Opamp still frying, and so is my brain...

[teemuk]

I would just rebuild the damn thing with proper circuit board.

Those vero boards are just endless resource for all kinds of issues.

While at it, I would seriously recommend improving the following:

- Power supply filtering: ICs almost always need some local decoupling. You use none.
- Stability: Gain stages almost always need some HF bypass in NFB to prevent oscillation. Additionally they need that local decoupling.
- Layout
- Noding. Yours is just horrible. Especially the common return currents seem to flow with no logical plan whatsover.
- Reliability: All inputs and outputs interfacing to outside world usually should have diode clamps to limit voltages below safe values recommended by the ICs. Again you use none.
- Impedance: <22K input impedance isn't exacty optimal for the intented applications.

I understand that the Rod Elliot design doesn't address most of these issues but overall I wouldn't call this an extremely well-though out reverb effect design.

--

However, none of this likely explains why your OpAmps keep failing.

And your schematics still aren't depicting clearly what you are actually doing with the switching scheme(s) that makes the OpAmps fail.

[thom]

I would just rebuild the damn thing with proper circuit board.

Those vero boards are just endless resource for all kinds of issues.

While at it, I would seriously recommend improving the following:

- Power supply filtering: ICs almost always need some local decoupling. You use none.
- Stability: Gain stages almost always need some HF bypass in NFB to prevent oscillation. Additionally they need that local decoupling.
- Layout
- Noding. Yours is just horrible. Especially the common return currents seem to flow with no logical plan whatsover.
- Reliability: All inputs and outputs interfacing to outside world usually should have diode clamps to limit voltages below safe values recommended by the ICs. Again you use none.
- Impedance: <22K input impedance isn't exacty optimal for the intented applications.

I understand that the Rod Elliot design doesn't address most of these issues but overall I wouldn't call this an extremely well-though out reverb effect design.

--

However, none of this likely explains why your OpAmps keep failing.

And your schematics still aren't depicting clearly what you are actually doing with the switching scheme(s) that makes the OpAmps fail.

I've tried a few schematics to power a spring tank, and sound-wise this is by far the best, and not so different from others (e.g.accutronics) in its conception.
I am well aware my layout needs improving, this is only my second one, and I'm learning as I go. However, I must say some of your advice goes right above my head.
I might be out of my depth with this project, but I'm not going to give up yet I know this design works well, and I'll start from scratch if no explanation can be found for what's going on.
BTW, what do you mean by "noding"?

[gjcamann]

What bugs me is that, as Sam said, Rod Elliot's circuits are usually bullet-proof.
If my layout is correct and the PSU delivers the expected voltage, it might be something with my build, despite the circuit actually working, up to a point (of no return).
Or maybe a faulty component? I've checked all the resistor, they seem fine.

George: do you mean a cap before R4?

Yes. I'd actually recommend a cap right at at the input for good measure. If you have a DC bias at the input, then you might get some unexpected DC current flowing through the input to your op-amp. This could explain why Rod's reliable layout is not working for you - it may usually work just fine depending on the application it's used for.

What are you using as a signal source?

I'll try with a cap. What value do you reckon?

What do you mean by signal source?

I suggest you try a 100n or 200n for starters. What are you plugging into this circuit? A guitar? Are there any pedals before the guitar and this? Does the guitar have active pickups?

[thom]

I use a Jet King with ACH3 & 4 humbuckers, and plug the reverb into the loop of a TSA15H valve head.
Nothing else right now.

[dwmorrin]

It isn't clear in the first post in this thread when the problems started to occur.
In your last thread, you said it started after you wired in the footswitch.
Have you truly undone everything you did to put in the footswitch?

For instance, I see two wires coming off the +15 line... a smaller red wire, which appears to connect to +15 through a resistor.  Is this the LED wire?  And then I see a larger orange wire... is this the +15 from the power supply?
The layout only shows one wire.

There's nothing about this that explains the problem, but it makes me think there's more simplifying you can do to get back to a stable working circuit.
There was no problem before the footswitch wiring, right?  Maybe you can clarify IF and WHEN it was working fine, and WHAT you changed when the first IC smoked.

I would hesitate to mod the circuit more (by changing caps, etc) as this may only add more variables and red herrings for you.

[thom]

I think that's good advice, David.

