All things where nice and smooth, sounding great even. But then tonight one of the channels on my Audio Sector kit blew up. The chip is cracked and has the wonderful smell we all know.
Assuming that my rails are nice and smooth (this will be the first thing I check) and the rectifier is in tact what would be the next step in fixing this?
I do not have any way to measure the caps but I can check the rest with my simple multimeter. I plan on picking up a new LM3875 tomorrow along with a couple new resistors. I may even build up a new rectifier board.
Would a short or cold joint cause this to happen? If so where would be the best place to look? I am thinking that it could have been the tiny RF killer cap I put between pin 7-8 on the chip. It is pretty tight, there might have been something there that I did not see.
It just seems odd that all was well and then it just blew. No physical movement. Nothing changed. In fact there was not even a speaker connected to that channel. The amp was running but at very low levels. No where near enough to get anything anything hot. The transformer is bran new and it is plugged into a regulated UPS.
Thanks for any advice.
Ya I hear ya. I am at a bit of a loss. It could have been something that simple. The board has taken a pretty bad beating now that I have taken the chip off. I am not sure if I trust the board 
I have spent the last hour checking traces. I have to say that by doing this I am gaining a much better understanding of what is going on with this thing. So I guess there is a bright side to all this.
I have spent the last hour checking traces. I have to say that by doing this I am gaining a much better understanding of what is going on with this thing. So I guess there is a bright side to all this.
No not that I could tell. It happened while I was doing some soldering and I thought the evil smell was from hitting some plastic. But then I quickly realized that it was something much worse. I quickly powered down the amp and unplugged it to give it a good sniff and there it was a nasty chip. It sat there blown for about three or so min. The heat sink was a little warm.
There is no Zobel network on the outs.
Could previous high temps have damaged the chip slightly then causing it to fail later? Or would it simply die at the time of overheating?
I also touched the transformer to see if it had gotten hot. It was ever so slightly warm. I have not felt the transformer before to check for heat so I am not sure what is "normal" for it.
I was tried to check the voltage coming off the rectifier but it looks like my multimeter is on the fritz. My numbers where all over the place. It was the same on the working channel so I am pointing to the meter. I am trying to get one tomorrow along with my new parts. Also now that I am aware of what pins are in use I think I may just break off the ones that are in the way to ensure that there are no shorts.
There is no Zobel network on the outs.
Could previous high temps have damaged the chip slightly then causing it to fail later? Or would it simply die at the time of overheating?
I also touched the transformer to see if it had gotten hot. It was ever so slightly warm. I have not felt the transformer before to check for heat so I am not sure what is "normal" for it.
I was tried to check the voltage coming off the rectifier but it looks like my multimeter is on the fritz. My numbers where all over the place. It was the same on the working channel so I am pointing to the meter. I am trying to get one tomorrow along with my new parts. Also now that I am aware of what pins are in use I think I may just break off the ones that are in the way to ensure that there are no shorts.
Of course it did.MJL21193 said:
A short between pins 7 and 8 will do no harm to the IC. That would mean 0 difference from one input to the other, hence 0 output.
The only shorts that could damage the IC can happen at the output. Either from output to ground, when a high signal is present or from output to either of the rails, when a small or no signal is present.
Oscillations could also heat the chip up, and they would happen exactly, when no speaker is connected and no Zobel is present. Then there will be no impedance to dampen them. So either add a Zobel network or make sure that a speaker is always connected.
Plug in your bulb tester.
Check the output of the transformer.
Then add on the rectifier and smoothing. Check the output again.
Add the Zobel to the output or even better add a Thiele Network to the output.
Add on a good channel of the amplifier. Check the voltages on the transformer, at the smoothing caps and at the amplifier output.
Check the output offset of the amplifier.
Remove the good channel and add on your repaired channel. Go through those checks again.
Has the bulb stayed off through all this testing?
Check the output of the transformer.
Then add on the rectifier and smoothing. Check the output again.
Add the Zobel to the output or even better add a Thiele Network to the output.
Add on a good channel of the amplifier. Check the voltages on the transformer, at the smoothing caps and at the amplifier output.
Check the output offset of the amplifier.
Remove the good channel and add on your repaired channel. Go through those checks again.
Has the bulb stayed off through all this testing?
