I noticed on some of my Bliesma M74A mids there was an intermittent buzzing sound around Fs (400 - 500 hz) which disappeared when I put pressure on the small WG front flange portion.
Underneath the foam flange sealing gasket on the under side of the driver, there are 4 small allen head screws. I've discovered some these screws have bottomed out in the holes before the full clamping force is imparted on the VC ring asy that is sandwiched between the two metal parts. I did find some excess powerdercoat inside the screw holes, keeping the screws from seating at full depth inside the holes. Chasing the threads with a bottoming tap would likely be the most correct way to fix this, but I don't have an M3/0.5 bottoming tap or thread chaser, so I had to improvise and came up with the following simple fix...
I've inserted a 0.5 mm stainless steel washer under each of the 4 flange screws, which still allows the head of the screw to sit flush with the flange surface.
Important - only deal with one screw at a time !!! You don't want to risk the VC going out of alignment, so be careful. These are precisely aligned components you're dealing with here.
There is a light thread locking compound on these threads, so go slowly and carefully when removing the screws to avoid thread damage. Tighten the screws back to about 4 - 5 inch lbs. (If you need an exact torque figure, this is about right).
You dont need thread locker if you tighten the screws down evenly. Once you mount the driver, it will automatically be compressed together by the baffle screws, so there won't be any chance of the 4 small screws backing out on their own.
The small buzz was completely cured with this fix. It was noticeable on 2 out of my 4 M74As. I'm likely being very picky here, but I didn't want to risk a VC misalignment or shifting with these screws having an uneven clamp load on the VC asy. Again, once you mount the driver, it should no longer be a concern.
Underneath the foam flange sealing gasket on the under side of the driver, there are 4 small allen head screws. I've discovered some these screws have bottomed out in the holes before the full clamping force is imparted on the VC ring asy that is sandwiched between the two metal parts. I did find some excess powerdercoat inside the screw holes, keeping the screws from seating at full depth inside the holes. Chasing the threads with a bottoming tap would likely be the most correct way to fix this, but I don't have an M3/0.5 bottoming tap or thread chaser, so I had to improvise and came up with the following simple fix...
I've inserted a 0.5 mm stainless steel washer under each of the 4 flange screws, which still allows the head of the screw to sit flush with the flange surface.
Important - only deal with one screw at a time !!! You don't want to risk the VC going out of alignment, so be careful. These are precisely aligned components you're dealing with here.
There is a light thread locking compound on these threads, so go slowly and carefully when removing the screws to avoid thread damage. Tighten the screws back to about 4 - 5 inch lbs. (If you need an exact torque figure, this is about right).
You dont need thread locker if you tighten the screws down evenly. Once you mount the driver, it will automatically be compressed together by the baffle screws, so there won't be any chance of the 4 small screws backing out on their own.
The small buzz was completely cured with this fix. It was noticeable on 2 out of my 4 M74As. I'm likely being very picky here, but I didn't want to risk a VC misalignment or shifting with these screws having an uneven clamp load on the VC asy. Again, once you mount the driver, it should no longer be a concern.
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I forgot to mention you'll need a narrow style washer that is smaller in diameter than the opening and screw head. I had some m3 x 5 x 0.5mm thickness washers in my hardware stash, but any reputable hardware store will have what you need.
You could use non-stainless washers, but I prefer to only use non magnetic fasteners around dome mids/tweeters. This is to avoid anything getting pulled into the front dome surface and potentially damaging it. These Bliesma drivers have very strong, heavily saturated magnetic gaps, so any small, stray ferrous item can become a liability.
You could use non-stainless washers, but I prefer to only use non magnetic fasteners around dome mids/tweeters. This is to avoid anything getting pulled into the front dome surface and potentially damaging it. These Bliesma drivers have very strong, heavily saturated magnetic gaps, so any small, stray ferrous item can become a liability.
A final critical observation -
When I got to dealing with the second M74A to incorporate washers behind the flange screws, I noticed that some of the screws were only hand tight, which also sat alot lower in their locations below the flange. After installing a washer, these screws were still a tiny bit lower than flush with the flange.
Some of the screws were already sitting level with the flange surface and didn't have room for a washer under the head. These screws were reasonably tight and exerted enough pressure to secure the front flange. I just reinstalled and torqued these as they were after checking the threads over.
There's clearly a casting or machining variance in the rear housing at each screw hole location, specifically where the bottom of each screw head rests when tightened. The screws that sit lower were all loose and didn't clamp the front flange adequately. Further tightening these.screws.didn't fix the issue, as they just bottomed out when they were further tightened.
So in the end, equalizing all of the screws in height to sit evenly.level with the flange fixed the issue. Screws which sat flush or very close to it were clamping fine. Only the ones sitting lower than the 0.5mm thickness of a washer were loose and not clamping enough.
I hope this all helps someone and explains the issue adequately in case it happens to them. It can be frustrating trying to figure out strange little issues like this. These are wonderful drivers, but I wish it wasn't necessary to worry about stuff like this on something so expensive.
I check every single driver I use before install for any weird, buzzing noises, rattles, etc. This way its caught before use and most of the time still covered under some warrantee.
When I got to dealing with the second M74A to incorporate washers behind the flange screws, I noticed that some of the screws were only hand tight, which also sat alot lower in their locations below the flange. After installing a washer, these screws were still a tiny bit lower than flush with the flange.
Some of the screws were already sitting level with the flange surface and didn't have room for a washer under the head. These screws were reasonably tight and exerted enough pressure to secure the front flange. I just reinstalled and torqued these as they were after checking the threads over.
There's clearly a casting or machining variance in the rear housing at each screw hole location, specifically where the bottom of each screw head rests when tightened. The screws that sit lower were all loose and didn't clamp the front flange adequately. Further tightening these.screws.didn't fix the issue, as they just bottomed out when they were further tightened.
So in the end, equalizing all of the screws in height to sit evenly.level with the flange fixed the issue. Screws which sat flush or very close to it were clamping fine. Only the ones sitting lower than the 0.5mm thickness of a washer were loose and not clamping enough.
I hope this all helps someone and explains the issue adequately in case it happens to them. It can be frustrating trying to figure out strange little issues like this. These are wonderful drivers, but I wish it wasn't necessary to worry about stuff like this on something so expensive.
I check every single driver I use before install for any weird, buzzing noises, rattles, etc. This way its caught before use and most of the time still covered under some warrantee.
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