Bi-amping using surround receiver?/Ohms and power output.

markmaxx

My first post. Hi all.

This is my first question. My receiver has what they call bi-amp capability is this right?

Pioneer wrote:

The VSX-74TXVi was designed to do just that and more. Building on a long history of innovations in amplifier designs the VSX-74TXVi's new Direct Energy Bipolar 7-Channel power plant delivers 140 Watts to all 7 channels with absolute detail, power, and efficiency.

So in my manual "Bi-amping your front speakers"

manual wrote:

Bi-amping is when you connect the high frequency driver and low frequency driver of your speakers to different amplifiers.(in my case the front and surround back terminals)

I do have a question it's comming.

Is this true bi-amping?


Next question. My speakers, Polk Audio Lsi 15s are 4 ohm. The specifications for 6 ohms on my receiver are 180w @6 ohms.(I assume 4 ohms would be even higher? How much power at 4 ohms? do I have? ) do I now have well over 360 w per channel?

Schwingding

Yes, it is "true" biamping, but not in the manner purists refer to as biamping. What you are doing is sending the same signal through two separate amps to the speakers. "Purist" bi amping involves breaking the signal out before it goes to the dual amps so that lows get amp'd through one amp and the highs through the other, then they get connected to the speakers respectively. You are sending more power to your speakers - a good thing, but you don't have the equipment to really take advantage of the other situation I described. My RTi10s opened up dramatically when I biamped them as you describe.

As for the other question - got me.

TN_Polk_Lover

My Denon 3806 receiver has a similar function. If you are not going to use the Rear Surrounds (in other words, you are going to setup 5.1 as opposed to 7.1 system) then you can use those rear surrounds channels as part of the front channel amplification.

Instead of having 140 wpc to drive each of your front speakers, you would have a total of 280 wpc for the fronts, assuming your front speakers have bi-amp capability. However, each of these outputs would be at full frequency range. There is no crossover function provided in the receiver itself. You would be sending full range output to each of your speaker sections. Your tweeter's crossover would just block the low frequencies, just the same as it does if you don't bi-amp.

As Schwingding pointed out, some people only consider it to be "true" biamping if you have an active crossover at the pre-amp stage to split the frequency ranges before being sent to the speakers. Of course, the active crossover would have to be adjustable so that you could match to the speakers you were using.

The $64,000.00 question is -- will the type of bi-amping you can do with your receiver improve the sound? You will probably have to experiment and just see for yourself. Many fellow "Polksters" here have tried various forms of bi-amping and /or bi-wiring. Some have reported hearing an improvement. Some have not. It depends on alot of different factors.

One thing to be aware of. Most receiver manufacturers are, shall we say, "optomistic" in the way they report output wattage on a 5 or 7 channel receiver. They may report the FTC compliant wattage as something like "140 watts per channel continuous rms @ 8 ohms 2 channels driven with 0.08% THD". They seldom report the FTC wattage with all seven channels driven continuously. I'm not knocking receivers, I have one myself. But I am driving my front mains through a seperate amplifier using the "pre-outs". This takes some of the load off of the other 5 channels.

As far as the 6 ohm vs. 4 ohm question. If your receiver is not specifically rated for 4 ohms, then using it with 4 ohm speakers can cause problems, probably depends on how loud you like to listen. My Denon is rated at 8 ohms and 6 ohms. I think I read somewhere on Denon's website something to the effect "Can be used with 4 ohm speakers, but may cause the protection circuit to cut off the receiver if used for extended periods of time." In other words, the 4 ohm load is too much for the receiver and it is overheating.

I hope this helps. Welcome to the Polk Forum. There are a lot of wonderful people here. Some of the "old timers" have posted over 5,000 times and have been here for years. So it is kind of like an online family. There are often local gatherings called "Polkfests" where members get together in person and bring equipment for listening tests, etc.

John K.

Mark, welcome. As to how much power your 74TXVi can put put into a 4 ohm load, you can make a rough extrapolation: 140 watts into 8 ohms, 180 into 6 ohms, so maybe about 220 into 4 ohms. An indication that this isn't far off is the fact that the next model down, the 72TXVi, lab tested here at about 212 watts into 4 ohms. This is using a realistic two-channels driven measure, rather than all channels driven at full power, which doesn't occur outside the lab in typical home audio use.

