Bi-amping and equalizers

Discussion in 'Home Theater Projects' started by Jack N, Dec 24, 2003.

  1. Jack N

    Jack N Agent

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    I will soon be bi-amping my Yamaha 3300 with (3) Yamaha M-85's, and an M-60 for the effects channels. I plan on using the 3300 to drive all the tweeters, and the power amps to drive the mids & lows. Rather than using the conventional method of hi/lo pass filters, I plan on putting a 12 band/channel EQ in each channel for the power amps. I'm thinking that this will accomplish the following: 1) It can be used as a tailorable high/low filter by dropping the frequencies not wanted 2) It would give tonal flexibility to each individual channel of each power amp, and 3) I would be able to match the volume levels of the power amps to the receiver's amp. Sound feasible? [​IMG] Opinions please. Thanks.
     
  2. Wayne A. Pflughaupt

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    Not a good idea, Jack. An equalizer is for changing frequency response. The individual sliders are bandpass filters and therefore do not function as a high or low pass filter.

    The problems here are myriad.

    First, the response curve you “draw” with the sliders is a poor representation of what is really happening in the “real world.” The filters on equalizers like this don’t make razor-sharp transitions, they cut a very wide path. For instance, if you pull one band – lets say 500Hz - all the way down to –12dB you may think you’ve notched a narrow area, but this is not the case. Frequencies on both sides of the slider are also drastically pulled down, as far out as 3-octaves in both directions, even if you don’t move those sliders. So even though you haven’t touched the adjacent 250Hz and 1000Hz bands, response at those frequencies will be down as much as 8-9db. The next adjacent filters at 125hz and 2000Hz are down at least 4-6dB. And so on.

    So you haven’t notched out a relatively narrow area, despite what the slider positions indicate. You’ve cut a hole in your response something like 4-6-octaves wide.

    Perhaps you can see now that you will never get anything resembling the action of a high- or low-pass filter like this. If you’re filtering for the tweeter and let’s say, leave the bands down to 4000Hz flat, and cut everything below that, you don’t get flat response to 4000Hz and a sudden drop below that point. Your response will actually start rolling out as high as 20kHz.

    Another situation: A real high- or low-pass filter continuously rolls out the signal at a rate of so-many decibels per octave. So even with a rather shallow 12dB/octave filter, the signal will be down 24dB at the second octave, and 36dB at the third. By the time response reaches say, 60Hz response is down more than 70dB. So there’s no way anything happening at 60Hz would ever be “hot” enough to affect the tweeter.

    This is not the case with the EQ-as-crossover scenario you’re trying to do. The maximum roll-out you’ll get is 12dB, period. Below that response will shelve flat. So as with everything beyond the first octave, response at 60Hz and even below will be down a mere 12dB. It’s a highly likely scenario that some explosive action flick on DVD will transfer high levels of low freq energy to the tweeter, because the bass signals will be boosted high enough to effectively eliminate (flatten) the 12dB roll off.

    An equalizer is not the best tool for level matching, either. Any bands you boost, you’re limiting the equalizer’s headroom at that frequency by the same amount you’ve boosted. Same if you try to use the equalizer’s master gain control: any (+) dB value you dial in to get more overall output is that many dB in lost headroom. This is especially critical when you’re using the EQ in-line between the pre-amp and amps, like you are, which can result in much hotter signals going through it than if the EQ were connected to a tape monitor loop (like most EQs for home use are designed for).

    Jack, I respect the fact that you want to do biamping right, with electronic filters instead of the speaker’s built-in passive crossovers. But my advice is to wait until you can get a good-quality professional electronic crossover. You will save on EQ's too, 'cause you won't need a whole stack of them, only one in front of the crossover.

    Regards,
    Wayne A. Pflughaupt
     
  3. Jack N

    Jack N Agent

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    Wayne,

    Thank you very much for taking the time to answer. I was aware of the "Bell Curve" effect of each slider and was going to try and compensate for that, but I wasn't aware that the curve was so wide. I especially want to thank you for bringing up the fact of the reduced headroom. Very good point. If I stop and think about for a second, it only makes sense. I've been keeping an eye out for good crossovers & filters just in case my idea didn't work, but I haven't found anything that I particularly care for. Can you make some recommendations as to where to look?

    Thanks,
    -Jack
     
  4. Wayne A. Pflughaupt

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    With the kind of equipment you’re using, you might look into either Rane, dbx or Ashly pro-audio crossovers. I’d avoid anything cheaper or lower-end than those. All three brands feature 24dB/octave filters, although you can find older Ashly products on eBay that have 12 or 18dB/octave filters.

    The various brands have unique and useful features.

