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, sparking 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 though nowhere near as often as before. (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 since the Bostons came back 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 returned (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 seem to have 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, sparking 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 though nowhere near as often as before. (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 since the Bostons came back 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 returned (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 seem to have 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!!)
Crackling sounds from thermal expansion of the case maybe ? (and contraction again when cooling after power-off) ... similar to the ticking noises you hear from a car exhaust after the engine has been shut off after running for a good time ?
My latest amp (SissySIT in ModuShop 5U/400 enclosure) does this once or twice every evening. It is a quite sharp and loud sound.
Hope you find the cause,
best regards,
Claas
My latest amp (SissySIT in ModuShop 5U/400 enclosure) does this once or twice every evening. It is a quite sharp and loud sound.
Hope you find the cause,
best regards,
Claas
I'm taking care that all bolts and nuts are properly tightened , also using split washers wherever I can squeeze them in
especially at transistors and diode bridges ..... practically every mounting ensemble where thermal cycles are present
especially at transistors and diode bridges ..... practically every mounting ensemble where thermal cycles are present
Yeah, 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 may slowly working loose again.
Seems odd to me thermal expansion would cause this much ruckus, yet it would go unnoticed/un-discussed so far, in relation to either the PA-7 specifically or other amplifiers more generally. But I also can't seem to find any reference to any other type of issue that might cause this kind of behavior. Especially where the output through the speakers isn't changing, seems to me it likely isn't a mechanical/electrical problem of some sort.
Anyone have any other ideas?
Seems odd to me thermal expansion would cause this much ruckus, yet it would go unnoticed/un-discussed so far, in relation to either the PA-7 specifically or other amplifiers more generally. But I also can't seem to find any reference to any other type of issue that might cause this kind of behavior. Especially where the output through the speakers isn't changing, seems to me it likely isn't a mechanical/electrical problem of some sort.
Anyone have any other ideas?
I once owned a Mitsubishi DA-10 amplifier that would make the strangest and loudest "ting!" noises, especially after a few minutes after it had been powered off. Very strange, but it continued to work properly. Like others said, thermal expansion/contraction is the most likely culprit.
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