Two pairs of speakers on a Yamaha AX-497

Discussion in 'Hi-Fi Stereo Systems & Separates' started by FrankG, Nov 27, 2010.

  1. FrankG

    FrankG
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    I have a question related to the topic of running two pairs of speakers with one amplifier. Whether it will be a wise thing to even try it, given what the amp manual says about minimum impedance.


    The amplifier is a Yamaha AX-497
    Specs: http://************/2594ktu

    I've just ordered a pair of Boston Acoustics VS 260 speakers

    Specs: http://************/25u2q3v

    I am currently using a pair of Yamaha Soavo 900M speakers.
    Specs: http://************/232vgqj

    I was thinking of using all four speakers to see how it sounds.

    The amp manual says that to use A+B, the impedance of each speaker must be 8 or higher.

    The impedance of the Yamaha speakers I am using is 6. The Bostons are 8.

    So the question is, will it be really bad if I try both to see how it sounds? Is it better to not even try it? Can I damage something by just trying?
     
  2. BlueWizard

    BlueWizard
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    This varies from amp to amp, but I've got a Yamaha amp (100w/ch) and ran one pair of 8 ohm and one pair of 6 ohm without any problem, as long as I kept the volume reasonably low, below 50%.

    The speakers were some 8 ohm DIY 3-way and the 6 ohms were Wharfedale Diamond 9.6.

    But I ran into a problem one night during a particularly demanding section of 'Lord of the Ring'; my amp shut down.

    Now I have a separate stereo amp (Onkyo 50w/ch) to drive the 8 ohm speakers, and my Yamaha drives my newer larger 6 ohm speakers.

    So, this can be done, but how reliably and consistently it can be done remains in question.

    Keep in mind for an 8 ohm speakers, the real frequency dependent impedance can vary from about 5 to 6 ohms up to about 50 ohms, with the lowest impedance just above and below the resonance frequency. Typically, the resonance frequency is about 30hz.

    But in the end, it gets down to current draw. Low impedance speakers mean a much higher current draw from the power supply and the power amps, and more current means more heat. If the current and the heat get too high, then the amp will (hopefully) shut down to protect itself.

    Most Yamaha amps have an impedance switch on the back, you certainly want this switched to the low impedance setting.

    While I have done this with both my Onkyo and my Yamaha amps, I can't make any guarantees for you. You have to use your own best judgement.

    Steve/bluewizard
     
  3. FrankG

    FrankG
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    Thanks for the advise and helpful comments.
    From what I've been reading, it looks like going below 4 ohms with this kind of amplifier is really not recommended. Since the room is not big, and I sit about 2.5 meters away from the speakers, I expect the Bostons VS260 to be quite adequate. I will give it a brief try with all 4 speakers just out of curiosity, but I don't want to overload the amplifier on a permanent basis like that. Also, I suppose it's highly unlikely that it will sound better with all 4 speakers. Well, I will find out in a day or two when I get the new speakers.

    There is an impedance switch, which is set to the left (4 ohms minimum). The other position is for 6 ohms minimum.

    This makes me wonder. Since I will be using a pair of 8-ohm speakers, do you think it makes any difference at all (performance wise) which position the switch is set to?

    Again, thank you for your help.
     
  4. BlueWizard

    BlueWizard
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    What the Impedance Switch does is up for and is highly debated. There are those that claim that all the switch does is lowers the voltage to the output stages to keep the current lower for low impedance.

    Most claim that to get full power out of the amp, you should always use the high impedance setting. However, in your case, because you are pushing the amp a bit beyond its recommended limits, it would be wise to use the low impedance setting just to be sure the amp doesn't overheat.

    Again, I didn't have any real problems, but it may be because the Diamond 9.6 are an easy 6 ohms. Another brand of speaker may qualify as a difficult 6 ohms. Again, the impedance of a given speaker is all over the place, but amp manufacturer's know this and it is take into account in their amp designs.

    The real problem is excess current draw, if the combined impedances of the speaker achieves excess current draw, then likely the amp will shut down.

    Since the impedance of a given speaker is somewhat all over the place, it is possible that the lowest impedance of the drivers for each speaker will NOT occur at the same frequency, and that means the combined real impedance at a given frequency is going to be higher. But, without plotting the impedance graphs of each speaker, this is a very difficult and unpredictable thing to know.

    Steve/bluewizard
     
    Last edited: Feb 8, 2011
  5. FrankG

    FrankG
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    That makes sense. Looks like it just put a limit to the maximum power and how loud you can play them.

    My normal listening is at the 9 o’clock position on the volume knob, or slightly under. 10 o’clock already feels much too loud for me with most sources, so power is not really a concern. And since I will almost certainly be using only the Boston pair, I suppose it doesn’t really matter which position the switch is set at. Right now it’s at the lower impedance level. I can’t detect any difference in sound between the two positions at my normal listening volume.

