Ultrasonic speaker or ??

[MrDEB]

Going to try some small amps I have but now can't locate a data sheet to figure out how to connect.
VM27AB L4950TA
I am pretty sure its a 7watt amp but data sheet??

 

[MrDEB]

found a data sheet
**broken link removed**

 

[audioguru]

The TDA1518 and TDA8560 amplifier ICs are not made anymore. They had the same output into 8 ohms (5W with a 12V supply) and 4 ohms (10.1W) as most other bridged car radio amplifier ICs. The high power amplifier is with a 2 ohm coil and magnet speaker.

The LM4950 comes in odd packages with the leads extremely close together. With a 12V supply its power at clipping into 8 ohms is also about 5W.

The frequency response of a piezo tweeter is not flat, it is all over the place due to resonances as I showed in a previous post. The sensitivity is at a peak but the average level is much less. With 5W then the loudness at a peak is about 101dB at a distance of 1m and is only 83dB at 8m (about 27 feet). 83dB is not very loud and might not scare away anything. Two tweeters will have an output of 86dB which is also not very loud.

 

[MrDEB]

Any suggestions as to amp or other alternatives.
One thing to remember is this is to be installed on an automobile traveling down the highway. Don't want a siren or the driver to be able to hear it. If to loud the local law enforcement might have something to say about it.
I am going with the assumptions that deer can hear it very distinctly and run.
Not sure of how loud it really needs to be.

 

[audioguru]

Some of those piezo tweeters are rated at 75W into 8 ohms. A bridged amplifier with an output of 75W into 8 ohms has a supply of 37V, not 12V.
If you mount piezo tweeters on a car and they play audible beeps then of course you will hear them. Oh, you are hearing impaired? Then maybe not hear them.

 

[MrDEB]

The tweeters are rated for 50w **broken link removed**
the amp is a stereo amp so 3w per speaker or feed both sets of frequencies into one input thus 7w output with 12v supply.
the second amp is 24wBTL (maybe add a mixer to combine the two sets of frequencies?)
working on a perlim schematic

 

[Hero999]

Some of those piezo tweeters are rated at 75W into 8 ohms.

As you know piezo crystal is high impedance, do you know how they get the impedance down to 8R?

Does it use a built-in matching transformer?

 

[audioguru]

Mr. Deb,
You do not know how to read the datasheet for an amplifier.

A piezo tweeter has no resistance because it has no low resistance coil. Its output level is rated as though it is driven from an amplifier that is driving an 8 ohm speaker.

The LM4950 amplifier has an output of only 1.7W per channel (not 3W) at clipping into 8 ohms with a 12V supply. Its output is 5.6W bridged into 8 ohms at clipping with a 12V supply and it can drive a few piezo tweeters in parallel.

The second amp has an output of 5W to 6W into 8 ohms at clipping with a 12V supply like any other bridged amp IC. Its output is 24W into 2 ohms (much more current than 8 ohms) when it has a very high supply voltage and a square-wave output (lots of power in the harmonics that a piezo cannot produce).

 

[Nigel Goodwin]

As you know piezo crystal is high impedance, do you know how they get the impedance down to 8R?

Does it use a built-in matching transformer?

No, it's just a capacitive device, no one ever suggested it was 8 ohms. The impedance varies with frequency, but is designed to be used with an amplifier that outputs 50W (or whatever) in to 8 ohms - but it's the voltage that matters.

 

[MrDEB]

I agree about the data sheet

I need to read between the lines for sure.
Not sure how loud this setup will be?
Here is a prelim schematic.
How do I know what the impedeance is or just wait until the tweeters get here?
Hopefully just connect?
Any guidance is welcomed.
Thinking of using two voltage regulators? as indicated on schematic.
I am learning by doing and asking for suggestions.

 

[Nigel Goodwin]

I need to read between the lines for sure.
Not sure how loud this setup will be?

Not very, about 2W to each tweeter - assuming you can use 'watts' with a piezo?.

