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Old 5th October 2011, 09:00 PM   #1
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Default input coupling cap => blown IC

I have built a simple LM1875 based amplifier according to Rod Eliott's schematic, but mistakenly ordered a 22uF electro bipolar as the input coupling cap. I figured it was no problem, except the fact that the amp will reproduce sounds for elephant's ears, but after testing it, i've turned it on, and it worked for about ten seconds with a signal then blew the fuse. Checked everything on the circuit, replaced the fuse, and when it started it it went into oscillation.

I've checked everything again, built it on the same PCB, but with a 2.2 uF cap and another IC, and it works perfectly (the old IC wold still oscillate however).

Just for scientific curiosity, i replaced the input cap with the 22uF on the working amp and voila, another blown IC.

Why did this happen???
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Old 5th October 2011, 09:18 PM   #2
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When you were "reproduce sounds for elephant's ears" how long did you let this go on? Did the heatsink get hot. Sounds like you were just pushing it to hard. It would also be usefull to measure the power supply rail (rails) while you were pushing it. The larger value cap will pass lower frequency audio which demands more from the chip amp. The power supply was able to put out more than enough poop so the chip took the fall. You could 1. linit the input drive, 2. measure, and if nessisary increase the heat sink size, 3, install some form of potection, like lower value fuse in the transformer primary. Best answer of all is to use the amp withing its limits.
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Old 5th October 2011, 09:25 PM   #3
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OK - I need to read more before I write. If it is allowed to oscillate, all bets are off. There was feedback between the output and the input which is nessisary for it to oscillate. The higher value cap did not pass the feedback and prevented the problem. Have you got a picture? Perhaps some middle ground value for the cap would be possible with optimized layout. A PC board is highly recomended.
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Old 5th October 2011, 09:32 PM   #4
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Thanks for the answer. I tested it with no signal and slowly brought up the voltage. I gave it signal with dual 16v dc, the pcb for the amp had a 1amp fuse that blew. The heatsink and the IC were both cold - it was the first thing i checked after the fuse blew. The amp was also connected to speakers, and the volume was quite low.

I figured it might have been pushed too hard, but theoretically the size of the input cap should not matter (it cannot be too large) - as long as the input is within the amplifier's limits, right?
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Old 5th October 2011, 09:42 PM   #5
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Here's a picture of the layout, it was quicker to get from the PC and it also shows the traces.

Also, it oscillates even with no input signal. The capacitor is connected to the output only through the negative feedback network's ground connection.

The value of 22uF for the amp is not justified at all, a value of 4.7uF would be more than enough to cause no distortion at all at 20Hz. However, since the afore mentioned events, i do not trust my logic anymore and am a little worried about putting 4.7uF in it.
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File Type: jpg LM1875.JPG (79.6 KB, 216 views)
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Old 6th October 2011, 08:43 AM   #6
AndrewT is offline AndrewT  Scotland
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Do not connect speaker to any amp that has not be thoroughly tested and measured to check it is not faulty or badly set up.

Do not power up a mains powered project direct from the mains. Use a bulb tester in the Live feed.

Short the signal hot to signal ground for all early testing.
Leave the output terminal without any load in all early testing. There should be a stabilising Network on the output terminal. This can be as simple as an R+C (Zobel)
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Old 6th October 2011, 12:35 PM   #7
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Thank you for the detailed answer, Andrew.
For early testing the project was powered up using a dual lab power supply, bringing up the voltage slowly. The signal was tied to ground, also the output terminal was left without any load.
The stabilizing network does exist, exactly how you suggested it, and it consists of C28 and R24 in the pcb attached earlier.

I did get the amp working fine, on the same pcb, with a 2.2uF input capacitor, so my question is more theoretical in nature: is there such a thing as a too large input capacitor? (except of course due to the inherent inductance and possibly ESR of larger than needed capacitors)
Also, i really would like to try a 4.7uF, but i am not sure if it won't repeat the same process all over again. :|
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Old 7th October 2011, 09:05 AM   #8
AndrewT is offline AndrewT  Scotland
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I think the input capacitor can be too big. But not for it's DC blocking performance. It's to do with filtering the signal before the amplifier is asked to process filtered signal.
There are a number of threads that have risen recently where this is being discussed.

I bumped into it reading D.Self about 20years ago when as far as I could tell he was the only designer talking about, rather than keeping it secret.
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Old 7th October 2011, 11:58 AM   #9
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just a note, I asume that you did the layout yourslf, and you are only showing one side. But I like to fill the entire top and bottem side ground with copper in non trace areas. Then connect the two top and bottom side grounds. Not everyone does this, but I find it helpfull to prevent oscillations. You cad software surely has a "fill"" function.
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Old 7th October 2011, 12:22 PM   #10
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Here is an example.
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