    I will give it a brief try with all 4 speakers, just out of curiosity. My idea was that since the Yamaha’s have 5 inch drivers, while the Bostons are 6.5 inch, I would get a more even coverage of all the frequency range. Kind of as if I were making a pair of floorstanding speakers out of 2 pairs of bookshelf speakers. However, what I’ve been reading suggests this arrangement may seem good in theory, but it seldom gives better resuts, and that somehow the sound quality and clarity is negatively affected.
    I will find out soon. The Bostons are supposed to be delivered today and I can’t wait to try them. I imagine I will end up either selling the Yamahas or saving them for a possible future when I may use them as surround speakers (which is what they are meant to be in the Soavo series) if I ever go beyond a 2 channel system. Somehow, I have never been convinced that music sounds better on multichannel systems than on a good 2 channel. And then there is the fact that having a lot of speakers in a relatively small room is rather impractical and cumbersome. So I will probably stick with this for the foreseeable future.

    Thanks again for your input.
    FrankG
     
  6. FrankG

    FrankG
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    Well I have been using the new Bostons VS 260, comparing them with the Yamaha Soavo 900M, and with all four of them together.

    I suppose is a bit too early to comment, since the Bostons need some time to break in. But so far I’ve noticed that some kinds of music sounds better in one pair, and some other kinds on the other. In general, I find that choir music (like gregorian chant or Rennaissance sacred music) sound definitely better on the 5 inch drive Yamahas. Whereas jazz sounds better on the 6.5 inch Bostons.

    I’ve played all 4 together for brief periods. I set de Bostons about 4.5 feet apart, slightly below ear level when sitting, and I am sitting some 9 feet away. The Yamahas are right next to them on the outside, about 5 inches away, and their base rests about 8 inches higher than the base of the Bostons. So the Yamahas are placed higher but not completely above the Bostons.


    I had read many comments in different places about how the sound would be worse when using to pairs of speakers in parallell, but I must say this has not been my experience so far. Most music seems to get definitely richer when I play both sets together, with no loss of detail from the place I am sitting, and I don't notice any blind spots or funny things, at least not yet. So this has been a bit of a surprise.

    Of course I don’t dare use both together too long, as I am afraind of overloading the amp, but I am going to keep testing this as I break in the Bostons, and if I think it’s worth it I may consider getting another amp to run one of the pairs.

    The Bostons come with posts that allow bi-wiring. On this subject I have read all kinds of contradictory comments, some swering it makes no difference, and other saying it definitely does. I used to have a pair of Bostons VR-M60, bi-wired. They were the best sounding bookshelf speakers I’ve ever had or listened to. My ex kept them. It never occured to me to test if there was any difference with or without bi-wiring. I don't have any bi-wire cable now, but it is something I will consider doing.
     
  7. BlueWizard

    BlueWizard
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    Here is an alternative you might try -

    Place one speaker on top of the other on a single speaker stand, but put the tweeters in the middle so they are close together. If we start from the bottom of the lowest speaker and move up, with "W" for woofer and "T" for tweeter, the configuration would be WTTW.

    This is very similar to the D'Apolito MTM (or WTW) design and might have some advantages.

    If you have both speakers in front, then wire them the same; Amp(+) to Speaker(+), but if later you try them with one pair in the front of the room and the other pair in the back of the room, wire the back speakers so they are Amp(+) to Speaker(-).

    Because the rear speakers are facing in the opposite direction, they are actually out of phase with the front speakers. So, wiring the rear speakers electrically backwards, brings them mechanically in-phase with the front speakers.

    As mentioned, I still have four front speakers, but the second set is driven by a separate amp. For basic music and casual listening, I just use large tower Diamond 9.6 speakers, but for movies, especially action movie, I kick on the second amp and the second set of speaker (12" 3-way), and I can really rumble the house. This is a very powerful experience.

    In my not so humble view, those who have bad experiences with two pair of speakers, are simply not doing it right.

    Again, you seem to have a nicely working setup, but, if you feel the need to experiment, consider some of the things I've suggested. If nothing else, if might just be fun to compare.

    Steve/bluewizard
     
  8. FrankG

    FrankG
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    Thanks for these suggestions. I may try some of these experiments, but my room is too small to put speakers in the back.

    I tried my speakers in a friends appartment that has a very large living room. My speakers are about half a foot from one wall, and the opposite wall is only 10 feet away, which is the width of the room. My friend has the speakers facing lengthwise across a very long living room, with lots of furniture and things on the walls.

