Wednesday, 9 August 2017

Korg Nutube 6P1 vs 12AX7 tube: Hybrid Amplifier (6)

Distortion analysis

A 1kHz 600mVpp sinewave is injected into the amp input, the waveform and spectrum at the output is analyzed for several gain values (0, 0.5, 1) to check the level of 2nd and 3rd order harmonics.

The volume level at 1kHz is set at 0dB for the maximum gain values (G1 = G2 = 1) and the relative attenuation is shown for different gain values combinations. The level in dB at 1kHz, 2kHz and 3KHz is measured. The relative attenuation of the second and third harmonic respect to 1kHz is also measured.

NuTube 6P1 results

With BOOST off:
G1 generates an attenuation of 1 and 4dB at G1= 0.5 and 0 respectively
G2 generates an attenuation of 4 and 19dB at G2= 0.5 and 0 respectively
Second harmonic has an attenuation of 20 dB (G2=1), 20 dB (at G2=0.5) and 21 dB (at G2=0)
Third harmonic has an attenuation of  35dB (G2=1), 36 dB (at G2=0.5) and 39 dB (at G2=0)
As we can see there is almost the same harmonic distortion at all gain levels.

With BOOST on:
G1 generates an attenuation of 0 and 1dB at G1= 0.5 and 0 respectively
G2 generates an attenuation of 4 and 19dB at G2= 0.5 and 0 respectively
Second harmonic has an attenuation of 10 dB (G2=1), 12 dB (at G2=0.5) and 16 dB (at G2=0)
Third harmonic has an attenuation of 21 dB (G2=1), 21 dB (at G2=0.5) and 23 dB (at G2=0)
As we can see there is a bit more harmonic distortion at higher gain levels.

Harmonic distortion is higher with BOOST on, this means that some distortion happens in the last JFET amplifier stage and some at the triode amplifier stages.

G1 G2 Boost Vpp (mV) Att (dB) 1kHz (dB) 2kHz (dB 3kHz (dB) 2nd /1st (dB) 3rd/1st (dB)
0.0 0.0 0 31 -23 -24 -44 -63 -20 -39
0.0 0.5 0 162 -8 -9 -29 -48 -21 -39
0.0 1.0 0 275 -4 -4 -25 -43 -21 -39
0.5 0.0 0 40 -20 -21 -41 -58 -20 -36
0.5 0.5 0 211 -6 -6 -26 -43 -20 -37
0.5 1.0 0 358 -1 -2 -22 -38 -20 -36
1.0 0.0 0 46 -19 -20 -40 -55 -20 -35
1.0 0.5 0 249 -4 -5 -24 -40 -19 -35
1.0 1.0 0 415 0 0 -20 -35 -20 -35
0.0 0.0 1 306 -20 -20 -37 -44 -16 -24
0.0 0.5 1 1747 -5 -5 -21 -29 -16 -23
0.0 1.0 1 2800 -1 -1 -17 -23 -16 -22
0.5 0.0 1 334 -19 -20 -32 -41 -12 -21
0.5 0.5 1 1937 -4 -5 -17 -26 -12 -21
0.5 1.0 1 3000 0 -1 -13 -21 -12 -21
1.0 0.0 1 350 -19 -20 -30 -41 -11 -21
1.0 0.5 1 1958 -4 -4 -15 -25 -10 -21
1.0 1.0 1 3080 0 0 -11 -21 -11 -21

12AX7 results

With BOOST off:
G1 generates an attenuation of 1 and 5dB at G1= 0.5 and 0 respectively
G2 generates an attenuation of 4 and 17dB at G2= 0.5 and 0 respectively
Second harmonic has an attenuation of 3 dB (G2=1), 8 dB (at G2=0.5) and 17 dB (at G2=0)
Third harmonic has an attenuation of 8 dB (G2=1), 12 dB (at G2=0.5) and 21 dB (at G2=0)
As we can see there is more harmonic distortion at higher gain levels, as expected.

With BOOST on:
G1 generates an attenuation of 0 and 3dB at G1= 0.5 and 0 respectively
G2 generates an attenuation of 4 and 18dB at G2= 0.5 and 0 respectively
Second harmonic has an attenuation of 4 dB (G2=1), 8 dB (at G2=0.5) and 19 dB (at G2=0)
Third harmonic has an attenuation of 11 dB (G2=1), 15 dB (at G2=0.5) and 21 dB (at G2=0)
As we can see there is more harmonic distortion at higher gain levels, which means that distortion and saturation depends a lot on tube grid input level.

Harmonic distortion is similar with BOOST off or on, since the BOOST happens in the last JFET amplifier stage this means that most of the distortion happens in the tube amplifier stages.

G1 G2 Boost Vpp (mV) Att (dB) 1kHz (dB) 2kHz (dB 3kHz (dB) 2nd /1st (dB) 3rd/1st (dB)
0.0 0.0 0 55 -26 -22 -39 -43 -17 -21
0.0 0.5 0 265 -12 -8 -26 -29 -17 -21
0.0 1.0 0 380 -9 -5 -22 -25 -17 -20
0.5 0.0 0 103 -20 -19 -26 -30 -8 -12
0.5 0.5 0 480 -7 -5 -13 -16 -8 -12
0.5 1.0 0 732 -3 -1 -9 -12 -8 -11
1.0 0.0 0 147 -17 -18 -21 -26 -3 -8
1.0 0.5 0 670 -4 -4 -7 -13 -3 -8
1.0 1.0 0 1060 0 0 -3 -8 -3 -8
0.0 0.0 1 248 -24 -21 -40 -42 -19 -21
0.0 0.5 1 1300 -10 -7 -26 -28 -19 -21
0.0 1.0 1 2025 -6 -3 -19 -22 -16 -19
0.5 0.0 1 442 -19 -18 -26 -32 -8 -15
0.5 0.5 1 2272 -5 -3 -12 -18 -8 -15
0.5 1.0 1 3280 -2 0 -9 -12 -9 -12
1.0 0.0 1 530 -18 -18 -21 -31 -4 -13
1.0 0.5 1 2705 -4 -4 -7 -17 -4 -13
1.0 1.0 1 4060 0 0 -4 -11 -4 -11

Conclusion

More harmonic distortion is observed in the 12AX7 tube than in the NuTube.
It is easier to obtain grid clipping and saturation in the 12AX7 tube than in the NuTube.

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