| Bing Yang |
I have a HK A300. The circuit is very traditional. I modified it to bypass tone control and balance. Since the old 1M volume pot was not well balanced after cleaning, I replaced it with a 100K cheap volume pot. It worked for a while but shorted out one day. The volume was loud even at the lowest dial.
This volume pot is between the selector switch and the 12AX7 input amplifier. I did add a 500K grid resistor to the ground and hoping that it will compensate the smaller value pot. I also listened only CDs.
After I learned more and obtained a record player, I rebuilt the phono stage. The phono section was a very simple two stage 12AX7 with feedback. Then I found out that I need a 1M pot to match the impedance. So I put the pot back and suffer the balance problem.
My questions are:
1. Can I replace the pot with 100K if I only listen to CD? Could it be possible that my cheap pot was destroyed because a 100K pot cannot be used in front of a 12AX7? I do have a blue 100K ALPS pot that is quite expensive. I would prefer to use it instead of buying a new 1M pot. They are difficult to find.
2. If the first answer is yes, can I add a MOSFET follower right after my phono preamp 12AX7 output and before the output capacitor? This should lower the output impedance so that I can use the 100K pot. Is it true?
3. Since the phono section gain is low, I can also use the empty tube space for tone control to add a one tube gain stage with low plate resistor value. This will give more gain and lower output impedance. I think a simple 12AU7 should do the job. Any suggestions? |
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