Altering a PA monitor - need wiring help. - diyAudio
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Old 29th October 2010, 04:02 AM   #1
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Default Altering a PA monitor - need wiring help.

I have an old Traynor PA monitor that has a 16 ohm Eminence 10" speaker and a Motorola piezo. It has a volume pot mounted on the jackplate. I really need it to be 8 ohm. What I'm looking to do is add a resistor in parallel to the piezo to make it also a 16 ohm load, add a cap before that to roll off the low end going to the piezo, and wire that parallel to the woofer - hopefully that will change the total speaker impedance seen by the amp to around an 8 ohm? I'd probably be feeding that monitor somewhere around 50 watts or so - does the resistor need to be rated 50 to 100 watts? What rating does the capacitor need? Part of the point of the exercise is to alter the speaker in a very cost effective way - if it will end up being more than 30 bucks or so I might as well just replace the speaker with an 8 ohm. Thanks!
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Old 29th October 2010, 12:42 PM   #2
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This isn't going to do what you want. First, you need to understand that speaker impedance isn't a single value, it varies with frequency. You will certainly change the load the amp sees for high frequencies, but it will have no effect on impedance of the Eminence driver. Unfortunately, there is no easy way to change the impedance of the Eminence. Your best bet is simply to replace it with an 8 ohm driver.
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Old 29th October 2010, 01:05 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

The point of a 16 ohm monitor is you can have two of them driven by
a single typical 8 ohm mono amplifier. There is no point in changing
the c/o to the piezo at all, leave it as it is. There is no real point
changing the driver either. Look for another 16 ohm monitor,
or simply use it as it is, it will be fine for the job.

rgds, sreten.

Read the piezo wiki here, you might be able to improve matters.
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Old 29th October 2010, 01:26 PM   #4
Xoc1 is offline Xoc1  United Kingdom
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There is some advice on wiring the piezo driver here...
planet_10 hifi
If you wire it up up with a resistor and capacitor on the input it will help with the powerhandling of the tweeter.
Apart from that there is nothing to do but try the monitor as is and see if it is suitable for your needs. If you have a single monitor, and a fairly small stereo amp you could aways try using the amp in mono bridge mode. This would deliver twice the individual 8 ohm rating per channel into the speaker.
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Old 29th October 2010, 01:37 PM   #5
Boofers is offline Boofers  Canada
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Hopefully I understand you correctly here… Let’s say you have a 16V signal into a 16ohm speaker. The current from your amp will be E/R = 16V/16ohm = 1A. The power on the speaker will be (E*E)/R = (16V*16V)/16ohm = 16W.

Now let’s say you put a 16ohm resistor in parallel with the speaker. With the same 16V signal on the parallel speaker & resistor, the speaker will still have 16W on it, and the resistor will have 16W on it. The difference is now your amp is putting out 2A instead of 1A, so you’re really just wasting power and possibly adding distortion, without any improvement in sound.

If you replace the 16ohm speaker with an 8ohm speaker and apply the same 16V signal, the power on your 8ohm speaker would be (16V*16V)/8ohm = 32W!
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Old 29th October 2010, 02:45 PM   #6
Xoc1 is offline Xoc1  United Kingdom
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Quote:
If you replace the 16ohm speaker with an 8ohm speaker and apply the same 16V signal, the power on your 8ohm speaker would be (16V*16V)/8ohm = 32W!
Basically this is correct but you never quite acheive the doubling on power.
By bridging the amplifier you can double the voltage so (32V*32V)/16ohm = 64W
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Old 29th October 2010, 03:53 PM   #7
ampimp is offline ampimp  United Kingdom
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Default Bridging???

Quote:
Originally Posted by Xoc1 View Post
Basically this is correct but you never quite acheive the doubling on power.
By bridging the amplifier you can double the voltage so (32V*32V)/16ohm = 64W
Who mentioned bridging????
Ian
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Old 29th October 2010, 03:57 PM   #8
ampimp is offline ampimp  United Kingdom
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Id say replace the speaker or get it reconed to 8 ohm's, And why do you require 8 Ohms ???
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Old 29th October 2010, 10:11 PM   #9
Boofers is offline Boofers  Canada
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Quote:
Originally Posted by Xoc1 View Post
By bridging the amplifier you can double the voltage so (32V*32V)/16ohm = 64W
Wicked!
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