High Sensitivity HeadPhones, new direction

WOW! The first speaker working like that I ever saw was while cleaning out the finished attic of an old house circa 1950. That speaker was built exactly like that. I recall it to be a large cone like you show, maybe a foot or more in diameter. The coil driver was just a big old relay coil-looking sort of thing. The lever and its fulcrum looked just like a relay armature; the cone driver just an extension rod on the other side of the armature bearing-shaft. This is an easy way to get more leverage, a mechanical impedance transformer if you will. That unit was obviously a refugee from an ancient AM radio from some where in 1920's. The former owner was a past Mayor of Wilkes-Barre, PA, Dan Hart... He was sai dto have had some wild parties up in that finished garret during Prohibition days.

Anyway, the important factors are the steps made to mechanically "impedance match" the electrical force available; a strong armature force which can only go a short distance over coil - to the cone wagging in the air presenting little reaction force, but should travel over a much longer distance.

Go for it... build one..

You could use an old high impedance relay, mounted with coil axis horizontal, on an inertial base (e.g. a brick) .

The mechanical transformer ratio is adjusted as the length of the extension rod.

It was about an inch or so long as I recall...

The cone drive point has an axial pin glued into the apex tip of the cone; use super-glue, I think.

Manila folder stock, painted with clear dope to stiffen it, may work as the cone. I'm not sure of the cone base support, but I suspect that any hanging arrangement will do; the axis of the cone is horizontal.

This could be a "Big-Gallows" speaker; completely impedance matched to air.

Ange

"Angelo Campanella" wrote in news:ibkoi9$jih$ snipped-for-privacy@news.eternal-september.org:

According to the diagram, the acoustic load seen by the speaker cone is a confined volume of air, the impedance of which is a purely reactive compliance. In order to match that load impedance, the source impedance would need to be a purely mass reactive, and the match would only exist at a single frequency. For a broadband (300Hz-3000Hz) impedance match, both the source and load impedances would need to be predominantly real and approximately equal in magnitude. I don't see the latter requirment being satisfied in the proposed design. What am I misssing?

Hi Ange, This is basically the same arrangement found in some of the older headsets. Although being the Gallows style it can be built big, so fat fingers can do it, also the size will allow many turns for impedance matching. The reason I went with a manufactured speaker assembly is because maybe the solved some of the matching problems that I'm clueless about. Although by sealing the speaker and adding the tube, I've probably messed up all the good design that the speaker had. I don't really like the fulcrum idea as drawn, but I have another idea (probably not new either) that I'll draw up later. Re:

It would be neat to have an adjustable fulcrum to fine tune the leverage, I don't know where the trade off is between cone displacement and the power available to overcome the leverage.

Now to find/build an iron core with a gap as drawn. Are the laminations thinner on audio transformers than on power transformers? Any tape wound cores used for audio? (might be easy to mill the gap) Mike

Well, an air gap would decrease coupling a lot and the effect i think would be the equivalent of a loss. Yes, audio (and modem) transformers have a lot thinner laminatins - so there is a lower loss at the higher frequencies. Never heard of tape wound cores for audio, but if you could get such with thin "laminations" that should work quite well. Forget gaps!

I need the gap to put the driver in. MikeK

Answerman, I have realized you are very intelligent and well educated in the subject of acoustics. I'm reassigning you to a higher position, you my friend are now the Chief Design Engineer of the High Sensitivity Crystal Radio Headphone Project. Now known world wide as the HSCRHP. The task at hand is to design a very efficient audio transducer with a minimum frequency response of 300Hz-3000Hz, a wider range would be ok. The transducer will be used with crystal radios in contest situations. There are two options for electrical impedance matching;

1) taps can be made on the transducer for matching. (Taps at 1 Megohm and less) 2) A transformer can be used, realizing losses of 1 to 3 db. Electromagnetic or piezo driver are acceptable. The transducer assembly must be produced from easily available parts and be able to be built by a skilled homebrewer without any special tools. Putting together a winder or a few jigs is acceptable. Congratulations on your promotion and thanks in advance for your dedicated work on the HSCRHP. MikeK

"Angelo Campanella" wrote in news:ibfs2k$sbn$ snipped-for-privacy@news.eternal-september.org:

If it isn't already apparent to everyone, you've spotlighted the two key requirements which are high electroacoustic sensitivity and high impedance so as to not load the detector. In order to achieve both simultaneously, the use of an impedance matching transformer between the detector and the earphone needs to be avoided. This is so because for every increase N^2 increase in impedance, there is a 1/N decrease in overall trasformer-earphone sensitivity. The ideal solution is to increase the earphone impedance as much as possible while still maintining the hightest possible sensitivity. Because of present mass- market requirements, the focus of current earphone technology is on high output and high efficiency, and not on ultra high impedance. Consequently, current technology is not the place where on is going to find an improved high sensitivity, ultra high impedance earphone that is optimized for use with a crystal radio. So the only solutionare to either build your own earphone from scratch or to modify an existing balanced armature. The good news is that it's already been done.

