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Old 12th August 2017, 12:03 AM   #41
lupi is offline lupi  Italy
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Hi lowmass
Thank you for replay
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Old 12th August 2017, 01:00 AM   #42
lupi is offline lupi  Italy
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Quote:
Originally Posted by bolserst View Post
@lupi,
What type of amplifier are you using when testing?
The rolled off high end suggests you are using direct drive without any series resistance. Have you measured the distortion contribution of the amplifier when driving this very low impedance load at each of the output levels?

Also, can you check the boxes below the plot to show the distortion curves for 2nd and 3rd harmonic to see what distortion type is dominant?

Another thought concerns using the swept sine stimulus for distortion measurement. You need to have the signal level near the top of the available range so that that there is enough "bits" available to calculate the distortion properly. You might consider trying to verify the distortion trend with volume level using the spectrum analyzer for a few discrete frequencies.

The amplifier test was the first thing I did. On a resistive load of 0.05 ohm (the ribbon has 0.075 ohms) and at the highest level I used on the ribbon, the THD is always below 50 dB. Lower levels even less. You're right: I do not use resistance in series

Those graphics by mistake I did not save them. I send you one similar graph at the 6.7W level. Distortion is almost the second harmonic.
ribb3-18_a.jpg
At the oscilloscope it seems (it's hard to see THD <4%) that the positive halfwave, and always that at all frequencies with high THD, is slightly crushed on the top. Not flat at all, but less sharp then it should. At 0.43W prevails second or third harmonic according to the frequency.

Yes, levels are close to the top, especially for strong signal measurements. I have a single frequency cancellation type distortion meter, and at the beginning I did comparisons with REW. With the bar levels just on the orange they get the best dynamic. In this particular case I did not consider it necessary to make a verification, as the THD is relatively high
lupi
Attached Images
File Type: jpg ribb3-18_a.jpg (117.4 KB, 252 views)
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Old 22nd August 2017, 03:37 PM   #43
bolserst is offline bolserst  United States
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Thanks for the new measurement, and confirmation that you are performing distortion measurements with REW using optimum dynamic range.

I noticed that the SPL for this new measurement is about 2.5dB higher than the 6.7W measurement in post#36. The distortion is also much higher...about +10dB in the midrange. Is it possible that your microphone was closer to the ribbon for this new measurement than for the older one? If so, I wonder if it is your microphone that is causing the unexpected increase in distortion with signal level. It is not uncommon for electret capsules to generate significant 2nd harmonic content with increased signal level depending on the internal JFET buffer configuration. I noticed this when performing near field woofer distortion tests with some microphones. You mentioned replacing the capsule in you microphone, so perhaps it is related.

Quote from Linkwitz website: System Test
"...When the cartridge is connected as described by Panasonic it produces fairly high distortion at moderate SPL's and is marginally suitable for serious recording and measurement purposes. The microphone itself is extremely linear, but the built in FET amplifier stage is not configured optimally..."

You might experiment measuring your ribbon (or another driver) at a fixed higher power level and moving the microphone closer and further away to see how the distortion trends with SPL at the microphone. If the microphone is not introducing level dependent distortion, the distortion levels should stay the same for all the distances.
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Old 25th August 2017, 12:49 AM   #44
lupi is offline lupi  Italy
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Hi bolserst
It is certainly possible that I have put the microphone at different distances. Always between 3 and 5 cm, but not always the same. I usually write the distance, but as I wrote those curves I did not save them.
A suspicion on the microphone came to me, and I had tried to move it away, but the THD at 6.7W did not diminish. From the last post I made 2 things at one time: I ordered a calibrated microphone, UMIK1, and I built another sandwich ribbon: Alu-depron-Alu, which I've already mounted on the engine. The UMIK1 has already arrived but I did just a few tests on the new ribbon, because of the work. From these tests I have the feeling that the blame of the high THD is to be attributed in part to the microphone and partly to the previous tape. As soon as I finish the job and clean the tables I will do other tests, including reassembling the old tape and making comparisons.
lupi
I read linkwitz, which refers to the WM61A capsule. I also ordered some of those capsules, but the delivery is scheduled in a few weeks
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Old 30th August 2017, 11:33 PM   #45
lupi is offline lupi  Italy
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To come up with the increase in distortion with the signal level I first bought a new microphone, UMIK1, and performed calibration. Specifics say 1% THD at 133 dB SPL and 0dB gain setting. I've been looking for a "gain setting," but once I load the calibration file, the gain of the sound card has no effect, so I assume I'm in the specifications and I can overlook the microphone distortion.
I made measurements on 3 ribbons, which I called risp. Ribb3, ribb4 and ribb 5.