    The current Rane models includes a control to time-align the high frequency driver with the others.

    Ashly models have a control that allows for changes from Linkwitz-Riley to Butterworth or other filter alignments. The effect for the end-user is adjustments to response around the crossover frequency. Ashly also has multi-channel models that would be desirable in a home theater situation.

    The dbx Driverack products might be of particular interest to you, since they include both filtering and various types of equalization in a single digital processor.

    However, generally I’m a little distrustful of pro-audio gear for hi-fi applications, so study the noise/THD etc. specs carefully before you buy, and make sure you have a generous return policy in case you find the crossover is as clean as it should be. The one and only time I used a pro audio crossover in my system (to divide the main L/R signal for subs) I found it audibly colored the sound of the main speakers. So you might want to spend some time listening to the one you choose before pronouncing it a “keeper.” I suggest doing this with speakers you’re familiar with, with the crossover in two-way mode and the crossover frequency set as low as possible (to get as much of the frequency spectrum to the mains as possible).

    Another option might be to find some out-of-production AudioControl Phase Coupled Activator Series III components on eBay or somewhere. I would trust them to be cleaner and more accurate than the mid-line pro brands. These units had a built-in sub crossover, but the frequency can be easily set anywhere. It’s not as user-friendly as the pro models, however; the crossover frequency is set by a plug-in module. AC sells them in various frequency values, or if you’re handy with a soldering iron you can easily make a module for any frequency setting you desire. But the nice thing is that once you have everything set up, your crossover settings are tamper-proof.

    Another alternative – perfectly functional if not quite as elegant - would be to use high-quality car audio crossovers from companies like AudioContol or Phoenix. All you need to power them is an AC adapter with the correct voltage and amperage rating. One nice thing about car audio crossovers is that you can get them in multi-channel configurations, which cuts down on the amount of hardware needed. The A/C crossovers are like the PCA III for home use that I mentioned earlier, using plug-in modules for the frequency settings.

    One thing you have to keep in mind, though, is that active bi-amping can be risky business, and it’s easy to accidentally damage or destroy the drivers. The crossovers, especially, should be secured so that the settings can’t be changed or (even worse) defeated with a “bypass” button. Fortunately there are security covers available for pro audio equipment.

    Plus there is always the danger of transients during power-up and shut down. This is critical IMO because the tweeters in home theater speakers are quite a bit more fragile than the high frequency horns used in PA systems. So I suggest that everything be connected to a power-sequencing device. It will activate the crossovers to make sure the tweeters are fully protected before the amps kick in.

    Regards,
    Wayne A. Pflughaupt
     
  5. Jack N

    Jack N Agent

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    Hi Wayne,

    Thanks for the tips. I'll definitely check them out. I'm wondering if you've had any experience with either of these: Harrison Laboratories FMODs, or Pyle Audio Pro PYX3X. They can be found at hlabs.com/technical/crossovers/ and pyleaudio.com/itemdetail.asp?model=PYX3X

    I've already started watching eBay to see what comes up there. And I hadn't given the car audio thing any thought, but you're right, all it would take is an adapter. I'll check into that too.

    One nice thing about the old Yamaha power amps, they have a built in delay circuit so that everything has a chance to settle down before any signals are allowed to go to the speakers.

    Like I say, I'll definitely check into your suggestions, however I would like your feedback on the Pyle and Harrison units. The Harrison FMODs are really intriquing to me because it's such an ingenious idea. They're simple, tamper proof, adjustable by adding what you need, and cost effective. Kinda makes you wonder if it's one of those "too good to be true" things.

    - Jack
     
  6. Wayne A. Pflughaupt

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    I’m familiar with the FMODs but I don’t have any personal experience with them. You can probably get more feedback on them from the DIY forum or one of the car audio forums. The only downside that jumps out at me is the inability to set relative gains of the independent high and low channels. But if your amps have gain controls, this is not a problem.

    I’m not familiar with the Plye pro products either, but I must say the price of the crossover you’ve mentioned is so low it’s a little scary. However, on paper its specs are on par with Ashly or Rane, so perhaps much of what you’re paying for with the latter is the rugged construction (which is obviously unnecessary for home use).

    Still, the S/N specs (around 90dB) and frequency response deviations (.5dB + or –) for all these products (including Rane and Ashly) is less than stellar – AC’s PCA III specs are far superior. Aside from perhaps the FMODs I think I’d be looking real hard at the better-quality car crossovers.

    Regards,
    Wayne A. Pflughaupt
     
  7. Jack N

    Jack N Agent

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    Wayne,

    I've checked out your suggestions and as many others as I could find. I've found that almost all of them use the XLR connections and not the RCA connections. I'm not familiar with the XLR connections at all. Do these connections use a different signal? Can RCA's be adapted to them some how? Any info you can throw my way would be helpful.
     
