12AX7/12AU7 Hybrid Amplifier circuit test and verification
Below a close picture of the 12AX7 tube powered on.
A picture of the whole board powered on.
Left orange LED is +24V power on, right orange 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 treble. Above the bass control is the input gain (it has no real effect on the 12AX7/12AU7 amplifier.
The 24-pins TSSOP device on the right is the 25W class-D power amplifier.
12AX7 tube is plugged into a socket.
|12AU7/12AX7 Hybrid 25W Amplifier|
Wave signal verification with oscilloscope
Summary of results:
- First JFET amplifier gain is 0 dB
- First 12AX7 tube stage gain is 10dB
- Second JFET buffer gain is 0 dB
- 3-band equalizer gain at 1kHz with Bass, Mid and Top = 0.5 is -17dB
- Second 12AX7 tube stage gain is 8.5 dB
- Third JFET amplifier gain with gain control and Boost Off:
- G2=0.0 =>-22 dB
- G2=0.5 => -8 dB
- G2=1.0 => -5 dB
- Third JFET amplifier gain with gain control and Boost ON:
- G2=0.0 => 3 dB
- G2=0.5 => 17 dB
- G2=1.0 => 20 dB
Maximum output level is 7880mVpp which corresponds to a maximum total gain of 22.4dB
|Input signal: Sine wave 600mVpp 1kHz|
|TP2 signal after first JFET buffer 600 mVpp (0 dB)|
|TP4 signal after first triode anode 1900mVpp (10 dB)|
|C16 signal after second JFET buffer 1900mVpp (0 dB)|
|C20 signal after 3-band equalizer 280mVpp. Treble=Middle=Bass = 0.5 (-17 dB)|
|TP21 signal after third JFET buffer 290mVpp (0 dB)|
|TP5 signal after second triode anode 760mVpp (8.5 dB)|
|C27 signal after fourth JFET amplifier 57mV, Gain 2 = 0 (-22.5 dB)|
|C27 signal after fourth JFET amplifier 300mV, Gain 2 = 0.5 (-8 dB)|
|C27 signal after fourth JFET amplifier 450mV, Gain 2 = 1.0 (-4.6 dB)|
|C27 signal after fourth JFET amplifier 1100mV, Gain 2 = 0.0. BOOST ON (3.2 dB)|
|C27 signal after fourth JFET amplifier 5630mV, Gain 2 = 0.5. BOOST ON (17.4 dB)|
|C27 signal after fourth JFET amplifier 7880mV, Gain 2 = 1.0. BOOST ON (20.3 dB)|