Measured results are a mixed bag at best. (I had high expectations for those and then quickly realized how compromised they actually were)

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As always, my 48 kHz sampling rate Unimik-1 USB microphone limit accuracy above ~16 kHz and cut solid at 22 kHz. As a result, I can't confirm the 50 kHz claimed upper limit of the tweeter :(

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Pros:

• Time domain performance is pretty much perfect. In fact, the very best I've ever measured.

• Very quick and breakup free tweeter. Exactly as expected from a mass-less coupler.

• Silent operation, with no hiss, and nothing audible. Just good treble.

• Other than a small trace amount on the initial start-up, there’s no ozone smell, so it appears to be ozone free as per claim. It’s actually barely noticeable, even with your nose positioned right at the horn exit.

• The unit reached operating temperature very fast, and as a result stopped emitting ozone.

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Cons:

• Distortion rises to totally unacceptable levels, and way too low in intensity to be considered usable for my personal need.

• The manufacturer opted to optimize the tweeter to play at lower intensity levels. Most people listen at 85 dB peak at the listening position, and in those circumstances, the tweeter does OK I guess but still somehow disappointing. Personally, it doesn’t come close to cutting it.

• Very inefficient @ ~185 watts power draw per tweeter assembly, to barely reach 90 dB output at an "acceptable" distortion level.

• For comparison, a typical bullet tweeter can play at ~107 dB’s with just 1 watt input and distort at less than 1% at such level, totally outperforming the plasma tweeter for less than 2% of the Acapella TW1S cost. It’s important to keep this in mind. 

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There’s no doubt that the ion technology offers tremendous advantages.

 

But as per Acapella TW1S offering, this model is under powered for me to consider buying a pair.

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Remember: my measurements were performed at 3" distance. So, the 100 dB curve is measured at 3".

This translates to approximately 87dB output at 1 meter, and around 84 dB at 2 meters away.

Hence, my complaint about lack of output capacity. 87 dB is far from loud and the distortion at this level is way too high to be considered audiophile grade. IMHO.

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If Acapella would do a "heavy duty" model, with a bigger flame, a bigger horn to support lower crossover point, a more powerful power supply to support the bigger flame electrical draw, and less distortion as result in more air displacement capacity of the bigger flame, I’d be very interested indeed.

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Unfortunately, I don’t know a "heavy duty" model from any other plasma Tweeter manufacturer. If such unit were to exist, it would likely draw upwards of 800 watts each, and radiate all that heat in the room, let alone, probably melting the horn. What a design challenge that would be!

No wonder nobody does it...

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Overall, it was a lot of fun to experiment with the TW1S.

I appreciated the opportunity to have them in house and hear what they can and specially can't do.

I took my sweet-ass time to evaluate them properly, unlike your average HiFi show where condition isn’t optimal, and crowds are always too noisy.

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At very low volumes (normal for some, background music for me), they’re a treat for the ears. Very special sound indeed. Quite unique I'll say.

However, at a realistic full orchestra level, they literally torture you. I mean they sucks big time, sorry, its really what I think.

Bottom line is that for me, they're just toys to play with. Once you're done, they go back in their boxes and may never come out again...

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Thanks to these loaners, it saved me quite a bit of time, effort and money, as they stopped my interest in further plasma tweeter exploration. 

Trivia fact: Audio Guru Mr. Nelson Pass also had a strong interest in plasma tweeters in his early days. He then understood and gave up as well.

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Afterwards, I joked with my friend about these tweeters. I called them "boner killers". 

It is like getting excited to date a supermodel, and then realize she farts in bed...

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Bottom line:

• A quality Bullet type super tweeter, lighter, more efficient and much cheaper will outperform the plasma tweeter any day.

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Update August 2021

 

The owner of the tweeter have blown one of his Acapella and send them back to me for repair.

I'm not sure what happened first but once the first component blew, the rest did blew as well like a domino.

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-the main PL509/PL519 tube was short

-the output transistor (BU526) was short, since it was obsolete, I've used a BUL510 in its place

-the driver transistor (BD410) was short, since it was obsolete, I've used a J3305 in its place

-2 signal transistor (BF259) were short, since it was obsolete, I've used a J3503 in its place

-2 resistors were blown and had to be replaced as well.

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-I've added some adhesive heatsink to reduce heat stress on the driver board power transistor and hopefully lengthen the longevity of the fixed tweeter

Here a few picture of the fix I've performed.

 

Note that the replacement transistors I've ordered aren't pin compatible! you have to place them the right way or it will blow up.

On picture 2, the gold 3K3ohms power resistor should be adjusted to be at +80VDC on the right pin as per ground reference (or against the chassis). To perform such adjustment, use the bottom right potentiometer (tan color with black adjustment knob) as seen on the same picture. If you adjust the potentiometer clockwise, the voltage will increase and the plasma flame will become bigger (Acapella call for 80Vdc and I've adjusted it as such) Going higher will likely impede reliability and I don't expect it to improve anything.