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Trying to determine proper resistance loading for my Pass Labs Xono and a Lyra Etna MC phono cartridge. What is the measured input capacitance of the Xono phono stage set to moving coil?I measure a total capacitance of about 725pF from the cartridge phono connectors on the right channel red and green tone arm leads (cartridge disconnected). So that total includes the tone arm IC cable (Discovery Cable?) in a Triplanar VII arm plus the Xono phono stage input capacitance that's connected to it and turned off.
This seems pretty high. The Lyra instruction table for cartridge resistance loading only goes to values of 600pF (It specifically states it's a combination of tone arm cable plus phono stage input capacitance) and notes that the pF of the phono stage should be minimal with most of the capacitance expected in the tone arm cable. Standard tone arm wiring capacitance should be 100pfpF per foot or less.
I double checked that the Xono cartridge dip switch was set to MC and all the loading capacitance value switches were off. I also zeroed out the effect of the capacitance meter test leads before I connected them to the tone arm connectors for measurement. I double checked polarity (shouldn't matter).
I can measure the phono stage alone, call Kent at Pass Labs, etc. but am wondering if I am missing something. Something here does not seem right.
Thanks for your help!
Edits: 11/26/15Follow Ups:
> but am wondering if I am missing something. Something here does not seem right.
It is not possible to measure input capacitance of a phono stage with a capacitance meter. You should make your measurement with the XOno disconnected. A solid state phono stage will normally have input capacitance in the neighborhood of 50-pF without any additional capacitors connected to its input.
Try measuring the wiring capacitance and adding 50-pF as an estimate of the phono stage capacitance. For example, the phono interconnects and tonearm wiring for my SME V measure 155-pF. Try making your measurement with both the cartridge and the XOno disconnected and then add 50-pF.
Good luck,
John Elison
Thanks John. The Ono/XOno schematics (I got from DIY audio) show a 100pF capacitor across the input of the MC stage so I guess that's my answer and I can measure the cable.
The coil inductance of the Etna is very low at 11uH. The electrical bandwidth of the system is sufficiently wide that it is largely insensitive to capacitive load. Indeed, taking your figure of 725pF, the bandwidth is 1.8MHz. Don't worry about it. Optimising the load resistance is more important and a good rule of thumb is 10x the output impedance of the cartridge.
Input capacitance on MC stages varies depending on the design, but the capacitance is being used as a filter for RF noise. For example the MF M1-ViNL is 470pF, plus the tonearm arm leads etc which in my case puts the total capacitance at 590pF.
Regards Anthony
"Beauty is Truth, Truth Beauty.." Keats
Many thanks. Understand your points and aware of the rule of thumb but it's hard to completely ignore the recommendations of the designer.
Lyra's J. Carr seems to have a different view of cartridge loading than many others as he prefers higher resistance loading all other things being equal. He's focusing on potential overload distortion of the phono stage well above the 20kHz region as you can see in this link:
http://www.whatsbestforum.com/showthread.php?15077-Cartridge-Loading-A-Misnomer/page4
This cartridge is still breaking in so the sound is changing, but I'm preferring ~200 ohm loading so far. I think Carr would argue for a resistance more than twice that (I forgot I have the Xono schematics so know the input capacitance).
In the end, I'll go with what sounds best. This Etna cartridge is really phenomenal, but it's a bit of a cameleon between break-in and different loadings. Like you said don't worry about it....
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