In case you don't know what an fm pick up is -
Weathers FM Cartridge
Anyways, it turns out Paul Weathers filed a patent for a stereo FM cartridge. View Patent
And here's an RF circuit for an RF Condenser Microphone which work on the same principal. RF Circuit Does the RF Circuit need to be modified for an fm cart?
Seeing as how people build custom microphones, why not a cartridge? Anyone feeling inspired?
PS Putting something like this together is over my head but building it looks pretty straightforward. Any ideas?
Weathers FM Cartridge
Anyways, it turns out Paul Weathers filed a patent for a stereo FM cartridge. View Patent
And here's an RF circuit for an RF Condenser Microphone which work on the same principal. RF Circuit Does the RF Circuit need to be modified for an fm cart?
Seeing as how people build custom microphones, why not a cartridge? Anyone feeling inspired?
PS Putting something like this together is over my head but building it looks pretty straightforward. Any ideas?
FYI: Figure 5 is for the more common stereo 45 degree record pressing.
The second design is for the less common "hill and dale" stereo.
It seems crosstalk was a problem with these carts? Any solutions?
SY, it would be great if you could share any valuable learning experience you gained from your attempt!
The second design is for the less common "hill and dale" stereo.
It seems crosstalk was a problem with these carts? Any solutions?
SY, it would be great if you could share any valuable learning experience you gained from your attempt!
To answer my own question someone suggested FM Multiplex technology on another forum, like radio stations use. No idea how that works.
Or could you use different frequencies for the RF circuit : like tuning into different radio stations and eliminate crosstalk altogether? More questions than answers, I know.
Again, way over my head.
Or could you use different frequencies for the RF circuit : like tuning into different radio stations and eliminate crosstalk altogether? More questions than answers, I know.
Again, way over my head.
Brainstorming!!!!!
True. But I think the idea was to apply it to the low noise oscillators. You need two in the same cartridge for stereo but they can't interfere with each other. Right?
-Maybe a simpler solution is if you can attenuate both channels with a simple foil enclosure then the foil would reflect any unwanted interference between left and right.
Basically, what's the best solution to this problem Paul Weathers ran into 50 + years ago?
Any radio hobbyists want to chime in?
True. But I think the idea was to apply it to the low noise oscillators. You need two in the same cartridge for stereo but they can't interfere with each other. Right?
-Maybe a simpler solution is if you can attenuate both channels with a simple foil enclosure then the foil would reflect any unwanted interference between left and right.
Basically, what's the best solution to this problem Paul Weathers ran into 50 + years ago?
Any radio hobbyists want to chime in?
Maybe
Could you keep the mass down by using a dynamically balanced tone arm?
Like here Scroll down half way down the link
Could you keep the mass down by using a dynamically balanced tone arm?
Like here Scroll down half way down the link
SY said:
Do you mean the moving mass on the stylus/cantilever assembly? You could always go to a higher polarization (excitation) voltage and reduce C.
What about nano-strain guages bonded right to the cantilever. Not sure about the heat from the excitation voltage, but diamond is a great heat sink.
SY, I may have an illuminating piece of information. It turns out the weathers also relied on a small brush to achieve it's one gram stylus pressure! 
Here's an excerpt from the original patent for the mono version:
"In one embodiment of the invention it may be in the form of a multiple number of bristles or hairs to form a relatively short flexible brush structure attached relatively flexibly to the pickup body and so proportioned that it effectively reduces the vertical pressure on the record surface of the stylus from 4 to 5 grams to approximately one gram value or any desired value. The multiple element, flexible, or resilient supporting structure is so designed that the stylus controls the lateral feed across the record. The total combined area of contact of the bristles or individual flexible elements in the supporting structure is sufficiently great to make the unit pressure on the record exceedingly small.
The action of the resilient supporting device damps both lateral and vertical swinging tone arm resonance, and additionally cleans the record ahead of the stylus when constructed of hair-like bristles of proper material. It has been found that by the pickup unit controlled in stylus pressure,rumble from the motor drive elements of a phonograph record reproduction system may be substantially eliminated as well as acoustic feedback from any associated loudspeaker. The dimension of the individual flexible elements of the multiple support structure are so chosen that the individual elements do not effectively engage the grooves to act as lead-in devices but ride along the record surface wholly as resilient supporting and damping means directly adjacent to stylus point. It has been found that a pickup as described, in accordance with the invention, will thus track even a 78 R.P.M. shellac record at one gram stylus pressure and not jump a groove even when operating in a moving vehicle."
read more here
Here's an excerpt from the original patent for the mono version:
"In one embodiment of the invention it may be in the form of a multiple number of bristles or hairs to form a relatively short flexible brush structure attached relatively flexibly to the pickup body and so proportioned that it effectively reduces the vertical pressure on the record surface of the stylus from 4 to 5 grams to approximately one gram value or any desired value. The multiple element, flexible, or resilient supporting structure is so designed that the stylus controls the lateral feed across the record. The total combined area of contact of the bristles or individual flexible elements in the supporting structure is sufficiently great to make the unit pressure on the record exceedingly small.
