Lets really look at this 4 vs. 8 ohm thing

[crashguy]

If I could, I'd like to ask a question. For all of those people that post questions regarding whether or not it is safe to drive a pair of 4 ohm speakers, or hook up two sets of 8 ohm speakers in parallel (making a 4 ohm load), I would like to ask all members to reply to this post (especially you experienced guys).

Has anyone ever had any problems relating to running 4 ohm speakers with a receiver or amp not rated for 4 ohm speakers?

Now I know we're not supposed to do this (please no one tell the CIA), but really; I have ignored the impedence ratings of any speakers I have owned for the last 15 years I have been an "audioholic" (and believe me, I've got it bad ). I've never had a problem.

I will be the first to agree that if you tried to run a set of Martin Logan electrostatics (which dip down to 2 ohms over much of their range) with a $150 el-cheapo receiver, you WOULD have a problem. But really, with decent quality gear such as Denon, Yamaha, H/K, Onkyo, etc (sorry if I left YOUR brand out, can't list them all), do we really have to worry???? I think not.

Clint, are you there? What do you electronics gurus think?

Opinions/comments????

 

[zumbo]

No problem here. Maybe this should be a poll.

 

[HowY]

What really seems to be missed by most with reguard to
lower resistance is it's actually more work for the amplifier.

Consider a straw and a 2"d pipe blow into the straw (hi resistance)
a little work produces a noticable flow of air and you really gotta
huff into the 2" pipe to feel anything (less resistance)

Had this arguement so many times and am always told less resistance
is a good thing but the reality is that it is *not* necessairly and especially
not to older head units set for 8 ohm loads.... (cars & old 'ceivers)

Car stereo . com explains this relationship best:
__________________________________________

what effect do these different wiring combinations have on the amplifier?

With a 25 watt (10 volt) amplifier with a single 4 ohm speaker we have:

Current = Voltage / Resistance = 10 / 4 = 2.5 amps

So when this amp is producing maximum power (25 watts) into a 4 ohm load, the load will draw 2.5 amps from the amp.

Now let us look at what happens when we connect two 4 ohm speakers in parallel (which gives us a 2 ohm equivalent load) to this amp:

Power = Voltage^2 / Resistance = 10^2 / 2 = 50 watts

This is seems great! Our 25 watt amp is now producing 50 watts but there are some complications. Let us see the current now:

Current = Voltage / Resistance = 10 / 2 = 5 amps

Even though our voltage is still the same (10 volts) our current has now doubled from 2.5 amps to 5 amps. If the amp has the capability to produce this much current and dissipate the heat that this will generate then everything will be fine.

One way to determine if your amp is capable of this is to look for power ratings that are given into 2 ohms in addition to the normal 4 ohm rating.

Further if the power doubles into the 2 ohm rating then the amp has ample current capacity. Another clue to tell whether the amp will work with 2 ohm loads is look for the phrase "2 ohm stable." Being 2 ohm stable only means that the amp will function with 2 ohm loads; it does not necessarily mean that the amp will produce more power into 2 ohms. If you attempt to use a 2 ohm load with an amp that cannot handle it a well designed amp will shut itself off or blow a fuse and a poor one could be permanently damaged.

 

[crashguy]

I thought about that Zumbo....I tried to edit it and turn it into a poll, but I can't. Maybe one of the moderators can do that for us. (?)

I realize driving a lower impedence load represents more of a load on the amplifier, but what I want to know is if there are people out there who have ever experienced a problem when driving a 4 ohm load with an amplifier not specifically rated to do the task. The reason I bring this up is because it seems a bit silly to me that my receiver that costs $1000 (CDN) or so is not rated to be able to drive a 4 ohm load, yet a $150 Panasonic receiver IS rated to drive 4 ohm loads.

Make any sense to you guys?

 

[zumbo]

I run Linear Power amps in car audio. They have tabs inside to move from 4ohm to 2ohm for 2ohm opperation. The trick with us Linear guys, is to keep the tabs on 4ohm. Not only that, but bridge a 2ohm load on that! 1ohm I have amps from '85 & '82, and they have been running this way for all these years with no problem!

But, this was one of the few brands that you could do this with. So I was told at the time.

 

[derekwwww]

im guessing amps are like computers in a way.... how u can overclock a computer to run faster but you need to make sure it runs cool. So its probably safe to run lower ohm loads on the amp as long as you get some fans or something to make sure it runs cool? A few years ago I used a couple old towers that had 2 15s in each tower and a horn in each rated at 4 ohms with an old telefunken receiver. If I would crank them up for long periods of time the receiver would shut down from heating up. A party came up that I had to use this setup for and have cranked at high volume (outdoor party) for he whole night. I setup two big fans pointing on the amp and ran them all night with it and the amp stayed cool to touch didnt warm up at all and ran good all night! So I would assume the same thing would apply here but you would use small fans not big things like i did.

