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Registered Member #2422
Joined: Tue Oct 06 2009, 02:41AM
Location:
Posts: 85
EDIT: Now the dc offset is down to ~0v, but no sound. I'm guessing I just did something wrong and this problem isn't solvable over the internet.
I'm making an amp along the lines of esp's project 127 using the TDA7293.
The left channel works fine, but on the right I get a dc offset (no input) of about -15v. I've looked a million times and can't find a problem anywhere, but I might just be missing something obvious. What I'm wondering now is where to look.
What I don't understand is how the dc bias can be so high unless I've fried something (resistors and such check out good, I'm not sure about the chip, but the implication seems to be that they are very rugged.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You have two channels that are meant to be identical. But one works and the other doesn't, therefore they must not be identical.
So, compare the voltages on all the IC pins between channels. Soon you'll find one pin that's different. Investigate.
If you have no DC offset and no sound, I'd suspect the mute or standby pin.
These ICs aren't quite as rugged as a good old-fashioned power amp made of TO-3 transistors, but they're perfectly good as long as you don't push them to the limit. Marshall made a guitar amp that tried to extract 350W from four of them, but it was so unreliable they had to stop making it.
Registered Member #2422
Joined: Tue Oct 06 2009, 02:41AM
Location:
Posts: 85
Ok here we go...
15 -VS (POWER) 14 OUT 13 +VS (POWER) 12 BOOTSTRAP LOADER 11 BUFFER DRIVER 10 MUTE 9 STAND-BY 8 -VS (SIGNAL) 7 +VS (SIGNAL) 6 BOOTSTRAP 5 CLIP AND SHORT CIRCUIT DETECTOR 4 SIGNAL GROUND 3 NON INVERTING INPUT 2 INVERTING INPUT 1 STAND-BY GND
On the working channel: 15: -11.6V 14: 0V 13: 15.1V 12: .46V 11: 0V 10: 15.1V 9: 15.1V 8: -11.6V 7: 15.1V 6: 10.8V 5: .5V 4: 0V (gnd) 3: 0V 2: 0V 1: 0V (gnd) and 0V between C1A and R1A
On the non-working channel: 15: -11.6V 14: -10V 13: 15.1V 12: -10.6V 11: -9V 10: 15.1V 9: 15.1V 8: -11.6V 7: 15.1V 6: -12.8V (this means there is a 1.5-2V reverse voltage on the bootstrap cap, so I'm hesitant to run this circuit more than necessary) 5: -10.2V 4: 0V (gnd) 3: -9.5V 2: -5.5V 1: 0V (gnd) and .7V between C1A and R1A
So it looks like the chip has burned out and shorted (more or less) most pins to -Vs (which is why it's sagging). I don't have a scope, though, so this is just my best guess.
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problem with gainclone-like amp
