X150 produces a "Pop" during power down

[kmtkwong]

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.

Kelvin

[Nelson Pass]

Since your email arrived yesterday and you have not received
a reply as yet, I imagine you won't mind if I respond publically
so that others can benefit.

Low transient noise on X amplifiers depends on two conditions,

1) the amplifier has been properly adjusted for both differential
and absolute DC offset values

2) that the inputs of the amplifier have comparable source
impedances at DC.

With single-ended operation , you use a shorting pin between
XLR pins 1 and 3 to short the negative input to ground. In
addition, there are versions of the X250 in which the inputs have
been furnished with input coupling capacitors with bypass jumpers.

In the single-ended circumstance where the input capacitors
have been bypassed and the preamp has an output coupling cap
with a high value resistance to ground (or no resistor to ground),
any DC offset tends to be amplified.

So first we want to check if the coupling capacitors on your amp
have been bypassed. Then we want to know what the
resistance to ground of the preamp is when it's operating but not
connected to the amplifier.

If the input coupling caps are bypassed, then remove the bypass
plugs. If there is high resistance to ground at the output of the
preamp, then consider using a 22Kohm or so resistor to ground
at its output and see what happens.

It is also possible that your amplifier channels need adjustment
for offset, but we don't want to undertake that task until more
information is known.

[kmtkwong]

Thanks for your kind response, Nelson!

I questions about the 2 suggestions you made:

"If the input coupling caps are bypassed, then remove the bypass
plugs......"
> so how can I find out if the input coupling caps are bypassed and what is the procedure to remove the plug for X150?

"If there is high resistance to ground at the output of the
preamp, then consider using a 22Kohm or so resistor to ground
at its output and see what happens."
> I am using a denon receiver as a preamp for now. From the manual, I couldn't find any info about the resistance to ground in the manual. I will contact Denon about it.

One thing I want to bring your attention: When I physically disconnect my receiver from the X150, there is still "Pop" noise during BOTH power-on and power-down, instead of during power-down only when receiver is connected. Does it rule out the role of upstream components (ie the receiver) in producing the "pop" ?

I am not that electronically inclined. Pls excuse me if I ask trival questions. Thanks alot again!


Kelvin

[emilg1955]

The problem turned out to be the DC offset was 8 volts on 1 channel and 5 volts on the other when it was used in single ended mode. I'm not sure how it got to that point as the unit had never been touched. Adjustment cured the problem and improved the sound quality as you might expect.

Note that this unit did not have the input coupling caps and had current production boards in it.

[emilg1955]

The problem turned out to be the DC offset was 8 volts on 1 channel and 5 volts on the other when it was used in single ended mode. I'm not sure how it got to that point as the unit had never been touched. Adjustment cured the problem and improved the sound quality as you might expect.

Note that this unit did not have the input coupling caps and had current production boards in it.

[Nelson Pass]

Sensitive the offset

becomes, happy that you had

vision to fix it.