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Cock-ups With Ceramics
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    Posted: 28 Mar 2023 at 12:35pm
How did the ceramic cartridge die? Were they really crap? 

Or was it because the "RIAA curve" wasn't understood?

One statement should have led to understanding, but it simply went straight above most heads.

"there is bass boost below 50Hz"

Take a look at all those RIAA curves and see if you can find it.

You cannot!

But back then in my early twenties, I was a mere beginner and thought the writers of electronics magazines and books knew everything, and I must be missing something - it had to be my fault.

It was not until an article by Gary A. Galo "Disc Recording Equalization
Demystified", originally published in 1996, that I understood it wasn't my fault after all.

The only way the replay RIAA curve can show the bass boost is by tipping it to the right by 45 degrees.

At that point, it can be seen that the response is shelved down between 500Hz and 2122Hz, and then it eventually clicks that the output of a magnetic cartridge must be rising at 20dB/decade.

But a ceramic cartridge doesn't do the same as a magnetic cartridge!
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Points: 16314 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 28 Mar 2023 at 10:02pm
What do we need to know to use a ceramic cartridge correctly.

1. the frequency response

According to the National Semiconductor Audio/Radio handbook (1980), the frequency response is constant amplitude, which means flat, so if the record has flat output the cartridge output will be just as flat.

It should be noted that the crystals used didn't have an extended high frequency response and often struggled to go beyond 10kHz. But how many musical instruments get higher than 10kHz? Probably only the higher harmonics.

Also according to the same book, the RIAA record amplitude falls between 500 and 2000Hz, and that is supposed to be a fall of 12dB.

But as everybody knows (??) the 500Hz and 2000Hz corners are not sharp corners. If the frequencies are to be believed, then at 500Hz there is -3dB compared to lower frequencies, and at 2000Hz there is +3dB compared with higher frequencies, so between 500Hz and 2000Hz there can only be 12dB - 3dB - 3dB = 6dB.

(a sheet metal worker will understand such bends as set-backs and bend allowances)

2. the source capacitance

According to my Peak LCR meter, the source capacitances of a Sonotone 9TA and a BSR ST20 are somewhere between 500pF and 600pF, so we could perhaps take that to be 550pF.

3. the RIAA record 50Hz boost

This is the flat portion of the velocity/RIAA response at the beginning of the plot on the left of the graph.

If tipped to the right it would be flicked up to its left. That indicates the pre 50Hz boost, so the response is up by 3dB at 50Hz.

With a 550pF source capacitance the input impedance of the preamp should be

1/2pi * 0.00055uF * 50Hz = 5.787 Meg-ohms.

Ever seen such a preamp?

Exactly.



Edited by Graham Slee - 28 Mar 2023 at 10:06pm
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This is a circuit from Practical Wireless in 1976 with component values for a Sonotone 9TA.

PW Ceramic PU preamp

It looks more like a converter to make it into the velocity/RIAA replay response.

PW Ceramic PU preamp response

But gave a better result than just loading it with 50k ohms like most DJ decks - see dashed green curve.

PW Ceramic PU preamp vs 50kohm load response

Still, the bass loss meant turning up the bass control. Even then the sound was bright.
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But, if you really want the cheapest and quickest trick to convert a ceramic cartridge to a velocity response suitable for a magnetic phono preamp, try putting a 3k3 resistor in parallel with the input.

ceramic PU response into 3k3
If it's too quiet, increase the resistance to say 10k, and if too loud, drop it to 1k.

Warning: I haven't tried this yet.
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Points: 16314 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 29 Mar 2023 at 6:29pm
I thought I'd give the simple load resistor a try, but I thought there wouldn't be enough output so instead of 3k3 I went for 13k, because I've rather a lot of them.

It's far too loud, so my original hunch of 3k3 might have been right...
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3k3 now, and the level is about right.

Bass is quite reasonable, but the upper shelf frequencies are a tad bright, so I set my Accession M to the "American" (NAB) EQ which tamed it a bit.

This suggests the 9TA cartridge has a rising response and that's shown in the plot on Roger Russel's site. It's up about 3dB.

In my ceramic preamp I went for just 6dB shelving boost instead of the 12dB the RIAA standard calls for, and I think that might be right, or at least in the right ballpark.
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Here's my nude Astatic 9TAHC (high compliance). Same load as above. Same NAB EQ. Much better sound quality.

Astatic 9TAHC "nude"
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