Gaetan,
Thank you for the kind words and for referencing to my paper and! However, the information you're quoting is partially incorrect and certainly not what I personally gave you. The resonant frequency of our ribbons is set to 16.5 Hz, not 12 Hz. The impedance values are in the right ball park, but I did not tell them to you specifically. I assume you took those figures from my paper, although the value at 20 Hz is 0.8 ohms instead of 0.9 ohms. You should also note that the impedance will vary quite a lot depending on the ribbon motor design, i.e. flux density in the gap, resonant frequency, dimensions and material of the ribbon, and so forth.
Best regards,
Julian
Hello Julian
Yes, those two number come from your AES paper, I don't know how come I wrote 12 hz, maby wen I read it I've note another number from ahother part of the paper, the 0.9 ohm are a typo error, I'm not at my best with the english language reading or writing, I'm french speaking.
Btw, I presume that making a transformerless ribbon mic would need too much gain from a pre-preamp and would have much more noise ?
Bye
Gaetan
Gaetan,
No worries! I just wanted to set those few facts straight. Regarding a transformerless ribbon mic: It's certainly something I've thought about and it could be a useful idea for a miniature ribbon microphone (since the transformer is a major size constraining factor), but I'm not a circuit designer and have done much research on this. Let me put it this way: It's not on the top of my priority list right now ;-)
No worries! I just wanted to set those few facts straight. Regarding a transformerless ribbon mic: It's certainly something I've thought about and it could be a useful idea for a miniature ribbon microphone (since the transformer is a major size constraining factor), but I'm not a circuit designer and have done much research on this. Let me put it this way: It's not on the top of my priority list right now ;-)
Julian, let me set you straight on why a transformer of some kind is optimum for this application. For a given source VOLTAGE, and the ribbon element is that, the lowest possible noise input from an active device (that I know of) is related to 1/(2Gm) with a bipolar transistor. At room temperature, this is about 13 ohms with 1 ma flowing through the bipolar transistor, a far cry from the resistance of the ribbon element. To get a 3 dB noise figure, we would have to run 13/.2 or 65 ma through an array of perfect bipolar transistors continuously. Fets, tubes, and even normal transistors would need even more current. However, IF you convert the 0.2 ohms to 13 ohms or more, then only 1 ma would be necessary for a 3 dB noise figure, or 3 dB worse than ideal. This would require a minimum winding ratio of 8 with a perfect transformer. More gain would make things even better, so a higher winding ratio would be optimum.
The problem is real, and there are pretty good transformers that can be used.
Don't feel too bad though. Back in 1967, they showed me the door at Ortofon, when I suggested the same thing to eliminate the phono transformer from their 2 ohm phono cartridge and do it in the same way as you suggested. They assured me that it was impossible. 10 years later, they claimed to invented the breakthrough.
The problem is real, and there are pretty good transformers that can be used.
Don't feel too bad though. Back in 1967, they showed me the door at Ortofon, when I suggested the same thing to eliminate the phono transformer from their 2 ohm phono cartridge and do it in the same way as you suggested. They assured me that it was impossible. 10 years later, they claimed to invented the breakthrough.
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Thanks for the data. From that 90uV/PA with syn08's .32nV preamp you could eek out maybe 66-68dB snr which is at best 26dB SPL self noise. This is at least 6dB worse than with the transformer. Even with the transformer they are not world beating for noise.
Those impedance plots show that you could get real damping effect from a virtual ground input, doing better than .2 Ohms is a challenge though.
Those impedance plots show that you could get real damping effect from a virtual ground input, doing better than .2 Ohms is a challenge though.
Gents,
I have a HUGE question/doubt around transformers in such low level circuits. I simply cannot believe that the magnetic domains will linearly switch state under such low level excitation - if they even switch state at all. They certainly work at the higher levels of excitation, but how many dB down do they still work - or do they really do what I hear - they IGNORE excitation levels lower than some abitatry level?
Even 16/44 PCM has a supposed dynamic range of 96 dB, so we would want to capture as much of that as possible - but 96dB down on the already very low peak level out of a ribbon is microscopically small. My hunch is that a transformer has well given up transfering signal by then.
Facts? Data? Opinions? Numbers?
Regards, Allen
I have a HUGE question/doubt around transformers in such low level circuits. I simply cannot believe that the magnetic domains will linearly switch state under such low level excitation - if they even switch state at all. They certainly work at the higher levels of excitation, but how many dB down do they still work - or do they really do what I hear - they IGNORE excitation levels lower than some abitatry level?
