What is 2 channels driven? - Page 3 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 12-11-14, 04:14 PM Thread Starter
Shackster

Join Date: Nov 2014
Posts: 17
Wow. Exactly what I needed. Soooo helpful. Thank you GCG and the rest. If there is more, please continue.hehe
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Old 12-11-14, 04:24 PM
Senior Shackster

Patrick

Join Date: Aug 2013
Location: Alabama Gulf Coast
Posts: 567
My System
Re: What is 2 channels driven?

I know we need to get to the OP’s question but I feel that insuring that everyone is at a similar level of understanding is important. Bear with me. I’ll be getting to power measurements, %THD thresholds, and the like soon (as long as the demands of the season don’t get in the way).

Ohm’s Law
Here’s a page that has the mathematical relationships between all the various components we discovered above. There is nearly no end of treatments on ohm’s law on the internet and it would be ridiculously redundant to add another. Please ask Uncle Google for a full explanation.

http://diyaudioprojects.com/Technical/Ohms-Law/

What isn't clear from the equations and many of the net treatments is the concept of the source and the load. The source in our case is the amplifier. The load is the speaker or more specifically the current drawn by it. Notice I said DRAWN. Make no mistake the passive downstream component, the speaker, can best be described as DRAWING current from the amp and the only thing you need to prove it is the breaker panel in your home. If the source had the capacity to limit the flow of current in a circuit we wouldn't need breakers, fuses or any other protection. If the resistance in a circuit goes lower, as long as the voltage remains constant the current will rise. *WITHOUT LIMIT* If you replace your 8 ohm speakers with 4 ohm speakers they will draw more current (x2) from the amplifier. That hurts because with more current comes more heat and heat is kryptonite to amplifiers and most all electronics. A short circuit (bare wire across the speaker terminals) is a resistance, too; a resistance of effectively zero. This will make the current try to go toward infinity. It will succeed only until it reaches the limits of the amplifier’s endurance and at that point the amplifier will die. This will happen more rapidly than you could possibly catch. If you want an example think about how fast a car battery cable will burn to a crisp, conductor and all, if it’s shorted to the car’s frame.

But voltage, current, and resistance are related. So, what happens to voltage when you go from an 8 ohm speaker to 4 ohms? It goes down. This helps because upper voltage is one of the limitations in any power supply. By reducing the voltage demand for a given power output more VOLTAGE head room is available before the amplifier goes into clipping (later, I promise). This Voltage head room is only of use if the amplifier is designed to be rugged enough to handle 4 ohm (higher current) loads. Most are not although most can sustain usable output given reasonable discipline on the part of the owner. You can usually see a notice below the speaker terminals on amplifier’s and AVRs that will state something similar to “Rated for 6 ohm – 16 ohm speakers” If you attach a 4 ohm speaker and play at high levels you will stress the amplifier and risk an overload trip or worse, damage. TAKE CARE when using 4 ohm speakers on non-4 ohm rated amplifiers.

Now we’re getting closer to understanding the complication in answering the original question. We have an idea of what is going on in the amplifier and how it is affected by load (speaker resistance) changes. But what puts limits on power. That’s next – Power Supplies.

{note: in the following I’ll be using the correct term for speaker load resistance – impedance. Resistance is a term used for DC (direct current) however audio is AC (alternating current) and must include allowances for the effect of frequency on the ultimate value. Each voice coil has a DC resistance specification determined by the wire gauge and the total wire length in the winding but its impedance will vary greatly from this value across its usable frequency range. The 4-ohm/8-ohm designation is a nominal value for the full range that is useful for matching speakers to amplifiers}

The equation wheel in the link from above shows power can be defined as a combination of voltage (V) and current (I). Take either one higher while the other remains the same and power goes up. In every amplifier is a “Power Supply” that converts house current (110VAC [volts AC] 60Hz US and others, 220VAC 50Hz Britain, Europe, and others) into various DC voltages depending on the make and model of the design. (DC stands for direct current in which the current is always flowing in the same direction and AC is for alternating current in which the current switches direction either 50 (Hz) or 60 (Hz) times a second). The components are chosen to withstand a certain limit in terms of current. These current limitations along with the voltage CHOICES combine to establish the power limits of a given supply. At no time can a power supply deliver more total power than is allowed by these constraints without risking damage. That said there are usually two specifications for this. One is the continuous rating. This is the limit for long periods. The other is the dynamic rating. Allowing for brief (<1 sec) bursts of increased output.

