I've just bought a nice Logitech Sound System for my home theater and it sounds great. However, I noticed that the two side frontal speakers, powerful as they are, are too small to convey good mid range frequencies. And the result is a sound that has way too much contrast in it for my taste. This because the subwoofer does an excellent job at low frequencies, and the rest of the speakers diameter's are not large enough to generate a sound that is warm enough for my taste.

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Anyway, to fix the situation, I bought a couple of 3-Way Sony Speakers to substitute the aforementioned frontal speakers, and the sound quality is now superb. At least for my mundane non-audiophilic taste.

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The thing is that the Sony speakers have an impedance of 6 ohms, whilst the Logitech's have about 4.2 ohms. I actually measured their value with a high quality multimeter.

I thought that shouldn't make much of a difference in practice, since a higher speaker impedance should demand less power from the amplifier, and therefore be gentler with the system itself. But it seems I was wrong. Whenever I turn the system on, it sounds great (better than with its two original frontal speakers), but after a while it begins to stutter a little bit (the sound cuts of for a fraction of a second, but then immediately comes back) and after about 15 minutes the amp goes mute and I have to turn it off and back on again so as to reset it have it working again. This happens regardless of how much volume I demand from the amplifier.

My guess is that the Logitech's amplifier is sensing an anomaly of sorts and is protecting itself.

Is there a way to trick the amp into believing that a 4.2 ohm speaker is connected instead of a 6 ohm one? I was thinking about placing a resistor in parallel with each speaker to obtain the desired impedance but something tells me that it's not that simple. Perhaps an external coil having both the desired resistive and inductive impedance would do the trick? Or is there such thing as a "speaker impedance converter" circuit out there?

 

Try the resistor first.
Do you have the same problem if you connect the other speakers back up?

 

No amplifier designed for 4Ω will have a problem with a 6Ω Load.
Don’t forget that a 4Ω speaker has a rising impedance that will probably be above 6Ω for any frequencies above 500Hz.

 

half a kilowatt audio output, one kilowatt peak. And Logitech say that's RMS. A very big amp. Your new speakers are only 100 watts max.

 

half a kilowatt audio output, one kilowatt peak. And Logitech say that's RMS. A very big amp. Your new speakers are only 100 watts max.

That’s the total. The individual satellites are only 67W

 

You might want to make an L-pad to match the speakers to the amp.

This article is really intended for RF but the section “lossy devices” starts with the L-pad and has the explanation and formulas.

 

You might want to make an L-pad to match the speakers to the amp.

This article is really intended for RF but the section “lossy devices” starts with the L-pad and has the explanation and formulas.

Don’t do that- speakers are never matched to amplifiers (in impedance). They need to be driven by as low an impedance as possible to keep resonances under control.
The minimum load impedance specification is nothing other than a thermal limit for the output transistors.

 

Don’t do that- speakers are never matched to amplifiers (in impedance). They need to be driven by as low an impedance as possible to keep resonances under control.
The minimum load impedance specification is nothing other than a thermal limit for the output transistors.

The “as possible” part is the question. Do you mean according to the native impedance of the driver?

 

The “as possible” part is the question. Do you mean according to the native impedance of the driver?

Damping factor= load_impedance/amplifier_output_impedance.
A good value is >100. The more the better.
Most of the ”output” impedance will be the cable resistance.

 

Try the resistor first.
Do you have the same problem if you connect the other speakers back up?

Will try that first thing today. That's a wise double check, thanks.

No amplifier designed for 4Ω will have a problem with a 6Ω Load.
Don’t forget that a 4Ω speaker has a rising impedance that will probably be above 6Ω for any frequencies above 500Hz.

I did not know about said rising impedance. If that's the case, maybe a 2 ohm resistor in series that would add up the total impedance to about 8 ohms is a better strategy than a resistor in parallel that would bring the impedance down to the 4.2 ohms registered in the original speakers.

It's very strange that this issue is a consecuence of demanding (theoretically) less power from the amplifier.

 

Listen to Ian. The problem is not likely to be the higher impedance of the speakers. Something else is wrong.

 

I still think that the only thing the amplifier will know about the power it is delivering is the temperature of its heatsink, and it will only be even the slightest bit concerned if starts to get too warm. I think that the problem is elsewhere.

