Capacitive Speakers

Thread Starter

MarcosKDC

Joined Sep 10, 2021
67
I bought a set of piezoelectric tweeters, I didn't know they were until I found they didn't give continuity at all...
I have 4 XTC PT1, and two bigger conelike XTC PT100.

The bad thing is that I don't really now which is their actual impedance, I searched some information and found this:

" Piezoelectric speakers have a complex electronic equivalent circuit but mainly they can be seen as a capacitive load with values between 10nF and 1µF. The capacitive value of the speaker is an important characteristic for the amplifier circuit of the speaker. “Piezoelectric means capacitive load” Most amplifier IC’s are developed for electrodynamic speakers. They can deliver high current variations and are limited to a voltage level of +/- 10 Vpp. Piezoceramic speakers demand voltage variations. The current consumption is extreme small and the voltage peak to peak level goes up to 60Vpp. "

I wired two XTC PT1 in series, then paralleled with the XTC PT100 , expecting that the little ones will see half the power of the big one, but i'm not sure if that's right..

I guess I don't have to put any capacitor also because of their nature... Somebody told me to put 4ohm resistors in series to protect the amplifier...

Please any information about this will be well received :))
 

LowQCab

Joined Nov 6, 2012
2,342
Some Amplifiers are not very stable,
and will not tolerate a direct Capacitive-Load on their Outputs.

Connecting Piezo-Tweeters in series will double the Voltage tolerance,
and reduce their Output by half, just like Electromagnetic Speakers will behave.
This will act like a Resistive-Voltage-Divider,
and they will share the Voltage pretty-close to equally.

The Voltage-Rating of a Piezo-Tweeter is not an absolute value,
neither is it a "guaranteed-performance" number.

Piezo-Tweeters can be "over-heated" by continuous operation at their rated maximum-Voltage.
This heating-effect gets worse as the Frequency gets higher.

If your Amplifier starts getting into some sort of ultra-sonic oscillations,
it will quickly destroy the Piezo-Tweeters "without making a sound", ( that You can hear ).
This is possible even with an Amplifier that can only supply maybe ~10-Volts AC max.
This can be largely mitigated by adding a NON-INDUCTIVE Power-Resistor
of around ~30-Ohms, in series with the Tweeter, ( 4-Ohms is way too low ).
This may also have the effect of smoothing the Frequency-Response of the Tweeter.

The "apparent" Impedance of Piezo-Tweeters gets lower, as the Frequency increases,
and vice-versa,
this is why they don't necessarily require a Crossover.
They can be less than 1-Ohm at Frequencies over 100khz,
but at Audio-Frequencies,
they generally only get down to maybe ~50-Ohms, ( at ~20khz ).

You can also strap an 8-Ohm Resistor across them and then
feed them with any standard Cross-Over-Network,
but it's the Crossover that needs the Resistor, not the Tweeters.

I used to have a Folder with all sorts of tricks for Piezo-Tweeters,
but that was decades ago, and I can't find it now.
Motorola was the original designer and manufacturer of Piezo-Tweeters back in the '60's,
then several other USA manufacturers kinda gave it their shot,
but I think the only manufacturers now are in China,
and most of them are absolute garbage, with very poor quality-control,
but I could be wrong, because I haven't built any Speakers in the last ~10-years or so.
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Last edited:

Ian0

Joined Aug 7, 2020
5,885
Completely capacitive loads will send some amplifiers unstable, so a series resistor is a good idea.
Putting the two small ones in series will give half the voltage on each. It tends to be overvoltage that kills piezos.
The PT100 is a copy of an old Motorola KSN1025A design, which looks electrically like a 330nF capacitor.
Audio quality is between poor and dreadful as you can see from the frequency response graph. The copies tend to be worse than the originals.
The original KSN1025A had a built-in series resistor to prevent amplifiers from going unstable, but don't for a second believe that the copies might have.
 

Thread Starter

MarcosKDC

Joined Sep 10, 2021
67
Thanks a lot for the complete info.

I will be mainly using car amplifiers, I'm not expecting to have more than 25V rms...

I did the series combination because I think I've read that the little ones would handle less power than the big one maybe I'm wrong...

I don't really know about the quality, I've heard about the motorola thing but I don't think these ones are original...

I'm just hoping them to give some more highs... Now I have two pionner 6x9 and a sub all enclosed in their respective boxes, and it sounds too bassy...

