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Old 19th October 2011, 07:15 PM   #1
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Default ACOUSTAT III

Hi all,
I have a problem with my Acoustat III.
Xover power rheostat was replaced with two resistors RCL of 30 W
Mounted on the plate 5 mm thickness.
When play loud the resistors are getting up very hot and I think is loading up on my PWR amp Odyssey Stratus as well.
The sound is good but I'm afraid that something will break down
This is a the modified circuit with a no rheostat.
Is something wrong with this set up?
How low gets the impedance of the spk and at what freq?
All advices and help are highly regarded.
Sincerely,
Williams
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File Type: jpg MODIFIED XOVER.jpg (23.2 KB, 133 views)
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Old 19th October 2011, 07:28 PM   #2
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A thirty watt resistors might be a bit small. I think fifty would be better. That said if you can press your finger against the resistor when it is playing as loud as you would normally do and hold it there for ten to twenty seconds or so you probably do not have an issue. If it is so hot that you cannot touch for more than a few seconds then use a higher wattage resistor. Hollow ceramic tube type resistors mounted on an angle to accomodate air flow will keep things cooler even a vertical mount of a solid resistor will help in cooling. Best regards Moray James.
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Last edited by moray james; 19th October 2011 at 07:30 PM.
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Old 19th October 2011, 08:06 PM   #3
kevinkr is offline kevinkr  United States
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Is that wired correctly? As configured in that schematic the resistors do not replace a rheostat.
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Old 19th October 2011, 09:55 PM   #4
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Quote:
Originally Posted by Williams Audio View Post
Xover power rheostat was replaced with two resistors RCL of 30 W
When play loud the resistors are getting up very hot...
The sound is good but I'm afraid that something will break down
Is something wrong with this set up?
Your setup looks pretty close to Acoustat's Mod-C recommendations.
The Factory modifed setup is shown at the bottom of attachment in this post:
Acoustat Mods

The shunt portion of the resistance in the Factory Modified setup has a minimum possible value of 13 ohm.
Your's is 10 ohm, so generating a little more heat than neccessary, but should work fine as the value is not real critical.
Anything between 10ohm and 15ohm will work fine. Heat generated goes down as resistance goes up.

The wattage, as Moray James already mentioned, was 50watts for the factory setup.
Remember also that the Wattage of your RLC resistors will be derated if your heatsink is smaller than recommended.
As reference, I believe for no heatsink, the de-rating is 50%. So your 30W resistor would only be good for 15W without overheating.

Last edited by bolserst; 19th October 2011 at 10:23 PM.
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Old 19th October 2011, 10:27 PM   #5
ehous is offline ehous  United States
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I personaly did this mod several times over the years, Your wattage is low and I always used 13 to 16 ohms as a guide. 12.2 ohms may be a little low, but should work just fine. I used The aluminum heat sinked vishay's in the 50 to 75 watt range. Good Luck.
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