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[SOLVED] LM1875 works on breadboard but not on Stripboard

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BobL3394

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Hello,
as mentioned in the title, i bought a LM1875 as i wanted to build an audio Amplifier. I first built the "standart" circuit ive read on the datasheet, on the breadboard. As everything worked fine, i welded it on stripboard but it didnt work anymore: the speaker begun the hum, the ic immediately got hot as soon i plugged in the power supply, all this even with no audio input.
its worth mentioning that i tried to rebuilt the circuit multiple times, to check, double-check and triple-check if every connection was right and to rebuilt it on breadboard thinking the ic might have passed out resulting in the circuit working again!
Does someone have a clue on what could be the problem?
 

It could be due to reverse connection of power supply or there could be short between the VCC and ground pins in the stripboard. please double check it.
 

build error on the strip board, not quite cutting tracks properly...
 

Ah that old Chestnut...a small Jewellers eyeglass would be very useful....useful for surface mount for me. Available for peanuts from Ebay sellers.
 

hobbyckts said:
It could be due to reverse connection of power supply or there could be short between the VCC and ground pins in the stripboard. please double check it.
Click to expand...

Done, but unfortunately thats not the problem here.

Its not the power supply cause i checked it again and there's no ground to vcc short. I tried also to mount the circuit on another stripboard cause i thought the maybe having the supply on the same board could somehow affect it but nothing changed so its not the stripboard.
 
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Do you solder each and every terminal with help of wire or it is a board which is routed already and you are just assembling the components?
 

Im soldering each component with wire
 

Hi,

you have to admit, there must be any difference between both designs.

So show us pictures of your strip board design, with a bit description of part values, voltages and so on.

***
Please measure all pin voltages (directely at the device pin, not at any solder joint) against GND and tell us the values.

Klaus
 

Are all the GND(0V) connections good, and connected together?

Capacitors connected the correct way?
Feedback filter good?
All decouple capacitors in place?
 

Put some method to it. Might start by simple IC-pin voltage
readings around the part, in the two build configs. Where
are you seeing discrepancies in detail?
 

the speaker begun the hum, the ic immediately got hot as soon i plugged in the power supply, all this even with no audio input.
Click to expand...

It's possible you have high-frequency oscillations. Beyond our hearing range.

These convenient audio IC's are easy to use, but not foolproof. Try installing a small value capacitor at strategic node.

Or try a different volume control network. I once had to do the same thing with a stereo IC amp I built. It required some experimentation, until I found an arrangement that did not cause oscillations. I have never figured out what I did wrong or what I did right.
 

So i tried to mount it again on breadboard and after working for a couple of minutes i touched the heatsink to feel if it was heating up and then it stopped working and again hummed and heated up. i tried to put some caps here and there but at least for now i didnt solve anything. I used an oscilloscope and checked the output and noticed that it has a average output of 15V and a Vpp of 4V. The output oscillates at 100Hz.
I also measured the supply voltages and noticed that while +Vcc=20V, -Vee=-15; thats strange as the voltage measured at the filter caps is +-26V( i measured it using another, fully working, ic: the TDA2005 10W).
 
Last edited:

Hi,

It seems there is a lot of voltage drop...means a lot of series impedance. This leads to oscillations.
But 100 Hz is a bit low, I expected a much higher frequency.

You need thicker and shorter wires. Low ESR capacitors next to the amplifier.

A picture could help..

Klaus
 

With a dual polarity power supply the output pin should be 0VDC with the signal causing it to go positive and negative. Since you measured 15VDC then an 8 ohm speaker will be conducting 15V/8 ohms= 1.9A continuously causing the IC to get hot.
 

Sounds like you've blown up some of the black bits... a new 1875 would be a good start...
 

BobL3394 said:
Im soldering each component with wire
Click to expand...

So there seems to be the problem, Take brand new components and just check the circuit once again in breadboard after that start assemble in the stripboard with quite carefully. Use short wires and not lengthy ones. Better you post a picture of the board which you are working already. That could help us to provide some more information by looking into it.
 

The odd thing is that building it on breadboard sometimes doesnt work so, after verifying that there arent misplaced components, i re-assemble the circuit and somehow it starts working properly again with no distortion and no overheating.
So id say that the problem cant be in the supply but must be either in the IC itself(fake, broken?) or in the disposition of the components it needs to work.
I think that its also possible to exclude the further need of caps at the AMPs power supply pins as when it works it does also with those recommended by the manufacturer.
Im using the Snubber i found on the datasheet.
 

So are you satisfied with your circuit working? or you want to find the root cause for it?
 

An LM1875 amplifier IC made by National Semiconductor (who invented it) or Texas Instruments (who recently bought National Semi) is "idiot proof". It limits the output current and shuts down if it gets too hot.
If it was made by a Snake Oil copy-cat manufacturer then it might not be idiot proof.
 

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