Adding power amp to receiver - technical question

[Goliath]

Greetings,

First, great site. I have a technical question I'm hoping you guys can help me answer. If I want to add a two channel power amp to a receiver (eg Onkyo 706), would the remaining channels have more power, since the two main channels are powered by an outboard amp? I would think so.

You guys are far more technical than me, so I look forward to hearing your thoughts.

 

[FirstReflection]

Well, it's a bit of a "yes and no" answer.

The amps in your 706 receiver all share the same power supply, but they have independent output circuits. Onkyo rates their amps according to the rated output of the individual circuits and then says "2 channels driven", which means that the total supply of power available to the amps is, in reality, only twice what an individual output circuit can provide.

Think of it this way, the "per channel" rating might be 100 Watts or some such. You might think, "ok, there are 7 amp channels and 100 Watts per channel, so this receiver can pump out 700 Watts at any given moment." Well I'm afraid, that's not actually the case. That "2 channels driven" rating means that the most the receiver can pump out at any given moment is 200 Watts, not 700. Furthermore, each individual channel can only pump out 100 Watts through its specific circuit. If you were only driving one speaker, it's not as though you could pump all 200 Watts into it. Only 100 Watts. But you can drive 2 speakers with 100 Watts each, or you can drive all 7 speakers, but each of the seven speakers will only be getting 28.57 Watts if they are all being drive equally and all at the exact same time.

So does using a separate amp to drive two of the speakers allow more power from the receivers amps to go to the remaining five speakers? Yes, in the sense that you have this 200 Watt "bucket" of power available that the remaining five speakers all share, so now there's more power to go around since two of the speakers are no longer dipping into that "bucket". But no in the sense that the most that can ever be delivered to an individual speaker is still the rated "per channel" Wattage of 100 Watts in this example. You don't suddenly get to dump more than 100 Watts into any given channel.

Hope that helps!

 

[Goliath]

So by using a power amp for the two channels means that the receiver won't be producing more power into the other remaining channels - it will still be the same power (ie 70 watts, or whatever the value is)? Sorry if I'm not understanding things, I'm relatively clueless when talking about stuff like this.

 

[anamorphic96]

Yes. They still remain at there rated output.

 

[Goliath]

Okay, so it's the same rated output, but the remaining speakers get a bigger proportion of that power? Would that be correct?

 

[markw]

Not Exactly.

They just get the benefit of the unused power supply current, not the output power.

It's like you have two people sharing a certain amount of food. If one stops eating and only one eats, that one doesn't become twice as strong, but he may realize some benefits.

Let's assume a receiver is rated at 100 * 5, or 500 watts.

It's not like that, since the unused watts (2 * 100) is divided among the remaining three channels (200 / 3 = 66 wpc), they just get the use of the unused power supply current so they might be able to produce a few extra watts, say 10 or 20, and that's just a rough estimate.

But, the downside is that (for most receivers) if all channels are driven in the first place, not every channel would be outputting 100 watts per channel to begin with. When you see "per channel" rating, it's usually for only two channels being driven at a time.In fact, it's generally much, much lower than 100 watts.

But that's not really a bad thing. In home theatre it's very rare for all channels to be called on for full output at any one time.

FR was pretty right on so this is jut a rewording of his concept. There's a lot more math, science, and variables involved so no exact answer is possible. This is just a rough example to, hopefully, get a point across.

 

[Goliath]

Thanks guys. I think I'm understanding this a little better.

 

[jonnythan]

The big benefit of adding an outboard amp is that the two channels hooked up to the external amp are guaranteed solid power full time.

 

[FirstReflection]

@Goliath

Just imagine that the total amount of power available to all of the channels in a receiver is a "bucket". All of the channels have to share from that one "bucket" of power. But each individual channel can only take a maximum of half of that "bucket" at any one time. No individual channel can take the entire "bucket" all for itself.

In the case of your 706 receiver, the "bucket" is 200 Watts of power. Any individual channel can dip into that "bucket" at any given moment and take as much as 100 Watts of power all for itself, but no more.

But do remember that the total "bucket" is only 200 Watts. So any two channels go dip into that "bucket" and take 100 Watts each, but then all of the remaining channels would get zero Watts and would have to be silent.

So let's imagine that 5 channels all want some power at the same time. All 5 of those channels would dip into the "bucket" of 200 Watts. Obviously, if all of those 5 channels get at least SOME power, they can't ALL dip in and get 100 Watts. The "bucket" just isn't that big!

