In the "Some agreement on..." thread we got into some discussion of phono stages and cascode. Instead of prolonging that thread on something slightly OT I decided to start a new one.
I had been designing a phono stage with two stage passive EQ using 12AY7 based mu-follower (with 12AU7 on top) followed by 12AX7 CC followed by 12AU7 CF. Our discussion made me think that a cascode first stage might be much better. So I played with LTspice a little bit and came up with the following basic layout. Note: the R-C values will need some tweaking as I took them from the original circuit with a couple of slight modifications. Thus the response is not spot on yet but I thought I would see what you thought.
The gain of this circuit using all medium mu triodes (6N1P) is within three dB of the original which used much higher mu tubes. It sims at about 42dB gain at 1K. Output is...
20Hz=14.2dB
50Hz=12.3dB
65Hz=12.2dB
1KHz=-4dB
2.2KHz=-7dB
20KHz=-24.6dB
I tried it with both UL cascode and traditional cascode hoping to see what happens with distortion but as usual my FFT results in LTspice were not helpful. Even bypassing the upper FB resistor with a cap in series with a small resistor only reduced gain with no discernible effect on the total HD or its spectrum.
I would like to see if the cascode works with LED bias but I need to find one that gives about 1V.
I had been designing a phono stage with two stage passive EQ using 12AY7 based mu-follower (with 12AU7 on top) followed by 12AX7 CC followed by 12AU7 CF. Our discussion made me think that a cascode first stage might be much better. So I played with LTspice a little bit and came up with the following basic layout. Note: the R-C values will need some tweaking as I took them from the original circuit with a couple of slight modifications. Thus the response is not spot on yet but I thought I would see what you thought.
The gain of this circuit using all medium mu triodes (6N1P) is within three dB of the original which used much higher mu tubes. It sims at about 42dB gain at 1K. Output is...
20Hz=14.2dB
50Hz=12.3dB
65Hz=12.2dB
1KHz=-4dB
2.2KHz=-7dB
20KHz=-24.6dB
I tried it with both UL cascode and traditional cascode hoping to see what happens with distortion but as usual my FFT results in LTspice were not helpful. Even bypassing the upper FB resistor with a cap in series with a small resistor only reduced gain with no discernible effect on the total HD or its spectrum.
I would like to see if the cascode works with LED bias but I need to find one that gives about 1V.
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Take a look at IR leds, there are quite a few with Vf in the 1.1 - 1.2V range.. (They also often have very low dynamic impedances as well)
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