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Old 30th May 2006, 08:33 PM   #1
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Default Amplifier-speakers

I WOULD WANT TO LEARN HOW BEHAVES A SYSTEM WHEN THE AMPLIFIER EXIT FORCE IN THE 8 OHM WHILE WE WE HAVE PLACED SPEAKERS IN THE 6 OHM.WHAT HAPPENS WITH THE FORCE OF THE SYSTEM?
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Old 30th May 2006, 11:41 PM   #2
lndm is offline lndm  Australia
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Hi Glazarou,
This might cause your amp to get hot, and it might cause your amp to fail. Still, I think you could do this if you do not turn the volume up too far.

By the way, you might want to switch your caps lock off.

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Old 31st May 2006, 12:19 AM   #3
CBS240 is offline CBS240  United States
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As Indm stated, it will work as long as you keep the volume not so high. This will still stress the outputs more and decrease efficiency because the impeadence isn't matched properly. If you are lucky enough to have a DIY amp that has been overdesigned, you might get away with it. You may find you will not be able to get more power out because the SOA of the outputs could be exceeded....then you get smoke. I wouldn't do it if the power amplifier is a chipamp. If it is, and you do, hopefully it has a overdrive cuttoff. They can be expensive to replace and repair.

How much power are you talking about?

You must also take into consideration the phase of the impeadence you are putting on the amp. For example, a sub-woofer may have a lagging(inductive) impeadence. This throws the voltage and current output of the amp out of phase. SOA, Safe Operating Area(determained by the datasheet) of any output transistor is a function of Vce vs Ic. A phase angle of a speaker load may cause a situation where Vce is high at the same time Ic is. BJT's are particularly sensitive to this, and sometimes have problems driving highly reactive loads. Compare SOA graphs of BJT's and Mosfets. There is quite a difference. Capacitive loads(leading phase angle) can be problematic as well for a different reason but typically these are the highs and acount for a small percent of power to a full range amp relative to bass. In a nutshell, it's really all about the SOA conditions placed on the output devices, as far as overload failure. There are more technical problems might arise, like higher current making the output's more non-linear and the rest of the circuit may or may not be able to compensate leading to higher distortion figures, typical crap like that.

It is best to have the correct impeadence on the circuit that it is designed for. Connecting multiple speakers in series/paralell circuits to achieve 8 Ohms is better for the amp than a lower impeadence if posible.

BTW: Welcome to DIY!
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Old 31st May 2006, 06:54 PM   #4
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Default amplifier-speakers

thanks "cbs240" and lndm" for the reply.exacly the speakers i have placed are 2 suround speakers and 1 central.when i get to a shop to buy the speakers the man there showed me a speaker packet with 2 suround and 1 central.i asked him for the power and the impendance and he told me that the speakers power is 80watts and the impedance is at 6ohms.i told him that my amplifier's output power for these channels is 70watts and the impedance is at 8ohms.finally he told me that <<if the amlifier gives 70watts at 8ohms so at 6ohms gives more power,so don't be afraid>>.by the time my system works good but i have observed that these speakers gives more loudness than the front speakers.

i want to change these speakers with some other with correct impedance but as far i know i can't find periferal speakers at 6ohms.


something else.is there somebody who knows a good program for speakers(calculate the cabinet,the filters,the drivers)?
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Old 1st June 2006, 03:02 AM   #5
CBS240 is offline CBS240  United States
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Hi glazarou

You have the same voltage across 6 Ohms instead of 8. Ohms Law states you will have more current and power for the same output voltage. I=V/R; P=VI
This means more current in the outputs with the same voltage across them and they will become hotter. What the man at the store told you is true(also in heat dissapated by the outputs) and you may get away with it here, but it is generally a bad idea to put a lower impeadence on an amp then what it is designed for.
If you have a way to turn down those channels to sonically match the others, you will be OK. Also make sure the airflow across the heatsinks(asuming convection cooling) is clear and dust doesn't build up too bad on it.
I'm thinking you could put a 1 Ohm 5W resistor in series with those speakers to normalize them a bit with the rest of the system if you can't adjust the gain. Seems to me it would work, wasting a few watts though no big deal. IMO, balance and quality is more important than power.
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Old 1st June 2006, 07:48 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi Glazarou,
go easy with the volume control.

I like the idea from CBS, use a resistor in series with the 6ohms speakers until you can replace them with the correct impedance. The higher impedance (6ohm+1r) will help stop the amplifier overheating and it will absorb a bit of the excess power making the speakers a bit quieter and finally it will reduce the phase angle of the load slightly making it easier for the amplifier to cope.

You may find that you like the sound of the series resistor fed speakers. It changes the Q of the speaker with the result that the new sound may better suit you and/or your room.
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