OTL Speaker

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OTL Speaker

Postby George Schmermund » Fri Jan 01, 2016 6:29 pm

Since I seem to have poked the hornet's nest over in the Tube 101 forum, I'll try testing the waters over here. I'm a DIY type of guy so I'm always looking for other DIY'ers to share ideas with. When folks suggest to me that they are into building their own speakers my naive and delusional mind assumes that they are building their own drivers. Then they have to explain that what they really mean is that they enjoy building boxes and putting store bought drivers inside them. OK, fair enough.

What I'd like to offer up is a way to make your own drivers. The kind that I prefer to build and listen to fall under the rubric of planar magnetic dipole radiators. Besides their unsurpassed sound quality they also come with the blessing of not having to build an enclosure for them. All that's needed is a rigid panel to support them in any configuration that pleases you.

The photos are of a sample prototype of one of the HF drivers that I designed and built way back when. The unit is now about 25 years old and shows some of it's age. It still works fine, though! The current project that I'm working on is a new form of driver that will have an unprecedented VC resistance of several K ohms. The reason for this high value is that when I get the 'Hot Rod 26' tube finished it will need to have a proper load to drive if I want it to be a useable output tube. I want to have the tube's plate directly drive the VC w/o the need for any transformer. This driver design will require a lot of thinking outside the basket (speaker pun). I'll use the diaphragm and frame of the planar tweeter as a testbed for the high impedance VC. I have some extra parts around here somewhere that will reduce the amount of new machining that will have to be done.

The diaphragm is .6 mil (.0006") polyester film. The original voice coil is one turn of 30 microinches (.000030") of copper with a resistance of 2.0 ohms. These were used in line arrays and the sum of the VC resistances could be added for whatever value was desired. Since the VC is only 1 turn and sits above the magnet in the focused fringe gap, it's impedance is basically a purely resistive load in the audio frequency range up to 40 Khz or more. A multi-fiber damping arrangement negated the need for electromagnet damping. These drivers make excellent tweeters in a 4 way system using active filters in front of each power amp. The companion midrange and upper bass drivers are larger versions of similar design.

I'll have to wind some high resistance test VC's for prototyping. To try and keep this project's description semi-coherent, I'll just give a quick explanation of where things started and where there headed. It'll take a couple of posts to get up to date.
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Re: OTL Speaker

Postby Geek » Fri Jan 01, 2016 6:49 pm

Creating/building drivers is well within most DIY tech's ability and should be encouraged.

Some forums need to be avoided with certain ideas ;) This one is one of the best all-rounders :))

I would love to hear it!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Re: OTL Speaker

Postby George Schmermund » Fri Jan 01, 2016 8:06 pm

Mr. Geek, your comment has left me nonplussed. You were the first to comment about my DIY triode in the other forum on this board and explained in no uncertain terms that it was a fools errand to even consider that a functional, reproducible triode could be DIY'd . The idea "is a pipe dream" only to those who are timid about what they think can actually do.

My offering in that forum is to demonstrate that anyone really interested in building a good triode can be successful in that endeavor. It's not rocket science (tubes were built well over 100 years ago) and is quite doable with only a modest investment in off the shelf stuff. I'm not saying that it's trivial, but why spend your life doing trivial things.

I know many people that already possess the basic tools that are need to travel the path. They just need guidance and the will to do it. We're not talking about Everest, we're talking about an advanced hobbyist project that can be done in the garage by those who profess to be very involved in their tube audio hobby.
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Re: OTL Speaker

Postby Geek » Fri Jan 01, 2016 9:29 pm

How odd... I encourage you and you are non-plussed.

It was not me that said you were on a fool's errand, I was just concerned with inconsistensies. Here was my post:
There's a few with the engineering setup to play around with that. Consistensy is a pipe dream, but it's for educational purposes mainly.

I remember back in the old Radio-Electronics some dude made his own diodes by re-purposing light bulbs by adding an anode. Efficiency sucked, but hey, an educational article came from it ;)

Check out YouTube.

Cheers!


The discouragement came from another user, not me.
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Re: OTL Speaker

Postby George Schmermund » Sat Jan 02, 2016 11:30 am

Thank you for clearing that up for me. I mistakenly thought you were expressing the opinion that it would be very difficult to make reproducible DIY triodes. I stand corrected.