If they all grabbed an equal amount of power from the "bucket", each of those 5 channels would get 40 Watts. Or it could be the case that 1 out of those 5 channels needs to play louder than the rest. So maybe 1 channel grabs 80 Watts, while the rest get 30 Watts each. Or maybe 1 out of the 5 channels needs to play as loud as it possibly can. So it grabs the maximum that any 1 channel can take, which is 100 Watts, leaving the other 100 Watts in the "bucket" for the remaining channels to share - so maybe the other 4 channels each grab 25 Watts.

Now, if you were to give 2 out of those 5 channels their own, separate amplifiers, now you only have 3 out of the 5 channels dipping into that "bucket" of 200 Watts at any given time.

So, once again, maybe 2 out of the 3 channels each grab 100 Watts, leaving zero Watts for the 3rd channel. Or maybe all 3 each grab an equal amount of power, so each of the 3 channels would get 66.66 Watts.

You can see in that example: when the receiver was powering all 5 of the channels, if all of the channels dipped into the "bucket" and took an equal amount of power, each channel only got 40 Watts. But now, with 2 out of the 5 channels having their own, separate amps, each of the 3 remaining channels can grab 66.66 Watts. So, in that way, using a separate amp to power some of the speakers increases the amount of power that remains for the other channels that are still being powered by the receiver.

But continuing, maybe 1 out of the 3 remaining channels grabs as much power as it can from the "bucket". So 1 channel takes 100 Watts, leaving 100 Watts for the other 2 channels to share - so each of them could take 50 Watts, for example.

Again, remember back to when all 5 channels were being powered by the receiver. If 1 out of the 5 took as much power as it could - 100 Watts - each of the remaining channels only got 25 Watts. Now, with 2 out of the 5 channels on separate amplification, 1 channel takes the max that it can from the receiver - 100 Watts - and the other 2 remaining channels can have 50 Watts each, instead of 25.

So you've got the "bucket" of 200 Watts no matter what. It's just a matter of how many channels are all dipping into that "bucket" and taking some of that power for themselves. When you add a separate amp, now you have fewer channels dipping into that "bucket". But at any given time, the most any individual channel can take out of that "bucket" is 100 Watts.

Hope that helps!

 

[Nestor]

I find it hard to believe the amp power supply is capable of only 200 watts total output.

According to the specs, current draw for the entire receiver is 5.9amps.

I realize power supplies are not 100% efficient, but this seems awfully low.

 

[PENG]

I find it hard to believe the amp power supply is capable of only 200 watts total output.

According to the specs, current draw for the entire receiver is 5.9amps.

I realize power supplies are not 100% efficient, but this seems awfully low.

We, the consumers really don't know how they rate the 706. Okay may be they did say 5.9 amps in the manual but under what conditions, continuous, peak, and if continuous, do they really mean "continuous" = from infinitely long or just 5, 10, 15 minutes etc? If in fact it is 5.9A continuous then what would be the rating for 0.01 second, 1 second, 1 5, 10 minutes etc? And then remember to translate 5.9 amps into average power in watts, you need to know the load power factor (cosine of the angle between the voltage and current) and other minor factors that I don't think I should get into but the power factor alone is a significant enough factor not to be ignored. That being said, if I have to choose just one between amperes (current) and watts (power) I would actually prefer the amps as I can then try to figure out the watts from the phase angle, and impedance characteristics of the speakers myself.

I can tell you even some power amplifiers including a few perceived by forums members here as having some "real" continous power rating, in fact cannot theoretically deliver their advertized rated current base on their specified tranformer rating. If you are interested you can do the math yourself but on the practical side I won't dispute their claims because I believe (inferred from reading a lot of specs of a lot of AVR/power amps) when they say continous they probably all mean more than a few minutes, may be even longer, say 10 to 15 minutes, but again, who knows? Face it, most enjoyable music rarely if ever play at its maximum/peak level continuously, or if they do then they don't have peaks. So "real" continuous or not is arguably a moot point. Lastly, keep in mind that power supply transformers typically can deliver current much higher than its "real" continuously = infinitely long rated value without sustaining damage.

 

[FirstReflection]

I find it hard to believe the amp power supply is capable of only 200 watts total output.

According to the specs, current draw for the entire receiver is 5.9amps.

I realize power supplies are not 100% efficient, but this seems awfully low.

