Dear Steve, If your subwoofers have built-in amplification designed to receive signal from a line-level source, you will have a big problem driving those amplifiers from the output of your main amplifier; it won't work. The signal voltage is much too high at the outputs of the main amplifier to drive an input designed for line level. REL specifically designs their subwoofer amplifiers to be driven by the output of a primary amplifier. If on the other hand you are thinking of just driving the subwoofers in parallel with your main speakers, from the same single amplifier, then in part you lose one of the benefits of using a subwoofer, which is that bi-amplification removes much of the burden from the main amplifier. No free lunch.
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I agree with the substance of Raul's advice. In certain circumstances, one can get away with a first-order hi-pass crossover, which can therefore consist of nothing but a small value capacitor in series with the input of the amplifier that drives the main speakers, affording the least possible coloration. Then you can add, for one example, an active lo-pass crossover to drive the woofer amplifiers, which can be much steeper in slope.
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Raul, I agree. For clean low bass, SS is best. Also, I like and respect Duke Lejeune very much, and I applaud his success with AudioKinesis. My problem when it comes to placing four subwoofers in my living room is where to place them.
But in my Beveridge system, I use a Threshold amplifier to drive my home-made transmission line woofers for all frequencies below about 100Hz, and the results are excellent. Dahlquist crossover receives the output from the preamplifier and provides the signal to the Threshold. All solid state.
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Tim, Was this last post intended to make the rest of us feel good about the prospects of installing and positioning four subs in a listening room? If so, it didn't work. In my case, the listening room is our living room. There is no video in the room at all. Apart from the physically huge audio system (Sound Lab 845PXs on the front wall and three turntables on the back wall, for example), the decor is art deco with a lot of antiques and original art works. It's a pleasant place to be away from video, a bit smaller than your room but similar in proportions. I would hate to have to watch out for tripping over subwoofers whilst walking around the room to change records, or when we are entertaining guests at parties. I'll talk to Duke. |
I've resisted using a subwoofer (or really a woofer based system, not really a "subwoofer" per se) with my Sound Lab 845PX speakers for several years. The reasons for my recalcitrance are as follows:
(1) I've never met an electronic crossover that did not color the sound in some objectionable and noticeable way.
(2) I am getting tremendous low bass response from my 845PXs with no added woofer (but see below).
(3) I ascribe to the idea that one-way systems are a virtue; I have revised the electronics in my 845PXs to eliminate the filtering that precedes the two audio step-up transformers. (This is another long story not suitable to this thread.) There is no crossover in my system anywhere and only one 0.1uF capacitor in the signal path, from the line level input of the preamplifier to the speakers.
(4) I envisioned that adding a woofer system would require introducing at least two large-ish and probably ugly cabinets into our living room, and I am or was loathe to do that. Esthetics matter to me.
However, the price I pay for this policy is in fact that I have too much bass. I do perceive suboptimal resolution and definition at the extreme lowest frequencies, where there is not so much a "peak" but a gradual rise in speaker output. I use Tube Traps behind each SL panel to ameliorate this problem, but it is not a perfect solution. I believe this is because the SL speakers rise in impedance as frequency goes down. (I've measured this.) Since I use OTL tube amplifiers, this results in a broad rise in speaker output, inversely related to frequency. (The OTLs love high impedance.) Lately, I perceive there are ways to overcome my reservations about electronic crossovers, either by using only a capacitor at the amplifier input, to roll off the 845PXs at low frequency, or by using any of a very few electronic crossovers that may be acceptable to me. In addition, one of you guys pointed out that a pair of woofers could be placed behind the panels, out of sight, and that the cabinets need not be huge.
Over the years, I have made this system better and better. Now I think the only thing left is to take a shot at adding a woofer supplement. Duke LeJeune probably knows more about supplementing SL speakers than anyone, and I intend to consult with Duke on the best course of action. |
Tim (Noble100)
We are on the same page. However, I would be doing it for the gain in bass definition and overall frequency balance, more than to relieve my primary amplifiers of any stress. The 845PX's normally use a 6db/octave passive crossover in front of two different audio step-up transformers that receive, respectively, bass and treble frequencies after the x-over. This configuration, however, creates a nasty impedance dip at mid-frequencies, in addition to sucking energy across the resistor used in the hi-pass filter. 2-3 years ago, I replaced the treble transformer with a very high quality full range transformer, which enable me to ditch the RC network that creates the hi-pass; I still use an inductor in front of the original SL bass transformer, albeit I replaced the OEM inductor with an 12-ga air core type. This mod hugely improved the efficiency of the speaker; I daresay it could be driven by less than 50W to very loud sound pressures. My OTLs can do about 100W, and they are loafing at all times. It also dramatically improved dynamics. I measured impedance v frequency across the audio range, after the mod, and I see 20 ohms or greater from about 100 Hz to 5kHz. Below 100 Hz, impedance is rising. Above 5kHz, it falls to about 6-8 ohms at 10 Khz. This is an ideal match for an OTL.
I did mention that someone here had pointed out that two of the 4 woofers could go behind the panels. Sorry for not remembering to credit you with that insight. I've known Duke for a long time; I bought my 845s from him. I will give him a call soon. On his website today, I do see the Swarm, but I don't see the Debra system. I'll look again. |
Thanks, Noble. I did find the Romeyn website for the Debra, by searching on "Audiokinesis Debra", oddly. As you note, they are similar in price and very similar in composition. So this begs the question, why are two different and so similar products co-existing? Do you have a feel for the audible differences between the two? And, a question I will address to Duke as well, does the set-up provide for a hi-pass filter to feed the amplifier that is driving the main speakers?
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Raul, I assume you’re referring to the electrical phase of the signal delivered to your subwoofers with reference to the electrical phase of the signal delivered to your main speakers. This is known to make a difference. But usually one can only alter the phase relationships by 180 degrees, by reversing leads or inserting a phase switch. Are you able to test differences in phase that are >180 or <180 degrees but more than zero degrees? If so, how?
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