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Trevor Wilson

On 6/05/2017 2:08 AM, wrote:
On Friday, 5 May 2017 08:12:27 UTC+1, Trevor Wilson wrote:
On 4/05/2017 9:58 PM, tabbypurr wrote:

Piezos are not the 'junk' some opine,

**Oh yes they are. Absolute crap.

that's not a sensible factual claim

**Absolutely factual. They're ****. They are down there with the worst
HF drivers available.

Rip one apart and you'll wonder that they even work at all.

I already know how they work, and have constructed experimental
speakers using piezo technology. I don't wonder that they work, the
principles are well known, and have been for a long time.

**Indeed. However, the reality is that the performance has always failed
to live up to the hype.

far from it. But they're not
top of the tree either. If you're not spending lots of $ then
piezos can deliver quite good performance at relatively low
price. Their shortcoming is the frequency response is a bit
uneven at the top end, but domes, cones etc aren't flat either.

**They have a number of serious problems that rule them out of any
high quality audio application.

'high quality' is a bit too vague there. They're good enough for the
average domestic hifi.

**I guess we have different ideas on what constitutes 'average domestic
hi fi'. One local manufacturer decided that piezo tweeters were a great
idea for his speaker systems. He crossed them over correctly and used a
parallel resistor to ensure the whole thing worked correctly. I used to
replace the piezos with a nice dome tweeter (after modelling and
correcting the crossover to suit of course). The result was far more
satisfying. The speakers cost (as I recall) around AUS$1,200.00/pair.
That is what I refer to as 'average domestic hi fi'.

Sound reinforcement is probably OK, because the listeners don't
give a ****.

They can be connected where your old tweeters were if you add an
8R resistor - piezos are high impedance devices, at least in
their operating range. Always put a series resistor on piezos,
10R is ok. Otherwise the amp sees a capacitive load, and some are
not ok with that.

**The resistor MUST be in parallel (when used with a crossover).

a resistor must.

The capacitance exhibited by a typical piezo driver is almost
inconsequential. Any amp that runs into trouble with such a load
will have exploded years ago.

Maybe you should tell that to the amp I had that couldn't drive them
stably without a resistor. It's clearly not factual.

**The average capacitance exhibited by piezo tweeters is between 100nF
and 250nF. Any amplifier incapable of dealing with those amounts of
capacitance (on the end of a speaker cable and crossover) does not
deserve to be called an audio amplifier. It is no better than a door-stop.

Any type of tweeter comes in good & not so good variants. If I
were buying blind (or deaf) for average consumer kit I'd be more
likely to get a happy result from piezos than domes.

Crossover frequency is typically 1 - 4kHz. Tweeters only handle
somewhere vaguely in the region of 20% of a speaker's input
power.

**Much, MUCH less than that.

Percentage does vary according to the music.

**Of course. It is still MUCH less than 20%. More like 5%.

However, as Phil stated, they still respond to bass frequencies and
thus benefit from a crossover.

NT

--
Trevor Wilson
www.rageaudio.com.au