For the most part AVRs and multichannel amplifiers have a single power supply that is shared by all channels. Each channel can deliver voltage UP TO the power supply’s voltage limits and current as determined by the supplied voltage and the load (4 ohm/8 ohm). When the voltage limits are reached the amplifier will “clip”. Clipping effectively flattens off the positive and negative “tops” of the waveforms. This is BAD. Clipping causes the amplifier to run at higher currents for longer (Google: duty cycle) increasing heat. It also creates potentially damaging harmonics that can destroy tweeters. Clipping is to be avoided. Since low impedances make for lower voltages, 4 ohm speakers are inherently less susceptible to clipping than 8 ohm; a plus for 4 ohm speakers. This is the reason amplifiers specify different power into different impedance speakers. The speaker can draw more current before clipping occurs if the amplifier circuit can handle the current. Since the voltage limit is the same and the current is higher – more power.

Before I go on I'll take time for questions - if any. Still to come is Frequency, Fundamentals, Harmonics, Voicing, and THD (Total Harminic Distortion)

7.1 System
Fronts - Infinity RSb (circa 1978)
Center - Infinity Video 1
Surrounds - Definitive Technology UIW 63/A (in ceiling)
Rears - Definitive Technology UIW 63/A (in ceiling)
Sub - Dayton Audio/Parts Express 15" Titanic Mk III kit
AVR - Pioneer Elite SC-71
Content source - WDTV Live Hub Media Server
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Old 12-11-14, 07:48 PM Thread Starter
Shackster

Join Date: Nov 2014
Posts: 17
Please go on. Very well written and understandable.
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Old 12-11-14, 08:33 PM
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Tony

Join Date: Sep 2007
Location: Edmonton, Alberta, Can
Posts: 14,816
My System
Re: What is 2 channels driven?

Patrick, just wanted to say that this is very informative and should be made into a dedicated thread that is made a sticky.
Lots of people can benefit from this info

Home theater:
Onkyo 805, Yamaha YDP2006EQ, Samson Servo 600 amp
3 EV Sentry 500 monitors across the front, 4 Mission 762i's Surrounds, SVS PB13U sub, Panasonic BDT220, Harmony 1100, Nintendo WiiU
Panasonic PT-AE8000 on a 120" 2,35:1 fixed screen

Living room system:
Sherwood/Newcastle R972, Mission 765's, SVS SBS02's, A/D/S MS3u sub, Yamaha YDG2030EQ
Yamaha KX-393 Tape deck, CDC 805 CD changer, Panasonic BD60, Sony turntable PS-T20
Panasonic TC-P50ST60, HD-PVR & WDTV Live, Harmony 900

tonyvdb is online now
Old 12-11-14, 10:27 PM
Senior Shackster

Patrick

Join Date: Aug 2013
Location: Alabama Gulf Coast
Posts: 567
My System
Re: What is 2 channels driven?

Is this the correct forum or would another be more appropriate?

7.1 System
Fronts - Infinity RSb (circa 1978)
Center - Infinity Video 1
Surrounds - Definitive Technology UIW 63/A (in ceiling)
Rears - Definitive Technology UIW 63/A (in ceiling)
Sub - Dayton Audio/Parts Express 15" Titanic Mk III kit
AVR - Pioneer Elite SC-71
Content source - WDTV Live Hub Media Server
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Old 12-12-14, 12:40 AM
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Wayne Myers

Join Date: Jun 2012
Location: Lincoln, NE
Posts: 4,609
Re: What is 2 channels driven?

We will create a new thread for it, probably a sticky, I am looking for the best place. In the mean time, continue on assuming it to be an instructional sticky thread, and we can copy/paste it into place when we find it a home.

Thanks again for taking the time, Patrick.
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Old 12-12-14, 07:09 AM
Senior Shackster

Patrick

Join Date: Aug 2013
Location: Alabama Gulf Coast
Posts: 567
My System
Re: What is 2 channels driven?

No problem, happy to help.

7.1 System
Fronts - Infinity RSb (circa 1978)
Center - Infinity Video 1
Surrounds - Definitive Technology UIW 63/A (in ceiling)
Rears - Definitive Technology UIW 63/A (in ceiling)
Sub - Dayton Audio/Parts Express 15" Titanic Mk III kit
AVR - Pioneer Elite SC-71
Content source - WDTV Live Hub Media Server
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Old 12-12-14, 10:01 AM
Senior Shackster

Patrick

Join Date: Aug 2013
Location: Alabama Gulf Coast
Posts: 567
My System
Re: What is 2 channels driven?