I'll try to keep my posts focused on just the project from now on.
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Re: OTL Speaker

Postby George Schmermund » Sat Jan 02, 2016 1:14 pm

I made some time intermittently this week and started playing with the speaker project again. Using some pieces of stuff from the junk box and having found my stash of high resistance wire, it gave me a head start towards getting something done. The magnetic circuit that I plan to use requires that a racetrack shaped coil be used. Setting up the pantograph to mill the coil former was a snap. Fortunately, there was a Sherline hand threading lathe lying about and in need of some exercise.

As the photos show, it was an easy job to lash together a simple coil winder. The wire is 2.75 mil enameled Karma with a resistance of 108 ohms/ft. I wound about 25 turns and should have somewhere in the 1 to 1.2 k Ohm range. The coil was lacquered while still on the former and it will be interesting to see how well the silicone spray mold release works when the lacquer is dry.
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Re: OTL Speaker

Postby burnedfingers » Sat Jan 02, 2016 6:12 pm

Geek wrote:How odd... I encourage you and you are non-plussed.

It was not me that said you were on a fool's errand, I was just concerned with inconsistensies. Here was my post:
There's a few with the engineering setup to play around with that. Consistensy is a pipe dream, but it's for educational purposes mainly.

I remember back in the old Radio-Electronics some dude made his own diodes by re-purposing light bulbs by adding an anode. Efficiency sucked, but hey, an educational article came from it ;)

Check out YouTube.

Cheers!


The discouragement came from another user, not me.


Sir....the discouragement came from me. Sorry I still see no need to re-invent the tube because they are still available at a decent price. Secondly, I seriously doubt that anyone can manufacture a tube that will be efficient. You need to conside that most either do not have a lathe and tooling on hand or simply are not a master machinist.

You do however have my attention with this product. Please include efficiency @ 1 watt 1 meter, frequency response graph and power handling.

I seem to remember an audio company about 30 years ago that was close by that took on the task of designing and developing a decent ribbon tweeter. The suceeded in making a wonderful product that unfortunately was extremely power hungry. I still remember the clearest cleanest music coming from them. Power be damned I have enough repaired amplifiers on my shelves to run anything. ;)
carry on.
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Re: OTL Speaker

Postby George Schmermund » Sun Jan 03, 2016 6:59 pm

The coil winder has been redesigned and can now wind spiral instead of helical forms. This allows for a very flat coil to to be bonded onto the Mylar film. I wound one yesterday that is twice the length of each of the first 2 helical coils and was able to get about 2.8K ohms in 40 turns. This geometry makes a single layer coil that is ~.010" thick and ~.065" wide. These dimensions include the glue that holds the coil together. This allows a nice fit when its in the focused magnetic gap. It's now possible to wind a coil of any reasonable resistance for flat driver voice coils.

Next is to setup some standardized testing arrangement to compare different coils, film substrates, and magnet structures. Since these drivers will be of the dipole radiator type the jig will be mostly to assure reproducible positioning of the panels and microphone and not to try and conform to typical dynamic driver testing. The numbers from these tests will be unique to this setup and are only intended to indicate any progress in construction details.

This project is aimed at designing and building an SET OTL amp and directly coupled speaker from basically found parts that have been lashed together. I now have the supreme luxury of setting up the testing specifications to be anything that I want. This system is being designed to fill in the gap between headphones and conventional room filling loudspeakers. I'm designating this new rubric to be that of a 'personal space' sound reproduction system. All other specifications are now off the table.
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Re: OTL Speaker

Postby burnedfingers » Mon Jan 04, 2016 4:30 am

George,

If I understand you correctly the fact that the speakers work will be the win win for you. You have no thoughts as to using standard testing evaluation methods that the industry has adopted in order to evaluate your fine work? Everyday I have to count on the efficiency of a product that I happen to spec/use in a sound reproduction situation. I for one would be interested in seeing the results of your work. Maybe your work actually has some merit other than I built a driver from scratch. If not your not any further ahead than if you visited a neighbors garage sale and purchased a used pair of Radio Shack speakers.
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Re: OTL Speaker

Postby George Schmermund » Mon Jan 04, 2016 11:53 am

The coil winding has been so successful to this point that it's time to start looking for the flies in the ointment. So far I haven't been able to get a meaningful inductance measurement from any of my LCR meters, but it's no surprise since the coils' resistances are so high. I could use an online calculator, if there was one for high resistance wire, but I'd rather be forced to dig out the audio test gear. Those instruments needs an occasional warming up, anyhow. I'll do a transfer function impedance curve which should be very telling.

