Short in K4004B

A problem has suddenly and unexpectedly developed with my audio amp (K4004B) after about 3 months of use without any problems.

As soon as powered-up, the fuse blows on the (+) (plus) side input. The fuse on the (-) (minus) side is not affected. This is not a problem with a fuse itself; I’ve replaced it 4 times as I tried to track down the source of the problem. This is not a problem with the mains; the power supply board (K4006) is not affected, other than sometimes its fuse on the (+) output side also blows. The short is in the amp: when the amp is disconnected from the power supply board, the (+) fuse on the power supply board doesn’t blow.

Looking at the amp board and the heat sink, all connections are intact, nothing is touching ground and there are no obviously burned components.

I would appreciate any suggestions as to how to get started in diagnosing and fixing this problem.

If you cannot see any obvious reasons why the fuse blows, we must suspect a defective amp module. This amp is very susceptible to overvoltage and to an interruption in one of the supply lines. We’d recommend to replace both amp modules with identical types.

Thanks for your prompt reply and advice.

I was/have been careful in construction and use of the amp. The power supply for this amp is the recommended K4006.

Can you advise me as to the following:
a. how can I obtain the ICs? I don’t see their number in the kit instructions.
b. I purchased the kit in June, 2007 from an e-tailer here in the US (electronics123). Is the kit under warranty? Will the ICs be replaced?

Going a bit further after the repair: you indicate the vulnerability of the ICs to power fluctuations as a possible cause.
a. are their alternative ICs without this problem?
b. I note that the K4006 power supply contained 2 large caps in the kit with room for 2 more caps. Would adding additional caps work?

Spare parts: Your Velleman distributor or for US residents call Velleman Inc at (817) 284 7785. Component reference:TDA1514A

Warranty: only if the kit is sent in for inspection. If our engineers decide that the problem is caused by a manufacturing error, the unit is repaired under warranty conditions.

This kit is not sensitive to power fluctuations. Abs. max input voltage is given in the manual. Surpassing this voltage can destroy the IC. Furthermore, slight interruptions of either power rail, V- or V+ will lead to the destruction of the IC.
There are no alternative IC’s. The supplied IC’s work fine if properly used. There is no need to add caps. For all further enquiries, please contact Velleman Inc. support dept. at the number above.

Thanks for the information.

I appreciate the offer to have Velleman engineers examine the item. Please tell me where to send it.

One other curiosity. You write, in part, "This kit is not sensitive to power fluctuations. …Furthermore, slight interruptions of either power rail, V- or V+ will lead to the destruction of the IC. "

Doesn’t the last part of your statement contradict the first?

For returns, contact Velleman as indicated.

There is a difference between a fluctuation and an interruption of the power supply. Fluctuations are minor changes and generally do not cause any harm. Interruptions cause a temporary drop to 0V. This usually destroys symmetrically powered amps such as the K4004B. Poor wiring is often at the cause of this.

Thanks. The folks at Velleman U.S. have been most helpful and the board will be returned to them for evaluation and repair at a minimal cost.

I am concerned about the following regarding the K4004B kit. The widespread (on the Velleman website and many websites selling the kit) general information that accompanies the kit, presumably put together by Velleman, states (in part), “…Universal, robust and compact are the words to describe this amplifier…”

While the word “robust” could be classified as advertising “puffery,” I am concerned that this advertising is misleading. From your thorough tutorial on the matter, there is susceptibility of the amp to transient severe drops in line voltage (assuming that is what you meant in your reply by “…0V…poor wiring”) that results in failure of the amp. That implies this amp should be run on an UPS, not merely to avoid interruptions in operation, but to avoid amp destruction. In my experience with all types of instrumentation, that requirement is unusual and, for a consumer audio amp, perhaps unprecedented.

Please forward my comments to the appropriate folks at Velleman for their consideration.

It is a physical property of ALL symmetrical powered amps (any brand, any make) that there is a big chance that the output section is destroyed if there is an interruption in one of the supply lines. There is no need to run the amp of a UPS. Small interruptions of the AC grid will cause BOTH supply lines to drop a little, which is not a problem. Furthermore, the caps will act as a backup to avoid large drops. However, if due to poor wiring or a bad contact one of the supply lines gets temporarily interrupted, then the amp might fail.

Thanks for the response and explanation.

With respect to power supply interruption, and the role of the big caps on the power supply board, you may recall that earlier in the chain I wrote:
“b. I note that the K4006 power supply contained 2 large caps in the kit with room for 2 more caps. Would adding additional caps work?”

You previously responded that this would not help. In order to prevent “0 Volts on one of the rails,” the transient supply condition that shorts the amp, you may want to reconsider your advice that adding these caps would not be helpful.

Read the above carefully. They are not helpful if one of the supply rail wires has a bad contact or interruption. I hope this clears things up so we can end this discussion.