The action of the resilient supporting device damps both lateral and vertical swinging tone arm resonance, and additionally cleans the record ahead of the stylus when constructed of hair-like bristles of proper material. It has been found that by the pickup unit controlled in stylus pressure,rumble from the motor drive elements of a phonograph record reproduction system may be substantially eliminated as well as acoustic feedback from any associated loudspeaker. The dimension of the individual flexible elements of the multiple support structure are so chosen that the individual elements do not effectively engage the grooves to act as lead-in devices but ride along the record surface wholly as resilient supporting and damping means directly adjacent to stylus point. It has been found that a pickup as described, in accordance with the invention, will thus track even a 78 R.P.M. shellac record at one gram stylus pressure and not jump a groove even when operating in a moving vehicle."
read more here
Stax built a stereo FM cartridge system many years ago. It was troublesome. I worked with them to explore designing an RF hybrid chip to make it work. We made some progress on paper but at the time (early 80's) the noise of the oscillator was a limitation. I was trying to move the frequency from 10 MHz (I think) to 1 GHz to get a much better s/n ratio. The capacitance of the stylus assembly of the Stax was around 1.5 pF and the delta much less.
The Sennheiser MKH microphones use a very similar principle with some very special magnetics in the discriminator to get a really good s/n ratio. The self noise is below that of the air moving on the diaphragm.
The Sennheiser MKH microphones use a very similar principle with some very special magnetics in the discriminator to get a really good s/n ratio. The self noise is below that of the air moving on the diaphragm.
Another couple ways of doing it, probably easier to diy-
Scanning probe microscopes use a tiny silicon stylus at the end of a flexible silicon cantilever. A laser is bounced off the back of the stylus onto a quad cell to detect motion. You could probably do something similar, but bounce the laser off a shiny spot on the other end of the cantilever. That way the laser and detector can be located in the middle of the cartridge body.
Ancient Motorola car radios were reluctance tuned- a ferrite slug was moved in and out of a coil to change the frequency, actually a whole gang of them. I've built position measuring devices on the same principle. It might be possible to mount two very small slugs of ferrite on the cantilever, each at the mouth of a small coil, changing its inductance as the ferrite moved. The coils become the tuning element of a high frequency oscillator, so you need a frequency to voltage converter to get audio. IMO, it's easier to do it this way, than by changing capacitance, and with the right design it can be very linear.
All way to tiny. I'll just keep buying cartridges and building amplifiers out of nice big parts.
Scanning probe microscopes use a tiny silicon stylus at the end of a flexible silicon cantilever. A laser is bounced off the back of the stylus onto a quad cell to detect motion. You could probably do something similar, but bounce the laser off a shiny spot on the other end of the cantilever. That way the laser and detector can be located in the middle of the cartridge body.
Ancient Motorola car radios were reluctance tuned- a ferrite slug was moved in and out of a coil to change the frequency, actually a whole gang of them. I've built position measuring devices on the same principle. It might be possible to mount two very small slugs of ferrite on the cantilever, each at the mouth of a small coil, changing its inductance as the ferrite moved. The coils become the tuning element of a high frequency oscillator, so you need a frequency to voltage converter to get audio. IMO, it's easier to do it this way, than by changing capacitance, and with the right design it can be very linear.
All way to tiny. I'll just keep buying cartridges and building amplifiers out of nice big parts.
I thought the Stax would have the benefit of lower tip mass and less interaction with the load. I was alo looking to get a DC output proportional to deflection that I could use to drive a servo. It was too much of a projects for too little (and there was no way to get $500 for a cartridge then). Today, being able to get $10K for such a solution may make it possible. But not necessarily an improvement.
There is a turntable system that tracks only with a laser (Finial Audio, now ELP ELP ) it may be the best way to solve the problem, BUT I have heard that it does have some tracking problems with inner grooves.
There is a turntable system that tracks only with a laser (Finial Audio, now ELP ELP ) it may be the best way to solve the problem, BUT I have heard that it does have some tracking problems with inner grooves.
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