Noob question, why is their a minimum impedance rating on an amp?

[Leo Shafai]

I noticed this message on the back saying "Caution: Use 8 ohm min speakers"

I've been running 4 ohm speakers on it for a few hours with no issue...

Is this only an issue when turning the volume knob up past half (I've never gone past about 1/4-1/3) due to increased current?

The unit im using is not at all modern and I can't find it on the internet

 

[lovinthehd]

A rating is meeting a particular standard at particular measurement points. Hard to simply generalize, but an amp "rated", i.e. specified by the manufacturer in the stated ratings at specific parameters for those ratings, hopefully comparable to others' measurements/ratings....but lower impedance ratings are a good thing, but you won't find a lot of amps "rated" for 4 ohm even if somewhat capable due various standards out there....so what is the make/model of what you've got?

As to volume controls and their relation to others, meh. Some are dB based and can be calibrated to a THX standard, but that's more relevant for movies that there's a standard of sorts for....

 

[Leemix]

Just be a bit cautious with the volume knob ye.
It might just be that your speakers are more honest with their resistance rating than most because “8 ohm” speakers almost never actually are 8ohm minimum (or according to some standard ok to be down to around 6ohm as 8ohm rated), most dip down to 5 or 4ohm even if it says 8ohm on the spec sheet. So if taken literally there are very very few speakers that could be connected to a lot of the 8ohm minimum very common AVRs
That said its always a good idea to be careful and check if the AVR gets really hot, both for longevity and your hearing.


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[PENG]

I noticed this message on the back saying "Caution: Use 8 ohm min speakers"

I've been running 4 ohm speakers on it for a few hours with no issue...

Is this only an issue when turning the volume knob up past half (I've never gone past about 1/4-1/3) due to increased current?

The unit im using is not at all modern and I can't find it on the internet

Amplifier manufacturers don't seem to adhere to the same standards (not sure if they even exist). In general, some manufacturers are more careful, and/or might have tried harder than others to minimize warranty claims due to users using their units beyond their capability.

The same can be said about speaker manufacturers. You can have 10 speakers specifying their impedance as 4 ohm nominal, yet some may be easier to drive than the others. You can even have 8 ohm nominal speakers that are harder on your amp than some 4 ohm nominal speakers.

It is sad but true that such cautionary message on the back are anecdotal to some extent, though not entirely useless.

Is this only an issue when turning the volume knob up past half (I've never gone past about 1/4-1/3) due to increased current?

As HD mentioned, the volume position is not always a good reference because there are also no fixed standards manufacturers follow so one amp may reach their output limit at 1/4 while another may be at 1/2 all else being equal (such as the source contents, signal input level..etc.). However, you are absolutely correct that with 4 ohm speakers you do need to lower the volume to keep the current down (again, there are other factors, it's complicated), versus using 8 ohm speakers, all else being equal.

I have to emphasize "all else being equal" for reasons I mentioned earlier.

 

[highfigh]

I noticed this message on the back saying "Caution: Use 8 ohm min speakers"

I've been running 4 ohm speakers on it for a few hours with no issue...

Is this only an issue when turning the volume knob up past half (I've never gone past about 1/4-1/3) due to increased current?

The unit im using is not at all modern and I can't find it on the internet

To answer the question that has been left alone so far, yes- current is the reason your receiver is rated for 8 Ohms or higher. The design used certain output devices (transistors or tubes) and those need to be cooled properly, or they will fail. The reason they fail is due to excessive heat, but the primary cause of that heat is current, a secondary cause is lack of air flow. If the unit is old and has dust on the inside, clean it.

The current will be excessive at high power levels but depending on the design, it could still fail at lower levels.

Adding a resistor will prevent the amplifier's demise, but the resistor would need to be able to handle its own excess heat (it needs to be the correct type and they get HOT) and using a resistor just wastes power.

 

[head_unit]

s this only an issue when turning the volume knob up past half (I've never gone past about 1/4-1/3) due to increased current?

Yup. Ohm's Law says V=I*R, or Voltage = current * resistance. To rearrange and reword that, you get
current ∝ volume ÷ resistance
OK the proportionality is by a square root, but anyway the point is more volume sucks more current out of the amp. So until the volume is somewhere near maximum, the amp can supply enough current just fine. I gotta run to work so I'll just post some boilerplate stuff:
Here are "Head_Unit’s Rules Of Protection" (speaking as a loudspeaker engineer who later worked in amplifier product planning and automotive testing):
1) If when things start to sound distorted or odd you TURN IT DOWN, you are unlikely to ever break anything.
2) If you constantly "turn it up to 11" you will break something.
3) The amp and speaker power ratings do not matter. Don’t bother “matching” the amp and speaker power. That is a seemingly sensible yet actually meaningless exercise, because:
- Speaker specifications are 92% useless, due to different manufacturers measuring the same specs in different ways on different setups. And then (shudder of horror!) the Marketing Department gets hold of them, yowZa!
- Specs for amps are not thorough since they are measured into resistors for pragmatic reasons and speakers are not resistors at all.
4) Amps' 4 ohm or even 2 ohm rating is the most meaningful even if your speakers are 8 ohms because again the speakers are not resistors Should be 20-20k Hz, distortion under 1% or it's baloney.
5) For amps "more" power means (IF specs are comparable) at least three times as much due to the logarithmic nature of hearing.
6) You are less likely to damage speakers with a big amp, since let’s face it everyone cranks it up sometime, and a small cheap amp is then more likely to clip and possibly put out DC and ultrasonics (This assumes the speakers are not tiny little pieces of poop).
7) Be merciful and get yourself one of these https://www.acinfinity.com/receiver-amp-cooling-fans/ because AVRs pack a LOT of stuff in one chassis, and every channel generates a chunk of waste heat even at low volume.

