is this 4ohm or 8ohm

Moze

Member
2012-12-02 11:40 pm
Need opinions, is the speaker in the pic a 4ohm or 8ohm speaker? Please note the actual impedance is 1ohm lower than is pictured. The cable impedance of 1ohm was not factored. Trying to build an amp and am not sure to treat this as an 8 or 4 ohm load. Thanks
 

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anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Moze,
Treat it as an 8 ohm speaker. You probably measure about 6 ohms with your meter. If you measure 4 ohms, call it a 6 ohm load.

The difference between 4 and 8 ohm loads really only matters when you're driving things hard. At that point, may as well design for 4 ohms, plus a safety factor. Marantz used to build their equipment 25% higher ratings than a 4 ohm load. Some even higher margins. So it depends on how hot you are willing to run things. That has an impact on how long it lasts before a failure - on average.

-Chris
 

Moze

Member
2012-12-02 11:40 pm
This is a B&W LCR600S3 that Im wanting to power with a chip amp. I remeasured with correct settings and the dip at 100 to 300hz is just below 4ohms....So I should design my rail voltages for 8ohms? It seems counter given the majority of the FR is 4ohms
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Moze,
Keep in mind that the speaker has a maximum dissipation limit too. Designing for 8 ohm voltages and using heat sinks good enough for 4 ohm would give you the best performance overall. If you later change to a different speaker you will be all set to go. Also keep in mind that the impedance of that speaker will rise when you have things turned up. You can lose 2 dB just in additional IR losses. Throws the enclosure tuning out if you made it too small. As opposed to the proper size given by B4 or sealed alignments. So a 7th order boom box is off in the weeds by the time the party just gets going. A B4 would be a little off, but not by a whole lot.

Look also at the efficiency of the speaker and what SPL level you want to have in your room. Sometimes you really don't like the answer. Better to have the answer at the design stage then when you show it off for the first time!

It's DIY. You can do much better than box store stuff.

-Chris
 

Moze

Member
2012-12-02 11:40 pm
Thanks guys - im trying to get 105db at my listening position which is 3 meters from spker. From 50hz to 2000hz, Im getting 90db (c) at 2.83V which I guess is 1 to 2 watts based on the imp curve, from 500hz to 20khz it drops down to 75db so since Im bi amping it, even 50w @ 8ohms should be more than enough for the woofer and mid..Its a small room so lots of gain in the low end