Pass X150 is producing "Pop" during power-down

Hello all,

I have been experiencing a thump or "pop" from my recently purchased X150 (5yr old) when I power down the amp using the front button. The previous owner didn't experience this. I am using the single-ended RCA input from my receiver for now. Also, I notice all the woofers of the speakers retract (ie get sucked in or move backward) upon the power on and stay retracted until I power it down which gives the thump or pop .

If I physically disconnected the preamp from the amp, I get the "pop" not only during power-down but also during power-on.

Interestingly, the turn off thump and woofer retraction phenomenon disappear totally if I remove the XLR pin1 to pin3 jumper to pretend having balanced input without physically having XLR cable plugged it. (For those who doesn't own this amp, there is a jumper connecting pin1 to pin3 of the XLR input if you are using single-ended input).

Pls excuse my long description. My questions are 1) Are these normal things to expect if I am using single-ended input? I am worried since the bass response seems compromised.

Thanks in advance for your kind input.

I have a Pass Aleph 3 (single ended) paired with an Aleph P and there is a very slight, single thump when powering down and a even less audible thump on power up. Neither is a problem at all.
Thanks for sharing your experience, Dill. Indeed the thump is not slight although I didn't use a sound level meter to measure it. But the most concerning thing is the persistent retraction of the woofer which, I think, causes the bass response degradation.

I am guessing that you don't suffer the woofer retraction problem right?


I think you have some DC coming out of your preamp. You might try measuring the preamp output with voltmeter. It's not good for the amp or the speakers.
few questions:
1)Is the amplifier running off some sort of a power strip or power conditioner? If so, run the amp directly from the wall outlet. No power conditioners/strips.
2)can you run balanced? Is your rpeamp balanced?
3)double-check all conncetions, speaker cables and everything.
4)is the power cord on the amp original? Ground pin intact?
5)how efficient are your speakers?
I also have a slight "pop" when power up and when power down my x150. You can also ask: