Ultrasonic actuator from Ceramic Phono Cartridge

[Emreth]

Hi,
I'm using a piezoelectric ceramic phono cartridge to produce high frequency displacements in the phono stylus. I'm driving it with a function generator that has an output of +-10 V sine wave . I connected the generator to a transformer that has a 1/10 reduction ratio to ramp up to voltage 10 times, and connected the output to the cartridge. I get about +-10 microns displacement at up to 5kHz but it's rapidly reduced to zero at 10kHz and higher. I need to get to 20-30 kHz. I get the same amount of displacement for lower frequencies so i don't think it's caused by the inertia of the stylus. Data for these cartridges claim that they can operate up to 20 kHz with a flat response (when the stylus is displaced by the record, it supposedly produces a signal which stays constant with changing frequency). I have a suspicion that it's a frequency dependent impedance problem either from the piezo elements in the cartridge or from the transformer itself. Any ideas on what the reason can be and how to fix it?
thanks

 

[Bobbywhy]

Having spent most of my career working with high power sonars your post intrigues me.

When used as phonograph pickups a typical piezo cartridge puts out around 6 or 10 millivolts at miniscule power levels. The reverse process-what you are doing-requires lots of energy to get reasonable mechanical displacement.

1. Can you say the make/model of the phono cartridge you are using? Is it for stereo...does it have two elements, one for each channel? Some piezo materials are hydroscopic and so their performance degrades with humidity.

2. How is it you measure the stylus displacement to be about +-10 microns?

3. Are you monitoring the actual voltage applied to the element(s)? Does it decrease as the drive frequency increases?

Your system seems to suffer from lack of sufficient power to drive the element(s). Typically an audio power amplifier is found between the signal generator and the driven elements.

 

[jim hardy]

Data for these cartridges claim that they can operate up to 20 kHz with a flat response

I'd double check that assumption

there's something called "RIAA equalization" that might come into play

i don't know and if you do, just ignore this post.

 

[AlephZero]

In a record player, the cartridge has a prescribed displacement at the input (the stylus) from the groove on the record, and the body of the cartridge and the arm of the record player form a seismic mass which nominally does not move, to create the strain in the piezo material and hence the output voltage.

If you are trying to move something by pushing it with the stylus, you will be changing the dynamic response of the cartridge by effectively adding more mass to the stylus tip, and you will get a roll-off of the high frequency response because of the larger inertia of the tip. You will be bending the "cantilever beam" that carries the stylus, instead of moving the end of the beam. Trying to overcome that by applying more input power may just result in breaking the stylus arm rather than getting a bigger output.

@jim hardy, I think the RIAA equalization is irrelevant here. That is applied to the audio when the disk is manufactured, to cut the low frequency content of the recorded signal and allow the turns of the spiral groove to be packed closer together, increasing the recording time. The playback pre-amp needs to reverse that, and also compensate for the fact that some types of cartridge produce an output proportional to displacement and others to velocity, but except in expensive systems you wouldn't tweak the frequency response to match a particular make of cartridge, stylus, etc.

 

[Emreth]

When used as phonograph pickups a typical piezo cartridge puts out around 6 or 10 millivolts at miniscule power levels.
1. Can you say the make/model of the phono cartridge you are using? Is it for stereo...does it have two elements, one for each channel? Some piezo materials are hydroscopic and so their performance degrades with humidity.
2. How is it you measure the stylus displacement to be about +-10 microns?
3. Are you monitoring the actual voltage applied to the element(s)? Does it decrease as the drive frequency increases?
Your system seems to suffer from lack of sufficient power to drive the element(s). Typically an audio power amplifier is found between the signal generator and the driven elements.

That output of 10mV is for moving magnet type coils, piezos give as high as 0.5 V and can directly drive speakers. The phono cartridge is a stock one on a Vibe turntable, a cheapo one and probably no brand. It's stereo so has two piezo elements (90 deg at each other and 45 with the cartridge), which when run in-phase move horizontally and run out-of-phase move vertically. It's a ceramic one so we can neglect humidity effects.
I run the stylus to scratch a CD on the turntable and measure the wiggles with a microscope. So the displacement is nearly steady up to 5kHz. About the voltage I'm pretty sure it's constant, as it comes from a nice function generator. It's possible that the power is not enough. The transformer I use is rated at mains frequency (50-60 Hz), so it might be suffering at high frequencies. I can't use an audio transformer though, the voltages are too high. Maybe best bet is to run a fast amplifier driven from the signal generator instead.

In a record player, the cartridge has a prescribed displacement at the input (the stylus) from the groove on the record, and the body of the cartridge and the arm of the record player form a seismic mass which nominally does not move, to create the strain in the piezo material and hence the output voltage.

If you are trying to move something by pushing it with the stylus, you will be changing the dynamic response of the cartridge by effectively adding more mass to the stylus tip, and you will get a roll-off of the high frequency response because of the larger inertia of the tip. You will be bending the "cantilever beam" that carries the stylus, instead of moving the end of the beam. Trying to overcome that by applying more input power may just result in breaking the stylus arm rather than getting a bigger output.

@jim hardy, I think the RIAA equalization is irrelevant here. That is applied to the audio when the disk is manufactured, to cut the low frequency content of the recorded signal and allow the turns of the spiral groove to be packed closer together, increasing the recording time. The playback pre-amp needs to reverse that, and also compensate for the fact that some types of cartridge produce an output proportional to displacement and others to velocity, but except in expensive systems you wouldn't tweak the frequency response to match a particular make of cartridge, stylus, etc.

I use the stylus to scratch the surface of a CD. Very little extra load is imparted on the stylus. I played with the contact load and it doesn't affect the amplitude. Also the "cantilever beam" that carries the stylus was a very compliant plastic, so I coated it with some elmer's glue to stiffen it (it's much stiffer now when i feel it with my hand), that didn't change anything though, so that wasn't the issue. I'm pretty sure it has something to do with the transformer I use. Is it possible to add some elements to the secondary circuit (piezo side) to reduce back EMF or other inductive loads from the coil or anything else that might be coming from there?
I probably need a fast and simple amplifier to connect to the output of the signal generator instead (+-10 V and output impedance 600ohms or TTL output). Anybody that can help me design one that would work at 20-40 kHz, output at +-100-200 V, input preferably 9-12 V max? The piezo acts mostly like a capacitor, i saw some values like 300pF, but the one i have might be different. It's probably very simple, i would just need info on fast enough power transistors and/or opamps?

 

[jim hardy]

The transformer I use is rated at mains frequency (50-60 Hz),

i'm surprised you got to 5khz with it. It's laminated for 60hz. You need thinner laminations.

you might see what an audio transformer would do
i'd think a tube output transformer would be just right.
Drive the low-Z side with a LM386 and see if it looks close to working, LM 386 available everywhere

Hammond still makes such transformers , many hobby sites carry them.
http://www.hammondmfg.com/125SE.htm
It's only rated to 15khz at full power to go higher you have to get a hifi version.
Also peruse thrift stores for an old vacuum tube TV or radio.

my favorite IC audio amp was LM383 . It's several watts compared to LM386's half or so. National obsoleted it but it's available in a TD part number from somebody else, Thompson i think. There's plenty of others, take apart any old powered computer speaker.

old jim

 

[Emreth]

I'll try that, thanks.