Phono stage for 12B4 amp [from previous thread] - diyAudio
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Old 2nd April 2009, 04:20 AM   #1
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Default Phono stage for 12B4 amp [from previous thread]

The 12b4 design is done--next is the phono stage since I designed the power supply to add it. I will start a new thread in this forum.

My desire is to do a passive RIAA design with the third corner (at 50KHz) added. This will be a new one for me. In the past I've copied the Hagerman Coronet design as one that works very well, but I'd like to match to the 12B4 line stage.

1. Standard MM cartridge gives about 5mv output. Assume 4mv
2. 12B4 stage needs 200mv to drive full 2V ouput or 34db of gain, giving about 3db of clean headroom.
3. Insertion loss for RIAA stage =~ 21 db.
4. Total gain needed is thus 45db.

Thus a dual 6DJ8 stage is a possibility although a 12AX7 phono gain section is more universal. Also, the power supply is matched for low current tubes. A better possibilty might be 6SL7??? Would the 6SL7 be too microphonic?

Here is my suggested starting point for the passive RIAA section to be inserted between stages one and two of the phono gain section. Assumes 100K input impedance of second stage. I'm setting .1 uF for the first rolloff because I have a box of .1uF Tektronix Select matched .01% precision oil filled caps (used originally for tube based oscilloscopes).

Looking forward to recommendations and design tips.
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File Type: jpg riaa.jpg (10.6 KB, 382 views)
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Old 2nd April 2009, 09:26 PM   #2
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Default Whoops

Make that 55db of gain. Can't add to well late at night.
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