Before I go on I thought I’d point out a couple of other things involving power and power supplies. I stated that going from 8-ohm to 4-ohm speakers can allow an amplifier channel to deliver more power within the constraints of the designed current limits. I also said that the power supply could only reliably deliver power up to a given limit and when that limit is reached, the power well runs dry. If, as is the case of multi-channel surround sound (5.1, 7.1, etc.), 4-ohm speakers are used for the fronts and 8-ohm speakers are used elsewhere and the fronts end up demanding more power at similar volumes then that dips deeper into the power well leaving less for the remaining speakers. This is easy to do and is in some cases hidden due to the way specifications for speakers are written to tell us how loud a speaker will play. Speaker specifications will usually state this as either Efficiency or Sensitivity. It is measured in db and in this case db is the measure of the Sound Pressure Level or SPL. It’s the same SPL the safety guy at work would measure to determine if hearing protection is required in a given work environment. The sticky point comes with the conditions under which the specification is measured. It’s usually stated by the following:

1 kHz @ 1 Watt @ 1 meter
..or..
1 kHz @ 2.83 Volts @ 1 meter

Meaning they run a 1kHz tone at a power of 1 watt (2.83 volts) to the speaker and measure the SPL from 1 meter away. I prefer the former and here’s why. Assume the conditions above are applied to an 8-ohm speaker. By the equation wheel we can see that for 8-ohm speakers they say the same thing.
Let’s run 1 watt to that speaker and see what voltage develops.

V = √(P*R)
V = √(1*8)
V = √8
V = 2.828427125 ~ 2.83

Now do the same for the 4-ohm speaker.
V = √(P*R)
V = √(1*4)
V = √4
V = 2.00 volts … a 41% difference that gives me an idea of how much headroom I’ll have – db for db.

Now let’s go the other way – let’s apply those 2.83 volts to the 4-ohm speaker and calculate the power being delivered.

P = V²/R
P = 2.83²/4
P = 8/4 = 2 watts … a 2-for-1 difference that betrays the otherwise hidden fact that, db for db, going by this spec I’ll be dipping twice as deep into my power supply well.

To me the first definition is more – apples to apples – than the second. It relates the Efficiency/Sensitivity more directly to the power supply spec of watts which is more useful in my opinion.

Another thing that may be of interest is using the specs to do a little detective work to ESTIMATE the limits of the power supply. I’ll use the specs for my AVR since I have them handy. By my signature you can see that’s a Pioneer Elite SC-71. You can go by the manufacturer’s spec but I prefer using independent lab tests if I can find them as I tend to believe they have less bias. My favorite is Sound and Vision. They did a full review of the SC-71 with a nice lab workup you can find here:

http://www.soundandvision.com/conten...ver-test-bench

First let’s figure out how deep my power supply well is. We see that the power limit into 8-ohms for all seven channels driven (Hey, chinzo, LOOK we’re finally getting to your question – sorta) @1.0% THD (I’ll be getting to that) is 106.8 watts.

106.8 * 7 = 747.6 watts (in the manual the spec is 560 watts – under spec’ing or conservative spec’ing is a good thing to find in a manufacturer in my opinion)

It’s deep enough for me (for now anyway). That’s the power supply total output but what about the voltage limits. When will this thing begin to clip? For that I need to push a single channel (or just two) up to its limit without draining the power supply well. When I find the limits in those conditions I’ll know a little more about upper voltage. Higher speaker impedance yields higher voltages so a limit pushing spec into an 8-ohm speaker is what I’m looking for. And I find it in the first paragraph under the Harmonic Distortion graph. That paragraph reads as follows:

Quote:
This graph shows that the SC-71’s left channel, from CD input to speaker output with two channels driving 8-ohm loads, reaches 0.1% distortion at 120.6 watts and 1% distortion at 146.0 watts. Into 4 ohms, the amplifier reaches 0.1% distortion at 204.3 watts and 1% distortion at 247.3 watts.
The spec is most likely to be pushing the envelope at the 1.0% point and beyond so I’ll use that. For 8-ohms that’s 146.0 watts. Again from the equations:

V = √(P*R)
V = √(146.0 *8)
V = √1168
V = 34.18 volts

But that’s RMS and I want the value for peak (I’ll discuss the difference later but peak is where clipping occurs) which is RMS*1.414 which comes to 48.33. So it’s safe to assume the power supply voltage limits are at least +/- 48.33 volts. An estimate of power supply current can now be made by using the max power we found before and this voltage value in our calculations.

