Integrating speaker-level sub with 5.1 receiver

Discussion in 'Speakers & Subwoofers' started by PeterSM, Oct 12, 2003.

  1. PeterSM

    PeterSM Auditioning

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    I have an old M&K Volkswoofer III (with a new driver) that requires speaker level input. It has a servo amp that picks up the speaker level input, performs active equalization, and drives the woofer. It uses passive low pass and high pass crossovers, also speaker level. I have used this sub in two channel arrangements and find it to be quite satisfactory. Good response, unobtrusive, no boom, and improving quality of mids and highs. I am considering using this sub in a 5.1 surround arrangement that includes an Integra DTR-5.4 receiver and 5 Paradigm speakers: 4 Atoms and 1 CC-170. None of this surround sound equipment is purchased yet. My proposed setup would be as follows:

    a. Tell the receiver that I have no sub and that the front L-R speakers are large. My understanding is that this will re-route all bass that would normally go to the .1 channel so it is split equally to the front L-R channels.

    b. Run the front L-R speaker outputs to the sub, where it is split and used to drive the woofer and the front L-R speakers.

    I have some questions about this.

    1. Apart from differences in amplification channels and crossover method, will I retain all the bass information this way?

    2. Would you expect any sonic problems to be caused by this rerouting of the bass signal?

    3. Assuming that the subs work fine in two-channel reproduction, is there any reason I would not like this setup?

    4. Conversely, apart from potential improvement in sub quality from buying a new sub (which is not the issue here) is there any reason I would find an equivalent pre-amp level sub powered by the .1 channel to be more satisfactory?

    Thanks in advance.[​IMG]
     
  2. Brian L

    Brian L Cinematographer

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    IMHO, you proposal should be fine, assuming the receiver correctly sends the .1 channel to the mains when set to No Sub.

    Compared to your 2 Channel set-up, you will simply be taking bass from the center and surrounds and sending it to the mains, where the sub crossover will then deal with it.

    However, since your receiver is capable of dealing with the crossover, I would probably go line level, just so the crossover frequencies are complimentary between channels. If they are mismatched, as they could be if sub and receiver frequencies have a gap, you could be loosing something.

    I know of no comparative tests between using pre-amp vs. speaker level sub connections. Perhaps other have some experience with that and can offer an opinion.

    One thing that may be an issue would be if you ever went with a hi-res (DVD-A and/or SACD) player. They connect via 5.1 analog connections, which would be problematic for your speaker level set-up.

    BGL
     
  3. Justin Ward

    Justin Ward Supporting Actor

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    Another thing you could consider is using an old stereo amp to power the sub from the line level out of the reciever.
     
  4. PeterSM

    PeterSM Auditioning

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    Thanks, all. I think I will try it in the proposed mode (sending the sub signal to mains) first. Then I will use an old receiver to drive it from the .1 pre-out and see if it makes a difference. If I like the amp approach better, then I will shop around for a used amp. I don't have any old amps lying around.
     
  5. Doug BW

    Doug BW Stunt Coordinator

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    I think the LFE (".1") channel is going to be a problem for you in your proposed setup. I'm not sure if your receiver is going to re-route the LFE to the front L/R speakers, but either way it's a problem. Obviously, if it doesn't re-route the signal, you lose the LFE track, which is bad.

    But if it does re-route the LFE, you're going to be sending a huge amount of bass to your mains, putting a big load on both the mains and the amplifier channels driving them.

    In a 5.1 system, normally the signal sent to each main will never be louder than 105 dB (this, assuming you're running at reference level). The LFE channel, however, can get a 115 dB signal. Since you plan to route ALL your bass to the sub, you can theoretically get 123 dB of bass being sent to your sub's crossover. Near the crossover frequency, you'll find that the crossover is sending 120 dB of bass to your sub, and another 120dB is being split between your two main speakers (117 dB to each speaker)!!

    I realize you may not listen at reference level, but no matter how you slice it, you're going to be driving your mains much, much harder with your proposed setup. I think using the separate receiver/amp is preferred way to go.
     
  6. PeterSM

    PeterSM Auditioning

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    Thanks Doug. That sounds pretty bad, no pun intended. [​IMG] I am confused. There are two things I don't understand about the concept of overloaded mains.

    1. Even if I had a separate amp for the sub signal, is there not a crossover involved somewhere in the signal processing, so above the crossover point the mains would still get the signal? It is hard to imagine that the sub signal is totally separate from the main signal - an expolsion covers a spectrum of frequencies, and all the speakers need to participate in it. What is the real difference between the crossovers in the two scenarios? Putting it another way, why does the processor's bass management system give the option of crossing over at different frequencies if there is no crossover?

    2. I would guess that the speaker level input to the sub draws very little power from the main speaker amp in the bass range. After all, the sub's internal amp is supplying the power to drive the sub's speaker, so there is no need for power to be supplied from the main amp. I don't know for sure, but I would guess that the sub presents a relatively high impedance to the main channels' amps in the bass range, so that all the main channel amps does is provide a voltage source for the sub's amp to track. If this is true, then the main amp would not be overloaded.

