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I have tried a a search of this Forum as well as a general Google and can't find the specific answer. (so, "mea Culpa" if I missed an obvious source for this info)What capacitance load does a cartridge "see" at the input of a PAS 12ax7 based phono stage?
I am using a cartridge know to be sensitive to load capacitance, an AT 440MLa.
I have read that a 12AX7 based phono stage can have "high" input capacitance caused by tubes Miller Effect. I have not found a solid definition of what "High" means in real numbers.
My current IC based phono pre-amp, a Gram Amp 2se, is spec'ed by the manufacture as presenting a 100pf on the input. I like how my cartridge/arm/cable sounds into that unit - is the PAS even close to this?
Edits: 01/18/15Follow Ups:
Here is the most comprehensive study of the PAS phono section that I have ever seen. I have not read through the whole thing yet, but it might provide some direction in addressing this interesting question. Maybe someone could write to George and ask him.
Edits: 01/22/15
I had seen that file before and have just now reread it.
Perhaps I am missing it but I do not see a statement of the phono input capacitance. To be fair that is not the intent of the article but I was hoping.
Very Interesting and suggests some "playing about" with riaa values may be worth the experiment to hear what I hear.
I will take your suggestion to ask the author if he knows the input capacitance of the circuit.
Please let us know what he says.
I don't recall ever reading that spec. either. I'm sure someone has measured it at some point, but that's no help to you unless you can find their results.
Speaking of measuring capacitance, I learned on the Vinyl Asylum that measuring this in regards to phono stages, cables, etc. can't be accomplished with a simple capacitance meter. See link below, and good luck. If you DO find the answer, please post it. I'd like to put it into my Dynaco PAS information folder.
Yes, does seem odd this data is not out there someplace. With this phono stage in production for so many years it is hardly an obscure circuit.
Sites that talk about "Miller Effect" in 12ax7 are all over the place on what the functional capacitance might be. Seems the topology/function of the gain stages have an impact on Miller Effect capacitance. None of the sites I can find discuss the 12ax7 in exactly the same circuit as the stock PAS phono with its negative and positive feedback.
I lack the gear to do this check on my own. Perhaps if I were more clever I could compute it.
Your phono cable is also in the capacitance equation. Might so small it's insignificant though. The cartridge spec sheet should tell you the total input capacitance. Whatever the wires measure subtract that from the spec sheet and just add in the difference.
Just looked up the specs and it says nothing about capacitance loading. The loading is 47K ohms on the input. Standard MM cartridge specs.
Thanks for getting back on this.Electrically the body of the 440MLa is very close to the 150MLX. They can mount the same styli. The 150 MLX specs a 100 to 200 pF load range. Audio Technica cartridges in general give the flattest response into capacitive loads under 200 pF.
Many of us AT 440MLa users find loading with 33 K ohm, placing a 100 k resistor in parallel with the pre amp input, tames a resonance in the top end of the frequency response. Others report that the 150 MLX does not need this fix and sounds fine into standard 47k, possibly a result of the more rigid boron cantilever.
Be good to know if I can get close to 200 pF with the PAS 12AX7 based phono board. I do understand that tone arm and cable are part of the equation.
Edits: 01/20/15
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