[Filburt]

Okay, so I've a problem with my PINT. The power supply is 18v (2x9v). I've checked polarity on everything, as well as alignment of the op-amps. I've also checked all the resistor values, and the soldering job is clean.

So the problem is, my ground channel op-amp (AD8397) heats up to burn-my-finger temperature within seconds. Presumably, this isn't normal. So, any ideas on what might be wrong? Is the chip just bad?

I had this problem on the last board, and desoldered the op-amps and mounted it on a new board with fresh parts minus the Alps pot and the rail cap, both of which I tested before using on this board. I have no idea what the problem is, and I'm finding this rather frustrating...

 

[MisterX]

do you have pictures?

do you know what the Dc-offset and current draw is?

 

[Filburt]

How do I measure those two things with my multimeter?

I can try to get some pictures tommorow. However, I checked maybe 10 or 15 times against the schematic as well as tangent's tutorial that uses a PINT prototype, and I checked my soldering repeatedly. It appears to all be correct, and solder job is quite clean...

 

[cire]

for DC offset: set multimeter to measure millivolts and stick in the probes in parallel with the signal output ground and left (then right).

current draw: set multimeter to measure at least 2 amps and stick probes in series with the battery to amp.

 

[MisterX]

The whole notion of showing us pictures is that sometimes we can see what you keep overlooking.
(a fresh set of eyes is always a good thing)


To measure current draw you have to insert the meter between the amp and the power source

maybe the picture I photochopped will help explain it?

To measure the DC offset:
Connect the amp to the source and with no music playing measure between
out ground (+) and inground (-)
out left (+) and inground (-)
and
out right (+) and inground (-)

 

[BavariaBarbarian]

Awesome pic, MisterX!

Can I extrapolate, that you answered this question repeatedly?

 

[NeilR]

Misterx,

Why do I care about the offset between input and output? I always measure offset between OR/OG and OL/OG. That's what the load sees.

If there were, for example, an offset between OG and IG, why do I care?

Regards,
Neil

 

[amb]

Quote:

Originally Posted by NeilR Why do I care about the offset between input and output? I always measure offset between OR/OG and OL/OG. That's what the load sees. If there were, for example, an offset between OG and IG, why do I care?

It's true that the load sees across from OL/OR and OG. However, if there is an unreasonable amount of DC offset between IG and OG, then that indicates there is something wrong with the circuit.

The output voltage of a unity gain voltage follower (which is what the ground channel output amp is) should be identical to its input (0V, assuming ideal opamp). A large discrepancy might point to oscillation, failed opamp, or bad soldering/wiring.

 

[MisterX]

Quote:

I always measure offset between OR/OG and OL/OG. That's what the load sees.

Remember we are not talking about a conventional amplifier here.
The active ground channel complicates things just enough that measuring "what the load sees" does little to indicate if there is a problem with the ground channel

Quote:

If there were, for example, an offset between OG and IG, why do I care?

Who am I to suggest what you should care about?

 

[NeilR]

Quote:

Originally Posted by MisterX Remember we are not talking about a conventional amplifier here. The active ground channel complicates things just enough that measuring "what the load sees" does little to indicate if there is a problem with the ground channel Who am I to suggest what you should care about?

OK... I was looking at it from the perspective of damaging/degrading headphone sound. I see your points regarding a difference as indicating an issue. I've never had a problem myself so I have never paid attention to it (although I check it on my new 3 channel amps when I do my smoke test).

 

[Filburt]

Pictures:




Yeah, that one resistor is a bit chewed up. I accidentally chipped off some of the covering post-build, but the problem was occurring before that happened, so that sort of isolates it from explanatory factors. The ground channel AD8397 is slightly disfigured on the top as well due to issues during its removal from the other board. It's not *too* bad, but the top part is slightly guffed up.

 

[Teerawit]

Are your battery connectors hooked up properly?

 

[Filburt]

Quote:

Originally Posted by Teerawit Are your battery connectors hooked up properly?

Yes, and one might note the diodes are in place to provide reverse supply protection, anyhow

 

[dhp]

C8+ appears shorted out to the opamp's top right pin on the bottom

 

[Filburt]

Those two things seem to be connected on the PCB itself. I'll try to clean it up but I'm not sure it'll make a difference...