 

[zumbo]

I have this one guys. This will keep it cool!

 

[dohanc]

The output device of an amplifier is a electronic component called a MOSFET. The MOSFET can handle a certain amount of current which generates a certain amount of heat. The heat the device can handle is what limits it. Heat sinks are used to extract the heat away. The better you can do that the longer the MOSFET will last.

The other limitation is the power supply. Only a certain amount of current can be provided by the power supply.

Can you plug a 4 ohm load into a 8 ohm amp? Yes. Keep the volume -3dB (half the power) lower than you would with a 8 ohm load to keep the power output equivalent. Eventually at a low enough load you will be requiring too much current even at a very low voltage.

 

[gregz]

dohanc's got it; if you run 4 ohm speakers on an amp rated for 8 ohms only, run at half power and you'll be fine:

Relationship between power, voltage and current is:

P=I*V

Resistance VS current and voltage is:

V=I*R


Going back to the quote in How Y's post, the REST of the story is:

25W amp at 8ohms means the amp is rated to put out 14V at 1.8 Amps.

Since most common amps can be considered voltage sources, if you attach 4 ohm speakers instead of 8 ohm speakers, that 14V across 4 ohms can draw up to 3.5 Amps at full power for 50W.

However, if you run the amp at only half power (7V), then you're only drawing 1.75 Amps, which gives you 25W output. No harm done, no strain, no extra current.

The problem with doing this is that you might not know when you're overdriving your amp and when you're not unless you have test equipment permanantly attached to the output. Since each CD is recorded at a different volume, you can't just mark a spot on the volume dial because it will change from source to source.

So there.

 

[mcwilson]

I've had one experience driving 4 ohm speakers (JAMO D870s and D8CEN) with an 8 ohm receiver (Yamaha RX-V2400). Under most circumstances, I had no problems. Watching movies, listening to music, everything sounded great. However, my receiver began to clip with a check speaker error if I listened to loud levels (like during battle scenes, etc.). I ended up upgrading to separates.

Just my one experience...

Mark

 

[crashguy]

I know that there are quite a few of the higher quality HT receivers out there that are fan cooled. I had a few old Technics receivers that had a fan that ran on output power, rather than being thermostatically controlled. I think most of the internal fans now come on only when the receiver heats up. At least that's the way my Denon 3802 worked, as does my new Yanaha 2500. This should look after the heat build up issue.

 

[Mudcat]

Consider a straw and a 2"d pipe blow into the straw (hi resistance) a little work produces a noticable flow of air and you really gotta
huff into the 2" pipe to feel anything (less resistance)

So, is there a picture you could post that shows you with a 2" pipe in your mouth?

 

[20to20K]

I'm running three 4 ohm speakers...

I've got a Denon 3805 (120x7) and I'm running two Polk LSi 15's for the fronts and a LSiC for the center and they're all 4 ohms. I'm running Fxi-3 for the rears that are 8 ohm...all through the 3805 and the Denon handles it. After watching a couple of movies or a long music listening session it does get
luke warm...but nothing outrageous.

Normally I have a Denon POA-2400 driving the fronts, but it's out for service right now. I was hesitant about putting all the Polks on the 3805 since the manual only recommends 6-16 ohms but I gave it a try and it has yet to shutdown or get excessively hot yet.

The Audioholic reviewer for the 3805 (Clint DeBoer) ran five 4 ohm speakers on them without a problem...that was one of my deciding factors in buying it.
This is an excert of his review:

The Denon AVR-3805 didn’t appear to break a sweat driving the 4-ohm impedance presented by the RBH Sound 1266-LSEs and 661-LSE speakers. In fact, the rated 120 watts per channel was sufficient enough to drive the Reference 3 System speakers to a steady 97dB SPL at the listening position without any audible distortion. For those of you who are wondering, this translates into approximately 105dB SPL (average continuous) from 1’ from the front of the speakers. In the Reference room, the Denon receiver was parked at -10 on the digitally-controlled analogue volume to produce these levels

I guess the bottom line is it depends on your receiver. I would think that receivers of an equal caliber of Denon (H/K, some Yammies, and Oinks)
could handle it as well.

On the other hand I do notice a dramatic lacking of bass and soundstage
missing without my high current 200 Watt RMS power amp, but the 3805 is doing a sufficient job in the interim...

 

[Lagger]

If I could, I'd like to ask a question. For all of those people that post questions regarding whether or not it is safe to drive a pair of 4 ohm speakers, or hook up two sets of 8 ohm speakers in parallel (making a 4 ohm load), I would like to ask all members to reply to this post (especially you experienced guys).

Has anyone ever had any problems relating to running 4 ohm speakers with a receiver or amp not rated for 4 ohm speakers?

