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Vinyl Asylum Welcome Licorice Pizza (LP) lovers! Setup guides and Vinyl FAQ. |
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In Reply to: RE: In that case I am curious . . . posted by Biff on August 16, 2012 at 07:32:27:
It's not the voltage or the plate resistor value. The cathode should be bypassed but the real problem is the load.
A 12ax7 will not drive (without getting into trouble) the impedance represented by the RIAA filter in that design.
Let's look at the RCA 12ax7 phono pre.
http://diyaudioprojects.com/Schematics/7025-12AX7A-Tube-Phono-Preamplifier-Schematic.htm
This is a simple phono pre with a "one shot" RIAA filter between the two triodes of a 12ax7.
R6 (R204 in the Cornet a 10k) is 470k.
The output is labeled 220k ohms min.
The operating point of the 12ax7 stages themselves are fine (well, not really. I would run twice the current but that's another story). It's the load that's the problem.
In the second stage of the Cornet the tube is playing into 3520 ohms at the high frequencies.
Again, it's not the operating point of the 12ax7 in the Cornet that is wrong. It's the load line. At the high frequencies the tube is being loaded down so you have the filter shaping the frequency response and you have the loaded down tube shaping the frequency response.
When a filter shapes the frequency response you get the response you want and the phase shift you want but you don't get added harmonic distortion.
When a loaded down tube stage shapes the frequency response you get added harmonic distortion.
If the filter is designed to not load down the tube, you get the phase and frequency response you want without the added harmonic distortion.
A phono preamp doesn't have to load down the tube to shape the frequency response. Think of a RC filter as a frequency dependent voltage divider.
The RIAA filter designer tool in the link below assumes that the gain stage has a zero ohm output impedance and is not being loaded down my the filter. The filter is doing the frequency response shaping all by itself.
If the values chosen, by the user, load down the tube then the frequency response will not be correct. I assume the designer of the Cornet took the fact that the tubes are being loaded down into account and adjusted the values of the RC filter networks accordingly.
That solves the frequency response problem but does not address the harmonic distortion problem.
I could be all wrong. The Cornet might be a great phono preamp.
I just don't understand why anyone would build it that way.
Tre'
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- RE: In that case I am curious . . . - Tre' 10:53:17 08/16/12 (0)
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