3 ribbons.JPG

All 3 have the same suspensions, both lateral and at the ends, and are divided into 4 strips electrically connected in series
--- Ribb3 is the one of the last measure I have posted, and it is: Alu thick 80 um + depron thick 3 mm transversally segmented, 0.075 ohm, left in the photo.
--- Ribb4: 30 μm Alu, 1.5 mm depron, 30 μm Alu. The upper Alu strips are welded to the corresponding Alu strips bottom, 0.098 ohms total. At the center of the photo.
--- Ribb5: Alu thick 80 um + depron thick 3 mm smooth, 0.072 ohm, right in the picture.
I did some photos while building ribb4:
making Alu strips x2.JPG
shaping lat susp.JPG
and ribb5:
attaching lateral paper.JPG
making silicone sheet.JPG
glueing silicone sheet.JPG

The mic-ribbon distance is for all curves 7 cm, the signal level is 0.18Veff, so the power on the ribbons is not the same but varies depending on the resistance. Electronic compensation is always present.
ribb3_umik-30.jpg
ribb4_umik-30.jpg
ribb5_umik-30.jpg

Attached Images
File Type: jpg making Alu strips x2.JPG (296.8 KB, 155 views)
File Type: jpg shaping lat susp.JPG (302.6 KB, 149 views)
File Type: jpg attaching lateral paper.JPG (251.9 KB, 142 views)
File Type: jpg making silicone sheet.JPG (285.4 KB, 143 views)
File Type: jpg glueing silicone sheet.JPG (209.6 KB, 48 views)
File Type: jpg ribb3_umik-30.jpg (109.3 KB, 53 views)
File Type: jpg ribb4_umik-30.jpg (108.9 KB, 30 views)
File Type: jpg ribb5_umik-30.jpg (108.7 KB, 29 views)
File Type: jpg 3 ribbons.JPG (276.0 KB, 62 views)
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Old 30th August 2017, 11:39 PM   #46
lupi is offline lupi  Italy
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for the 3 graphics below the signal was 0.72Veff
ribb3_umik-18.jpg
ribb4_umik-18.jpg
ribb5_umik-18.jpg
The following graphs compare the SPL and THD of the 3 ribbons in the case of signal = 0.72Veff. Green = ribb3, red = ribb4, blue = ribb5
SPL-18 compare.jpg
THD -18 compare.jpg
Ribb4 seems to me the best, especially under 70 Hz, so for the final version I will work for the type of sandwitch construction, though it is substantially more laborious.
Attached Images
File Type: jpg ribb3_umik-18.jpg (109.8 KB, 50 views)
File Type: jpg ribb4_umik-18.jpg (109.2 KB, 13 views)
File Type: jpg ribb5_umik-18.jpg (109.2 KB, 16 views)
File Type: jpg SPL-18 compare.jpg (102.4 KB, 19 views)
File Type: jpg THD -18 compare.jpg (106.5 KB, 18 views)
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Old 22nd September 2017, 01:34 AM   #47
lupi is offline lupi  Italy
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To conclude the speech of the increase in distortion by increasing the signal level:
I spent a lot of time comparing the graph made with 2 different microphones, but I did not find significant differences in the THD, so I have to assume it's not a microphone artifact but it's a feature of the ribbons. The reason for this behavior has not been highlighted by the numerous tests I have done on various ribbons, and at this point I am temporarily off topic and continue with the beginning of the construction of the final, 112 cm long and 30 mm wide ribbons. It may be that drastically changing both motor and ribbon the above reason comes out.
I decided to reduce the width from 40 to 30 mm for less directionality, and I compensated for the surface reduction with the increase in length, which I initially placed at 92 cm.
First, I have refined the magnetic field simulations by designing polar expansions as close as possible to reality, that is, with the curves resulting from sheet bending, and adjusting angles, taper and wings length for better uniformity of field . However, I had to choose a compromise, since to improve uniformity over a certain point we have to accept a reduction in the intensity of the field. The picture below is what I decided to accomplish

ribd46real1.pdf

The following 3 charts are for Bx along the 8mm below, center, and 8mm above lines. The maximum Bx excursion across the whole of area is about 8.5%, or, if you prefer, +/- 4.25%.
Bx30mm-8mm.jpg
Bx30mm_centro.jpg
Bx30mm+8mm.jpg
The uniformity of Bx is greatly influenced by even small changes in the geometry of the wings of the polar pieces, and knowing the precision of the mechanics I realized I should still be within 10% of the total. Going to the realization I ordered to a workshop the U with the angular wings. Initially I made asymmetric angles of 58 and 65 , based on previous simulations.

U from off.JPG

Later on, based on the latest simulations, I returned to the workshop and corrected the angles at 75, symmetrical. I have tapered the U wings not exactly as by design but virtually, leaving the constant thickness and cutting V-shaped slits so as to minimize also the shadow effect. It's not the same thing but the differences should be small (I hope). I did not control it with a 3d simulation because it would take too, too long
zoomU.JPG
V-cuttings I made them by means of a mold I built for this purpose, mounted on a small manual pressdie
stampo.JPG

In the end I have the 4 finished polar expansions, painting aside

U finite.JPG
Attached Images
File Type: jpg Bx30mm-8mm.jpg (59.4 KB, 23 views)
File Type: jpg Bx30mm_centro.jpg (53.3 KB, 21 views)
File Type: jpg Bx30mm+8mm.jpg (57.5 KB, 19 views)
File Type: jpg U from off.JPG (284.0 KB, 36 views)
File Type: jpg zoomU.JPG (376.1 KB, 42 views)
File Type: jpg stampo.JPG (353.5 KB, 54 views)
File Type: jpg U finite.JPG (314.5 KB, 50 views)
Attached Files
File Type: pdf ribd46real1.pdf (146.5 KB, 12 views)
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Old 22nd September 2017, 12:23 PM   #48
lowmass is offline lowmass  United States
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Interesting stuff lupi

Things that I would try...

