LM4562 Destroyed on Power Up, Audible Hum from C2 Coupling Capacitor Surge

162 11

#1 21675853 05 May 2016 08:29

Charles Johnson Charles Johnson

Anonymous

Post #1
21675853 05 May 2016 08:29

The attached circuit has two issues.
(1) When the B+ to the plate is applied, it produces an initial audible hum.
The hum indicates to me that a surge through C2 has taken place.
(2) When the circuit powers up, one of two things take place.
(A) The hum disappears and the circuit performs normally.
(B) The LM4562 is destroyed.
The coupling capacitor does not appear to be doing the job.
Is there another solution?
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#2 21675854 05 May 2016 16:29

richard gabric richard gabric

Anonymous

Post #2
21675854 05 May 2016 16:29

A and B are mutually exclusive, you can't say it behaves normally but it blows up!
My guess is you will be putting excess voltage on the opamp non inverting input at turn on until it discharges to ground through R6. Diodes are the most common way of preventing this, their placement and the resistors used to limit the diode current depend on the circuit configuration.
By the way, I don't believe you have the opamp configured correctly either, R7 serves no useful purpose,
Cheers,
Richard
#3 21675855 05 May 2016 17:11

Charles Johnson Charles Johnson

Anonymous

Post #3
21675855 05 May 2016 17:11

Hi Richard,
I've learned that if I don't know what I'm talking about, I listen instead of pretending to know something. You should do the same. You do serve a purpose to this community though. You're an example of someone who should be ignored.
Thanks so much for your contribution...C
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#4 21675856 10 May 2016 21:24

Kevin Angelo Ma Kevin Angelo Ma

Anonymous

Post #4
21675856 10 May 2016 21:24

Hi,

Let's not make complications here. Can we try making this forum as productive and fun for all? If not, please leave this forum.

Thanks
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#5 21675857 10 May 2016 22:00

Dean Franks Dean Franks

Anonymous

Post #5
21675857 10 May 2016 22:00

You didn't indicate how the power is being switched in your schematic. Assuming that it is a switch on B+ and B+ is a low impedance voltage source you could get a very fast edge on power on. The capacitor would couple this fast edge and R6 is too large to damp the pulse (as Richard alluded to).

If this is the case, then a resistor (or series inductor) in series with the battery (to create a RC/LC low pass with C1 (which might need to be increased) will solve the problem. A pair of ESD diodes between the output the blocking cap and the V+/V- rails would also be a good addition as well as a series resistor on the input of the opamp. The series resistor will not affect the gain and will limit the current sinked by the opamp internal ESD diodes when the pulse arrives. You might use a pair of fast zeners (head to head) between the output of the cap and ground in combination with the series resistor instead.
#6 21675858 11 May 2016 05:20

Charles Johnson Charles Johnson

Anonymous

Post #6
21675858 11 May 2016 05:20

I apologize Kevin.
But I stand by the comment about pretending to know something.
When I'm asked something and I don't know, I say so.
I don't try to dazzle them with brilliance, or Buffalo them with bull****.
From now on I'll keep my negative response to myself.
BTW... Dean Franks makes sense...C
#7 21675859 11 May 2016 05:32

Charles Johnson Charles Johnson

Anonymous

Post #7
21675859 11 May 2016 05:32

Hi Dean,
Thanks for the response Dean.
Your assumption about switching
the B+ is correct. Your logic is
good, and I'll give those ideas
a try.
Bob Cordell was kind enough to
suggest a pair of anti-parallel
diodes at the + side of the IC
to ground.
Your thinking is similar to his.
Both of which I'll try.
Thanks So Much...C
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#8 21675860 28 May 2016 05:36

Sambath Kumar Sambath Kumar

Anonymous

Post #8
21675860 28 May 2016 05:36

Hi Charles Johnson,

See my attachment it is self explanatory.
#9 21675861 28 May 2016 08:09

Charles Johnson Charles Johnson

Anonymous

Post #9
21675861 28 May 2016 08:09

Hi Sambath,
Thanks for the response.
I'm not able to download your illustration.
Can you send it to: netminer(_at_)juno.com?
Thanks...C
#10 21675862 31 May 2016 03:22

Sambath Kumar Sambath Kumar

Anonymous

Post #10
21675862 31 May 2016 03:22

Hi CHARLES

using your mouse right click over the image now copy the image ,open word and paste it on that .now go to view and zoom the image to 200 or 150 .
#11 21675863 07 Jun 2016 00:38

Sambath Kumar Sambath Kumar

Anonymous

Post #11
21675863 07 Jun 2016 00:38

Hi CHARLES,

You can down load the image as i told you . what happened pl reply.
#12 21675864 07 Jun 2016 06:20

Charles Johnson Charles Johnson

Anonymous

Post #12
21675864 07 Jun 2016 06:20

Hi Sambath,
I was able to download the schematic.
It makes sense to me.
I haven't tried it yet, but will soon.
I had quite a few more projects ahead
of that one on the bench. I'll send the
results when I have them. Thanks...C
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Topic summary

✨ The discussion addresses a circuit issue where applying B+ voltage to the plate causes an initial audible hum, indicating a surge through the coupling capacitor C2, and sometimes results in destruction of the LM4562 op amp. The hum may disappear during normal operation, but the op amp can be damaged due to excessive voltage at the non-inverting input during power-up before discharge through resistor R6. Suggested solutions include adding diodes to clamp input voltage spikes, placing a resistor or series inductor in series with the power supply to create an RC or LC low-pass filter with the input capacitor, and increasing the input capacitor value. Additional recommendations involve using ESD protection diodes between the coupling capacitor output and the op amp supply rails, and adding a series resistor at the op amp input to limit current through internal ESD diodes. Anti-parallel diode pairs and fast zener diodes were also proposed to protect the op amp from voltage surges. The importance of proper power switching and input protection to prevent damage to the LM4562 was emphasized.

