Greetings all! So I have this awesome Nak PA-7. One-owner, essentially, so I know its history - that is to say, none. It’s never had any issues, or ever been to any amp doctor for a real problem (the closest being when it blew its fuse for what may have been the first time, for no apparent reason).
It sure seems to sound beautiful, though I haven’t had many amps come through here so I can’t compare it. But I hear no issues through the speakers, whatsoever, ever.
BUT! The poor thing has started making this...cracking sound. Not through the speakers, mind you - it comes from the unit itself. At no regular interval or pattern, and every once in a while the sounds come in pairs (or, occasionally, in triplicate). Only when it’s on, of course, though sometimes the noises will happen a time or two after the power's been shut off. (And that's at the power strip where it gets cut off, not via the unit's own power button.)
It sounds scary AF; a violent, sharp crack. Like a big, powerful spark just shot off inside there. There’s nothing visual, nor does anything come through the speakers when it happens, at all.
In response I took my PA-7 in to my local shop, about six months ago now, and they were quite puzzled at my description of the sound. In the hour or so I was there we couldn’t get it to do it, of course, and the tech had no good idea(s) as to what it might be.
But, he got to work anyway adjusting the bias pots (they had drifted a bit over time) and bench-testing the rest of the amp. DC offset was within spec, though he set it back to zero anyway. Everything else checked out, including the power caps, so I just took the amp home with orders to try and record the suspect noise if possible.
After this service the noises stopped for a time. Temperatures on both sides' heatsinks were comparable to one another and not very hot - to this day I can stick my fingers down in-between the fins and leave them there, tolerably. But lately the noises have been been starting back up again, with somewhat increasing frequency. (Non-scientific sample collection there though.)
So I was thinking about what to do, but in the meantime swapped out speakers. Out went my pair of RBH 641-SEs and back in went my Boston Lynnfield 500Ls, fresh off a nice refinishing job.
I didn't expect this to make any difference, but over the last couple of weeks the PA-7's cracking/sparking sounds seem to be much reduced. It's a totally non-scientific sample, but the amp itself went from several cracking noises per evening to zero, maybe one, since the Bostons came back (and it still makes plenty of the good kind of noise, i.e. the speakers sound excellent).
The RBHs are rated at 4 ohms, while the 500Ls are rated at 5 ohms. I know resistance changes across the frequency band, and I haven't graphed out or seen a resistance graph for either of these speakers. But the amp's unsettling noises clearly changed in response to the swapping. Slightly higher resistance on the 500Ls might allow the PA-7 to run a little cooler, I suppose, but it never got "hot" under the RBHs and it's in a setup with way, way more than adequate airflow. Plus, we're talking about a change of a single ohm, rated.
What is going on? Has anyone ever heard anything like this before? Is it somehow...normal? Or should I freak out?!
If there is a problem with my amp, any ideas what it could be? Or what I should look for if I tear it out and try to bench-test it?
Help! (And thanks in advance!!)
It sure seems to sound beautiful, though I haven’t had many amps come through here so I can’t compare it. But I hear no issues through the speakers, whatsoever, ever.
BUT! The poor thing has started making this...cracking sound. Not through the speakers, mind you - it comes from the unit itself. At no regular interval or pattern, and every once in a while the sounds come in pairs (or, occasionally, in triplicate). Only when it’s on, of course, though sometimes the noises will happen a time or two after the power's been shut off. (And that's at the power strip where it gets cut off, not via the unit's own power button.)
It sounds scary AF; a violent, sharp crack. Like a big, powerful spark just shot off inside there. There’s nothing visual, nor does anything come through the speakers when it happens, at all.
In response I took my PA-7 in to my local shop, about six months ago now, and they were quite puzzled at my description of the sound. In the hour or so I was there we couldn’t get it to do it, of course, and the tech had no good idea(s) as to what it might be.
But, he got to work anyway adjusting the bias pots (they had drifted a bit over time) and bench-testing the rest of the amp. DC offset was within spec, though he set it back to zero anyway. Everything else checked out, including the power caps, so I just took the amp home with orders to try and record the suspect noise if possible.
After this service the noises stopped for a time. Temperatures on both sides' heatsinks were comparable to one another and not very hot - to this day I can stick my fingers down in-between the fins and leave them there, tolerably. But lately the noises have been been starting back up again, with somewhat increasing frequency. (Non-scientific sample collection there though.)
So I was thinking about what to do, but in the meantime swapped out speakers. Out went my pair of RBH 641-SEs and back in went my Boston Lynnfield 500Ls, fresh off a nice refinishing job.
I didn't expect this to make any difference, but over the last couple of weeks the PA-7's cracking/sparking sounds seem to be much reduced. It's a totally non-scientific sample, but the amp itself went from several cracking noises per evening to zero, maybe one, since the Bostons came back (and it still makes plenty of the good kind of noise, i.e. the speakers sound excellent).
The RBHs are rated at 4 ohms, while the 500Ls are rated at 5 ohms. I know resistance changes across the frequency band, and I haven't graphed out or seen a resistance graph for either of these speakers. But the amp's unsettling noises clearly changed in response to the swapping. Slightly higher resistance on the 500Ls might allow the PA-7 to run a little cooler, I suppose, but it never got "hot" under the RBHs and it's in a setup with way, way more than adequate airflow. Plus, we're talking about a change of a single ohm, rated.
What is going on? Has anyone ever heard anything like this before? Is it somehow...normal? Or should I freak out?!
If there is a problem with my amp, any ideas what it could be? Or what I should look for if I tear it out and try to bench-test it?
Help! (And thanks in advance!!)
Thermal expansion causing mechanical cracking - not all the bolts fully tight?
Or actual sparking? Not good, if this is happening it could fail at any time - what are the rail voltages on this beast?
Worth checking for floating metal debris, try inverting the box with the lid off and see if anything loose?
Or actual sparking? Not good, if this is happening it could fail at any time - what are the rail voltages on this beast?
Worth checking for floating metal debris, try inverting the box with the lid off and see if anything loose?
Hmmm...all good thoughts. We checked for loose detritus when it was in for servicing the first time, and there was something rattling around in there. Turned out to just be the cut-off end of a nylon zip tie. After that was extracted we heard nothing else.
I’m rooting for the thermal-expansion theory. All connections were checked during the servicing and nothing was loose, but we tightened them all anyway. I suppose one or two bolts are slowly working loose again.
I don’t hear any kind of sizzling bacon sounds, or the like. Nothing that would indicate a set of sketchy contacts. This is always a single, sharp crack or pop. Nothing that sounds like a rattling connection that’s no longer tight, for example. I’d love for it to be something like that, but it’s never been a crackling sound - is always isolated, singular, and sharp.
I’m rooting for the thermal-expansion theory. All connections were checked during the servicing and nothing was loose, but we tightened them all anyway. I suppose one or two bolts are slowly working loose again.
I don’t hear any kind of sizzling bacon sounds, or the like. Nothing that would indicate a set of sketchy contacts. This is always a single, sharp crack or pop. Nothing that sounds like a rattling connection that’s no longer tight, for example. I’d love for it to be something like that, but it’s never been a crackling sound - is always isolated, singular, and sharp.
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