Now I know we're not supposed to do this (please no one tell the CIA), but really; I have ignored the impedence ratings of any speakers I have owned for the last 15 years I have been an "audioholic" (and believe me, I've got it bad ). I've never had a problem.

I will be the first to agree that if you tried to run a set of Martin Logan electrostatics (which dip down to 2 ohms over much of their range) with a $150 el-cheapo receiver, you WOULD have a problem. But really, with decent quality gear such as Denon, Yamaha, H/K, Onkyo, etc (sorry if I left YOUR brand out, can't list them all), do we really have to worry???? I think not.

Clint, are you there? What do you electronics gurus think?

Opinions/comments????

My setup is similiar to what your are describing. A mid-fi receiver (Yamaha rated 20-20,000hz, 100 watts x 5 Channels @ %0.06thd) pushing a relativly low impendance (4ohm) low sensitivity (86 db) speaker, DynAudio 52's.

The Yammie has a switch on the back that allows you to alternate between 8 and 4 ohm loads. Running an 8 ohm load, it produces highs and lows well, but the midrange is lacking, as in missing. The 4 ohm load brings the midrange back to where it should be. So obviously I leave it on the 4 ohm setting. My guess is that it provides slightly higher current/voltage than regular (8 ohm).

My only problem is I have to turn the volume UP, way UP. In 2 ch mode I have the volume 1/2 way up most of the time for comfortable listening levels. I guess at those volums it is loud, however, this is were I feel the midrange is presented accurately. D.D. and DTS is a little lighter on the knob, but still........I feel the need for an external amp. I haven't really felt my amp for overheating issues after a long session. So I cant comment on that.

My understanding is that these higher end a/v rec's produce more watts per volt. Even though 2 rec's might be rated the same, it doesnt state power consumption, cooling effectiveness, etc...

So my conclusion is that if you are planning on running low impendance, low sensitivity speakers, either buy a high-end rec. or seperates. Peace_

 

[zumbo]

My setup is similiar to what your are describing. A mid-fi receiver (Yamaha rated 20-20,000hz, 100 watts x 5 Channels @ %0.06thd) pushing a relativly low impendance (4ohm) low sensitivity (86 db) speaker, DynAudio 52's.

The Yammie has a switch on the back that allows you to alternate between 8 and 4 ohm loads. Running an 8 ohm load, it produces highs and lows well, but the midrange is lacking, as in missing. The 4 ohm load brings the midrange back to where it should be. So obviously I leave it on the 4 ohm setting. My guess is that it provides slightly higher current/voltage than regular (8 ohm).

My only problem is I have to turn the volume UP, way UP. In 2 ch mode I have the volume 1/2 way up most of the time for comfortable listening levels. I guess at those volums it is loud, however, this is were I feel the midrange is presented accurately. D.D. and DTS is a little lighter on the knob, but still........I feel the need for an external amp. I haven't really felt my amp for overheating issues after a long session. So I cant comment on that.

My understanding is that these higher end a/v rec's produce more watts per volt. Even though 2 rec's might be rated the same, it doesnt state power consumption, cooling effectiveness, etc...

So my conclusion is that if you are planning on running low impendance, low sensitivity speakers, either buy a high-end rec. or seperates. Peace_

It should be set to 8ohm. Read this.

 

[crashguy]

The Yammie has a switch on the back that allows you to alternate between 8 and 4 ohm loads. Running an 8 ohm load, it produces highs and lows well, but the midrange is lacking, as in missing. The 4 ohm load brings the midrange back to where it should be. So obviously I leave it on the 4 ohm setting. My guess is that it provides slightly higher current/voltage than regular (8 ohm).

I'm sure by now you have followed the link to learn that setting that silly switch on 4 ohms actually reduces the amount of power your amp can put out. I don't know how to explain you perceiving it sounds better that way, as sound preference is individual. I think there are others out there who also believe the switch somehow boosts power for 4 ohm loads, when in fact it does the opposite.

Lagger, have you ever had a shut down or overheat while running your 4 ohm speakers? If you set the switch back to the 8 ohm setting, will you give it a test drive, and let us know if you experience any heat or shut down issues?

 

[gregz]

I'm sure by now you have followed the link to learn that setting that silly switch on 4 ohms actually reduces the amount of power your amp can put out. I don't know how to explain you perceiving it sounds better that way, as sound preference is individual. I think there are others out there who also believe the switch somehow boosts power for 4 ohm loads, when in fact it does the opposite.

Hey, guys; if that switch DOES reduce the output voltage, then it allows the amp to run 4 ohm speakers without worrying about drawing more currrent than it was designed for. You'll get your full rated power into 4 ohms.

Conversely, if you leave it in 4 ohms setting while connected to 8 ohm speakers, then you're hamstringing your amp at half its rated power.

