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Post by precaud on Jan 6, 2021 1:05:59 GMT
Nice forum, I like the test and measurement emphasis in the analysis.
I'm in the process of reviving a Fontek electret headphone setup that I bought new in the 1980's, but have been sitting dormant for some 20 years (see 1st attachment). I have two set of headphones; the original set which has weak output on both sides, and a set bought a few years later that is good with minor issues, most of which I have addressed.
As I've worked on them, I noticed something which puzzles me. All the capsules have a small piece of metal tubing held in place by what looks like brown duct tape (We'll talk about the tube's purpose later). Here is is, removed from one of the bad capsules.
Continued next post to overcome the 3-image limit...
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Post by precaud on Jan 6, 2021 1:14:15 GMT
The difference is, the old capsules have smaller pieces of tape, like this:
While the newer capsules have large pieces of tape that cover fully 1/3 of the rear of the capsule:
I've worked with various tranducers for decades and I've never seen anything like this. Why would the designer be loading the rear of the diaphragm like this? And in such asymmetrical fashion? What is the purpose? Do other electret headphone capsules (i.e. Stax) have similar treatment?
Any input would be appreciated.
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solderdude
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measureutternutter
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Post by solderdude on Jan 6, 2021 17:37:25 GMT
I have never seen this before either. The few electrets I have seen usually have foam or some type of paper to damp the rear.
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Post by precaud on Jan 7, 2021 2:16:30 GMT
That's what I had expected to see. So what do you think is the purpose of doing this? What is the effect or rear-loading the diaphram like this? If it were a dynamic driver, it would surely raise the resonant frequency and restrict the bass output.
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solderdude
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Post by solderdude on Jan 7, 2021 17:31:11 GMT
Maybe the pipe is not important and only the taped-off surface is. Maybe the 3 studs are not rigid enoungh and they taped a rigid piece of pipe to it.
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Post by precaud on Jan 8, 2021 3:52:49 GMT
The pipe + tape thickness applies some pressure on the rear of the capsule assembly when assembled into the rear housing. I removed them from one of the capsules which has low output, comparing the sound before and after. It definitely increases the bass output. I don't understand why, but it does.
But the main question is about the large piece of tape loding the rear of the capsule, and its effects. I only have one good set of headphones, so I'm a bit chicken to experiment with them unless I have some measure of understading what the impact might be. This forum is the only one I found that seems to have any technical savvy with 'phones.
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solderdude
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Post by solderdude on Jan 9, 2021 21:03:09 GMT
What happens to the sound of the softer driver without the metal rod and then only compare with and without the tape. Usually, with dynamic drivers, adding more damping or taping things shut it lowers bass response. That metal pipe with one side open remains a mystery. Maybe it is supposed to act as a tuned resonator being damped a bit by the tape ?
Maybe even cover 2/3 of the driver to exagerate the effect. Not easy to do without a measurement rig.
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Post by precaud on Jan 10, 2021 14:39:15 GMT
I think the metal pipe was just something convenient and cheap that was the correct thickness.
I agree, adding damping or restricting opening usually lowers bass response. But then the pipe raises it. My guess is the tape affects a broader frequency range but that's only a guess. With a dynamic driver, one could the effects in the impedance curve. But not with an electret.
Comparative measurements would be the way to go. I do have a good measurement rig (for speaker systems) but am not set up for headphones. Not sure it's worth the trouble making one just for this, though it's a pain having to completely disassemble them to make even a subtle change.
Unfortunately I just blew some transistors on the amp that drives these, while adjusting the bias. Sigh. So I have to set this aside until I fix it.
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Post by precaud on Feb 16, 2021 15:19:58 GMT
So the amp situation is fixed and I'm back working on these. Have been listening to them for a while and, while I like the relatively clean, low-distortion quality, the tonal balance is definitely "thin", analytical, lacking richness and warmth. My guess is, the tape covering the back of the capsule is responsible for some of that.
I decided to have a closer look at the coupling transformers. The input impedance is below. Except for the null just above 20kHz, these are a very easy load to drive. That notch is most likely the interaction of the capsule + wiring capacitance with the transformers inductance.
Then I measured the transfer function through the stepup transformers. The coupler input has a 22R resistor to ground to load the amp, and a 4R7 in series to drive the xfmrs and set the minumum impedance. The yellow curve shows that result. Maybe not bad for a 50x stepup device, but definitely tipped toward the bright side. And the 20kHz notch is there too.
So I increased the series R to 5R2 to see if it lowered the treble some. Yes, it did, but it is also taking down the low bass too. To be expected from a transformer, I suppose.
Next steps: I may experiment with a RL || R driving the transformer, maintain the lows but trim down the highs. And then see how much capacitance the headphone wiring has. Maybe replace it with Teflon insulated. If that notch can be moved higher in freq, that would definitely help matters.
Attachments:
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Post by precaud on Feb 21, 2021 2:05:46 GMT
There's doesn't appear to be much interest in this, but for completeness I thought I'd wrap it up. More experimments with the series and shunt R's on the input of the transformers showed that they indeed make a large difference in the sound. The series R is particularly impactful, changes of 0.1 Ohms being quite dramatic, way beyond the measured freq response change would suggest. By lowering the series R to 4.5 Ohms and raising the shunt R to 24, I got a more balanced sound, but still with that midrange-strong "quacky" quality. So I decided to put trepidation aside and experiment with the rear diaphragm loading. In the 1st pic, you can see that the front of the diaphragm is itself "loaded" by the front aluminum ring over about 1/3 of its surface area; inside, it's actual diameter is near that of the casing's. The loading is all around the edge.
So perhaps all that tape on the rear (see pic in 2nd post) is to equalize the acoustic loading on both sides. But they went overboard. The tape extends well beyond the triangular plastic piece underneath it. So I carefully trimmed it back to the plastic piece with a razor blade, and listened comparing to the other capsule. The aggressive mids were definitely lessened.
So I went further and opened up some area on either side along the perimeter.
Sorry for the blurry pic, but you get the idea. The result was wonderful - the quacky quality is gone. The bass is a bit rounder/smoother, but that is as much from its upper harmonics being tamed down as anything. So I did the other capsule to match, and voila. The results are really excellent. The overall sound is maybe just a tad warm, but it's a very nice, seemless sound from top to bottom with nothing objectionable sticking out, wonderfully detailed and very engaging. Mission accomplished.
I'm tempted to pick up a pair of Stax electrets and experiment with them. My hunch is, like the Fonteks, their tuning components were chosen using the standard power resistor values available back then, and the optimum values are somewhere in between them.
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