How low do you think I could go?
Yeah that's kind of the problem I'm looking into now. Preferably I'd like to run them low, need to find one that can.
Like input capacitance and such. Those power mosfets have NF of input capacitance. Input capacitances don't matter much when there's crap for voltage appearing at the terminals. Neither does linearity.
If 1A/80db is the typical performance do I really need that much? Isn't 85db pretty loud?
I would be concerned about this if my design wasn't a current source amp.
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Wikipedia offers some sane insights...
"The internal resistance of an ideal current source is infinite. An independent current source with zero current is identical to an ideal open circuit. The voltage across an ideal current source is completely determined by the circuit it is connected to. When connected to a short circuit, there is zero voltage and thus zero power delivered. When connected to a load resistance, the voltage across the source approaches infinity as the load resistance approaches infinity (an open circuit). Thus, an ideal current source, if such a thing existed in reality, could supply unlimited power and so would represent an unlimited source of energy.
No physical current source is ideal. For example, no physical current source can operate when applied to an open circuit. There are two characteristics that define a current source in real life. One is its internal resistance and the other is its compliance voltage. The compliance voltage is the maximum voltage that the current source can supply to a load. Over a given load range, it is possible for some types of real current sources to exhibit nearly infinite internal resistance. However, when the current source reaches its compliance voltage, it abruptly stops being a current source."
and
"Conversely, a current source provides a constant current, as long as the load connected to the source terminals has sufficiently low impedance. An ideal current source would provide no energy to a short circuit and approach infinite energy and voltage as the load resistance approaches infinity (an open circuit). An ideal current source has an infinite output impedance in parallel with the source. A real-world current source has a very high, but finite output impedance. In the case of transistor current sources, impedances of a few megohms (at DC) are typical.
An ideal current source cannot be connected to an ideal open circuit because this would create the paradox of running a constant, non-zero current (from the current source) through an element with a defined zero current (the open circuit). Also, a current source should not be connected to another current source if their currents differ but this arrangement is frequently used (e.g., in amplifying stages with dynamic load, CMOS circuits, etc.)"
+
"Because no ideal sources of either variety exist (all real-world examples have finite and non-zero source impedance), any current source can be considered as a voltage source with the same source impedance and vice versa. These concepts are dealt with by Norton's and Thévenin's theorems."
//
"The internal resistance of an ideal current source is infinite. An independent current source with zero current is identical to an ideal open circuit. The voltage across an ideal current source is completely determined by the circuit it is connected to. When connected to a short circuit, there is zero voltage and thus zero power delivered. When connected to a load resistance, the voltage across the source approaches infinity as the load resistance approaches infinity (an open circuit). Thus, an ideal current source, if such a thing existed in reality, could supply unlimited power and so would represent an unlimited source of energy.
No physical current source is ideal. For example, no physical current source can operate when applied to an open circuit. There are two characteristics that define a current source in real life. One is its internal resistance and the other is its compliance voltage. The compliance voltage is the maximum voltage that the current source can supply to a load. Over a given load range, it is possible for some types of real current sources to exhibit nearly infinite internal resistance. However, when the current source reaches its compliance voltage, it abruptly stops being a current source."
and
"Conversely, a current source provides a constant current, as long as the load connected to the source terminals has sufficiently low impedance. An ideal current source would provide no energy to a short circuit and approach infinite energy and voltage as the load resistance approaches infinity (an open circuit). An ideal current source has an infinite output impedance in parallel with the source. A real-world current source has a very high, but finite output impedance. In the case of transistor current sources, impedances of a few megohms (at DC) are typical.
An ideal current source cannot be connected to an ideal open circuit because this would create the paradox of running a constant, non-zero current (from the current source) through an element with a defined zero current (the open circuit). Also, a current source should not be connected to another current source if their currents differ but this arrangement is frequently used (e.g., in amplifying stages with dynamic load, CMOS circuits, etc.)"
+
"Because no ideal sources of either variety exist (all real-world examples have finite and non-zero source impedance), any current source can be considered as a voltage source with the same source impedance and vice versa. These concepts are dealt with by Norton's and Thévenin's theorems."
//
I just said it didn't matter.You should distinguish between average SPL and peak SPL. The ratio of peak/average is often thought to be 10. It can be higher.
You want your amplifier/speakers/etc to be able to sustain the average SPL you desire and have headroom for the peaks. You don't design for average and let everything above average to clip. You design for worst-case or maximum peak SPL.
85 dB is not loud for some music. It maybe too loud for other music.
The sensitivity of your build is unknown until you build it and measure it.
What you plan to build is what I call a planar magnetic full-range speaker. I think of a ribbon as a free-swinging skinny diaphragm that is attached only at the top and the bottom. Magnets and/or pole pieces are in the plane of the diaphragm. This type of driver is kinda sorta push-pull. Certainly not push-pull like an electrostatic speaker can be but much more push-pull-ish than a planar magnetic speaker.