Try a ribbon with no suspension at the sides. I suspect some if not much of your distortion issues are related to the suspension. Yes if your going to take it low in frequency you will have to get the gap between ribbon and magnet down to around 2mm or less.

The thicknesses of the materials you are using are quite thicker than a typical ribbon. Most good plastic backed ribbons have backing around 10 microns, adhesive around 10 microns, foil around 15 microns. These are approximate values that depending on construction could be 1/2 of this BUT not much more than this or the sound not as good.



People talk on and on about getting a uniform field across gap. I have found that while its a good goal it does not make a huge difference in end result with plastic backed ribbons. Better to get the ribbon itself right. In a practical wide magnet gap arraignment this usually means build a ribbon that is light, well damped, and quite flexible across its width ( not a stiff large pleat design). If you look at Apogee mid/treb ribbons they are All very small pleat designs that are made to flex easily across their width. They are quite well damped by virtue of the combination of Kapton backing and visco elastic adhesive and foil lamination. This flexibility and good damp works well with a non perfect uniform feild strength. Ribbon can flex without causing too much issue in sound. The resonances are mostly benign this way. The wide ribbon, larger pleat designs that are usually "FO" ( foil only) will usually be used below about 3 khz as the resonances of that stiff undamped ribbon can be ugly above that. You might think that a super uniform field strength would help a lot with this BUT it only helps a little.
I have only made one stiff across the width ribbon design that is well damped enough to get the best of both worlds BUT it is quite a bit more difficult to build.

I too have experimented with the suspension thing in the past. At one point to reduce the suspension issues I was thermo forming polyester film of about 5 microns to make the half roll suspension to reduce mass. As well I cut a number of slits in the half roll all along its length to suppress the resonances along its length(that suspension is very stiff along its length without the slits to free up the ribbons action). This helped BUT regardless of what the distortion and freq response says , my ears told me that the suspension is just dead weight on a very low mass free swinging transducer. In other words the simple ribbon done well and made large enough to go as low as I wanted just always sounded better than one with a suspension.

Last edited by lowmass; 22nd September 2017 at 12:49 PM.
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Old 22nd September 2017, 02:39 PM   #49
WrineX is offline WrineX  Netherlands
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check your umik to, as in the previous post linked by bolsert to my thread it was only 10db off soo.....in the higher domains, compared to my new measurement mic
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Old 23rd September 2017, 12:44 AM   #50
lupi is offline lupi  Italy
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Quote:
Originally Posted by lowmass View Post
Interesting stuff lupi

Things that I would try...

Try a ribbon with no suspension at the sides. I suspect some if not much of your distortion issues are related to the suspension. Yes if your going to take it low in frequency you will have to get the gap between ribbon and magnet down to around 2mm or less.

Hi Lowmass
I also tried ribbons without lateral suspension, more for testing than otherwise, but the trend of the THD with signal level did not change appreciably. Using the side suspension was one of my project's assumptions, since, in my opinion, it is the only way to reach woofer's frequencies. In fact, I have not seen around ribbons with side gap falling below 100 hz, or that to put the ribbon in a closed box, everything must be airtight, otherwise you have a vented box.
All this without touching the argument that according to many peoples lateral gaps are sources of distortion due to the Venturi effect, and that to THD fall below some% the gap width should be no more than a few tenths of millimeter, what is hardly obtainable on 1m in length

The thicknesses of the materials you are using are quite thicker than a typical ribbon. Most good plastic backed ribbons have backing around 10 microns, adhesive around 10 microns, foil around 15 microns. These are approximate values that depending on construction could be 1/2 of this BUT not much more than this or the sound not as good.

I use ribbons with Alu very thick to increase the mass without greatly reducing the efficiency. The mass is necessary to extend the low-frequency response in closed box. I am aware that the Alu 80 μm is hard to work and it is not elastic, but with small thicknesses and dead weight added the yield would be too low


I too have experimented with the suspension thing in the past. At one point to reduce the suspension issues I was thermo forming polyester film of about 5 microns to make the half roll suspension to reduce mass. As well I cut a number of slits in the half roll all along its length to suppress the resonances along its length(that suspension is very stiff along its length without the slits to free up the ribbons action). This helped BUT regardless of what the distortion and freq response says , my ears told me that the suspension is just dead weight on a very low mass free swinging transducer. In other words the simple ribbon done well and made large enough to go as low as I wanted just always sounded better than one with a suspension.
The polyester film you used is too rigid, even though it is thin 5 μm, and longitudinal cuts break the air tight and therefore make the side suspension useless, apart the other advantage of the side suspension that is it greatly reduces rattle problems, to which are subject long ribbons
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