manufacturer at Acoustic Reality and see what they suggest. Hope this helps and solves your problems. Sean
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Thanks both of you Sean and Tom. You have been tremendously helpful. You are very knowledgable Sean, you must have a technical background I suppose.
Well, the Acoustic Reality has an 8 kOhm input impedance, which is low. An Accuphase representative stated that that wouldn't be a problem with the C-290V preamp I am currently using. But when splitting the signal at the end of the interconnect the amp's input impedance is going to be... 4 kOhms, which is very low indeed. How can I raise the input impedance of the amp? Would be using two pairs of interconnects a solution? The preamp has two pairs of outputs. I've been told though that the preamp actually splits the signal that goes to the amps, instead of driving the two outputs actively. So it will also "see" halved input impedance. |
Tom brought something up to me privately that i'm hoping all of us didn't forget to address. It is so simple that it is easy to overlook as we tend to take this kind of matter for granted. That is, the jumpers that connect the woofer section to the high frequency section should have been removed. If that wasn't taken care of, i would highly recommend doing so pronto : ) Sean
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You can unscrew the rear terminal panel and the entire crossover will slide out a tract..In this way you could diagram and inspect the crossover..These babies are ripe for component crossover upgrades..What a suprise lurking..Tom |
Dazzdax: I can't tell you what's going on, but you have obviously changed the impedance that the preamp sees. That is, you're dividing the signal between two amps rather than just feeding one. This lowers the impedance that the preamp sees by 50%, effectively cutting it in half. This in turn will draw twice as much current from the output stage of the preamp, which could account for yet another reason that the system sounds loaded down or bass heavy. Most preamps are horribly under-designed in terms of their output stage, hence designers trying to keep the input impedance of the amp up. Your system is now making your preamp work twice as hard and it may not like it at all.
The way that you are splitting the signal also creates different nodes in terms of voltage to current distribution within the cabling itself. This too could be a factor in why things sound differently than they did with just one amp per speaker. Optimally, the signal should have been separated at the preamp. Not only does this introduce less line loss by increasing the surface area, the amps also receive the benefit of better electrical separation from one another.
By splitting the signal closer to the amps, the potential for crosstalk from one amp to another is drastically increased. Given that most all amps suffer from their output stage modulating their input stage to some degree, the woofer amp may be modulating what the tweeter amp sees. Using separate interconnects all the way back to the preamp would have provided better isolation between the amps with the longer pathway acting as a buffer.
You've got a lot of variables going on here with the electronics in the system, not so much with the speakers. While it is possible that the speakers have an internal wiring problem inside of them as Tom suggests, you would literally have to pull the speakers apart and make a diagram of the crossover network. While Dunlavy was a good engineer, that doesn't mean that some knucklehead in production couldn't have made a mistake.
As a side note, we ran into this with my Father's Legacy speakers a while back. Sometime during construction of the crossover of these speakers, one leg of the midrange circuit was tied to the woofer circuit. Given that my Dad had never bi-amped these speakers ( only bi-wired them ), this wouldn't have created a problem since the entire circuit was all driven by the same amp anyhow. Having said that, had he tried to bi-amp the speakers, this simple mistake in production could have severely damaged both of the amps. That's because the mis-wired midrange would have electrically tied the two channels of the individual woofer and tweeter amps together. Sean
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PS... For as much money as some of you guys are spending on gear, you really need to find a technically competent dealer to work with. Buying and assembling a few components to work together and produce sound is completely different than building a complex audio system at the cutting edge of music reproduction. From what i've seen though, this might be more difficult than i've thought.
Most of the high priced dealers are either lacking in technical chops and / or more interested in profit margins than anything else. Keeping the customers on the equipment carousel i.e. "flavour of the month club", whether on purpose or due to lack of technical understanding, surely isn't going to hurt their income levels. That is, until the customer has had enough and either gives up entirely or learns to do for themselves.
