Impedance matching

netster403 · 10th July 2007, 04:09 AM

Ok, I have a receiver (6 channels x 100w) The manual says to ensure I utilize 6-8 ohm speakers. Just my luck, the speakers I ordered are 4ohm.

I know they don't put that warning there for no good reason, is there a way to modify the impedance of the speakers (i'm guessing i couldn't be as simple as a resistor on each channel)

any advise would be GREATLY appreciated.

Thank ya

OzMikeH · 10th July 2007, 05:39 AM

you should be fine as long as you don't crank it up more than half of full power.

Be warned: MOST of the time full power is significantly less than full clockwise on a volume control, a very shifty marketing trick.
"It's really loud at 3, imagine how loud it would be at 10"

If you want to idiot-proof it you can stick an auto 12v stop or indicator lamp in series with each speaker. That will limit the current, but only when there is enough current to heat the filament. Just don't be a fool and turn it up just to see the lamps glowing.
Never exceed a dim glow! at standard brightness (21 watts average) the filament is 6 ohms. when cold it's much lower.

netster403 · 10th July 2007, 11:38 AM

You are awesome, thank you for the tips ^_^

Really I'm not too worried about the speakers, they are some cube Samsung home theater speakers I got for $25.

I just want to make sure I don't blow out my receiver. Maybe I will go with the indicator lamp, sounds like a solid haxx until I can afford some really good (6-8ohm) speakers.

OzMikeH · 10th July 2007, 11:06 PM

Don;t forget to set your speaker size to "Small" in your HT box.

the thing that is at risk is not necessarily your speakers but your amp.

Imagine it as a little engine coupled to a water pump designed to work against a certain resistance, the pump at "full throttle" against that resistance runs at say 2000 RPM which is an acceptable RPM for that motor.

What happens when you remove half of that resistance (restriction in the pipe), you don't burst the pipe, the pump over-speeds and goes bang.

this is why you can run a lower resistance, but don't use "full throttle"

It's to do with the impedance of the speaker limiting the amount of current that flows through the output transistors, half the impedance = twice the current. and your amp is clearly rated for 6-8 ohms so it can't handle the current.

Moderator: this thread might be more appropriate in electronics.

Cal Weldon · 11th July 2007, 12:06 AM

Solid state it is.

10th July 2007, 04:09 AM	#1
netster403 diyAudio Member Join Date: Jul 2007	Impedance matching Ok, I have a receiver (6 channels x 100w) The manual says to ensure I utilize 6-8 ohm speakers. Just my luck, the speakers I ordered are 4ohm. I know they don't put that warning there for no good reason, is there a way to modify the impedance of the speakers (i'm guessing i couldn't be as simple as a resistor on each channel) any advise would be GREATLY appreciated. Thank ya

11th July 2007, 12:06 AM	#5
Cal Weldon Speakerholic diyAudio Moderator Join Date: Jan 2004 Location: British Columbia	Solid state it is. __________________ Next stop: Margaritaville Some of Cal's stuff \| Cal Weldon Consulting

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10th July 2007, 05:39 AM	#2
OzMikeH diyAudio Member Join Date: Mar 2007	you should be fine as long as you don't crank it up more than half of full power. Be warned: MOST of the time full power is significantly less than full clockwise on a volume control, a very shifty marketing trick. "It's really loud at 3, imagine how loud it would be at 10" If you want to idiot-proof it you can stick an auto 12v stop or indicator lamp in series with each speaker. That will limit the current, but only when there is enough current to heat the filament. Just don't be a fool and turn it up just to see the lamps glowing. Never exceed a dim glow! at standard brightness (21 watts average) the filament is 6 ohms. when cold it's much lower.

10th July 2007, 11:38 AM	#3
netster403 diyAudio Member Join Date: Jul 2007	You are awesome, thank you for the tips ^_^ Really I'm not too worried about the speakers, they are some cube Samsung home theater speakers I got for $25. I just want to make sure I don't blow out my receiver. Maybe I will go with the indicator lamp, sounds like a solid haxx until I can afford some really good (6-8ohm) speakers.

10th July 2007, 11:06 PM	#4
OzMikeH diyAudio Member Join Date: Mar 2007	Don;t forget to set your speaker size to "Small" in your HT box. the thing that is at risk is not necessarily your speakers but your amp. Imagine it as a little engine coupled to a water pump designed to work against a certain resistance, the pump at "full throttle" against that resistance runs at say 2000 RPM which is an acceptable RPM for that motor. What happens when you remove half of that resistance (restriction in the pipe), you don't burst the pipe, the pump over-speeds and goes bang. this is why you can run a lower resistance, but don't use "full throttle" It's to do with the impedance of the speaker limiting the amount of current that flows through the output transistors, half the impedance = twice the current. and your amp is clearly rated for 6-8 ohms so it can't handle the current. Moderator: this thread might be more appropriate in electronics.

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