LM3875 Voltage

Puffin

Member
2006-02-19 8:15 am
U.K
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 ?
 

BWRX

diyAudio Moderator Emeritus
2005-01-17 5:29 am
Pennsylvania
Puffin said:
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

Member
2006-02-19 8:15 am
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.
 

BWRX

diyAudio Moderator Emeritus
2005-01-17 5:29 am
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.
 

Puffin

Member
2006-02-19 8:15 am
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!
 

Puffin

Member
2006-02-19 8:15 am
U.K
Thanks all for your input. Not knowing enough about these things I am just concerned that any over voltage could mean that if the chip reacts, blows up etc, that it will take my speakers with it ?!

If the chip runs farly cool and I cannot hear any protection circuits cutting in, can I assume that it is working within it's limits and not under any stress ?
 

valleyman

Member
2008-02-07 7:21 pm
I'd say chances are it will be fine. You aren't running hugely above spec and manufacturers do tend to quote conservative figures for things like that to cover themselves.
I wouldn't like to say that the chip isn't under stress however, any chip running close to or at its absolute max is under stress. It shouldn't be an immediate problem but if you want the chip to keep on running for years why not add some turns to the primary winding (or remove some from the secondaries). It wouldn't take long, is essentially free and might save you the headached of a broken chip or worse