Ruby Wall-wart Power

[ildar]

Why did using a 12v wall-wart power supply blow the 100uf filtering cap (twice)? The power supply is tip negative, 12vDC  700ma output, and has the proper size barrel. I just installed a DC jack and lost the battery clip. It worked fine with 9v from a battery.
Any ideas?

[ildar]

Well, a possible cause is a change of IC, of sorts. I forgot that I had changed out the 386-1 with a 386-4 and never tested the Ruby after that. Is there some difference in the 2 chips that would cause this? AFAIK, the 386-4 handles higher voltage, and that's the only (?) difference.
I should note that after I blew the 2nd cap, I replaced it, put a battery snap back in, and the circuit still didn't work until I put the original IC back in. I don't dare attempt wall-wart power again until I figure out what the problem could have been.

[Mark F]

Please don't take this comment as condescending but what voltage was the cap rated for? If it was rated for less than 12 VDC that would explain it.

[dano12]

I've build a butt-load of Ruby-type amps (over 30 at last count) and the only time I saw this happen was when the power cap was installed backwards. 9v isn't enough to fry it, but 12vDC at 700 mA probably would.

[ildar]

The 1st cap was 16v, the 2nd was 50v. The negative pole was to ground, the positive coming from V+.

[dano12]

The 1st cap was 16v, the 2nd was 50v. The negative pole was to ground, the positive coming from V+.

Ok, that is strange. Have you read the voltage and amperage coming off the AC adpator in a non-loaded condition?

[ildar]

No, I haven't done that.
I can't help but wonder about the chip, though. The fact that the 386-4 didn't work, even with a 9v battery puzzles me, and perhaps that's at the bottom of the problem.
Or not... 

[dano12]

No, I haven't done that.
I can't help but wonder about the chip, though. The fact that the 386-4 didn't work, even with a 9v battery puzzles me, and perhaps that's at the bottom of the problem.
Or not... 

I think your problem lies not in the chip, but somewhere else. I've used the 386n-1 and n3 across several different builds. Never used an N4 before...

[ildar]

Not to over-emphasize a point, but why would it work with the n-1 under battery power, but not with the n-4 under battery power?
I guess I want to blame the chip. 
Would the mA produced by the power supply pose a problem, or does the circuit draw only what it needs?

[coitmusic]

I've got two suggestions:

Double check that the adapter is 12vdc and not ac. I've made this mistake in the past. If it dc then double check the polarity. Look into adding a protection diode.

Since you're simply replacing the battery with a power supply, I'm assuming that you haven't added a voltage regulator. You would need a 7812 to regulate the voltage at 12v. Just because the power supply says 12v that doesn't mean that it always supplies 12v. Usually it's much more (maybe 15v+) Check around at amz and geofex for how to regulate the power supply.

Good luck!

[dano12]

Not to over-emphasize a point, but why would it work with the n-1 under battery power, but not with the n-4 under battery power?
I guess I want to blame the chip. 
Would the mA produced by the power supply pose a problem, or does the circuit draw only what it needs?

Not sure. The 386 datasheet lists the N4 as accepting a minimum voltage of 5v+, so technically it *should* work. But again, I wonder if the cap popping and chip issue are some how related. I'd also double check the AC adaptor output with my meter, you'll likely find that unloaded it is pumping out 14 or 15 volts. I'm working on a LM386 amp with a 12AU7 pre-amp, and I've started using 7812 regulators on all my builds just to account for the vagaries of AC adaptors.

[ildar]

I've got two suggestions:

Double check that the adapter is 12vdc and not ac. I've made this mistake in the past. If it dc then double check the polarity. Look into adding a protection diode.

Since you're simply replacing the battery with a power supply, I'm assuming that you haven't added a voltage regulator. You would need a 7812 to regulate the voltage at 12v. Just because the power supply says 12v that doesn't mean that it always supplies 12v. Usually it's much more (maybe 15v+) Check around at amz and geofex for how to regulate the power supply.

Good luck!

Did  double-check the adapter, triple-checked, actually, and it is indeed 12vdc, with negative tip.
As per Runoffgroove, a voltage regulator isn't indicated, as the Ruby has been verified to run on 12v battery power. The only caveat there was to check the IC for voltage rating. Mind you, I popped 16v and 50v caps. The adapter works fine, it's from a cheapie Yamaha keyboard, and has caused no problems with that.

Not sure. The 386 datasheet lists the N4 as accepting a minimum voltage of 5v+, so technically it *should* work. But again, I wonder if the cap popping and chip issue are some how related. I'd also double check the AC adaptor output with my meter, you'll likely find that unloaded it is pumping out 14 or 15 volts. I'm working on a LM386 amp with a 12AU7 pre-amp, and I've started using 7812 regulators on all my builds just to account for the vagaries of AC adaptors.

When I get home I'll check the adapter output, but again, let's say it is putting out 15 volts, why did a 50v cap blow? Weird...

[coitmusic]

"As per Runoffgroove, a voltage regulator isn't indicated, as the Ruby has been verified to run on 12v battery power. The only caveat there was to check the IC for voltage rating. Mind you, I popped 16v and 50v caps. The adapter works fine, it's from a cheapie Yamaha keyboard, and has caused no problems with that."

You're right, it doesn't say anything about a regulator, but it also doesn't say anything about using a wall wart adapter. The voltage from these supplies seems to be very unstable and the only real problems I've had with cooked chips and the like seemed to be related to using unregulated supplies. The yamaha keyboard you mentioned probably has its own 7812 and associated components.

but you're right...it still shouldn't be cooking that cap. I'd try removing the power jack from the ruby, plugging in the supply and then checking the polarity of the pins. I've screwed this up plenty of times, too.

[TheBigMan]

I'd try it with a standard 9V adapter and see what happens there.  The adapter does sound suspicious to me, although I run my Ruby off an 18V adapter for maximum clean headroom.  Never blown any caps, I'm using LM386-N4s from RS.

A very useful little thing to have is a standard Boss type barrel plug wired to a battery snap.  Handy for checking what happens with a battery on things that don't have a snap, or things that are a PITA to open up and install a battery into just for a quick test.

[ildar]

The wall wart measures 15.29 output with no fluctuation. Not any, or not most anyway, of the projects spec a wall wart supply, but it's sort of an assumed add-on, I guess. Does this project not apply? Can't see why not.
However, I'll go to a 9v supply with lower mA draw and see what happens