If the results can be believed, they are impressive. Specifically, an AC Impedance at 1 kHz of 56000 Ohms (pair) and a threshold sensitivity at 1 kHz of 0.00028 picoW (pair). For someone with young-adult normal hearing, the latter translates approximately to 35dB SPL for 1pW electrical input. So these mofified headphones have a sensitivity that is comparable to that of the Sennheiser HDA-280, but with an impedance of

56000 ohms instead of 35 ohms. Consequently, no matching transformer is required and the associated loss in overall transformer-earphone sensitivity is avoided.

I'm not understanding this this. Do you mean the voltage to the driver is divided by the turns ratio?

This bit of research is not worthy of the big promotion I gave you! The key words on the page you reference are; "Since his pair of phones has a 56k impedance at 1 kHz, a matching transformer can be eliminated for SOME crystal set applications."

This design is for a contest quality transducer, thus we can assume a high quality crystal radio. A good inductor can have Q of 900 at 1500khz or a loss resistance of 2.5 ohms (R1). A capacitor capacitor can have a Q of 5000 at 1500khz or a loss of 0.45 ohms (R2). Using the formula Z = (2 x pi x F x L) ^2 / R1 +R2 the impedance at resonance is

1.725 Megohms. Now we install a diode (detector) that matches the 1.725 Mohms at this signal level, (the diode resistance changes with signal levels). Now I don't know exactly where the impedance is at this point, but it is much higher than 56,000 ohms. So, the 56,000 ohms headphones are not that good of a match to a high quality crystal radio. So I'm back to asking for at least 1megohm transducer impedance. Thanks for your interest, Mike

Oh...seems like the construction would be like a speaker's magnet. Outer Circle North and inner circle South with small gap in between. Seems would be a lot of work to duplicate that geometry, when you could "steal" the whole thing intact and just modify the cone: rewind the driver part with as many turns of #80 wire (do NOT sneeze!) for highest Z and use speaker to "drive" an acoustical horn. Do not know what can be done with rear audio.

Do not forget that a piezo transformer may be usable..

It may be possible, I don't think even with #80 we could get enough inductance to make the high impedance we're looking for. Plus the nightmares caused by #80 wire. Interesting about the rear audio, I wonder if we could seal the backside of the speaker and use it for the other ear? 180* out of phase. Thanks, MikeK

Thanks for that, I know of them, wouldn't know how to implement its use. You have included it on our list of tools for possible use in the HSCRHP. Thanks, MikeK

On Nov 9, 9:31=A0am, "Tim Golden BandTech.com" wrote:

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Been thinking about this some and it makes sense that the coil pair would be part of the same magnetic circuit, so that for a magnetic diaphragm the area between the two coils was conducting (magnetically), with the loop passing down one coil core and out the other one. This is the best means to a short magnetic path through the light diaphragm, at the position of greatest motion. This should be easily verified with an old pair of headphones, for the cores of the two windings will be a U of iron. To ease the weight of the new high efficiency diaphragm this could be a single transformer plate cut down and attached to a lighter weight diaphragm. Those old phones had a circular thin ferrous plate. Ahh... how much does the mass really matter? If at resonance no heating takes place then the mass is a tank circuit, and merely rings a bit. Yes, this does matter quite alot, for the peak intensities of the signal will become subdued, and no doubt these are critical portions in terms of intelligibility at the limits. I believe that you want small cores and small magnetics for low level signals, so that eddy losses are minimal. This style will not do well with big signals, but that is OK, right? This is magnetic saturation kicking in, and that should set a smallest dimension. This is also consistent with selecting the smallest diaphragm magnetic conductor, so that a light weight motion is possible. I believe that the low power constraint actually leads to a small device; one that may fit in the ear. Still, the photo is an excellent prototype platform to work from. Be careful with these things! It would be a shame to blow your ear's sensitivity with a bad design. Mine already are abused, and I am more careful about loud noises now.

Jeeze, I've got to get to some magnetics this winter.

- Tim

What a revolting development! I gave a promoted Answerman to Chief Design Engineer and he hasn't shown up to work for 4 days. Oh well I'm tired, time for a break. MikeK

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Transducer loading doesnt exactly help with q. Also the shape of the sensitivity curve is all wrong. No agc, very low sensitivity, etc

NT

Look at small 120VAC motors for clocks and settable appliance timers (24 hr types); i just measured 45K DCR of the coil from something like that (ripped it out ages ago so do not remember). Diameter of the enameled wire appears to be about 0.0015 .

I used to have one for my crystal set but don't ask me how they work rick ps why is it called Gallows?

"amdx" wrote in news:298bd$4cd0a8fc$18ec6dd7$ snipped-for-privacy@KNOLOGY.NET:

For whatever reason, you are still not listening. You can not talk about building a high-sensitivity headphone when the headphone is to be used in a passive circuit such as a crystal radio. This is so because the load impedance of the headphone affects the terminal voltage of the circuit to which it is connected and because the source impedance of the circuit to which the headphone is connected affects the acoustic output of the headphone. It is because of this interaction that the sensitivity of a headphone is specified in terms of the pressure that it produces in specified aoustic load (eg IEC-318 coupler) per applied volt when driven from a low-impedance voltage source.

So if I have two high impedance headphones assembled by different manufacturers with different materials and 30 years between build dates, are you telling me that they will have the same SPL output with equal input? Or might one be better than the other? Even, more sensitive? MikeK