13v + Lm3875 = ?

[doomsweek]

Is it alright to power that LM3875 chip with only 13v?

I just found a (BIG!) transformer from my old National XE550 equipment, though I haven't checked its output voltage yet. Loking at the label, the equipment can be powered by 13v worth of battery, so I assume that the transformer has an output voltage of 13v. The datasheet for the LM3875 states that the minimum voltage range is 20v. But, I think I've read here somewhere that someone has driven the chip below 20v. I just want to be sure.

[Nordic]

As long as you have high efficiency speakers, and possibly keep gain to the minimum to prevent clipping....

[doomsweek]

I'm a beginner in this stuff.

So, I'm planning to use the speakers that came with the old national xe550 equipment. How do I know the speakers are of high efficiency? Only values found at the label are Impedance (6Ohms) and Peak Input (165W).

Thanks!

[Nordic]

Sensitivity is normally qouted in how many decibels you can create with 1 watt at a meter. I would say high efficiency designs would have figures of at least 90bd/W and hopeuflly more.... but this normaly comes at a sacrifice of power handleing with the exception of some PA equipment.

So I would bet your 160W speakers are not all that efficient.

I would say build the amp, give it a listrn with the tranny you have then get a proper one, you can even save a 10er or so a month, times speeds up as we age, and age we do. So in no time You can get what you need with some patience... I would save the tranny you have now for another project.

I have run LM3875 at with 11V trany for testing before, worked just fine. Sure it would not be a good idea to open up the volume...

[SpittinLLama]

13V may not work. The 20V in the datasheet is conservative but you may not be able to get down to 13. I have gotten as low as 16V on some LM3886 (very similar) but not lower.

Good luck,
---Troy

[Minion]

I believe I read in the Lm3886 datasheet that it has some form of undervoltage protection which needs something like 11v to disable it ,so there is a Posibility that you can run it off of 13v...

[doomsweek]

Originally posted by Nordic

I would say build the amp, give it a listrn with the tranny you have then get a proper one, you can even save a 10er or so a month, times speeds up as we age, and age we do. So in no time You can get what you need with some patience... I would save the tranny you have now for another project.

I have run LM3875 at with 11V trany for testing before, worked just fine. Sure it would not be a good idea to open up the volume...

I"ll try the 13v transformer first, then if it wont work ( which I think is really going to happen ) then I guess I"ll have to save. Thanks for the replies everyone!

[doomsweek]

Just found out that the transformer has 4 wires for the secondaries. I dont know which is which but the 4 wires are being paired by 2. 2 wires are colored blue and has an output voltage of 16v while the other pair ( colored orange ) produces 27v. Got the voltage using my analog multitester.

So, is this one alright to use for the LM3875 chip? Planning to use this for two LM3875 monoblocs.

[Nordic]

Assuming it can handle a few ampere it should be nice and even give you a nice supply for a buffer stage...

[doomsweek]

How can I know if it can handle "a few ampere"? How do I "calculate" for the transformer's VA?

Sorry for asking all this noobie questions.