Didn't anyone see my post on the first page of this thread? There's no magic going on, current flow is just output voltage divided by impedance. Power (in Watts) is current multiplied by voltage. If you cut the impedance in half (i.e. going from 8 ohms to 4 ohms), then you double the power demand unless you ALSO cut the voltage in half.

Since the maximum voltage output of a common steady state amp is constant, the only way you can ruin an amp is by trying to draw more current than it's designed to give, and the mechanism of destruction is heat.

 

[crashguy]

Hey, guys; if that switch DOES reduce the output voltage, then it allows the amp to run 4 ohm speakers without worrying about drawing more currrent than it was designed for. You'll get your full rated power into 4 ohms.

Conversely, if you leave it in 4 ohms setting while connected to 8 ohm speakers, then you're hamstringing your amp at half its rated power.

Didn't anyone see my post on the first page of this thread?

I have to respectfully completely DISAGREE with you on that gregz. Whether or not you get your "full rated power" with that silly switch set to 4 ohms depends on what exactly the manufacturer has designed it to do. I have read many a test in S&V magazine where the silly switch cuts the power by 2/3, and I've been a subscriber since 1992, when it was called STEREO REVIEW. If need be, I can look one such test up, and post the lab results here. There are many reviewers much smarter than I am that tell you to do with this swtch what they do with it, and that is IGNORE IT. The only way I can see the silly switch being of any use at all is to prevent shutdown so immediate that it causes your receiver to conk out as so as you hit play, such as in my example of an el-cheapo unit driving speakers that dip to 2 ohms. This is of course very unlikely, hence the name "silly switch."

The reason I started this thread in the first place was to ascertain whether or not there are any real world cases where running a 4 ohm speaker with a receiver or amp not specifically rated for it has EVER caused anyone any problems.

I guess the underlying theme here is; I think there are so few cases of problems that is relatively safe to say that running a set up as I describe is OK to do. I of course have been following my own advice on this topic for 15 years, and never had a problem. I want to find out if I'm just lucky, or if this thing is overhyped as an issue anyone (meaning the average guy with mid-fi gear such as myself) has to ever worry about, within reason of course.

 

[PENG]

I have to respectfully completely DISAGREE with you on that gregz.

I am not sure if gregz is all that wrong. A manufacturer can in fact use that "silly switch" to reduce voltage just sufficiently to give roughly the same rated power into a 4 ohm load as it would into an 8 ohm load.

In practice though, I agree with crashguy that the switch would probably reduce the voltage enough to make sure the current at rated power (for an 8 ohm load) is not exceeded, or only slightly exceeded. In that case, you end up with less power output. Example: say the d.c. rail is at 40V in the 8 ohm position. I=V/Z=5A, P=V*V/Z==40*40/8=200VA.
If voltage is limited to 30V in the 4 ohm position. Current I=30/4=7.5A (50% higher than the 5A into an 8 ohm load). In this case, power into the 4 ohm load, P=V*V/Z=30*30/4=900/4=225VA. If voltage is limited to 20V (half), then current I=20/4=5A (now it is same as the current at rated power into 8 ohms), and in this case, power, P=V*V/Z=20*20/4=100VA, that is, only half of the power rated for an 8 ohm load.

So it depends on how much the voltage is reduced by the "silly switch". You could end up with either the same power rated for an 8 ohm load, slightly less, or only half if the switch (in the 4 ohm position) limits the current into a 4 ohm load the same as the current into an 8 ohm load when in the 8 ohm position.

 

[gregz]

crashguy, I'm not sure what you're disagreeing with but I didn't make up Ohm's law so I won't be offended. My point was just to dispell some of the conjecture and misinformation about what low impedance does and doesn't do and why a switch that cuts the output voltage for 4 ohm loads actually makes sense from a design standpoint as far as maintaining the same design margins.

But don't let me spoil the party. Rock on!

************ 1/23/05 Edit ********************************

My earlier calculations were, in part, due to having consumed too many chocolate bars in too short a time.

As stated by PENG above, 50% voltage into 4 ohms does NOT maintain equal power. To keep power the same going from an 8 ohm load to a 4 ohm load, output voltage is required to drop to by roughly 28%.

This is because of the increase in current per volt due to the drop in impedance. Power (in Watts) is a product of current x voltage, so while current doubles from 8ohms to 4ohms at a fixed voltage, cutting the voltage in half will keep the current constant - which will cut total power in half.

This actually brings up an interesting point: Because no amp is a perfect voltage source, that 28% drop in voltage will vary a bit, which could explain why that Audio Magazine test showed the switch reduced output voltage to between 61% and 75% of normal. (I'm typing this from memory)

Anyway, none of this changes the idea of the discussion, which was "Damn the switch, full steam into 4 ohms!"