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In Reply to: RE: "Those discs are just cosmetic" posted by dandaroy on October 26, 2020 at 20:02:46
It's hard to think of a worse type for buffer duty than the 12AX7. The triodes in that type "can't fight their way out of a wet paper bag".
My hunch is triode voltage amplification and SS buffers. FWIW, I did that in the tweaked RCA setup.
Eli D.
Follow Ups:
I claim total ignorance on tubes or their use in a audio circuitry. Here is a snippet from a magazine review of the unit. May be you can translate in layman's English:
"The Pro-Ject Tube Box DS2 phono preamplifier has two 12AX7 dual triodes. It is designed with the first triode as the input stage and is cascaded with a 2SK170 J-FET transistor. The gain of this stage is about 70. The second triode is used as a classic gain stage biased into pure class A."
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I. Dandaroy
I need to see an accurate, honest, schematic.
Frankly, I don't trust the magazine. Of course, the 2nd gain stage is Class "A". All Class "A" means is that the device conducts 100% of the time. That sort of operation is mandatory for single ended audio work. The "aroma" of marketing BULLSHIT is intense.
The late Allen Wright is known for advocating SS/triode cascodes at the input of phono preamps. However, cascode topology and 12AX7 sections are not what I expect. The 'X7 triode exhibits very low transconductance (gm) and that's opposite of what's desirable in a cascode.
Yes, there's a HUGE difference between cascade and cascode. That tweaked RCA setup I previously provided employs cascaded triode gain stages, with passive RIAA equalization between those 2 gain stages. The output is buffered by a MOSFET voltage follower.
I want to know how the vendor meets the requirement of driving the 10 Kohm IHF "standard" load. Commercially sold items must (IMO) comport with the IHF standards, as well as complying with local legal standards (FTC, etc. in the U.S.)
Eli D.
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