 

However, I noticed that the two side frontal speakers, powerful as they are, are too small to convey good mid range frequencies.

Is that insure for home theater mode or for listening to music. For 5.1 (3D audio as Logitech calls it) should be for home theater with dialog (mid-range frequencies) mostly directed to the center channel. If listening to stereo music, you should switch to 2.1 or 4.1 audio to define left and right channels and make music fill the room more naturally. I find that turning audio effects to off sounds the best for music.

 

I’ll tell you what I told every DJ during my 25 years in professional audio and lighting when they complained about intermittent speakers: check your cables.
(All DJs have an overinflated opinion of their soldering abilities, and none of them looks after his leads)

 

Listen to Ian. The problem is not likely to be the higher impedance of the speakers. Something else is wrong.

Agreed.
Look for the problem somewhere else.

 

I’ll tell you what I told every DJ during my 25 years in professional audio and lighting when they complained about intermittent speakers: check your cables.
(All DJs have an overinflated opinion of their soldering abilities, and none of them looks after his leads)

The cables are fine ... they all are single-piece, and are firmly attached to their spring loaded terminals at the ends.

 

The cables are fine ... they all are single-piece, and are firmly attached to their spring loaded terminals at the ends.

You’ve probably looked after them better than DJs do.

 

Found the culprit ... and as expected, it was a rather stupid thing.

After following @sghioto 's advice, I plugged the old speakers back in and the problem persisted. So my first thought was "did I damage the darn thing when I connected the old speakers"? ... I was sure that wasn't the case because I had been extra careful . ... and then I remembered my kid's favorite tv show: Blue's Clues ... and started singing "Go back go back go back, go back to where you were!"

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... and Voilà ... I remembered that at the same time I installed the speakers I also installed a cheap-o crap-o Toslink optical fiber cable splitter between the tv set and the sound system so that I could also use a set of Bluetooth headphones when I wanted to watch something without bothering everyone else in the house.

The crappy thing works passively and has too much loss for it to be a reliable component. That was what was causing the problem.

So I got rid of it, and bought an active Toslink splitter in the hopes that the problem never presents itself again.

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And here's an epilogue:

I talked to a friend of mine who specializes in audio electronics, and told me that some pretty sophisticated equipment out there actually measure the amount of current going into the speakers, and if said current is too low when compared to the level of amplification that was set, it then infers that the speaker's impedance has gone up because the speaker's coil has overheated. So then the amplifier automatically shuts itself down to protect the speakers ... this was obviously not the case, regarding my equipment, but I found it interesting nonetheless ... this has been an interesting discussion, thanks everyone for all your help.

 

So I wasn't so far off when I suggested checking the cables!

I talked to a friend of mine who specializes in audio electronics, and told me that some pretty sophisticated equipment out there actually measure the amount of current going into the speakers, and if said current is too low when compared to the level of amplification that was set, it then infers that the speaker's impedance has gone up because the speaker's coil has overheated. So then the amplifier automatically shuts itself down to protect the speakers ... this was obviously not the case, regarding my equipment, but I found it interesting nonetheless ... this has been an interesting discussion, thanks everyone for all your help.

Not to sure about the above. It would only work if the frequency content of the music remained identical throughout. If the average frequency increased the current would fall because the impedance is higher at a higher frequency, and the "clever" electronics would switch off your speakers.
It might even fail if there was a big bass note that corresponded to one of the bass impedance peaks of your speakers.
This is a typical impedance plot of a 170mm speaker.

The resistance will increase by 25% between 25°C and 100°C, but the inductance will stay much the same. Above 1kHz, the impedance is dominated by the inductance. If it is looking for a 25% decrease in current, all that has to happen is for there to be a bass note at 100Hz, and the current falls by 75%.

 

So I wasn't so far off when I suggested checking the cables!

No, you weren't far off at all. My mistake was focusing all my attention to the system's output instead of also considering its input!

As to the speaker protection scheme. If I understand your explanation correctly, in order to protect the speakers from overheating (other than directly measuring their temperature) one would have to simultaneously measure the current and frequency applied to the speakers to more or less calculate their temperature ... I doubt there's a commercial equipment out there with such capability.