A planar magnetic speaker has magnets behind the speaker diaphragm. This imparts a square law non-linearity. As the diaphragm moves relative to the stationary magnets behind it, the magnetic field experienced by the diaphragm changes with the square of the distance between the magnets and the diaphragm.
Lots of people love Magnepans and Apogee planar magnetic speakers. I have owned Magnepans and have heard Magnepans owned by friends. They can sound very good.
And how can you make a thin ribbon low resistance?
Without superconductors, by making it thick.
And heavy.
How easy is it to make a heavy ribbon move 20,000 times a second?
There's practical limits on electrodynamic drivers. We can only make so-much magnetic field strength (limited by saturation of iron or magnet-face). Conductor mass is related to conductivity. Iron and Aluminum are very good from DC to hundreds of Hz and then fall-off 6dB/octave. It takes more energy to force current through the conductivity against the mass than we get in mass motion. We get 25% efficiency an octave above the mass/conductivity limit. Taking 1KHz (optimistic) we have 25% eff. at 2KHz, 6% at 4KHz, etc. If we want "flat to 10KHz" we want to be 1% efficient over most of the audio band. (Most wide-range speakers are near this efficiency.)
Rather than ask a bunch of questions, I will wait to see what you build when you post pictures and measurements. Have fun building.
Like I showed earlier. Many small ribbons in parallel. Who says a ribbon needs to be one long strip?
I can parallel as much as I need. The bigger issue is probably the interconnects. Although if I do as I said before and make the speaker structure itself conductive they may not be an issue.
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Maybe I’ve missed something in this thread, but the ribbon is apparently considered as a pure resistive element so far. Since it is a moving conductor inside a magnetic field, its EMF has to be covered by the amplifiers output voltage. What can you expect here from the mentioned ribbons? The required currents plus EMF headroom could make an amplifier for this kind of load a bit more complicated I guess.
I doubt that a true ribbon has an appreciable inductance that would cause an appreciable back emf. The one-turn loop defined by ribbon and the wiring would be the inductance.
That said, it would be instructive to measure and remove all doubt.
Wouldn't using thick wire and parallel ribbons reduce the total inductance to stupidly low levels?
I just received the ribbon module I made in the mail. Sadly it was not made to spec and thus I got limited use out of it.
I wasn't able to parallel the ribbons but I was able to connect a single 4" x 0.5" ribbon to the amp I designed running at 3A idle 1.5 A swings. Due to the wiring the load was about 1.3 ohms.
The amp worked perfectly and the ribbon was surprisingly loud for the small size. Even if I reduced the ribbon to 2'' length it still was very loud.
I ended up destroying both the ribbon and the module holding it because of the failure of the printing service to meet my spec I had to get creative to get it to work right.
I'm in talks with them for a redo.
I have to ask, does SPL scale with size if the same amount of current is used?
For example instead of 1 small ribbon, if I got 1 big ribbon using the same current would I get more, less, or the same SPL? The magnetic field of the current would be the same which makes me think I would get more SPL, at least that might be true for a single long ribbon but if I'm paralleling ribbons I feel like I would end up at the same SPL since the current is splitting. What do you think?
I'm still designing based on the possibility of using 40A since I don't know where I will end up. What do you think of this mosfet for low voltage high current operation?
http://ixapps.ixys.com/DataSheet/DS100134B(IXFN230N20T).pdf
The inductance may be in the noise with only 1 ribbon. Paralleling many may have an unmeasurable outcome. Try to make a measurement and see. Much more satisfying than speculating.
I do not have experience making large area planar magnetic speakers. The experience I do have is with large area ESL panels. From this experience, my educated guess is that a large panel will play lower in frequency and not give you more SPLs per-se. The thing that may happen is the all of the closely coupled ribbon moving as a unit may roll off at high frequencies because of larger mass.
Your IXYS part looks like a good one. I chose the 660N04 because it has almost 10X lower RDSon than the part you posted. Just my preference.
I hope you get your metalwork issues sorted out and a working transducer.
cheers
How would I measure this? The average voltage peak was 2v with absolute peaks at 2.2v, this is within the expected power output for the load resistance, that should mean no appreciable EMF right?
But I'd be paralleling many small ones, so the individual mass of each ribbon will be small.
Check this one out BSB013NE2LXI Infineon Technologies | Mouser look at the input capacitance! Not too bad!
Inductance is measured on an un-powered speaker with an inductance measurement instrument.
Back-emf can be observed by viewing the current with an oscilloscope when using a voltage amplifier or observing the voltage if using a current amplifier.
Your assertion concerning the mass of the speaker may or may not be true. You can know by measuring a single ribbon and comparing that to a collection of ribbons used a large planar mid-woof.
This you can be certain of, even if every individual ribbon of the collection of ribbons used as a mid-woof had no high frequency roll-off, you would have doppler distortion of that high frequency content created by large mid and woof excursions and you would want to roll of the highs and use a separate ribbon tweeter. How is that for a run-on sentence?
The MOSFET you posted appears to have no way to mount it on a heatsink. I suspect it is intended for low power PCB applications.