Once you learn to do for yourselves and can understand basic electronics, it makes it much easier to cut through the marketing hype and snake oil. This is why i encourage folks to learn on their own, but at the same ti me, it is also why finding and supporting a good dealer is important to the survival of high end audio. Without them, we have no visible means of entry into this hobby to the general public. |
The pairs of Dunlavy drivers are 8ohm wired in parallel except of course the tweeter which is a single 4ohm driver. When I biamped my SC4s I thought the mids and highs stuck out more than the bass. I never used an RTA to measure this perception. Sean is correct about the crossover freq..of 200 hz. it uses a 8.2mh inductor to roll the highs out of the woofer and a 200 mic cap to roll the lows out of the mids. Could there be a dropping resistor in the mid crossover that some how gets bypassed when bi-amped?..That would be wierd..Tom |
Hi Sean, thx for the thorough explanation. I'm not a technician, but wouldn't a well designed loudspeaker system comprise drivers with the same impedance? Otherwise the system would not sounding balanced through out the frequency spectrum. But it "sounds" like there is some impedance mismatch (as I stated earlier) between the two halves of the cross over networks. What I hear is an almost caricatural reproduction of music, with a far too loud bass and recessed and undynamic midrange/treble. Btw, I'm also using the same speaker cables and a pair of interconnects, splitted by an Y-splitter to provide both monoamps with the same signal (in biamp configuration). |
I can't say for certain, but breaking the speaker load up into segments can surely alter the power transfer characteristics of how each amplifier in a multi-amp system responds. This is in comparison to how one of the same amps driving the entire speaker full-range would respond.
If one factors in that each amp will have differences due to production tolerances, and whether or not each amp is up to the task to begin with, only adds further variables to the equation. This is not to comment on Dazzdax's specific amps and / or installation, but to say that not all amps are created equally regardless of what the paper spec's say and that some simply aren't suited for driving specific types of loads.
Having large multi-driver acoustic suspension speakers with a similar crossover point ( appr 200 Hz ) that i've passively bi-amped, my experiences are probably not that far off from what Dazzdax experienced. The difference here is how we interpreted the changes in presentation and the type of gear being used.
The reduction in distortion that takes place with such an installation can sometimes be quite fooling. Not only does this alter our perceived level of volume, but also of dynamics. By increasing the headroom and reducing the demands on the amplifier, transient response is improved, distortion is reduced and in some cases, the audio spectrum shifts slightly.
The loss in volume is due to the way that our brain and ears process distortion. Distortion is "grating" to our ears, telling our brain "this is loud". Removal of that distortion removes that agitant, resulting in less perceived volume. This is why many folks find themselves playing their system louder than they used to prior to a component change, although it doesn't sound as loud to them at the time. Less distortion equals less "apparent volume". Extreme levels of distortion become more apparent for what they are, telling us "this is not necessarily loud, it is just extremely distorted".
This also factors into both the high frequency dynamics and the shift in the audible spectrum. Due to a reduction in distortion and improved transient response, there is less smearing involved. Smearing is a by-product of non-linear harmonic distortion ( THD ), slewing induced distortion ( SID ) and ringing ( poor transient response ), all of which manifest themselves in a more prominent fashion as frequency rises. If you doubt this, take a look at almost all distortion curves and figures in a Stereophile review. As frequency rises, the distortion by-products do too. The wider the bandwidth of the amp ( faster transient response ), the more level the distortion is across the audible frequency range.*
By removing these aspects of high frequency output that one would normally hear, the midrange and treble region becomes both more subdued and less of an "irritant". Our brain processes that as a lack of dynamics, even though the output is cleaner and ( theoretically ) more dynamic.
In terms of increased bass output, this can be contributed to several factors. The first is that the amp can now concentrate all of the available power that it can develop over a much narrower bandwidth. If the amp was current deficient and / or lacking in power supply reserve for reproduction of longer duration notes, we've now freed up more energy to deal with these problems. The end result as we perceive it would be greater output in the low frequency region than what was there before. This output didn't just show up somehow, it was always there. The deficiencies in the previous installation simply didn't allow it to shine through.
This is one of the biggest problems that i've found with "digital" or "switching" type amps. That is, the bass lacks both intensity and duration. What is there sounds very fast and articulate, but much of that has to do with the lack of duration due to the power supply "pooping out". By reducing the duration of the note, it sounds both "faster" and "sharper" but lacks weight ( duration ) and impact ( intensity ). I've commented on this before as it was something that i ran into with several different types of "new technology" amps, including my Sunfire Sig's.
In most speakers that suffer from poor damping ( aka vented designs ), this can actually sound more natural, even though it is a distortion of the original signal. In this case, one positive error ( excess bloated output from the speaker ) combined with a negative error ( reduced truncuated output from the amp ) sums to a relatively flat presentation. Some would call this "system synergy" whereas others would call it "complimentary colourations". In this specific case, they are basically one and the same.
In Dazzdax's specific case, his speakers don't lack damping ( highly damped low Q sealed design ), so this is not the case. Instead, the increased output capacity of the amp was directly translated into increased output at the speaker. Since this speaker is a relatively linear transducer of energy, more input equalled more output. Combine that with the simultaneous loss of "apparent" mid and high frequency energy and you end up with a completely different presentation.
