Is my X26 cutting head loud enougH?

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Lima Victor
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Joined: Sun Feb 16, 2014 3:43 pm

Is my X26 cutting head loud enougH?

Post: # 48770Unread post Lima Victor
Sat Dec 23, 2017 5:06 pm

I am currently restoring a Wilcox Gay Recordio 6b40 and everything seems to be working pretty okay except for really low recording levels.
I know this type of record head often suffers from defective crystals but mine is still producing some sound altough not much can be heard when playing back the record.
We have hooked up the recording head to an amplifier with an output voltage of approx. 25 volts and also tried the internal amplifier, but both lead to the same result.
In the video below you can hear the amount of sound the cutting head makes
Can anyone tell me if this seems normal or if it should be louder?
I want to avoid opening up the head.
It's kind of hard to estimate the sound levels from the video, obviously. But it's soft enough to be drowned out by someone typing on a quiet computer keyboard.
That said, using a homebrew piezo transducer solution doesn't seem to be producing that much more sound but then I'm not sure if that's a great reference either.

Thanks in advance!

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markrob
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Location: Philadelphia Area

Re: Is my X26 cutting head loud enougH?

Post: # 48774Unread post markrob
Sun Dec 24, 2017 9:38 am

Hi,

You are very lucky that the head is still working after all these years! 25 volts drive level seems a low to me. You really can't drive a crystal head with a standard solid state power amp and get a decent level. They need to see something more like 100 Volts AC rms and up to get moving. These are very high impedance heads. You can drive the cutter head from a modern solid state amp if you use a transformer to match the impedance of the head to the amp (see the attached PDF's of a similar head). You would need an 8 ohm to 60000 ohm matching transformer to make that happen. Note that the transformer has to be able to pass about 10-15 Watts. The step up ratio is related to the impedance ratio by the square root. So 60000/8 = 7500:1 impedance ratio. That is the same as about 87:1 voltage ratio (square root of 7500). One poor man's way to get this is to use a power transformer in reverse to act as a step up. Looking at the Digikey site, I found this possible match:

https://www.digikey.com/product-detail/en/triad-magnetics/F-301X/237-1960-ND/5032162

This is a 120/240V to 2.5V step down power transformer. If you wire for 240V, the ratio is 96:1. That should work pretty well. The power handling is a bit low, but it should get you started. The 240V side connects to the head and the 2.5V side connects to the power amp. You can also wire for 120Vac operation if the head loads down the amp too much. That reduces the voltage ratio by half and increases the load seen by the amplifier by a factor of 4.

Hope you find this helpful.
Astatic X26 Crystal Cutter Head 2.JPG
Astatic X26 Crystal Cutter Head 1.JPG
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Lima Victor
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Re: Is my X26 cutting head loud enougH?

Post: # 48776Unread post Lima Victor
Sun Dec 24, 2017 5:05 pm

Sadly, i found the reason why there was sound but no vibration at the stylus. There is a fracture in the crystal plate so only the piece connected to the audio input is still producing sound.
Vibrations aren't coupled through to the stylus piece.

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Lima Victor
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Joined: Sun Feb 16, 2014 3:43 pm

Re: Is my X26 cutting head loud enougH?

Post: # 48777Unread post Lima Victor
Sun Dec 24, 2017 5:48 pm

It seems i cannot edit my posts. Is that correct?

I have now inspected it a bit further. How does the head even work? I would expect the stylus to move up and down, instead of left to right, if you look at the way the crystal is in there. Can the glassy plate even move from side to side?

I can also see there's enough space to build a small coil into. Maybe a flat coil from a laptop HDD. That would require modification of the drive circuit though.

My first experiments in cutting records were with a standard piezo transducer with the stylus mounted on the side, so it would move side to side. I used a small amplifier with a step up transformer indeed, to get 60-120v out. Above 60v the modern piezos tend to flash over...

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markrob
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Location: Philadelphia Area

Re: Is my X26 cutting head loud enougH?

Post: # 48778Unread post markrob
Mon Dec 25, 2017 10:16 am

Hi,

Too bad about the broken element.

I don't believe that the stylus moves side to side. Instead it twists. So the crystal element operates in a torsional mode. I would think that a typical piezo tweeter would not create enough deflection to work well in a cutter head design. You need to move +/- 0.010" or +/- 150um or so to cut lower frequencies. I've never payed with one of these heads, so maybe somebody with more experience can make a suggestion. Off the top of my head, I 'm wondering if the piezo element in one of the barbecue grill igniters might work. They generate a a 1000 volts or more after being struck by a mechanical trigger.

Mark

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Lima Victor
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Re: Is my X26 cutting head loud enougH?

Post: # 48779Unread post Lima Victor
Mon Dec 25, 2017 10:48 am

Torsional makes sense!
I have now resorted to using the flat voice coil and the magnet of a small HDD. Ample movement, but not a lot of power. Still have to finalise the design and replace the rubbers, but i think it will work. Will add a step down transformer to the amplifier.
If i could obtain strips of piezo material instead of the standard circular things i could maybe also fabricate a torsional system. I asked westtech about supplying me with a crystal but he doesnt send crystals. I wanna prevent spending a ton of postage to send the whole head to him for work i can do myself....

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