    The difference in sound, listening from about 9 feet away from the speakers, is enormous, and it can only be due to the room acoustics, as the amp is the same. I have just realized a key element that, if it is not dealth with, will ruin any system no matter how good. The wall facing my speakers 10 feet away has to be causing very strong reverberation which muddies the sound. The sound at my friends place is So much clearer and clean and real.
    I’ve done some research and found some good discussion on the topic of room acoustics such as this. It’s an eye opener.
    http://www.nnacoustics.com/pdf/room_acoustics.pdf

    I am now faced with a dilemma. I kind of like my walls pretty bare. But the only chance I have to prevent sound from bouncing back and forth is to put something on the walls that absorbs it, so that the sound I get from the speakers does not get dulled by its own reflections. I am going to have to think hard about this, how to reconcile the room’s visual aesthetics with an efficient acoustics.

    The most depressing part is that, apparently, the reflectivity of the front wall is just harmful. The article I linked above says:

    “Sound reflecting objects like LCD and Plasma TV should not be placed between loudspeakers or near front wall (the wall that you are facing from the listening position). If the front wall is mainly or entirely glass surface, than the only thing to do is to cover this surface with mobile absorbent screens. These extremely reflective surfaces drastically diminish the depth perception of your sound stage. The sound picture will be shallow, everything will be happening in one plane.”

    But, where else can one put the television except somewhere between the speakers AND near the front wall??

    It does look like, no matter what one does impriving the equipment and setup, everything will be for nothing if the walls of the room keep bouncing the sound back and forth.

    Regarding the tv being in the middle, of the speakers, I suppose one way to improve it is not to have it straight in front of you, rather a bit to one side, so the speakers will not be perpendicular to the wall, that way the reflection of the sound from the back wall will not go straight onto the surface of the tv, and from there back to your ears.

    I am now convinced that reducing the room's reverberation is absolutely essential
     
  9. FrankG

    FrankG
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    p { margin-bottom: 0.08in; } Well now that I´ve managed to significantly reduce the echo in the room, the speakers sound much better, and I have to say it definitely sounds better with both pairs. I have them all in front as I don´t want to be running cables all over the room.
    I haven´t had any problems at all with the amp driving both pairs, even if I am some 0.6 ohms below the recommended minimum of 4. But the amplifier doesn´t even get warm in the least, let alone hot. Then again I never listen to music very loud. So I suppose it should be okay.


    I thought of connecting them serially, but from what I´ve read the result is not as good, and the volume drops considerably. Besides, I am not even sure how the serial connection should be done. It seems you need to connect the positive posts on one pair with the negative on the other, which sounds odd.


    I am very happy with the sound of this setup, though sometimes I still worry about the 3.42 ohm total. Maybe eventually I will sell the Yamahas and get another pair of 8 ohm speakers with a similar size woofer. But then I think I am probably being overcautious. The amp does not get warm at all, so it either has a very efficient cooling mechanism or I am not using it at anywhere near the power needed to create a problem.


    One thing that surpises me is that the music in my favorite FM station sounds definitely better through the tuner than through the cable box. Who would have thought!
     
  10. formbypc

    formbypc
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    Connect one of each speaker in series on output A or B to give you a 14 ohm load?
     
  11. FrankG

    FrankG
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    I wonder what the effect on total impedance be if I were to try using only the low pass on the Bostons VS 260 (in other words, taking out the bridging plates and connecting only to the lower posts).

    Since I am using two pairs of speakers, I figure this way I would eliminate the tweeter redundancy (having two pairs of tweeters) and I would be using one set of 6.5 inch woofers on the Bostons plus the 5-inch woofers on the Yamahas with their 1 inch tweeters.

    But would this have any effect on total impedance?
     
  12. BlueWizard

    BlueWizard
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    Here is a graphic indicating how to wire two speakers in series -

    [​IMG]

    Ignore Speaker-3.

    Part of what I'm trying to show is that the series wiring can be done back at the amp using normal speaker wires. Just take the two appropriate wires twist them together, and cap them with a wire nut and some tape to secure it.

    When you connect two speakers in series, the voltage is divided between them. Using two identical speakers, having two speakers gives you twice the acoustical power, but with each speaker getting half the signal we have half the acoustical power. So, in a sense you have +2x - 2x, for a net gain of Zero.

    In other words, two identical speakers wired in series are exactly as loud as a single speaker, however, each individual speaker will now handle twice as much power, and have a much lower excursion. Both very positive things.

    With unequal speakers, in your case, 6 ohm and 8 ohm, the voltage doesn't divide evenly between them, so that produces a very slight imbalance in the output levels.

    I would suggest setting the speakers as close together as you can. The problem that occurs in using two speakers like this is an odd phase conflict that occurs at frequencies with the same wavelength as the distance between the centers of the speaker. Keeping in mind that the wavelength of bass is very long.