FAQ

TL;DR: Startup damage and hum come from a surge through the coupling cap; the thread reports “one of two things take place” and 2 outcomes total. Add input clamps and slow the B+ edge. [Elektroda, Charles Johnson, post #21675853]

Why it matters: This FAQ shows how to stop LM4562 inputs from seeing destructive startup transients in cap‑coupled tube/solid‑state hybrids.

Quick Facts

Symptom: audible hum at power‑on implies C2 surge into the op‑amp input. [Elektroda, Charles Johnson, post #21675853]
Root cause: fast B+ edge couples through C2; large R6 cannot damp the pulse. [Elektroda, Dean Franks, post #21675857]
Protection: use a pair of ESD diodes from the cap node to the rails, plus an input series resistor. [Elektroda, Dean Franks, post #21675857]
Alternative clamp: anti‑parallel diodes to ground at the + input were also suggested. [Elektroda, Charles Johnson, post #21675859]
Topology note: R7 “serves no useful purpose” in the shown configuration. [Elektroda, richard gabric, post #21675854]

What causes the LM4562 to blow on power‑up here?

A fast B+ turn‑on edge couples through C2 into the non‑inverting input. With R6 large, the pulse is not damped, so the input sees an overvoltage and the op‑amp can fail. Adding a clamp and limiting the pulse current fixes this. [Elektroda, Dean Franks, post #21675857]

Why do I hear an audible hum from C2 at startup?

The hum flags a surge current through the coupling capacitor during B+ application. That transient current injects a pulse into the op‑amp input until the node discharges through R6. Once it settles, the hum disappears. [Elektroda, Charles Johnson, post #21675853]

How do I protect the op‑amp input from the surge?

Clamp the cap’s output node with a pair of ESD diodes to the V+/V− rails and add a small series resistor before the op‑amp input. This limits current and keeps the input within safe limits during the pulse. [Elektroda, Dean Franks, post #21675857]

Where exactly should I place the ESD or clamp diodes?

Connect the ESD diodes from the node after C2 to V+ and to V−. Keep leads short. This shunts the pulse energy away from the input. A quoted tip: “A pair of ESD diodes… would be a good addition.” [Elektroda, Dean Franks, post #21675857]

Do I need a series resistor on the op‑amp input, and will it change gain?

Yes, add a series resistor between C2 and the op‑amp input to limit surge current. It does not change the closed‑loop gain when placed before the non‑inverting input. It just protects during transients. [Elektroda, Dean Franks, post #21675857]

Should I add resistance or inductance in the B+ line to slow the edge?

Yes. Put a resistor or small inductor in series with B+ so C1 forms an RC/LC low‑pass. You can also increase C1. Slowing the edge reduces how much of it C2 couples forward. [Elektroda, Dean Franks, post #21675857]

Is R7 actually doing anything useful in this schematic?

No. As drawn, R7 does not contribute to a valid op‑amp configuration and should be removed or repurposed. Recheck the gain network and biasing. [Elektroda, richard gabric, post #21675854]

What does “B+” mean in this context?

B+ is the high‑voltage supply feeding the tube plate. Switching B+ on creates a fast edge that can capacitively couple into the op‑amp front end through C2. [Elektroda, Dean Franks, post #21675857]

What is a coupling capacitor, and why can it surge?

A coupling capacitor (C2) passes AC while blocking DC between stages. When B+ steps quickly, the capacitor momentarily passes a large transient until the downstream node discharges via its resistive path. [Elektroda, Charles Johnson, post #21675853]

Quick 3‑step fix to stop startup destruction?

Add a pair of ESD diodes from the C2 output node to V+ and V−.
Insert a small series resistor before the op‑amp input.
Add series R/L in B+ and consider increasing C1 to slow the edge. [Elektroda, Dean Franks, post #21675857]

Can I use back‑to‑back zeners instead of ESD diodes?

Yes. Use a pair of fast zeners head‑to‑head from the C2 output node to ground, plus the series input resistor. This clamps the transient similarly to rail‑clamps. [Elektroda, Dean Franks, post #21675857]

Why did the circuit sometimes work and sometimes kill the LM4562?

The startup pulse amplitude varies with conditions, so outcomes alternate. The thread reported two outcomes: normal operation or failure after the hum. Clamping removes that variability. [Elektroda, Charles Johnson, post #21675853]

Do I need to change C1 or C2 values to help?

Increasing C1 helps form a stronger low‑pass with added series R/L, reducing the edge. Keep C2 as designed for bandwidth, but protect its output with clamps and a series resistor. [Elektroda, Dean Franks, post #21675857]

Is there a simple ground‑referenced clamp option?

Yes. Anti‑parallel small‑signal diodes from the op‑amp + input to ground were suggested. This provides bidirectional clamping at that node. [Elektroda, Charles Johnson, post #21675859]

What edge case should I watch for with slow‑ramp supplies?

If B+ ramps slowly, the transient shrinks, and failures may appear to vanish. However, a later fast restart can still exceed limits. Keep the input clamps. [Elektroda, Dean Franks, post #21675857]

Was forum etiquette addressed in this thread?

Yes. Participants asked to keep discussions constructive, then the original poster apologized and proceeded with technical fixes. Stay focused and share results. [Elektroda, Kevin Angelo Ma, post #21675856]