    Anyone have any comments on my speculation?
     
  7. Justin Ward

    Justin Ward Supporting Actor

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  8. Doug BW

    Doug BW Stunt Coordinator

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    Hi Peter. The "overloaded mains" problem will only occur if your receiver redirects the LFE channel to your mains when you have "sub=no". Let's assume your receiver does that.

    In a "traditional" configuration with "sub=yes", your receiver sends the entire LFE signal to your sub, and NONE of the LFE signal to your mains. The sub signal is NOT sent through a high-pass filter with the filter's output being directed to your mains.

    In your proposed configuration, your receiver is comibining the full LFE signal with your mains' full range signal and sending the combined signal to your sub's crossover. The signal then goes through the crossover's high-pass filter, with the filter's output being sent along to your mains. So in this case, a considerable portion of your LFE signal will find its way to your mains.

    Remember, crossovers aren't "brick walls". At the crossover frequency, the output of the high-pass and low-pass filters is the original signal reduced by only 3dB. Depending on the slope of the filter, you'll still be getting plenty of output from the high-pass filter well below the crossover frequency.

    This also answers your second point. You're correct that your receiver's amps will not be driving the sub. But your receiver WILL be driving the portion of the LFE that makes its way through your sub's crossover's high-pass filter to your mains.
     
  9. Doug BW

    Doug BW Stunt Coordinator

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  10. PeterSM

    PeterSM Auditioning

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    Thanks Doug. I understand what you are saying regarding non-brick-wall and agree all you said it as far as it goes. But I think you have missed the main point of my questions / comments, which obviously I did not phrase too well. I'll clarify my points.

    Regarding #1, the LFE signal has an origin in the source material, say a microphone. That source signal must be crossed over in the source processor in order to generate the LFE in the first place. Some of the explosion must go to the main signal because an explosion has a wide bandwidth. That fraction going to the main signal in the source separation will subsequently be blended if you chose sub = no. But then it will be separated out again by the sub. So, it seems to me that if the source separation is non-brick-wall, like the sub's separation, and if the crossover frequency and rolloff slopes are the about same at the source and at the sub, then the net output to the mains should be the same either way. As I see it, the mains will see a different signal from that intended only if the source processor and sub have different crossover characterics.

    And regarding #2, I still think that power demand is probably not the entire story. I am not an electrical engineer, so I cannot do all the calculations off the top of my head, but I'll give you my layman's feel for it. Please correct me if I am wrong. As I understand it, power transfer has to do with impedence matching. Consider in the limit the case where the speaker presents an infinite impedence. That's bad for tube amps and transformers due to the high voltages generated, but I believe a solid state amp will pass no current and no power under these circumstances. So, I would think that trying to drive a load (the sub) that presents a high impedence in the low frequency range (because the sub is designed to use its own amp, and that's the most efficient way to do it) and an 8 ohm impedence in the range that is passed through to the mains should result in less LF power being generated by the main amp. It would be nice to have an electrical engineer check this. [​IMG]
     
  11. PeterSM

    PeterSM Auditioning

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    Continuing on with the #2 example, what would happen if you put a Y connector in a full-range preamp out and connecteed two amps to it. One amp would drive a tweeter with a high pass filter in front of it, so the tweeter would not see signals in the bass range. The second amp would drive a woofer with a low pass filter in front of it, so the woofer would not see signals in the high frequency range. Neither filter has any resistance. It is only by impedence mismatching that the power transfer is prevented in the frequency range that is not of interest. So, when a speaker does not see the frequencies, the corresponding amp does not see the load. Is this correct or not?
     
  12. Doug BW

    Doug BW Stunt Coordinator

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    Hi again. I think it might help if we discussed this statement of yours:

     
  13. PeterSM

    PeterSM Auditioning

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    OK, Doug![​IMG] I finally understand it and appreciate the distinction. I will take all that into account. The system in question is actually for my bedroom, which I know I will not be playing at reference levels. So, I'll play it in crippled mode as described for a while. In the meanwhile, I have my main living room system to design as well. That one will be far more critical. So, I'll probably buy a new sub for that system anyway, wereupon I can test same in the bedroom and see if it makes any sonic difference that I would care to pay for.[​IMG]

    One last question. How do you know when you are playing at reference levels?

    Thanks again.
     
  14. Doug BW

    Doug BW Stunt Coordinator

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    To determine reference level, you need to calibrate your system. Basically, you use an SPL meter and some test tones that are played at a known level. For example, the test tones on the Avia DVD are recorded at a level that should register 85 dB on your SPL meter at the prime listening position.

    If you leave your master volume at the same level as when you calibrate, then you're listening at reference level. Be warned that this is quite loud. Most people don't like it that loud, and a lot of speakers can't play it that loud without distorting. So a lot of people will turn their master volume level down 10 or 20 dB (or more) from "reference" level.

    And, by the way, "reference level" only applies to movies. There's no such thing as a standard volume level for music CDs.
     

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