6SJ7 Distortion Level

[jan.didden]

This is my version of line input attenuator for power and line preamp. The output level will be limited as clamped by zeners to about 1.2Vp accepted by line input, DC blocking capacitor is used to avoid passing DC. Some sound cards (in the low end PC) has phantom spectrum even without input so be aware.

I would decrease R5 to max 10k (and increase R3 accordingly). The zeners will alway leak a bit and with 100k R5 that will cause distortion even far below the the zener voltage.
It may not make a very big difference as the circuit distortion is already relatively high, but it will help at zero cost.

Jan

 

[drteming]

I did this little test under a very specific set of conditions in preparation for a new build. The output voltages for the 47k plate resistors were in the 110s and for the 100k, in the 120s, both with plenty of gain for what I want to do. The load was 100k throughout, as that was my main concern for this exercise. I initially drew up a 6SN7 cathode follower to drive the 100k load, and I was trying to get rid of it in order to simplify the build. It's going to be a SE amp, so nothing interesting.

 

[drteming]

2 weeks later...

Left Channel:

Right Channel:

I have a pair of JJ 6CA7 in there, but I can run any of the 6L6/EL34 family tubes. Adjustable fixed bias, regulated screen voltage in pentode/beam tetrode mode. Shunt feedback nested in GNFB. Sounds pretty dang good!

 

[fredeb]

I did this little test under a very specific set of conditions in preparation for a new build. The output voltages for the 47k plate resistors were in the 110s and for the 100k, in the 120s, both with plenty of gain for what I want to do. The load was 100k throughout, as that was my main concern for this exercise. I initially drew up a 6SN7 cathode follower to drive the 100k load, and I was trying to get rid of it in order to simplify the build. It's going to be a SE amp, so nothing interesting.

Your research is fascinating to a less knowledgeable fellow like me , thank you for posting . Quite amazing how easily odd and even order harmonic dominance change with incremental changes to the circuit , and that you were able to find a balance between low distortion and vanishing odd order harmonics . Well done !

Would you mind sharing your schematic ? Did you measure the amp upon completion , or will you still measure ?

Your build looks neat , and I am assuming the OPT's are those 25W rated Edcors .

 

[drteming]

My approach after a build is to listen to it first. If I don't like it, then I'll monkey with it until it sounds good. The amp sounded great, so I didn't bother measuring it. The OT's are Edcor 25Ws. The plate voltage is higher than in my tests because of the shunt feedback.