{I have to do an edit here. The power supply peak voltage calculation is mixing RMS and peak values which is a NoNo. }
{Corrected using a value of 1057.1 watts peak vs. the 747.6 watts RMS , sorry for the mix up.}
I = P/V
I = 1057.1/48.33
I = 21.87 amps peak or 15.47 amps RMS

Just for grins let’s calculate the voltage using the specs for a 4-ohm speaker. At 1.0% distortion the channel was pumping 247.3 watts.

V = √(P*R)
V = √(247.3 *4)
V = √989.2
V = 31.45 volts RMS or 44.47 volts peak

That’s less than what was calculated for the voltage limit for the power supply and we haven’t reached the power supply max power limit so the best safe assumption is this limit is driven by the current limitations of the amplifier stage itself.

7.1 System
Fronts - Infinity RSb (circa 1978)
Center - Infinity Video 1
Surrounds - Definitive Technology UIW 63/A (in ceiling)
Rears - Definitive Technology UIW 63/A (in ceiling)
Sub - Dayton Audio/Parts Express 15" Titanic Mk III kit
AVR - Pioneer Elite SC-71
Content source - WDTV Live Hub Media Server

Last edited by GCG; 12-12-14 at 10:25 AM. Reason: I goofed
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Old 12-12-14, 10:55 AM
Senior Shackster

Patrick

Join Date: Aug 2013
Location: Alabama Gulf Coast
Posts: 567
My System
Re: What is 2 channels driven?

I went searching for examples of speaker manufacturers that use the 2.83 volt spec for Efficiency/Sensitivity with a 4-ohm speaker and found this right away on the PSB site.

(1W (2.83V) @ 1M, IEC-Filtered pink noise, C-weighted)

!!! Into a 4-ohm Speaker !!!

We just showed that 1 watt into a 4-ohm speaker yields 2 volts and 2.83 volts will yield 2 watts. This shows one of four things about a manufacturer.
1. They don't know what they're talking about. (unlikely, after all, they do MAKE speakers)
2. They want to confuse you. (I doubt it)
3. They left the writing of the spec sheet to an under-informed noob. (possible)
4. Nobody proof read the spec sheet before it was published. (most likely)

There is no way to know what the real conditions were to establish this spec. This alone might cause me to take a speaker off my short list.

7.1 System
Fronts - Infinity RSb (circa 1978)
Center - Infinity Video 1
Surrounds - Definitive Technology UIW 63/A (in ceiling)
Rears - Definitive Technology UIW 63/A (in ceiling)
Sub - Dayton Audio/Parts Express 15" Titanic Mk III kit
AVR - Pioneer Elite SC-71
Content source - WDTV Live Hub Media Server
GCG is offline
Old 12-12-14, 12:29 PM
HTS Moderator
Reviewer

Wayne Myers

Join Date: Jun 2012
Location: Lincoln, NE
Posts: 4,609
Re: What is 2 channels driven?

Quote:
GCG wrote: View Post
I went searching for examples of speaker manufacturers that use the 2.83 volt spec for Efficiency/Sensitivity with a 4-ohm speaker and found this right away on the PSB site.

(1W (2.83V) @ 1M, IEC-Filtered pink noise, C-weighted)

!!! Into a 4-ohm Speaker !!!

We just showed that 1 watt into a 4-ohm speaker yields 2 volts and 2.83 volts will yield 2 watts. This shows one of four things about a manufacturer.
1. They don't know what they're talking about. (unlikely, after all, they do MAKE speakers)
2. They want to confuse you. (I doubt it)
3. They left the writing of the spec sheet to an under-informed noob. (possible)
4. Nobody proof read the spec sheet before it was published. (most likely)

There is no way to know what the real conditions were to establish this spec. This alone might cause me to take a speaker off my short list.
Not sure what model you pulled up. I glanced at the Imagine T3 and T2 Towers...

The Imagine T3 Tower is nominal 8 ohms, minimum 4 ohms, and they specify the rms test voltage as you have shown for the sensitivity. The Imagine T2 Tower is nominal 6 ohms, minimum 4 ohms, and they conveniently left out the rms test voltage. Do you remember which model you looked at? Just curious.
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