Even 16/44 PCM has a supposed dynamic range of 96 dB, so we would want to capture as much of that as possible - but 96dB down on the already very low peak level out of a ribbon is microscopically small. My hunch is that a transformer has well given up transfering signal by then.
Facts? Data? Opinions? Numbers?
Regards, Allen
Magnetic domain "switching" is not necessary. A guy in Australia wrapped 300,000 turns of wire around some rebar to make a Shumann resonance antenna. There we're talkin' pico-Teslas, and the signal was right there above the noise. The B field inside a high-mu material is not quantized, do you have even one reference to this ? (Barkhausen noise/etc. is another issue). You can make a not very good transformer with a vacuum core, it still works.
Thought experiment, put a resistor in the primary and look at the noise in the secondary. Does the transformer output nothing?
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With enough turns on the primary the magnetic domains will switch with that low a signal for any high quality core, but for practical purposes a MuMetal or Supermalloy core should be used since they have the highest permeability at low induction. Microphone transformers such as the great UTC HA-100X have a very large number of primary turns as well as a Permalloy core.
You would have to do any calculations after having designed the transformer.
John
Yeah, I'm not pretending to know much about this stuff. All my fiddling with cores has been mainly about trying not to saturate them. But the people that make them sure have a real good idea what goes on down there at the bottom.
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Well the one difference with recording heads (as opposed to playback heads) is that they are subject to AC recording bias levels that are usually significant relative to the audio signal driving them.
I assume that magnetic recording tape itself would be subject to the same phenomena Allen was wondering about should it exist. I wonder if such a phenomena exists would it be as I suspect below the effective noise floor of the heads, tape, and electronics?
Ferrite RF transformers in some instances work with fractions of a uV input across the primary.
I don't know much about this stuff either, haven't observed any issues in practice and use transformers in a lot of my designs currently.
I assume that magnetic recording tape itself would be subject to the same phenomena Allen was wondering about should it exist. I wonder if such a phenomena exists would it be as I suspect below the effective noise floor of the heads, tape, and electronics?
Ferrite RF transformers in some instances work with fractions of a uV input across the primary.
I don't know much about this stuff either, haven't observed any issues in practice and use transformers in a lot of my designs currently.
I simply wonder why one's intuition would be that magnetic domains switch in a transformer. I would at first assume it's continuous through zero, the dead zone would have easily measured distortion even though I have never seen this reported in any reference.
The tape heads are complicated by the bias, how about moving iron carts?
The tape heads are complicated by the bias, how about moving iron carts?
Those are some very interesting questions about what happens in a magnet core at very low levels of field intensity.
I would guess that any discretization (quantization ?) would not be measurable at macroscopic levels. Probably there is some "magnetic noise" generated by the thermodynamic "vibrations" at room temperature. Even if some discrete jumps existed, I presume they should be completely masked by the "magnetic noise".
Any physicist around here knowledgeable about these fenomena ?
I would guess that any discretization (quantization ?) would not be measurable at macroscopic levels. Probably there is some "magnetic noise" generated by the thermodynamic "vibrations" at room temperature. Even if some discrete jumps existed, I presume they should be completely masked by the "magnetic noise".
Any physicist around here knowledgeable about these fenomena ?
Did a search for Schumann resonance but didn't find the story of the 300k turns antenna.
Do you have a link ?
That fellow seems to have removed his site, here is another from Italy (only 140k turns).
www.vlf.it - Large Induction Coil for ULF monitoring
Also check out the minimal loop link.
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I checked out the links, interesting DIY hobby they have, hehe. 😀
Since they have to battle against the noise, they must be fans of your AD797, right ? 😛
I'm certainly no physicist, but I've spent a fair amount of time on studying ribbon transformers for my masters thesis. The behavior of the magnetic domains highly depends on the structure of the core material, which is why some manufacturers offer transformers with amorphous (metallic glass) or nanocrystalline cores instead or in addition to standard iron-nickel laminated cores. Refer to Lundahl's LL2913 vs. LL2914 for an example.
There also is a disaccomodation or relaxation loss (also called "Nachwirkung" introduced by N.W. McLachlan) that is to some degree comparable to the slew rate of opamps. While this can be mostly neglected of iron-nickel alloys, it is known to have some significant for amorphous soft magnetic alloys (Boll, Richard: Weichmagnetische Werkstoffe / Gayford, Michael: Microphone Engineering Handbook).
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