lm1875 ic

[revenant]

Hi to u all,i recently build a circuit with tda 2050 for an old amplifier.the tdas work fine with split supplies of up to 25 volts but the amplifiers'transformer has a split supplies of 28 volts.a friend ask me to upgrade with lm1875 which has similar characteristics but can withstand a maximum of+/-30 volts and that the pcb is fine for the 1875.the problem is that upon switch on the 1 ohm resistor at the output (boucherot network ? ) of the 1875 burn out.what may be the problem?

 

[Mooly]

upon switch on the 1 ohm resistor at the output (boucherot network ? ) of the 1875 burn out.what may be the problem?

I take it that this resistor is in series with a small cap of around 0.1uF. If so then the only possible cause of the burning is high frequency instability and oscillation.

Why is a different matter. You would have to check the application is suitable for the LM1875.... without looking up the relevant data sheets I don't know off hand what the differences are.

 

[revenant]

Thanks Mooly.the 2050s did fine but unfortunately for me i couldn't accomodate them with their transistor regulators built on a large pcb inside the amp.

 

[Mooly]

I've just looked at the data sheets and there are no obvious problems that I can see. You would need to look at this with an oscilloscope and try and see what was happening. It may just need a small amount of compensation adding to the feedback network and/or perhaps a small output inductor adding.

All guesswork without a scope though I'm afraid.

 

[revenant]

National semiconductor datasheet effectively prescribe the use of an inductor and with an output resistor of 10 ohms to remedy to the situation.

 

[Mooly]

That has to be worth a try then. Output inductors are often a necessity for many amps. The value is not normally critical but it should be air cored.

 

[johnr66]

The inductor might help, however, I've never had an issue with the LM1875 oscillating even at near maximum supply voltage. It could be a feedback or circuit layout issue. All that is covered in the datasheet.

 

[Arty]

or a really bad cap at the bucherot cell...
allso you can scrape off a bit of voltage from the supply rail if you like, with a few series diodes.

 

[revenant]

lm1875

I would change all the components of the boucherot cell and let you know how it goes.Any way it is strange that the tda 2050s worked well on the same pcb without needing extra components like an inductor and output resistance.only lack of space for the transistor regulators prevented me from using them in the amp.

 

[danielwritesbac]

Try 100k feedback resistor and 2k7 feedback-shunt resistor.

And, at the amplifier board put standard diodes (0.7vdrop) in series with the DC cable, with a diode for V+ and a diode for V-. You can do that separately for each channel to reduce crosstalk.