LM3875 at 60 volts?

[nige838]

Hello,
I'm retrofitting a pair of LM3875's into a 1970's Realistic STA-77 amp. The amplifier power supply uses a center tapped power supply at 60v. Is this too much for the IC? I have been told that this is too much, but the datasheet states that it can take 85 volts.

The power reads

60------0------60
|-------|
30v

If it's too high, could I just use the center tap to half the voltage?

Thanks,

[AndrewT]

read the datasheet again !!!!!

[nemonis]

You mention a 60V power supply. How is this measured? I can think of 4 possibilities, only one of which would work well.

1. The power supply has dual DC rails, one +30V DC and one -30V DC, giving 60V DC between the rails. This setup would work well for an LM3875.
2. The power supply has dual DC rails, one +60V DC and one -60V DC, giving 120V DC between the rails. This setup would be completely unworkable for an LM3875, which is designed for an absolute maximum supply of +/- 42V DC.
3. The power supply transformer has a 60V AC center tapped secondary. This means that measured from the center tap to either end of the winding is 30V AC. From one end of the winding to the other would be 60V AC. This setup is really not a good idea with an LM3875, as it would give about +/- 42V DC once rectified. This is the absolute maximum for the LM3875; any increase due to transformer regulation would cause the chip to shut down, or even fry something.
4. The power supply transformer has a 120V AC center tapped secondary. This means that measured from the center tap to either end of the winding is 60V AC. From one end of the winding to the other would be 120V AC. Run, don't walk away from this one. That setup might work well for a tube amplifier, but is totally inappropriate for any solid state amp of reasonable wattage.

A few more details would make it easier to identify what you are trying to do, so that your question can be accurately answered.

To summarize, the LM3875 is really designed for a dual-rail DC power supply. The power supply rails should be equal but opposite voltages, with the TOTAL voltage between them being between 20V DC (slightly pointless) and 84V DC (pushing things WAY too hard). Really, you should read the datasheet at http://www.national.com/ds/LM/LM3875.pdfand determine your supply voltage based upon your desired output wattage and your speaker impedance.

Good luck!

[johnr66]

The STA-77 is rated 16 watts per channel into 8 Ohms. So the output from the DC side of the supply is probably more like 30-0-30 with light or no load. If the original power amp section is still working, I wouldn't bother changing it out for IC's. The ICs might get you slightly more output power that is not really noticeable. If the original amp is not functioning, I'd be more inclined to repair it.

[nige838]

Originally Posted by nemonis

You mention a 60V power supply. How is this measured? I can think of 4 possibilities, only one of which would work well.

1. The power supply transformer has a 60V AC center tapped secondary. This means that measured from the center tap to either end of the winding is 30V AC. From one end of the winding to the other would be 60V AC. This setup is really not a good idea with an LM3875, as it would give about +/- 42V DC once rectified. This is the absolute maximum for the LM3875; any increase due to transformer regulation would cause the chip to shut down, or even fry something.

I think that this is how it is. Only, it measures 60volts between the two hot contacts and 30volts between the hot and middle.

The final end was burned out, that is why I'm attempting to repair it using the chipamp. The original amp used the 2SD313 Transistor, the datasheet can be found here:

http://www.datasheetcatalog.org/data...NGS/2SD313.pdf

It states 60 volts as well, so I think that this is giving out 60 volts...

I went ahead and hooked it up, it ran for awhile and the chips didn't seem to get too hot, except one channel was around 10X louder than the other.

[KatieandDad]

The LM3875 can operate up to 94V + to - ie + 47V, However, it is recommended to keep the rails a bit lower than that at + 35V.

[nemonis]

Originally Posted by KatieandDad

The LM3875 can operate up to 94V + to - ie + 47V, However, it is recommended to keep the rails a bit lower than that at + 35V.

According to the datasheet, the absolute maximum voltage is 94V (+/-47) non-operating and 84V (+/- 42) operating.

[KatieandDad]

Yep, but read the Datasheet. Above +/- 35V the device runs into protection mode too easilly.

[AndrewT]

Hi Katie or her Dad,
just in case my memory is playing tricks with me, I downloaded a new copy of the 3875 datasheet direct from National.
I have had a read through reminding me of many features that National draw our attention to. I cannot find the reference to

runs into protection mode too easily.

if the supply voltage is outside +-35Vdc.
Could you highlight which paragraph and/or which diagram you are referring to that imparts this information?

[Leglandu]

Depending of the driver...

I find this diagram very important as it explains how you have to chose your supply voltage according to your driver.
Or, in the case of nige838, how to adapt the driver : insert a resistor at the output of the amp (~2 ohm or more) if your driver lowest impedance is less than 8 ohms... but 42V seems really out of range