Sunday, 30 July 2017

Korg Nutube 6P1 vs 12AX7 tube: Hybrid Amplifier (4)

Korg Nutube 6P1 Hybrid Amplifier circuit test and verification

Below a close picture of the Korg NuTube 6P1 powered on.

Korg Nutube 6P1
A picture of the whole board powered on.
Blue LED is +24V power on, greeen LED is +5V power on.Top sliding switch is +24V on/off, just below is the +5V on/off switch. To the right is the Gain Boost on/off for the power amp 8ohm speaker section, just below, the blue potentiometer, is the output gain control, below is the Gain Boost switch for the headphones/line-out section, below it is the power amp volume, below at the bottom is the headphones/line-out volume.
On the left at the bottom are the three controls for the 3-band equalizer, based on a Fender tone stack: bass, mid and trebles. Above the bass control is the input gain.
The 24-pins TSSOP device on the right is the 25W class-D power amplifier.
NuTube 6P1 is plugged into a 2mm pitch socket resting over 2 rubber bumpers to avoid vibrations that could pick microphonic noise.
Korg Nutube 6P1 Hybrid 25W Amplifier

Wave signal verification with oscilloscope

Summary of results:
  • First JFET amplifier gain with gain control 
    • G1=0.0 =>-2.7 dB
    • G1=0.5 =>  0.1 dB
    • G1=1.0 =>  1.6 dB
  • First NuTube 6P1 triode stage gain is 14dB
  • Second JFET buffer gain is 0dB
  • 3-band equalizer gain at 1kHz with Bass, Mid and Top = 0.5 is -18dB
  • Second NuTube 6P1 triode stage gain is 15dB
  • Third JFET amplifier gain with gain control and Boost Off:
    • G2=0.0 =>-20 dB
    • G2=0.5 => -5 dB
    • G2=1.0 => -1 dB
  • Third JFET amplifier gain with gain control and Boost ON:
    • G2=0.0 =>-2.5 dB
    • G2=0.5 => 12 dB
    • G2=1.0 => 15 dB
Maximum output level is 12750mVpp which corresponds to a maximum total gain of 26.5dB

Input signal: Sine wave 600mVpp 1kHz
A 1kHz sinewave 600mVpp is used at the input
TP2 signal after first JFET buffer 440 mVpp. Gain 1 = 0.0 (-2.7 dB)
TP2 signal after first JFET buffer 610 mVpp. Gain 1 = 0.5. (0.1 dB)
TP2 signal after first JFET buffer 720 mVpp. Gain 1 = 1.0 (1.6 dB)
TP4 signal after first triode anode 2180mVpp. Gain 1 = 0.0 (13.9 dB)
TP4 signal after first triode anode 2910mVpp. Gain 1 = 0.5 (13.6 dB)
TP4 signal after first triode anode 3440mVpp. Gain 1 = 1.0 (13.6 dB)
C16 signal after second JFET buffer 3440mVpp. Gain 1 = 1.0 (0 dB)
C20 signal after 3-band equalizer 450mVpp. Gain = 1.0, Treble=Middle=Bass = 0.5  (-17.7 dB)
TP21 signal after third JFET buffer 380mVpp (-1.5 dB)
TP5 signal after second triode anode 2170mVpp (15.1 dB)
C27 signal after fourth JFET amplifier 220mV, Gain 2 = 0 (-20 dB)
C27 signal after fourth JFET amplifier 1230mV, Gain 2 = 0.5 (-5 dB)
C27 signal after fourth JFET amplifier 1950mV, Gain 2 = 1.0 (-1 dB)
C27 signal after fourth JFET amplifier 1620mV, Gain 2 = 0.0. BOOST ON (-2.5 dB)
C27 signal after fourth JFET amplifier 8810mV, Gain 2 = 0.5. BOOST ON (12.2 dB)
C27 signal after fourth JFET amplifier 12750mV, Gain 2 = 1.0. BOOST ON (15.4 dB)

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