Audax HD3p tweeter - design ideas?

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my Audax HD3p seem to be a capacitor load on the end of an air cored step up transformer with a 4R resistor in between the amp output and the transformer input.

Q. has anyone try to check the step ratio of the transformer?

A design thought? What if one deleted the resistor and transformer and used a high voltage power amp to drive the tweeter directly?

Q. What is the maximum peak voltage that the HD3p can run to?

Q. What load /amps /volts should the amplifier be designed for?

All this might suit high voltage mosfets (500v) or even valves (tubes) without the step down transformer that is reputed to be the bugbear in valve amplifier design.
transformers wanted

(sorry for my english)
I'am interested too, because my friend have purchased from e-bay six tweeters HD 3P but he had a bad surprise : the tweeters are without
the air-transformer!!! :(
If someone could get me any type of information about the trafo I will appreciate a lot.
( I asked Audax for original spare parts or technical-drawing of the trafo but no answer ,at this time)
Hi guys
I seem to recall that the stepup ratio was 6 or 9. This sounds about right, given anecdotes of mild electric shocks received upon touching the dome with music playing.

Should be fairly easy to do a direct drive amp, at least, compared to ESLs!

Inertial, I would love to purchase a pair or swap for tubes etc if your friend can spare them.

Thanks for the reply CV,
I am sorry but my friend want to assembly 3 pairs of loudspeakers
About the taper ratio etcetera, in your opinion, if I get one original
audax trafo to a expert man about tubes amplifiers maybe he will can
to copy it. Right?
I have read that these are out of production and getting very difficult to obtain secondhand.
I would not want to experiment & blow very expensive tweeters that cannot be replaced nor to have to lose all that money!
Step up ratio of 6 to 9 suggests a peak drive voltage of between 240Vpk and 360Vpk less an allowance for losses in the transformer and crossover, maybe 150Vpk to 225Vpk.
The amp would need to be able to drive 4r*6*6 =144ohms reactive to 150Vpk(100Vrms). That gets me closer to a design.

Can anyone confirm these values or give alternatives they have found?
measuring the original transformer

Hi intertial,

Yes, if you get an original transformer, it should be quite straightforward to measure it and build a replacement.

Could work out the turns ratio v. easily by removing the transformer and apply a low signal (say 1v rms) at 10kHz to the primary and measure the secondary voltage.

Then determine the primary inductance "Lp"and build a ferrite transformer with same or higher Lp.

Take a look at this thread for transformer info:

I'd be inclined to go with direct drive myself though...

Hi Cv,
thanks for the confidence " go with direct drive".
But how do you find the safe drive parameters without risking damaging the tweeters?
The tweeter system appears to be a 4ohms impedance with an 4ohm resistor in series.
Is the 4ohm resistor there primarily to adjust the volume to match other 8ohm speakers?
Is the 4ohm resistor there to adjust the phase angle and give the drive amp an easier job?
Allo Andrew,

Well, I'm just guessing here - hard to say without seeing an exact schematic etc, but I'd expect the thing would be a capacitive load, so by sacrificing sensitivity etc we can add the 4ohms to make it more like a resistive load. Which is pretty much what you said :)

I guess the best thing to do is actually measure the voltage across and current thru across the piezo element at various frequencies while driving normally, ie through the x/o. Then you should get an idea of what the thing really needs and what the EQ requirements might be.

I know it's messy but I suspect that if you were to EQ in your direct drive amp you would find that much less power was required, which should really ease the amp design.

Hope this is of help.

Wish I could find a pair of these things somewhere... then again, probably easier to buy the piezo film (I found a source a while ago) and roll me own...

the peizo dome encloses a volume of pressurised gas. It may be that prodution of this and coping with warranties thereafter that convinced Audax to abandon production. They are there to make money out of their customers after all. If it was economic I would expect HD3p availability to be ongoing.

Q. Can this piezo element drive a small HF horn? Is there enough drive power to get it to work effectively?
Hi again,
Yeah, I suspected they had too many problems with the dome losing pressure and decided it was a warranty timebomb? Who knows...

As for the horn idea, no idea, but you'd need an elliptical throat there; if you had the capability to make something suitable, you could probably make your own HD3Ps :)

I could easily turn a wood horn for treble with undersized throat and then hand finish the throat section out to elliptical merging into circular over the first 10mm to 20mm or so.

Q. wood (would) the throat need to be elliptical if it simply formed a front chamber with 1 to 2 throat to Sd ratio? Would that need more driving power?

Even with a dismantled HD3p laid out in front of me could I even imagine the production tools and jigs needed to make and assemble that piezo system.
Hi again,
Really have no idea if the horn would work at all.
What are you trying to achieve anyway? If it's more low end, it *might* be worth pursuing, but if it's more efficiency, I think it's a misadventure.

Also, if you're going to use a compression chamber, you'll need a phase plug.

What with the superb frequency response and waterfall, whacking the thing on anything other than a well designed, true elliptical horn is probably going to do more harm than good.

Not that I want to discourage you from expermentation, and these are just my guesses anyway...

I would think that the gold foil is plated onto the piezo element and forms the electrode that charges up one face of the piezo element. The back face will similarly have a conductive surface but it might be aluminium to save weight.

It might be gold leaf, but I would NOT try to burnish on a patch for fear of damaging the piezo element.
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