PENG is right, there's a considerable amount of complexity when it comes to electrical specs vs. reality and measured power output. But even in EXTREMELY simplified terms:

5.9 amps x 115 Volts = 678.5 Watts

Only about 40-50% of that actually gets turned into useable Wattage to drive the speakers in a Class A/B amplifier, so somewhere from 270 to 330 Watts.

There's around 60 Watts going to just the pre-processor section, and dynamic power of as much as 120 Watts to a single channel. Sum it up and you've got around 300 Watts of useable power - which is right in line with the rated power consumption of 5.9 amps on a standard 115 Volt circuit with something around 50% efficiency from the Class A/B amps. Nothing hard to believe about that at all

 

[pcsolutionz]

Well, it's a bit of a "yes and no" answer.

The amps in your 706 receiver all share the same power supply, but they have independent output circuits. Onkyo rates their amps according to the rated output of the individual circuits and then says "2 channels driven", which means that the total supply of power available to the amps is, in reality, only twice what an individual output circuit can provide.

Think of it this way, the "per channel" rating might be 100 Watts or some such. You might think, "ok, there are 7 amp channels and 100 Watts per channel, so this receiver can pump out 700 Watts at any given moment." Well I'm afraid, that's not actually the case. That "2 channels driven" rating means that the most the receiver can pump out at any given moment is 200 Watts, not 700. Furthermore, each individual channel can only pump out 100 Watts through its specific circuit. If you were only driving one speaker, it's not as though you could pump all 200 Watts into it. Only 100 Watts. But you can drive 2 speakers with 100 Watts each, or you can drive all 7 speakers, but each of the seven speakers will only be getting 28.57 Watts if they are all being drive equally and all at the exact same time.

So does using a separate amp to drive two of the speakers allow more power from the receivers amps to go to the remaining five speakers? Yes, in the sense that you have this 200 Watt "bucket" of power available that the remaining five speakers all share, so now there's more power to go around since two of the speakers are no longer dipping into that "bucket". But no in the sense that the most that can ever be delivered to an individual speaker is still the rated "per channel" Wattage of 100 Watts in this example. You don't suddenly get to dump more than 100 Watts into any given channel.

Hope that helps!

I am looking at the amp below on Craigslist to add to my Yamaha RX-V765. I am curious if it would be worth it with my current setup in my Signature. It would be nice to get more power to my speakers than the AVR provides and also take some of the load off. It gets pretty hot when I crank movies. Do you think this amp would help and how many watts would I have available for my system. I'm planning on bridging the amp for three channels (Front and Center). Any help on this would be greatly appreciated, I'm planning on grabbing it tomorrow for 150 bucks.


Rotel RB-956AX 6/5/4/3 Channel Power Amplifier

SPECIFICATIONS

Continuous power output: 30 watts* per channel, min. RMS both channels driven into 8 ohms from 20 to 20.000 Hz with no more than 0.03% THD.
DIN Output: 50 watts per channel (1kHz, 4 ohms, 1% THD)
Peak Current: 20A (0.1 ohms, 10 usec, 1 pulse)
Power Output (BTL): 90 watts (bridged) min. RMS driven into 8 ohms from 20 to 20.000Hz with no more than 0.03%THD.
Total Harmonic Distortion (THD): No more than 0.03% (continuous rated power output).
(20 to 20.000Hz): No more than 0.03% (continuous 1/2 rated power output).
No more than 0.05% (1 watt per channel power output, 8 ohms).
Intermodulation Distortion: No more than 0.05% (continuous rated power output).
(60 Hz: 7kHz = 4:1) No more than 0.05% (continuous 1/2 rated power output).
No more than 0.05% (1 watt per channel power output, 8 ohms).
Output (Speakers): 8 ohms min (bdrigded mode); 4-16 ohms (stereo mode)
Damping factor: 180 (20 to 20.000 Hz, 8 ohms)
Input sensitivity/Impedance: 1.0V/27 kohms
Overload Level (THD 0.5%, 1kHz): 5V
Frequency Response: 10 to 80.000 Hz, +1 dB, -1.0 dB
Signal to Noise Ratio (IHF, A network): 105 dB

GENERAL
Power Requirement: 120 V/60 Hz, 230-240 V/50 Hz (depending on destination)
Power Consumption: 400 watts
Dimension (overall): 440 (W) x 92 (H) x 396 (D) mm
Weight (net): 10.5 kg/23.1 lbs