Strange 5534 thermal shutdown

Hi

I just built this simple unbalanced mic pre with two 5534 for a friend; noninverting mode, 20 to 60 dB. After 30 min or so, both channels shut down probably due to overheating. The opamps run on +/-19 V and it is a tight case without opening. Could this alone cause the shutdown? Or is there some low-level oscillation going on? I had issues with stability (coupling from out to in as everything is so tight), but it looks like these are solved now. Now 'scope at hand right now, unfortunately, but they sound good as far as I can judge with the cheap dynamic mic it is supposed to be used.

I can post a schemo if this helps, nothing special, however.

Thanks for you help!
Samuel

i have lots of 5534s running at +-18v and they don't get warm at all..? now my OPA stuff gets a bit warm though..

please post a schematic!

:thumb:

Don't know how hot things are getting, but which type of 5534 you're using ? NE ? SE ? SA ? They have different temp-ratings. NE has max Tamb of 70'C.

I am running TI opamps at 18V rails without problems also. Try to find out who made them. These days there are at least five different sources for NE5534s. Gotto look at the square wave response on the scope to tell if there is HF ringing going on. You could try increasing the compensation capacitor to 33p along with the feedback resistor bypass cap (>100pF) and see if it helps.

I'd never heard of thermal shutdown in 5534 parts, but I'm glad that whatever is going on is nondestructive at least.

I'd suspect a bias shifting to the point that the outputs are stuck at a rail.

The ambient max temps are usually not so much where devices cease to work, but where they go way out of spec.

Talking about one guy's 5534 that gets hot at +/-15V rails and another guy's that stays cool at +/-18V is a pointless comparison unless you take into account what sort of loads these ICs are driving.

[quote author="NewYorkDave"]Talking about one guy's 5534 that gets hot at +/-15V rails and another guy's that stays cool at +/-18V is a pointless comparison unless you take into account what sort of loads these ICs are driving.[/quote]

Not necessarily; it sounds like the original poster's chips are shutting down even when remaining at idle. It shouldn't matter what load they're driving if there isn't any signal.

Like others who have posted, I run 5534s from higher-voltage supplies, up to 20.5V on each rail, in closed cases, and while the cases get warm to the touch, I've never once had an IC go into shutdown or otherwise fail.

I'd look first for oscillation, then offset. Borrow an hour with a scope someplace, maybe a technical college?

Peace,
Paul

Talking about one guy's 5534 that gets hot at +/-15V rails and another guy's that stays cool at +/-18V is a pointless comparison unless you take into account what sort of loads these ICs are driving.

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I also side with Pstamler on this. While it may seem pointless because each person's setup is different, it is still a clue as to what might be happening overall. If one person can run at a higher rail regardless of the load, then we know the ICs will work at those higher rails ok. While we already *know* that these ICs can and will run at such higher rails, the proof is now tangible. Now we must look at the load that is being driven and how much gain this was designed for. We should just work from easiest to hardest and we will eventually figure it out.

I second hooking up a scope to the unit and watching it over time to see what starts to happen. We could guess all day long but it would be much more efficient to observe it.

:thumb:

Thanks for all answers!

I'd suspect a bias shifting to the point that the outputs are stuck at a rail.

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This was a good one. The opamp outputs slowly fall towards the negative rail. I guess I did something wrong with the design of my circuit, so here the schemo: [removed]

Opamps are NE5534AP from TI, C103/C203 are not fitted. DC path from noninverting input to ground is 11k trough the PAD.

The only thing I can see is that there is no DC path from the opamp output to ground; I usually provide one with a 10k resistor but omitted it here for space reasons. Would adding two resistors solve my problem?

Thanks for further input!

Samuel

My point was that we're making blind guesses and broad assumptions without seeing the circuit. But now we can see the circuit. I don't see a DC ground path for the inverting input, which means the feedback is decoupled at DC. That could be part of your problem...

Doesn't the opamp output count as DC ground?

Samuel

There is no need for output resistors to ground.

The opamp should be happy with 10k from output to inverting input. I suspect something going on with the assumption that the n.i. input is really 11k d.c. to ground. I'll bet the zero atten pad switch position has the n.i. input merely tied to C102/202---although that is also odd since the current out of the inputs of the 5534 is positive, isn't it? So that scenario would wind up pinning the output to the positive rail.

Note that for negative signal swings at the output the 1000uF caps get reverse-biased. If the gain is very high and the circuit working correctly this reverse bias will be small. But if the reverse bias gets a little bigger then the whole circuit starts to have d.c. gain and any net offset gets amplified. The process accelerates as the C leaks more and provides more d.c. gain in doing so.