Here's what I did last night:

disconnected the footswitch
replaced C5 (33nf film capacitor) with a 4.7nf as in original instructions on Rod's site
replaced C3 (100uf rated 25v) with one rated 50v

It goes against debugging rules to make several changes at once, but I've only got one working  opamp left.
It seems to work fine at the minute, with steady voltage on pins 4 and 8 and no smoke on the horizon.

You're right, the wire attached to a resistor coming off the +15v line is for a led, but it's not connected, and the larger one is from the PSU.

I'm not sure the changes I've made should make any difference, but we'll see how it goes...

[thom]

Post-2013 update:

So I've just received a bunch of new NE5532 and checked them against the working one in the circuit.
Except there was no 5532 in it, but an LM358 I must have put by mistake when I thought it was my last working 5532.

So I swap it with on of the new 5532s, all works fine until... power off, on again, and this time a loud bottom-E bass sound, a distinctive smell, and nothing.

I'm not knowledgeable to even begin to understand why one chip works while the other fries in that particular setup, but it's a shame the former is the 358, because it does not sound as good as the 5532

[dwmorrin]

There were some great suggestions in this previous post quoted below.  Did you exhaust all of these possibilities yet?

Have you checked the warnings on the datasheet:
http://www.ti.com/lit/ds/symlink/ne5532.pdf
Page 2-bottom notes.
A couple highlights from here:
Max Vcc+/- 22V
Max Input voltage (on IN1/2+/-) Vcc+/- Note 3 - never exceed Vcc+/- (12V in your case)
Max Input Current +/-10mA - Note 4 - Excessive current will occur if IN1-IN2>0.6V, unless limiting resistance is used.
From these items I think there's a good chance that damage could occur due to the inputs to this chip - and less likely that it's due to power.

Here are some thoughts:
1. Do you have an input capacitor to this circuit?
2. Maybe try a resistor on U1A-pin3.
3. Try disconnecting the inputs to this chip - or putting a 10k resistor across them and then see if you can fry it with by turning on/off power.
3b. Try disconnecting the reverb tank and try frying it with on/off.
4. Could the reverb tank be putting out some large outputs at power on.
5. On your bad chips - is it possible that once one of the Amps is blown - that would narrow down source of the problem
6. Try grounding the inputs (i.e no sound going into circuit) and try blowing a chip with on/off.


Could it be possible that the chip is getting an overvoltage on the power inputs (Vcc+/-)?
Some likely useless suggestions to protect the power input to the chip:
1. zeniers across the Vcc inputs to the IC, this would guarantee they don't get overvoltaged.
I've noticed with regulators that without any load the voltage up on their output can be sort of high - because without any current they can't stabilize their output.
The spec sheet indicates a minimum 5mA to regulate.
2. 2k resistor to ground on the output of the regulators so they can be regulating before you connect your circuit.

[R.G.]

If one opamp works and one doesn't the differences must be in the details of the opamps' abilities to withstand whatever abuses the circuit is doing to them.

Under this heading will fall all kinds of issues with decoupling, wiring, ground currents, input common mode ranges, and so on.

The loud bottom-E sound is a fairly good description of motorboating, a low frequency oscillation. This is usually a problem with power wiring, power decoupling, or mis-connection of the circuit itself.

Before I wasted another dozen 5532s trying to see if the next one acted just like the previous one by burning up, I'd check the circuit enough to be **SURE** I knew how it was wired, and rebuild it if needed. Unless you're getting truly bad fake 5532s, the problem is in the circuit or wiring

... hit send too quickly ...
Frying opamps is one of those situations where an oscilloscope can be really, truly needed. Modern opamps are pretty proof against output shorts, but high frequency oscillation can still take them out.

[thom]

I'm going to try a new build as I can't see anything obviously wrong with the current one, assuming the layout is correct.

I'll try some of the suggestions along the way, thanks.
Where should I put the input cap, before VR1 or before R4 (or right after the input) ?

[thom]

Final update for everyone who tried to help me 

I ended up drawing a better organized layout and it now works flawlessly.
It sounds great, with loads of "boing" when needed. I've added decoupling caps until I ran out of space and a footswitch.
The led is on the bipolar PSU board.
Thanks again to all of you, and here are a few pictures of the (mostly) completed project (not sure about the finish, I kinda like it as is):