Audio Technix 400.1 strong input signal triggers protection

Audio Technix 400.1 strong input signal triggers protection with or without a load connected to the speaker terminals.

The amp uses a small class D type 8 board in the output section.

The amp plays fine at low gain (below 60 percent) but when I increase it to 75 percent the amp will go into protection within about 10 seconds. Cycling the power to the remote resets the amp.

I tried the amp again with the gain at 50 percent and tried increasing the signal in and the amp behaves the same way as it does with the gain control. It does this regardless of wether or not the amp is driving a load or not. Cycling power to the remote resets the fault.

I haven’t seen this phenomenon before. I have check some of the components on the output driver board and the ones I’ve checked test good ( SMD transistors and diodes) I will have to pull board to investigate it further. Before I do that I wanted to know if there is some components of the main board I need to rule out first, or signal checks I need to do on the ICs on the driver board.

I checked the op amps in the preamp section with my DMM and I didn’t discover any unusual readings. I also check the diodes in the preamp section and the resistors in the audio chain.

The amp uses P channel and N channel FETs in the output.

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It is similar to the AQ2200, just scaled down significantly. I replaced the transistors you mentioned and the amp still exhibits the same symptoms.

The amp has a master and slave switch and when I run the amp in master mode, the problem persists. If I run the amp in slave mode the problem goes away. So I no longer think the problem is on the driver board. I think the problem is in the preamp section.

Have you seen this issue before? Where would you narrow your search? Do these ceramic caps go bad very often in the preamp section? Not sure how to test if they are good or not.



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Perry Babin

Paid Member
2003-11-20 11:01 am
If the slave mode bypasses part of the preamp section, I'd suspect excessive DC voltage on the output of one of the op-amps.

As a side note, those transistors should be replaced any time an amp like this comes in with the original transistors. They have a high failure rate and if they haven't failed yet, they may fail soon after the amp is returned to the owner.

Ceramic caps rarely fail.
So after much troubleshooting and reading through Perry’s manual I finally found the culprit to why this amp would go into protection with a high input signal or the gain above 50 percent.

It turns out that Q14 was leaky. It is a PNP transistor marked 2D (mmbt3906) is what I went back with. I had to pull the output card to replace it.

Perry, It was your note about the output predrivers on these boards, to check, recheck, and check again and that you said most of the troubles with these amps are driver board related that made me refocus my efforts on the driver board verses the preamp section. That and the fact that I could find a distorted, weak, or clipping signal anywhere in the amp.

Thanks for the help,