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AndrewT 22nd August 2005 08:36 PM

Audax HD3p tweeter - design ideas?
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.

inertial 23rd August 2005 04:17 PM

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)

AndrewT 24th August 2005 08:03 AM

Hi Inertial,
the air cored transformer is built into the little board attached to the tweeter. It is only a few coils thick about 2 mm from memory.

inertial 24th August 2005 08:49 AM

Thanks AndrewT,
but i need the exact schema-drawing of the winding to perfect replicate it! I am not familiar with trafos :(
The crossover is not a problem for me . :)

CV 25th August 2005 10:01 AM

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.


Luke 25th August 2005 10:27 AM

I would like to purchae one two if thats possible.

inertial 25th August 2005 10:39 AM

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?

AndrewT 25th August 2005 04:57 PM

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?

CV 26th August 2005 11:13 AM

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...


inertial 26th August 2005 04:58 PM

Hi CV and thanks a lot for the info :)

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