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Old 6th April 2005, 06:29 PM   #11
docali is offline docali  Germany
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Hi,
input voltage was 5mV in order to resemble a phono cartridge. Output level is nearly 500mV or less because the mu of 12ax7 is 100 and the mu follower has an amplification of nearly 100, decreasing with decreasing intermediate resistor.

Changing the load resistor to smaller values is a good idea! I will test it...
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Old 6th April 2005, 09:20 PM   #12
EC8010 is offline EC8010  United Kingdom
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Docali, you really should try substantially increasing R5 (and the HT voltage to suit). As a rule of thumb, R5 should drop about 100V.

As it happens, I was testing some 6SL7 today with an active load (similar to a good mu-follower) and typically measured about 0.15% 2nd harmonic at an output level of 14VRMS. All other harmonics were better than 20dB down on the 2nd. Since distortion is proportional to level, I would expect 0.005% 2nd harmonic to be achieved at your test level of 500mVRMS.
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Old 7th April 2005, 06:54 AM   #13
docali is offline docali  Germany
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Ok, some variations for the mu follower without coupling capacitor:

First, I lowered the load resistor R4 to 100k as proposed and I tried larger intermediate resistor R6. It should be mentioned that for every load resistor the voltage was atjusted to give nearly the same current and the only difference is the voltage to be dropped across the intermediate resistor. This procedure was also used for all previous investigations.

But the result is the same. The second order distortion undergoes a minimum.

What could we learn from this? The mu follower with coupling capacitor with respect to distortion should have an intermediate resistor as large as possible but if we remove the coupling capacitor of the lower stage things change and we have an optimal area of intermediate resistors with respect to second order distortion.
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