So, you have plenty of power for nearly any circumstance, but you can't get even more by using that form of biamping. When you mention possibly getting over 360 watts(rather than 180)this indicates that you may have the misimpression that the procedure would double the power available. The 74 has one power supply section which delivers its power to the output transistors in whichever channels are operating at that moment in time. If you connect a speaker through two sets of output transistors(one previously being unused)rather than one, this obviously can't increase the capacity that the power supply section has; it has to remain exactly the same as it was before, but simply is being fed through two routes. The transistors have no power of their own, they're just valves which measure out the needed amount of power which they get from the power supply section of the receiver. So, although you can't double the power or increase it by any amount in this way, you should already be in fine shape to enjoy your excellent speakers.

bchivers

\\\\

markmaxx

John K. wrote:

Mark, welcome. As to how much power your 74TXVi can put put into a 4 ohm load, you can make a rough extrapolation: 140 watts into 8 ohms, 180 into 6 ohms, so maybe about 220 into 4 ohms. An indication that this isn't far off is the fact that the next model down, the 72TXVi, lab tested here at about 212 watts into 4 ohms. This is using a realistic two-channels driven measure, rather than all channels driven at full power, which doesn't occur outside the lab in typical home audio use.

So, you have plenty of power for nearly any circumstance, but you can't get even more by using that form of biamping. When you mention possibly getting over 360 watts(rather than 180)this indicates that you may have the misimpression that the procedure would double the power available. The 74 has one power supply section which delivers its power to the output transistors in whichever channels are operating at that moment in time. If you connect a speaker through two sets of output transistors(one previously being unused)rather than one, this obviously can't increase the capacity that the power supply section has; it has to remain exactly the same as it was before, but simply is being fed through two routes. The transistors have no power of their own, they're just valves which measure out the needed amount of power which they get from the power supply section of the receiver. So, although you can't double the power or increase it by any amount in this way, you should already be in fine shape to enjoy your excellent speakers.

Thanks:) Well written!

liordra

Schwingding wrote:

Yes, it is "true" biamping, but not in the manner purists refer to as biamping. What you are doing is sending the same signal through two separate amps to the speakers. "Purist" bi amping involves breaking the signal out before it goes to the dual amps so that lows get amp'd through one amp and the highs through the other, then they get connected to the speakers respectively. You are sending more power to your speakers - a good thing, but you don't have the equipment to really take advantage of the other situation I described. My RTi10s opened up dramatically when I biamped them as you describe.

As for the other question - got me.

If one setups the surround back speakers as "Small" would it not mean that the receiver will cross over the highs (the lows remain as full range channels because the fronts are defined as "Large") ?

McLoki

liordra wrote:

If one setups the surround back speakers as "Small" would it not mean that the receiver will cross over the highs (the lows remain as full range channels because the fronts are defined as "Large") ?

You are not using the rear channels when in bi-amp mode. I assume you will not be able to bi-amp if you have the rear channels enabled and you will not be able to set a crossover for the rear channels without having the rear channels enabled....

Reguarding bi-amping. It may help you some if you play at loud levels. (I would not recommend playing LSi's at high levels when using an AVR not rated at 4 ohms)

Anyway - this is how it could help. This quote is from this article on clipping - agree or not, this section describes my though process on bi-amping:

Let us assume the desired musical waveform is essentially a loud sustained sinusoid at a frequency of, say, 500 Hz, and that the loudspeaker has a crossover that splits HF from LF at about 1 kHz so that any signal components above 1 kHz are directed to the tweeter. Let us also assume that we are using a 100 Watt amplifier, and the speaker is also rated in musical terms as being “ 100 Watt”, but uses a tweeter than can only endure around 10 Watts for periods longer than a few seconds.

If we now turn up the volume so that the amplifier is clipping the waveform by a factor of around ´ 3 or more, then the harmonic power this creates may be greater than the tweeter can endure for an extended period. Hence the tweeter may then fail as a result of the unintended high levels of HF power which the clipping created.

Basically saying that your bass frequencies pull most of your power and when your amp runs out of steam, it causes a power surge at a higher harmonic that will end up blowing your tweeter. If you use a different amplifier for your woofer and tweeter, that power surge will get absorbed by the crossover and never make it to the tweeter.

I do not think this benefit is worth running out and getting another amplifier, but if you already have the capability to do it, you might as well make use of it.

Michael