 

[Leo Shafai]

ok thanks for the help everyone, makes a lot of sense.

Seems its at the same level as "dont open to prevent shock" and is just there to be foolproof in case some guy shuts off the source but left the amp on at max volume and its inside a wooden box with no airflow under their tv and the company doesnt want to be responsible for when that guys house starts to catch fire.

7) Be merciful and get yourself one of these https://www.acinfinity.com/receiver-amp-cooling-fans/ because AVRs pack a LOT of stuff in one chassis, and every channel generates a chunk of waste heat even at low volume.

Yeah I already have 2 intake and exhaust pc fans into my tv stand or whatever its called since I also have all my old game consoles in there which I don't want to ever break (also took the bottom off of my atari flashback ii since it was just a sealed plastic case and it used a inefficient ttl based chip).

I came up with that idea when I had a really old blu-ray player from my dad that drew enough power when it was "off" (so its internal fan wouldn't be on) that it would get warm and it was a little annoying to have to plug it in and unplug it all the time since everything was behind my tv which is wedged into a corner (so from the reference point of the speakers they would be in a triangular room with no wall behind it, instead of a rectangle with a wall on the right side but not the left, which I was guessing would cause things to sound different on one side.
Also made running speaker cables easy since I could just tuck them under the room edges at the bottom (dunno what they are called) since it had carpet and I could squish it down to fit the cable in there).

 

[Leo Shafai]

Oh right I should add I was just using this unit temporarily as a sanity check when since I was trying to fix another old thing (which I could actually find the model of online, at least for the non canadian version).

This one sounds terrible and one channel is super quiet, and If it change to balance to compensate I get greeted with some super loud noise as I turn the knob and that channel which is now louder will clip and a bunch of frequencies. They are a separate dac and amp I guess you could call them, Im not sure which of the 2 has the problem and I haven't just used just used something else with them to test hence the "I guess you could call them" (they were from the 70s and Im guessing they are made only to work with each other, the types of controls you find on an amp today are partially moved onto this "dac", etc). The most likely case is the amp has the channel issue since the balance knob on that can change things, but that leaves the dac with the possibility of making things sound bad so I just not gonna bother. Which is the reason my dad dug up this other old receiver which ive been trying to fix (its great besides a 60/120 hz leak to the output (technically 60 hz, "technically" 120 hz in my opinion & in whats actually audible (same way lights turn on and off at 120hz with "60hz" power to them (unless they are a diode))).

as for that issue I remembered my dad was thinking of upgrading his sony receiver from the 90s to a modern denon one, and the 2019 models with all the feature he wanted now are not "new" so I noticed the cheapest one with everything we wanted is also on sale on the american site. hopefully it will also go on sale on the canadian site this black friday. so ill just use the old(er) sony one on my stuff instead of replacing every single capacitor in the old one hoping to get rid of the noise, since Ive tried everything else at this point)

 

[Leo Shafai]

This is probably the last time I'm gonna be here since I'm mainly a headphone guy and the old stuff I got from either my dad, grandpa, or uncle, is pretty great for me already. (only stuff Ive ever really gotten "passed down" to me since no one else wanted it)

I might come back for advice since I know my dad at some point might want to get more than 3.1 speakers, and he cant just find more of those same little bose ones (that come with that big sub that is half empty (might be intentional from not a marketing standpoint, especially since they were sold along things that were small), since those were like the only good speakers bose made from what Ive been told (and heard to some degree). And they are now very expensive to buy since anyone who wants good speakers in a small form factor gets that model form that year I guess.

So goodbye for now! Thanks for all the help!

 

[Verdinut]

To answer the question that has been left alone so far, yes- current is the reason your receiver is rated for 8 Ohms or higher. The design used certain output devices (transistors or tubes) and those need to be cooled properly, or they will fail. The reason they fail is due to excessive heat, but the primary cause of that heat is current, a secondary cause is lack of air flow. If the unit is old and has dust on the inside, clean it.

The current will be excessive at high power levels but depending on the design, it could still fail at lower levels.

Adding a resistor will prevent the amplifier's demise, but the resistor would need to be able to handle its own excess heat (it needs to be the correct type and they get HOT) and using a resistor just wastes power.

Using a resistor in series with a speaker is never a good idea for a woofer or a full range loudspeaker. It will annul the amplifier's damping factor with poor transient response as a result.
However, it is sometimes used in series with mid-range and high frequency drivers as part of the design of a crossover.

 

[highfigh]

Using a resistor in series with a speaker is never a good idea for a woofer or a full range loudspeaker. It will annul the amplifier's damping factor with poor transient response as a result.
However, it is sometimes used in series with mid-range and high frequency drivers as part of the design of a crossover.

Yes, I know- I was trying to preempt any questions about that by the OP and to do it correctly, the resistor before a mid or tweeter really should be part of an L-pad configuration, to preserve the proper impedance for the amplifier unless its value is small.