Since I'm lazy by nature and don't want to start wrestling with those tests today, I've decided that a DC breakdown test of the wire's insulation would suffice. Using the fixture that was left over from the construction of the now defunct Pickletron, it was relatively easy to do this test. Fortunately, my shaky hands get very calm after a few beers. So do I. Working with wire that I can hardly see is a real test of its own these days. As Tubalcain says " if it weren't for my OptiVisors, I'd be spending my final days in a rocking chair".

Anyway, the test indicates that the insulation is good for at least 500 volts DC at room temperature. Much more will need to be done to confirm that the dielectric coating (whatever it is) will hold up when it's put to it's task. I can always vacuum pot the coils if necessary
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Re: OTL Speaker

Postby George Schmermund » Mon Jan 04, 2016 1:58 pm

I got to do some quick tests on the new speaker project. The testing was done using the HP analyzer in the FFT mode to do a typical network analysis. This uses a transfer function to evaluate a device for it's impedance characteristics. I chose 2 tweeters of the garden variety, a 3k ohm metal film resistor and the 2.8k ohm spiral wound voice coil. The resistor is a close enough match to the VC resistance for testing purposes. The new voice coil is installed on a blank diaphragm of the previously posted planar magnetic frame and magnet structure. The test frequency range was from 1Hz to 50 Khz.

The results were as I suspected they would be, but still were quite amazing. The plots of the 2 tweeters were typical of any dynamic speaker; big hump at resonance and a rising impedance with frequency. The transfer plot of the new coil/speaker and the resistor were identical. I then connected the 3K ohm resistor in series with the conventional tweeters. The impedance curves of the tweeters were basically the same as before, just lower in level.

These quick tests confirm that the new hi-Z driver is ostensibly non-inductive from 1 Hz to 50 kHz.
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Re: OTL Speaker

Postby George Schmermund » Mon Jan 04, 2016 6:06 pm

Now that the basic proof of concept has been established for this new speaker it will be very interesting to continue the testing of prototypes as they emerge. As far as I know this is the only planar magnetic loudspeaker design that incorporates a multi-kohm voice coil with, what appears to be, an almost pure resistance for an output tube to drive. This will allow any SET amp to work into it's optimum load w/o the need for any reactive components in the final signal path. Good-by caps and transformers!

Since I'm still building the newly designed triode driver for this speaker I'll have to simulate an output tube with an arrangement of bench instruments. I'll evaluate most of the tests through the agency of the HP Signal Analyzer. A GR 1311A Audio Oscillator is being used as the proxy output source. The 1311A gives me a 160V PP output and can source 40 ma. My goal is to use a much higher output voltage when the new triode is ready, but it's 'any port in a storm' for now.

Initial tests at spot frequencies in the 1khz -10kHz (the coil is on a tweeter frame) show a distortion level of ~ 0.5%, which is in the area of what the 1311A claims to be good for. I guess that the only surprise so far is that it produces much more acoustical output than I had anticipated for such a crude prototype.

I'll be setting up a test stand for the SPL levels in the days to come and we'll see what the numbers amount to. Dipole radiators aren't the same critters as typical boxed dynamic drivers so this will be an interesting set of tests. The choice of designing this system for 'personal space' use lends itself very well to Dipole drivers.
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Re: OTL Speaker

Postby George Schmermund » Tue Jan 05, 2016 7:08 pm

Gentleman,

Thank you for your gracious acceptance of the ideas that I have tried to share with you. Your patients and helpfulness in these forums has been an inspiration to me. As much as I'd like to stay and hang out with you, I fell that it's time to move on. I shall continue to travel the road least taken.

Best wishes to all of you in this New Year!

George
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