Given that the electrical characteristics of the speaker remain the same on the whole i.e. separating the top from the bottom doesn't change the nominal impedance or transient capabilities of the speaker itself, the only thing that did change is how the amplifiers themselves would load up.
Bare in mind that this is kind of a "crash course" on this subject. It should help some to understand why they might have run into specific situations when trying to passively bi-amp, but in no means can i hope to cover all the variables involved. Not in one post or a hundred posts. Sean
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* Negative feedback can be used to try and flatten the distortion curve on an amp, but even then, the distortion will typically rise as frequency climbs. While a completely different subject, slewing induced distortion ( SID ) typically increases as more negative feedback is used due to a reduction in response time of the amp. In plain English, more negative feedback means a slower amp. This can result in increased glare and high frequency stridency i.e. "Solid State sterility". The spec's on paper look good in terms of distortion, but that's only because all of the spec's aren't available to fully interpret the performance of the circuit on the whole.
This is why i've always stressed high speed, wide bandwidth designs in every aspect of the system. This approach naturally combats distortion without the need for "band-aids" that only introduce other problems into the equation. |
I experimented with angled panels sitting atop my SC4's several years ago. The panels that I made could be angled backwards at varied setting,15 degree tilt at the top seemed best in my room..The bass was not only better but the upper midrange also became more focused..These same panels also worked with various Thiel models as well, same sort of results with more focus..My panels were made of heavy fiber board and covered with black trunk mat..lightweight and easy to remove..Having 4 identical amps..the bias may not be the same...so there could be some variation between all 4..3db is a whole lot of variation between 4 amps that are the same..Same cables on all 4, same IC's, speaker cables, and power cords, some hooked to different ac outlets or powerconditioners?..Tom |
Good question, Sean we need you! |
I started this thread because I did biamping before with four identical Acoustic Reality ICEpower amps (each with 500wpc), but when biamped there is a reinforcement of bass (some 3 dB or more) while the midrange/treble were a bit subdued and less dynamic. The whole picture is also less coherent. I still don't understand why such gross tonal shift occured. Some friends informed me about a possible mismatch between the bass and the midrange/treble cross over, but why should such a designer like John Dunlavy give each half of the devided cross over network another impedance? |
If you want to improve the performance of your Dunlavy's, rig up and secure as large as possible of a flat piece of wood on top of your speaker. This will increase bass coupling and improve loading into the room. You can vary the size, placement and amount of overhang in front of the cabinet to vary the amount of loading that you want and / or best suits your room.
The Dunlavy design is excellent, but one of the major variables that he couldn't control was the top woofer to ceiling distance. Using this approach, you've taken that factor out of the equation and can fine tune for best results.
Some may find that this somewhat limits vertical dispersion, but these speakers already have limited vertical dispersion as it is due to their design. If this is a concern, rather than use a flat baffle on top of the speaker, angle the board up and out from the baffle. This will still allow complete vertical dispersion into the room while offering improved bass reinforcement and better room loading than in stock form. This is kind of like using an angled "baffle extension" or "woofer beard" below a stand mounted monitor, but in reverse fashion above the cabinet. Like the above, changing the surface area and angle of the baffle extension / sounding board will allow you to fine tune for optimum results.
Other than that, you need GOBS of power with these speakers for best results. Wide placement on the long wall with no to very little toe-in also works best. If passively bi-amping, most speakers will work best with identical amps running the high and low frequencies. If actively multi-amping with a versatile electronic crossover, different amps can be used to great effect so long as one is willing to tinker quite a bit. Sean
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I'm not sure what passive bi-amping is, but I bi-amp my Dunlavy IVa with two Plinius SA-102 amps, one for the top posts, and one on the bottom. I have never heard Flmlamb's comment. I have talked to John Dunlavy many times over the years, he helped set me in the direction I went, so maybe I'm just not understanding your question. For my system the two amps are a significant improvement in bass definition and slam. With the fives it may be even more important. I would not go back after bi-amping with the last three amps I had, all on Dunlavy speakers.
My greatest concern is where will I ever find a better speaker for the cost once these are old and tired. |
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Dunlavy recommended NOT bi-amping, as he set it up to play with jumpers in. However, you can. The crossover is set lowenough however(around 200hz), that you shouldn't have any problems using a good "bass amp", like the Parasounds (for cheap on the bottom posts, and better amp on top!
I used to sell Dunlavy, but never biamped these. You'll have to try, which will really not be anything more than a small investment..then you'll know!
This is what being an adiophile is about really, tweaking and trying yourself you know...
good luck |