SimonKit said:
Ya I was working on the positive terminal on the MAINS while having a few drinks and playing some serious pounding techno! Like I said I have no idea what happened it all seemed so cool
OK so back to reality. Thanks to you all for the feedback.
pacificblue: Thanks for the info about the shorts. I am pretty sure that the outs where all clean. That is pretty straight forward stuff. I will keep extra attention to this as I look things over.
AndrewT: At the risk of sounding naive I have not looked into what it is to have an amp oscillate. I know what it is in terms of programming synthesizers quite well but in terms of a bad thing in an amp, no idea. I am sure that very little digging will tell me all about it so I wont pain you all by asking about it. I have checked the outs of the rectifier board and they are right where they ought to be. As for a Zobel I am hearing different things. I have to say that considering that I went with Peters kit I am generally inclined to go with his advice on his boards.
I do appreciate your step by step testing process and intend to use it.
Peter: I know from reading your building thread at least a dozen times that you are a less is more kinda guy. I like the idea of this very much as I am not looking for ANY tone in my amp at all. I want it to be as absolutely true to the source as it can possibly be. And it seems to me that the more there is in the path the further away you get. But is the level of minimalism in your amp putting the stability and reliability of the amp at risk in any way? A more general Gainclone question would be with absolutely no protection on the outs how likely is a blown chip going to take out my speaker?
I only have a couple of new things to add. I took a close look at the blown chip and noticed that some of the black coating on the chip had warn away around the mounting hole. I did not have any insulation between the chip and the heat sink. Both chips are mounted to the same heat sink and it is not grounded. Could this have killed the chip?
I was listening to music on the amp when it happened but only through one channel. Is there any odd behaver that I should know about regarding gainclones? Like only use the amp when two speakers are attached. Only turn the amp on when speakers are attacked? These are pretty odd things from where I am standing. Anything that is so obvious that no one is saying it?
Mike-Toronto said:
Quite likely. If the output transistors go open circuit, you are lucky. If they short, there will be DC at rail level across the speaker terminals, which can damage the speakers.Mike-Toronto said:
The black coating is the isolation. No further isolation is needed.Mike-Toronto said:
In what way is it worn away on your chip? Molten? Scratched? Squeezed?
The same rules apply to chipamps as to any other amplifier.Mike-Toronto said:
Oscillations in an amplifier are different from those in your synthesizer in that they are uncontrolled. A better comparison is the feedback you can produce with a microphone and a speaker, which produces a screeching noise, when the circuit starts to oscillate. Something like that happens in amplifier circuits above the audio range. The usual counter-measure is to introduce a low impedance in the frequency range, where oscillations tend to ocurr. That is, what a Zobel circuit is, a low impedance at high frequencies. In some cases the relatively high impedance of a speaker is enough to avoid oscillations. Sometimes the oscillations are weak enough to produce no detectable secondary effects, which could lead you to believe there are none and you can get away without a Zobel. It is better to have one all the same. Just like a fuse. You may never need it, but you just don't skip it, whatever the sonic improvement that could bring.
pacificblue said:
pacificblue said:
Ah, in the digital synth world the equivalent would be anti-aliasing filter. Basically a LPF that is above the effective audio range to filter out the undesirable effects that occur when you try to produce sound above a given sample rate.
I my kit did not come with the Zobel parts but I believe Peter will send if requested. I will be doing that.
In looking the amp that blew I noticed that there may have been a cold solder on the bottom of the board. The side that is used of course
When I take the positive out of this I am very glad it happened because I am learning this amp pretty well. And that is a good thing considering that plan on building at least one more of them.
I agree with Peter. If the amp is functioning correctly, with a properly implemented star grounding scheme, then a zobel network will not needed. In my opinion, the zobel network has a detrimental effect on the sound quality, giving colouration in the upper mid region.
And as for soldering with the power on
Regards,
Chris.
And as for soldering with the power on
Regards,
Chris.
That is a romantic notion. The vast majority of amplifiers has a Zobel network, and you should not assume that the vast majority of audio engineers has no idea about amplifier design.zebra100 said:
Oscillations have a detrimental effect on the sound quality and reliability. They can destroy the amplifier and the connected tweeters. A Zobel network can help against that.zebra100 said:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.