One of the benefits of living in an apartment building is the "yard sales" where people are just looking to get rid of stuff before they move.
Today, I picked up a pair of MB Quart 220 bookshelf speakers for $30. I don't know a ton about speakers, but I know they are probably worth more than $30.
Question -- they are rated at 4 ohms. I have an Onkyo receiver which is rated at 8 ohms. Is this a huge problem or a small problem? The receiver feels a bit hot to the touch, and the manual only says that during extended usage and at high volumes, the system's overload circuit may kick in. I just don't know what their metric is for "extended usage" or "high volume." I'm guessing that I will not approach either of those.
Am I running the risk of damaging my equipment, or starting a fire?
It's been too long since I've taken physics and learned about electronic circuits. I know you can go up in impedance without problems, but I'm not sure what the real risk is with going lower, for moderate use.
Small problem.
Since the speakers are a marginally lower impedance (4ohm to 8ohm isn't a big jump), they will draw a bit more current (creating more heat) from your amp. The built in protection circuit should prevent any damage (or fires), but you may get there quicker than if you had a properly matched set of speakers.
Ideally, you want the speaker impedance to match the output impedance of the amp for peak efficiency.
The volume control will also be non-linear with non-matched speakers.
Not a big deal, but if you use them with a home theater system (3D sound image) and have another pair of speakers on the other channel (with a different impedance), you will probably notice this. I had a similar situation at home on my receiver while I saved up the cash for a new (matching impedance) set of rear speakers.
If you are using them as a stereo pair only, this isn't an issue.
BBb Tuba with 4 Rotors -
TE-2110 (2009) + TE Rose
Mack 210 (2011) + Bruno Tilz NEA 310 M0
G. Schneider (Made in GDR, 1981?) + Conn Helleberg 120S
I earn my living as an Electrical Engineer - Designing Power systems for buildings
These will be stereo only, so no concerns there. Heck, I don't even have a second room to put these right now, they are really for next year, when we move to an apartment with, well, more than one room.
If you really wanted to find out what your stereo amplifier would do with a 4-ohm load, find out what the part number is on the two output power components in your stereo amplifier, and look up the engineering "white page" on it. It will tell you what you want to know as far as its rated impedance load and other operating characteristics.
I have a small guitar amplifier that I did that to. I wanted to plug in another speaker with the internal speaker to run a combined 4-ohm load on it, although the amplifier speaker jack was an either/or and said 8-ohm load only. When I looked it up, I found that the particular MOS-FET used as the power output component in this particular guitar amplifier was actually rated for better performance with a 4-ohm load, both in actual power output and overall lower output distortion. (i.e. it would get louder without breakup, so I could use it for my intended purpose: small gigs where I did not want to take more horsepower) So out came the soldering iron and I bridged the either/or finger of the speaker jack so I could plug in the second speaker. Being a small amplifier, which produces practically no heat in any operating parameter, it is obvious that the warnings were put on the amplifier to keep from having to honor warranty claims.
I agree it probably is not much of an issue with a straightforward home stereo speaker arrangement. Even a moderately efficient speaker system will produce a fairly high dB level with only a few watts, so unless you still enjoy rattling windows and drowning out the neighbors, it should work fine. (I, OTOH, still have my Marshall head and a couple of cabinets for the ongoing rare occasion that I still want to do that. In the meantime, my small amplifier does just fine, and is much easier to carry around!)
Usually, when the components are rated for 4ohm, but the system is rated for 8ohm, it's a result of the heat sink not getting optimal airflow. I bet that's the case with typical small guitar amps. (Evidenced by the fact that it doesn't noticeably heat the cabinet while it's running.) Operating with an 8ohm load reduces the internal heating and the resulting component failure rate. As long as you don't run it near peak volume continuously at 4ohms, you should be fine. Hence, warranty protection at 8ohms, but not at 4ohms.
For home stereo in the same scenario, if it's anywhere near peak output for long enough to be an issue, the neighbors have probably already called the cops...
BBb Tuba with 4 Rotors -
TE-2110 (2009) + TE Rose
Mack 210 (2011) + Bruno Tilz NEA 310 M0
G. Schneider (Made in GDR, 1981?) + Conn Helleberg 120S
I earn my living as an Electrical Engineer - Designing Power systems for buildings
NCSUSousa wrote:Usually, when the components are rated for 4ohm, but the system is rated for 8ohm, it's a result of the heat sink not getting optimal airflow. I bet that's the case with typical small guitar amps. (Evidenced by the fact that it doesn't noticeably heat the cabinet while it's running.) Operating with an 8ohm load reduces the internal heating and the resulting component failure rate. As long as you don't run it near peak volume continuously at 4ohms, you should be fine. Hence, warranty protection at 8ohms, but not at 4ohms.
For home stereo in the same scenario, if it's anywhere near peak output for long enough to be an issue, the neighbors have probably already called the cops...
The output MOS-FET has a large heat sink for its size, so heat dissipation is not an issue. But you're right, in the bigger picture. Guitarists are notorious for shoving their amps against a wall where there is no ventilation, especially in small bars. That's how I got my Marshall head so cheap: a rebuilt meltdown. Literally. It is a first year of issue 2203, and it had gotten so hot that the plastic grill on the top of the amp melted into a trough. That's a good thing, actually, because now there is a place to put a slide, extra preamp tubes, etc., for a gig.
Most amps handle 4ohm loads well... unless you intend to run the thing wide open. Keep in mind the rating is an average not an absolute, so while it averages 4ohm it may drop down to 2ohm at some frequencies or jump up as high as 16 or 20 at other frequencies.
The only time I've had it become and issue was running a gig for a garage band at a pool party with 'custom' (garage built crap) speakers with no true impedance rating or anything in the cross-over to normalize the impedance. This was around '83 and the flames only came up out of the Biamp powered mixer about 3 inches...
Just don't run them in line with any other speakers and you should be just fine.
Ricko wrote:The only time I've had it become and issue was running a gig for a garage band at a pool party with 'custom' (garage built crap) speakers with no true impedance rating or anything in the cross-over to normalize the impedance. This was around '83 and the flames only came up out of the Biamp powered mixer about 3 inches....
