How to calculate maximum cartridge output voltage?

martyh

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Paid Member
2004-02-01 2:58 am
Wisconsin
I’m playing around trying to covert an old theatre photo soundhead preamp to an MC RIAA. If you are not familiar, these usually have a high ratio transformer on the input on the order of 1:100 turns. This brings up the question of overloading the input tube on transients. I’ve searched but I can’t find a formula for calculating the maximum output of the cartridge given the output specification.

In this instance I would like to use a DL103R and the given specification is .25mV 1kHz @ 50mm /sec. horizontal. Is 500mm/sec. a safe value for maximum acceleration considering groove damage? Is the relationship between velocity and output voltage approximately linear? I.E if [email protected] 50mm/sec. then 2.5mV @ 500mm/sec.

Thanks
Marty
 
For what it's worth, many years ago, I measured levels up to 35 mV peak at the output of a moving-magnet cartridge while playing a rather loud dancehall reggae record with lots of treble. Assuming a sensitivity of 5 mV at 5 cm/s, that would boil down to 35 cm/s or 350 mm/s peak velocity.

Much longer ago, Tomlinson Holman measured levels up to 100 mV peak at the output of a high-output moving-coil cartridge with step-up transformer, probably with a cartridge sensitivity of 1.5 mV at 5 cm/s and with a 1:7 turns ratio, playing record ECM 1060. If I interpret his data correctly, that would boil down to 476.2 mm/s peak velocity.

Holman published his findings in two AES articles, "New factors in phonograph preamplifier design", Journal of the Audio Engineering Society, vol. 24, May 1976, pages 263...270 and "Phonograph preamplifier design criteria: an update", Journal of the Audio Engineering Society, vol. 28, number 5, May 1980, pages 325...330. I only have the latter article, which is a bit unclear about exactly what cartridge was used when measuring the 100 mV peak.
 

PRR

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Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
There are several kinds of "theater soundheads", but I can't imagine using a 1:100 ratio input iron on any of them.

The 47k standard phono interface assumed a vacuum tube grid. NO transformer. For MM just run the needle into the first grid.

For MC needles you want 1:10 input transformation (1:5-1:30). A 1:100 input transformation would put input impedance near 10 Ohms. (Which is why I have doubts about it as a soundhead input.)
 
Typically you should aim for 20dB overload at the input. That is, if cartridge is specified as 0.5mV (at 1kHz, 5cm/s), expect 5mV peaks. Also keep in mind, it's another 20dB in the treble region due to equalization. Now you're getting close to 50mV at HF.

That's why I put the LF turnovers first in a phonostage, I want to pull down HF as much as I can early, to prevent overloading of second stage.