Ungratefully I'm a newbie in the audio thing... I'm basically tryna start with the electronics then move to acoustics... But missing the basics of speaker design and that makes it quite difficult to get what I want at first try :')
 

Ya’akov

Joined Jan 27, 2019
6,275
From a reproduction standpoint piezo tweeters are really crappy. That is, if you care about the original concept of “high fidelity”. Wthi electronic, studio produced music the “hifi” reproduction is going to be based on what the artist mixed using whatever monitors they used.

A long time ago when I was involved in the recording studio business, the stated of the art for a serious (rock/pop) studio were Yamaha NS-10Ms for the near field and UREI coaxial monitors for high level playback.

1658393580221.jpeg 1658393541951.jpeg
NS-10M Pro and UREI Coaxial Monitor

So, unlike the BBC using an orchestra and a tape to A/B test the LS35A designs, the test today is one set of speakers against another. But almost no one cares about such comparisons. Instead a smooth power response over the reproduction range with low distortion at high volume is considered “high fidelity” as far as anyone cares.

Before this narrative grabs a hold of my brain and takes it on a long journey requiring thousands of words to complete, I will get to my point:

Even given the relativistic nature of what is considered good reproduction in the current non-acoustic music environment, piezo tweeter stink on ice delivering shrill highs with a noticeable discontinuity from the mids to the highs in most cases. The distortion produced by a piezo tweeter is very unpleasant as well but can be very subtle since it can occur at frequencies at the edge of perception of even in the audible range as interference.

I would expect that if you can do, ditching those piezos in favor of dome or electrodynamic horn tweeters would make you listening life a happier thing.
 

Ian0

Joined Aug 7, 2020
5,885
From a reproduction standpoint piezo tweeters are really crappy.
I did come across something potentially worse. It was an automotive tweeter, which consisted of a piezo transducer attached to an autotransformer, wound on an open bobbin of the kind used for DC inductors, or it may even have been air-cored.
Needless to say, I didn’t do any measurements to find out just how bad it was.
 

MisterBill2

Joined Jan 23, 2018
12,704
First question is if the piezo tweeters are the only load for the amplifier or not. That matters because the amp sees the combined load.
The comments about the very peaky response ignore the type of stuff being played through the system, some of it is improved by high distortion, (over 10%THD).
The rap played on the multi-thousand watt car systems is much better when the distortion is at least 50%, in my opinion. At higher percentages it becomes no worse than a simple loud exhaust sound.
 

LowQCab

Joined Nov 6, 2012
2,342
Interesting side note ............
The ( Old ), ( Motorola or CTS ), 2X6 Piezo-Tweeters were
far superior to the 3X3 "Super-Tweeters".
When used in a Line-Array of ~6 or more they would smooth out nicely,
and legitimately cover 2khz to 20khz with excellent Fidelity.
And, they're also a heck of a lot cheaper than regularly replacing ~$300.oo JBL Slot-Tweeters.
( 3 out of 4 DJs are complete idiots, especially when they don't have to pay for blown Drivers ).
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MisterBill2

Joined Jan 23, 2018
12,704
Do you recall the record player, I think it was a Zenith brand, that had an electrostatic high frequency speaker. It was driven by the plate to plate voltage of the push-pull 50C5 output tubes. I am not sure how the polarizing voltage was derived. No clue about the frequency response of that one. Probably much smoother.
How would one avoid the resonance of the tweeter in a piezo package?? That may be why they don't sound so perfect.
 

LowQCab

Joined Nov 6, 2012
2,342
"" How would one avoid the resonance of the tweeter in a piezo package??
That may be why they don't sound so perfect. ""


The resonance of the Piezo-Element is too high to hear,
but it has a Paper-Cone which has a relatively imprecise, and loose mounting arrangement,
and a poorly designed Horn.

The Horn is only efficient down to about 8khz,
but very few people use them with a properly designed Crossover-Network,
they usually just connect them straight across the Woofer.
So it's actually the Horn being used out of its designed
Frequency-Range that gives them such a bad reputation.

They have what's called "Horn-Honk",
but its at a particularly annoying high-Frequency, ( around 4K ).

Add to that, the rear Chamber of the Driver contains NO sound absorption materials,
and therefore resonates like crazy.

All of these problems can be easily fixed,
but what do You expect for ~$8.oo ???
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Thread Starter

MarcosKDC

Joined Sep 10, 2021
67
I don't understand most of what you've said, but yes, this speakers were cheap, so I guess they're not gonna sound amazing.. But well, I'm just tryna learn a bit with cheap equipment before getting broke xD
 
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