HiFi Subjectivist ¦ Audio Forum ¦ NVA User Group

Lurcher300b wrote: ↑Wed Nov 06, 2019 9:06 pm

So 690 views and 40 odd posts and I'm still no more educated than when I started (Bar the one sensible post)

All you will get from your question is other peoples opinions, if you want facts, then you will need to spend five years learning electronics to understand the answers, by which time you will not need the question answering anyway.

savvypaul wrote: ↑Tue Nov 12, 2019 11:19 am I'm mildly amused at the 'bill and coo' over photos of component innards that seems to be the norm on some forums and user groups. I doubt that 99% of the people commenting have a reliable understanding of what they are looking for...and the 1% who do have a proper understanding know that a photo alone cannot give them all the information they need to reach any deep conclusions.

As Nick (Lurcher300b) recently commented (elsewhere)..."Electrons don't get OCD"

Sometimes, a 'little' knowledge is a dangerous thing.

Lurcher300b wrote: ↑Mon Nov 11, 2019 7:48 pm Here is a tail of a odd failure of a pair of monoblocks I had just built and was running in. Working fine, sounding great, suddenly, they both go into protection. Hmm, well, they both failed, and stay failed, so for them both to go, it must be a common source. Must be excess voltage out of the phono stage. Is that broken?, no, that's fine. Take amp cover off, one of the rail fuses has blown (+ve). Take other amp top off, the other rail fuse (-ve) has blown. Replace, fuse, try on bench, fuse blows again. Disconnect power supply, power up amp on bench supply (current limited, assuming it will let me see where the failure is). Works just fine, thrash amp in every way I can think trying to break it, nothing. Hmm, maybe a bridge rectifier has gone. Shouldn''t have, but maybe. Try power supply with no amp connected, works fine, change bridge rectifier anyway thinking it may be failing under load. Nope still works fine on its own. Connect amp, it fails. Only thing different is the amp is powered from the mains when it fails and amp is connected to power supply and the chassis is connected to earth, but is floating when on bench supply. Eventually Track down problem, one of the TO220 driver transistors has shorted past its mica washer and shorted one of the power rails to earth. Cause of the problem was I forgot to widen the hole around the fixing screw in the heatsink, so there was not much insulation to keep the two apart. Replace mica and drill out hole, its working fine.

But its a mechanical fault, my fault, but still mechanical, so why did the other one go at exactly the same time? Repeat process with second amp, exactly the same problem, but this time in the driver transistor for the other rail. Same cause, not opening the hole enough, but why the hell would they both go at the exact same time?

The only theory I have is that one failed, shorted +ve rail to ground, blew fuse, shorting the rail to ground will have briefly pulled up the earth to a +ve voltage. Other amp, having its ground pulled up, will have seen a larger voltage between its case and its -ve rail (ground moving up, so further away from -ve rail). Increased voltage caused the insulation to pick that moment to fail under increased voltage.

It sounds unlikely, but its the only reason I can think they both went at the same time (well one a few miliseconds after the other maybe), and the other clue was that one went in the +ve rail, and the other in the -ve one.

In both cases the protection circuit disconnected the loudspeaker, shut the power supply down and flashed the leds on the control board indicating a DC issue.

That is an excellent example of intelligent troubleshooting, something I have found rare amongst some, so called, professional engineers.

I am guilty of always wondering what something else will sound like when really i should be finishing the diy stuff i have started and concentrating on listening to music.

Daniel Quinn wrote: ↑Tue Nov 12, 2019 12:55 pm You sell your wares. It will necessarily involve a different ethos.