Ampeg PF50T popping noise on line out

[Tone Meister]

Local pro has TWO of these amps and both are popping ONLY from the XLR PREAMP OUT and XLR TRANSFORMER BALANCED LINE OUT. Popping cannot be heard through bass speakers, only in the two XLR outputs.

We have eliminated the following:

It is not the XLR cable - tried several new ones
It is not motor noise from AC, coolers, fans,fridges, etc
It is not a doppler effect from any type fan
It is not a cell phone
It is not static electricity generated at the pickguard(s)
It is not the bass (tried several)

I suspect clipping that does not affect the power amp but distorts the line out section. But the input gain control is at 9 o'clock and master is about 11 o'clock. He is aware of his technique and feels he isn't overplaying the notes.

He bought another one this week and it pops in exactly the same way.

Yamaha refused to send me the service manual, so I have a schematic request in for that. What do you guys think is happening here?

[Enzo]

Are all these amps playing into the same thing? In other words have we made sure it is not the mixer or whatever that has the issue? Try sending the XLR to some other system.

If into a mixer, is the phantom power turned on? If so, turn it off and see if that solves it. Yes, I know all the phantom mics wil stop working then, but we are ONLY trying to solve the popping at the moment.

[The Dude]

It could also be a ground loop. Do both the amp and whatever it's feeding share power, or could they be on different legs? Also, it's recommended to clip the ground on the send end of the connecting cable to prevent ground loops.

http://www.rane.com/note110.html

[Tone Meister]

Are all these amps playing into the same thing? In other words have we made sure it is not the mixer or whatever that has the issue? Try sending the XLR to some other system.

If into a mixer, is the phantom power turned on? If so, turn it off and see if that solves it. Yes, I know all the phantom mics wil stop working then, but we are ONLY trying to solve the popping at the moment.

Different systems each show, so yes different mixer. Sometimes he hears it clearly at soundcheck only to have it "disappear" in the mix during the show. We already covered all the XLR caveats, phantom power included. Never hears it through the power amp and speakers, only the two XLR outputs.

[Tone Meister]

It could also be a ground loop. Do both the amp and whatever it's feeding share power, or could they be on different legs? Also, it's recommended to clip the ground on the send end of the connecting cable to prevent ground loops.

http://www.rane.com/note110.html

The systems he's on use professional power distribution. I already tried lifting the amp's XLR ground lift first, then tried using a modded XLR with the shield lifted at the XLR-M which takes the amp's line out to the snake, with and without the amp's ground lift disengaged.

[Tone Meister]

I *suspect* the direct outputs may be to some degree digital and not have quite the headroom the analog power amp has. Certainly sounds like digital clipping to me, and since it seems to occur at random, that might suggest he is playing just hard enough to clip the line outs but not distort the power amp. After all, it IS a 50-watt bass amp, and he is used to playing other Ampegs with 100 and 300 watts, respectively.

In absence of the schematic, I have suggested he try the -15dB input and adjust the gain accordingly. That should help determine if it's indeed digital clipping, or something else.

I'm still open to any and all ideas though.

[Enzo]

Digital?

Is there DSP in this amp? I am trying to imagine them going through A/D just to make a DI.

[g1]

Agree with Enzo, if it's going through digital and distorting there, that would show up on all outputs as they would not have a digital stage that is only for the DI out.
But you may be on to something about the glitch being something overloading in the DI out section.
Doctor contacted you about your schem. request in the other thread.

[itchymusic]

Hi, I know this is a pretty old thread, but did anyone ever come up with a fix or even a reason for this behaviour?
I have the same amp and the same problem. Have been looking at it with my tech without luck so far. I've only had the issue when playing live shows with in ear monitoring. Not a problem through the bass cabinet and not a problem when recording WITHOUT a bass cabinet attached. But the clicks/pops are there on both the preamp- and transformer out on our live shows.
It seems I can most times provoke a pop by playing a loud note in the low register, but in general the pops appear pretty randomly - though only when I'm playing. The sound is not too far off the kind of static sound you can get from touching the strings when there's a ground issue.
I've tried swapping tubes (just not the power tubes) and did the humbuster mod (the one from the talkbass member).

Hoping someone who's been there will chime in.
KR itchymusic