  8. Wayne A. Pflughaupt

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    XLR connectors are typically for balanced audio signals that are commonly used in pro-audio applications, where the signal (-) is separated from the shield (which isn’t the case in home audio).

    If you double-check, you will probably find that the same components also have 1/4” connectors. You can use those instead. It’s pretty easy to find 1/4”-RCA adapters at Radio Shack or a guitar shop.

    Regards,
    Wayne A. Pflughaupt
     
  9. Jack N

    Jack N Agent

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    Wayne,

    Thanks for the info on the 1/4” plugs. With that info in mind, and your advice to stay away from the low end stuff, I find myself leaning towards the DBX units – the 223 or 234. From what I’ve read, they seem like a pretty good buy for the money (in my case I need to buy 3 units). Electrically, my speakers are 2-way (various models of Definitive Technology) so the 223 should suffice. For $30 each more, the 234 would be good insurance for the future in case I should change to 3-way. Because of that, I find my myself leaning towards the 234, but I’m not sure how I’d hook up a 3-way crossover to a 2-way speaker. Opinions? Info? Thanks.

    Speaker question: Because all of my speakers are made to be bi-ampable, do I still need to disconnect the internal crossovers? I’m unable to hear any bleeding at all to unintended drivers when using any given set of binding posts.
     
  10. Wayne A. Pflughaupt

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    Quote:
    but I’m not sure how I’d hook up a 3-way crossover to a 2-way speaker. Opinions? Info? I assume for the time being you’ll be using the receiver’s crossover for the sub? Just make sure the crossover point between the lows and mids of the three-way goes lower than the receiver’s low pass. Shouldn’t be problem - I think most of these crossovers go a low as 40Hz.

    For connections use the high-out and mid-out. Setting the mid/low crossover point to its lowest setting should put you in business.

    Quote:
    Because all of my speakers are made to be bi-ampable, do I still need to disconnect the internal crossovers? Absolutely. There’s no advantage to active biamping with all those passive elements in line. Just make sure you use the same crossover frequency as the original passive crossover. (Actually, considering that most passive crossovers have 12dB/octave slopes and that most professional electronic crossovers have 24dB/octave slopes, you mwy well be able to safely lower the crossover frequency a full octave.)

    In addition you will find that with the internal crossovers bypassed the speakers will be louder – passive crossovers rob power considerably.

    Which brings up another point –active biamping doesn’t require near the power that passive biamping does. In fact, some info I saw from Harrison Labs years ago suggested that active-amping a three-way speaker would only need about 25 watts per driver (75 watts total) to get the same output that you would get with its passive crossovers in place and driven with about 250 watts.

    Regards,
    Wayne A. Pflughaupt
     
  11. MikeCTM

    MikeCTM Auditioning

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    hi i use the dbx 234 as my crossover currently. i'd say the sound quality is excellent. that's practically all that matters to me. real interestingly also, i first used my audio control DXS digital/elec crossover to do the job. i had to go to radio shack first ofcourse i get the proper $15 power supply. i didn't like that set up at all. in that arrangement i was getting noise in my system. could have easy been something i was doing wrong though. i then shopped and got the dbx and it ofcourse, like i said, kicks ass. i hate dis-agreeing, but if someone asked me, i'd say to not incorporate a audio control unit into the home audio system.

    also i'm sure wondering what high-end crossover to eventually get, when i have the money, to compare the dbx to. i've been looking at the Bryston 10B, the Marchand XM9 or XM44, or the IRD Audio PT-2.
     
  12. Wayne A. Pflughaupt

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    Mike,

    First, welcome to the Forum!

    I would expect that the power supply was solely the cause of the problems you had with the DXS. You didn’t say what kind of power supply it was, but it’s not uncommon for those things to deliver less-than-pure DC. If the power supply had any AC artifacts it’s a safe bet you would be getting some kind of noise.

    Audio Control’s products are among the best of their kind in every category they participate in, especially their equalizers and crossovers. I’ve been using their PCA III as a crossover and the C-131 1/3-octave equalizers for eight years now, and they have performed flawlessly. I did quite a bit of research on both professional and consumer products before buying, and found that no one could touch their specs, especially for S/N ratio and THD. For instance, the C-131’s specs easily bested professional equalizers costing three times as much. The PCA ‘s S/N ratio, for example, is fully 16dB better than the dbx 234.

    It seems a bit unfair to dismiss their entire home line because of a misused application.

    Regards,
    Wayne A. Pflughaupt
     

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