If it's inside the car it would be audible, but probably wouldn't scare a deer even if it was inside the car.

Here is a prelim schematic.
How do I know what the impedeance is or just wait until the tweeters get here?

No impedance, it doesn't matter - it's NOT a voice coil.

Hopefully just connect?
Any guidance is welcomed.
Thinking of using two voltage regulators? as indicated on schematic.
I am learning by doing and asking for suggestions.

Why regulators at all? - isn't that a car chip anyway? - google doesn't seem to find much about it.

 

[MrDEB]

I am generating the randoim frequencies using a PIC (see schematic) so 5v is needed.
This unit is placed outside the car in behind the grill.
As for impedence, BLT SOUND (purchased 6 tweeters from) sent me an Email suggesting if connecting to an amp then put an 8ohm (thinking 2ohm) resistor in series with tweeter. About a 5-10watt resistor. sounds kinda big??

 

[Nigel Goodwin]

I am generating the randoim frequencies using a PIC (see schematic) so 5v is needed.

Feed the PIC via a regulator, but not the amps.

This unit is placed outside the car in behind the grill.

I presumed as much, but I suspect it's FAR too low powered.

Have you seen the 'anti-kid' systems that have sprung up to keep young kids from hanging round shop doorwars etc.?

These keep kids (with excellent high frequency hearing) a few metres away, and run lot more power than you're looking at.

As for impedence, BLT SOUND (purchased 6 tweeters from) sent me an Email suggesting if connecting to an amp then put an 8ohm (thinking 2ohm) resistor in series with tweeter. About a 5-10watt resistor. sounds kinda big??

The webpages I supplied the links to suggest a 47 ohm 3 watt resistor in series with the tweeters, it's also fairly common to use a potentiometer to feed the tweeter(s) so you can adjust the level to match the rest of the audio system (not a worry in your case though).

 

[MrDEB]

Would a 1 transistor amp between the PIC outputs to the "power amp" help any or just spinning my wheels?

 

[MrDEB]

add from PIC out to amp input

to increase the output of the PIC?

 

[audioguru]

Mr DEB,
You have the LM4950 amp as a stereo amp so its output is very low (1.7W). Instead you should use it as a bridged amp so its output is much higher (5W to 6W). Its bridged output can drive a few piezo tweeters.
The Piezo tweeters are capacitors so the 470uF output coupling capacitors are not needed. A bridged amp also does not need an output coupling capacitor even with a normal coil and magnet speaker.

If you have a 50W amplifier then 5W to 10W resistors should be in series with the tweeters. A 2W resistor is big enough with your low power amplifier.

 

[audioguru]

Would a 1 transistor amp between the PIC outputs to the "power amp" help any or just spinning my wheels?

The resistors in the circuit for the LM4950 amp sets its gain so an extra transistor is not needed.
But your schematic is missing the negative feedback resistors so it won't work.

 

[MrDEB]

results of 555/tweeter connection test

Got my tweeter horns on Saturday **broken link removed**
then connected a 555 to see how much the circuit draws etc.
used a 8uf cap so I can hear - I get sound, GREAT no smoke!
used .01 cap and just a clicking upon connection, no sound but LED is lit so must be outputting.
disconnected LED, inserted DMM in line with tweeter and 555 pin 3
set meter on 4ma scale and get .o23ma
awfully low??
is this right??

 

[MrDEB]

wish I had a frequency meter

 

[Nigel Goodwin]

Got my tweeter horns on Saturday **broken link removed**
then connected a 555 to see how much the circuit draws etc.
used a 8uf cap so I can hear - I get sound, GREAT no smoke!
used .01 cap and just a clicking upon connection, no sound but LED is lit so must be outputting.
disconnected LED, inserted DMM in line with tweeter and 555 pin 3
set meter on 4ma scale and get .o23ma
awfully low??
is this right??

Output from a 555 is going to be low, but you can't use a multimeter in that way, it won't read anywhere near accurately so doesn't give you any idea what the current might be.

You also don't require a capacitor in series with a piezo tweeter.