Also: if the pad engaged puts ~1k to ground at the n.i. input, there is a small differential developed due to input bias currents flowing into different resistances, which makes the output go a little negative and also then begins to reverse bias both the 1000uF caps and the output coupling caps.

I'd use bipolar caps for C105/205 and maybe C106/206, in any case, and check them for leakage current beofre installation.

Sorry, my head wasn't screwed on right this morning. Yes, as drawn, with the shunt leg of the feedback network decoupled at DC, the circuit should have 100% feedback at DC and therefore a DC gain of only 1X. So it's probably some parasitic effect causing DC gain to rise, as bcarso points out.

Thanks for the answers.

C105/205 are the smallest 'lytics I could find, I guess these leake a bit more than bigger ones. C106/206 are bipolar allready as shown.

Not sure what to do about it, as I will not find other 'lytics with 1000 uF that fit the space need. I'll probably remove them and add a 2k resistor for the + input. This will give some hundred mV output DC, but this is better than -17 V.

Samuel

Ah yes---my printout garbled the right-hand plusses on the 106/206 caps so they looked more like minuses.

You could bias each - end of the 1000uF parts with a bit of negative voltage---move the 10 ohm R to the other side of the cap to ground, and run in some current from the negative rail to the cap/R junction. Simpler than doing a d.c. servo, and probably quieter too.

Still something fishy about all of this. I wonder how the two rails come up---maybe things are off to a bad start and go downhill (literally and figuratively) after that.

What is U301? Does it also feed the 5V regulator? What does the 5V rail supply? Would you end up with the positive rail loaded down too much over time?
By the way C303 IS drawn upside down on the schamtics. I don't know if it was installed that way in the real circuit, but electrolytic caps don't like that.
Also, I would make C305 and C306 from rail to ground to inject crap into grond instead of the opposite rail.

Still something fishy about all of this.

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Agree.

I wonder how the two rails come up---maybe things are off to a bad start and go downhill (literally and figuratively) after that.

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The rails settle faster to +/-19 V than my DVM can measure. And the opamp out is @ < 5 mV after a few seconds. It starts rising after a few minutes with some mV/s.

What is U301?

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DC-DC.

Does it also feed the 5V regulator?

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Yes, as shown.

What does the 5V rail supply?

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PlugIn power (phantom power for unbalanced mics).

By the way C303 IS drawn upside down on the schamtics.

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Oups, thanks for mentioning this. I wonder why it did not blow up yet. Not enough current draw, probably.

Samuel

Finally solved - but it is a strange one! C102/C202 (Panasonic SU) seem to put out a growing voltage. If I measure the DC voltage across a desoldered cap, my DVM shows some mVs which are rising with a few mV/s, the speed I observed at the opamp output. My DVM switched off after some dozend minutes, where the voltage was still rising. In contrast, a Panasonic FC shows a few hundred mV, but rock steady. Looks like I bought active caps. :grin: Anyone observed this as well? Note that the R101/C101/... node sits @ 0 V.

Solution was a 1k to ground at the noninverting input. This seems to load the cap enough to not let it put out funny voltages.

Thanks for your help!
Samuel

That's wacky and wonderful. But what was the impedance to ground at the n.i. input to begin with (i.e., what are those pads that are not shown on the schematic)? A 1k to ground is brutal, and is attentuating and loading down the mic capsule something fierce. If properly biased 'lytics required that kind of bleedoff we would all be in big trouble.

(Also---how can the R101/C101 node sit at zero volts? Aren't you pulling up on a FEt drain in the electret capsules?)

Electrolytics of course will show a lot of dielectric absorption, and there's no reason to suppose that a bipolar won't show it as well. So when you model them as two back-to-back parts, the assumptions about leakage current and capacitance of each half will enter in as to how they charge and discharge, and it's plausible that they will show an evolving potential difference just sitting there after sustaining a given equilibrium under bias. For a single polar lytic the voltage recovery is always of the same polarity as the applied proper-polarity voltage.

Note that it would seem you require both mechanisms in this case: the potential generation of the cap and the reverse-bias of the big d.c. isolation cap, which accelerates things by leaking and thereby no longer blocking d.c.

But what was the impedance to ground at the n.i. input to begin with?

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The PADs are 10k+1k shunt if out and 10k series/1k shunt if in.

A 1k to ground is brutal, and is attentuating and loading down the mic capsule something fierce.

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I don't like it either, but I didn't want to try several values, so I just used one that will work. However, one would still think that 11k should be well enough, but it's not.

how can the R101/C101 node sit at zero volts? Aren't you pulling up on a FEt drain in the electret capsules?

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PlugIn-power is switchable.

Samuel