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Old 15th March 2008, 08:57 PM   #1
Puffin is online now Puffin  United Kingdom
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Default LM3875 Voltage

I am using a 30-0-30ac tranny with just bridge rectifiers. Each rail is measurring +45 -45vdc. This seems to be the max voltage the chip will take. Speakers are 96db 8ohm. The heatsinks 3"x 4" just get a little warm.

Some posters have been scared to take the DC voltage above 35vdc per rail, but this seems to be o.k ?
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Old 15th March 2008, 09:04 PM   #2
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Puffin,
Have you measured the voltage under load? Is it still +/- 45?


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Old 15th March 2008, 09:07 PM   #3
Puffin is online now Puffin  United Kingdom
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Not strictly "under load", but from the chip + & - pins whilst idling.
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Old 16th March 2008, 05:57 PM   #4
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Old 16th March 2008, 06:20 PM   #5
BWRX is offline BWRX  United States
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Default Re: LM3875 Voltage

Quote:
Originally posted by Puffin
Each rail is measurring +45 -45vdc. This seems to be the max voltage the chip will take.

Some posters have been scared to take the DC voltage above 35vdc per rail, but this seems to be o.k ?
The datasheet (where you should always look first for answers to these questions) says +/-42V is the absolute max rating with an input signal. Anytime you are operating near or outside that range you are taking a risk, simple as that.
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Old 16th March 2008, 08:00 PM   #6
Puffin is online now Puffin  United Kingdom
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BWRX. When you say "risk", do you mean :-

a) Risk of blowing the thing up
b) Risk of operating the over-voltage mechanism (reduction in sound quality or shut-off to avoid damage ?)
c) Risk of damaging speakers

Just beacuse the thing only gets warm, that is no indication that things are o.k ?

Your help is appreciated.
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Old 16th March 2008, 08:27 PM   #7
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Based on a quick glance at the data sheet, it looks like your saving grace is the 8ohm load... coupled with the possibility that you're dropping a couple of volts under load.
I'm curious, what is your tranny VA rating and what are you using for supply filtering?


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Old 16th March 2008, 08:33 PM   #8
BWRX is offline BWRX  United States
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Again, just have a look at the datasheet.

From page 3:

Absolute Maximum Ratings (Notes 1, 2)
Supply Voltage |V+| + |V−| (No Signal) 94V
Supply Voltage |V+| + |V−| (Input Signal) 84V

Operating Ratings (Notes 1, 2)
Temperature Range
TMIN ≤ TA ≤ TMAX −20C ≤ TA ≤ +85C
Supply Voltage |V+| + |V−| 20V to 84V
Note: Operation is guaranteed up to 84V, however, distortion may be introduced from the SPiKe Protection Circuitry when operating above 70V if proper thermal considerations are not taken into account. Refer to the Thermal Considerations section for more information. (See SPiKe Protection Response)


From page 4:

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is functional, but do not guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions which guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit is given, however, the typical value is a good indication of device performance.
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Old 16th March 2008, 08:50 PM   #9
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seventenths. VA rating 160. Only supply filtering is the 1,000uf caps on the chip. I am trying to copy the original Gaincard.

BWRX. Thanks for that. It seems then that sound quality may suffer if voltage is too high. The heat sink seems to be sufficent as it only gets warm. I am concerned not to blow my speakers up!
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Old 16th March 2008, 09:05 PM   #10
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If you want to be sure, you should have the supply voltage within the limits, otherwise only time will tell if it is going to work.
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