    The wavelength is the Speed of Sound divided by the Frequency

    Speed of sound = 343.2 Meters per second, or 1126 ft/s

    Here are some sample frequencies -

    100hz = 11.26 feet
    1,000hz = 1.126 feet
    5,000hz = 0.2252 feet (2.7 inch)
    10,000hz = 0.1126 feet (1.35 inch)

    So, there can be some slight inconsistency at certain high frequencies, but this is going to be small and slight. Again, the reason behind my suggestion, assuming bookshelf speakers, of placing one on top of the other with the tweeters as close together as possible.

    I use two pair of speaker, as mentioned previously, and while I would like two pair of better speakers, the speakers I have make of some seriously devastating Action Movies.

    Not really necessary for music, but powerful video watching.

    Also, keep in mind that your amp likely has a Speaker-A/Speaker-B switch. If connect them in this fashion, you should be able to selectively turn each speaker set on and off, and use the speaker best suited to a given kind of music.

    Just a few thoughts.

    Steve/bluewizard

    Ste
     
  13. BlueWizard

    BlueWizard
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    It doesn't really effect the impedance at low frequencies, and these are the most critical. Above the resonance frequency, which is in the bass range, the impedance of a speaker, whether woofer or tweeter, begins to climb.

    However, he phase errors I mentioned in my previous post are likely to be at higher frequencies.

    Since your amp seems to drive both speakers OK, you could try it both ways. Use the Bass on both and the highs on only one, with one then the other speaker providing the highs.

    You could also try a single bass speaker with dual High sections.

    Typically the resonance point of a tweeter is outside its working range, so the impedance should be pretty stable. Just to illustrate, a tweeter might have a resonance of 700hz, but the lowest rated frequency might be 1600hz. So, we typically don't need to worry about an impedance dip for the mids and highs.

    I think ultimately, my solution was best, just get a second amp to drive the second set of speakers. Drive the second amp from the Pre-Amp Outs of your main amp.

    The two amps I use are the Yamaha RX-797 100w/ch and the Onkyo TX-8255 with 55w/ch. I got both at outstanding prices.

    Steve/bluewizard
     
  14. FrankG

    FrankG
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    Thanks much for all the advise and explanations.
    So far the amp hasn't shown any signs of having any trouble driving both pairs of speakers. Then again I never listen at a very loud level.

    I do have the speakers of each set as close as possible to the speakers of the other set. About one foot from center to center. I can't exactly put them one on top on the other because the Bostons have an irregular curvy shape. I do have A/B switches. Most music I like better with both sets, sounds "fuller", though sometimes imaging seems better, neater, with just one set.

    I wonder how the low pas/high pass separation works. I really don't know much about any of this. Do the low posts connect only to the woofer and the high posts only to the tweeter? Or is there some filtering system, such that input from each set of posts is taken only in a certain range of frequencies? When connecting only to the lower posts, I still hear faintly some high frequencies (nothing on the tweeters, I think) but for example most of the drum brush and cymbal work disappears.
     
  15. BlueWizard

    BlueWizard
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    Certainly there are crossovers inside the speaker cabinets to direct only low frequencies to the woofer, and only high frequencies to the tweeter.

    Also keep in mind that at the crossover frequency the sound from, say, the woofer, doesn't just stop, that is the point were it starts to fade out. Though it fades pretty quickly. So, it would not be uncommon to hear higher frequencies when only the bass driver is playing.

    In a 2-way (woofer, tweeter) system, the crossover is probably up around 2.5khz. That is up around middle-C on a piano, and from there the woofer starts to fade out, and the tweeter starts to fade in.

    As to how these are connected to the LF and HF terminals, this is typically LF (low frequency) to the Woofers only. The HF (high frequencies) is connected to the Midrange, if there is one, and the Tweeters.

    One complication is the location of the crossover frequency. If you are using the bass section of both speakers and the tweeter section of only one (or the opposite, two high, one bass) the two speakers might not crossover at the same place.

    Still that certainly won't harm anything, and whether it will sound good, is uncertain, it could be great, or it could be so-so. But I don't see any potential for harm in any of the arrangements as long as the amp is not shutting down.

    I happen to have two amps on hand, so that was an easy solution for me.

    As to speaker positioning, every aspect of Audio is a compromise. It seems that side-by-side is about the best you can do, and it seems to work fine. We could potentially do some detailed testing on the arrangement and find a flaw here or there, but that is true of any system.

    With my setup, I could play happily at about 1/3 (10 o'clock) with not problems at all. I ran this way for a few weeks. When my amp did shut down, was during Lord of the Rings, and the volume control was creeping up over 50% (12 o'clock) at the time.

    I think at modest volumes, you should be OK.

    Steve/bluewizard
     
  16. FrankG

    FrankG
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    Thanks again, Steve. These explanations help a lot. Checking the specs I saw that the crossover frequency for the larger speakers (the Bostons) is 1.9 kHz, while for the Yamahas it is 3 kHz.
    After trying it different ways, I am back at the original configuration, using woofer and tweeter on both sets. To my ear, it sounds better that way.
     

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