LM3875 Voltage

Puffin · 15th March 2008, 08:57 PM

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 ?

seventenths · 15th March 2008, 09:04 PM

Puffin,
Have you measured the voltage under load? Is it still +/- 45?

7/10

Puffin · 15th March 2008, 09:07 PM

Not strictly "under load", but from the chip + & - pins whilst idling.

Puffin · 16th March 2008, 05:57 PM

Bump

BWRX · 16th March 2008, 06:20 PM

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.

Puffin · 16th March 2008, 08:00 PM

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.

seventenths · 16th March 2008, 08:27 PM

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?

7/10

BWRX · 16th March 2008, 08:33 PM

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.

Puffin · 16th March 2008, 08:50 PM

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!

peranders · 16th March 2008, 09:05 PM

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.

15th March 2008, 08:57 PM	#1
Puffin diyAudio Member Join Date: Feb 2006 Location: U.K	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 ?

16th March 2008, 08:33 PM	#8
BWRX diyAudio Moderator Emeritus Join Date: Jan 2005 Location: Pennsylvania	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. __________________ Brian

16th March 2008, 09:05 PM	#10
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	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. __________________ /Per-Anders (my first name) or P-A as my friends call me

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15th March 2008, 09:04 PM	#2
seventenths diyAudio Member Join Date: Apr 2007	Puffin, Have you measured the voltage under load? Is it still +/- 45? 7/10

15th March 2008, 09:07 PM	#3
Puffin diyAudio Member Join Date: Feb 2006 Location: U.K	Not strictly "under load", but from the chip + & - pins whilst idling.

16th March 2008, 05:57 PM	#4
Puffin diyAudio Member Join Date: Feb 2006 Location: U.K	Bump

16th March 2008, 08:00 PM	#6
Puffin diyAudio Member Join Date: Feb 2006 Location: U.K	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.

16th March 2008, 08:27 PM	#7
seventenths diyAudio Member Join Date: Apr 2007	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? 7/10

16th March 2008, 08:50 PM	#9
Puffin diyAudio Member Join Date: Feb 2006 Location: U.K	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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