[Photo] Motorcycle of Opel M100KIT Amplifier Kit

1. Reconnect the EL34 curtain grid resistor to the screen electrode, the resistance value is 100 ~ 220Ω, the purpose is to prevent self-excitation.
2. Change the cathode resistance of the two power amplifier tubes (EL34) from the original 10Ω to 430Ω (above 5W). Do not connect the cathodes of the two tubes (the place marked with × in Figure 1).
3. Connect the two 100kΩ gate-drain resistors to the terminal that was originally connected to the negative voltage, and connect them to the ground of the two 430Ω resistors.
4. In addition to changing the filter capacitor to 330μF / 450V, add a 220μF / 450V electrolytic capacitor and a 56Ω10W resistor to form the RCπ filter circuit. The purpose of this is to make the power supply voltage drop to 400V, and more importantly, it can greatly reduce the power supply ripple voltage. Observed by an oscilloscope, the ripple voltage at 420V is sawtooth, and the peak-to-peak amplitude is 6.4V. After π-type filtering, the ripple voltage at 404V is approximately sinusoidal, and the peak-to-peak amplitude is only 0.5V. The copper foil side of the original machine's printed circuit board faces the panel, and it can be installed in reverse so that this side faces the main printed board. In this way, the welding is convenient, and the wiring is also short. Consider the heat dissipation of the 56Ω10W resistor, and do not get too close to the electrolytic capacitor.
The modified circuit can work in principle without debugging, but in order to reduce distortion and hum, the output tubes must be paired. The method is to first plug in four output tubes, but without adding input signals, and then measure the DC voltage of their cathodes to ground, which should be about 28.6V. You can choose a pair of tubes with the same or similar cathode voltage as the output tube of a channel. Of course, the four tubes are better matched. After the pairing is completed, connect the cathodes of the pair of output tubes of the two channels respectively. Don't forget this step, otherwise the output power will be greatly reduced due to the negative feedback of the AC current generated by the cathode resistance.
In the case of sinusoidal signal input, this machine has used an oscilloscope to observe the waveform on the cathode resistance of the power amplifier tube. When the amplitude of the input signal does not reach the peak clipping distortion of the output signal, the waveform at “×” is a horizontal straight line, indicating that it works in pure Class A state and there is no distortion. When the maximum output power is approached, you can see a waveform with a smaller amplitude and a nearly sinusoidal shape.
In some classic self-biased power amplifier circuits, the common cathode resistance of the two tubes is combined with electrolytic capacitors at both ends. But in the experiment, it is found that the distortion of the bypass capacitor with or without overload is different: hard distortion when there is a capacitor, and soft distortion when there is no capacitor. Therefore, no bypass capacitor is added in this circuit.
The front stage of the M100KIT kit uses the SRPP plus long tail inverter circuit. Since the cathode resistance of the 6N8P is 16kΩ, it is not too large, so the symmetry of the output signal of the inverter stage is not ideal. Pole resistance.
Interested audiophiles can change to a differential inverter circuit (essentially long tail) according to the circuit in Figure 2. In order to improve the symmetry of the inverted output signal, it is best to change the common cathode resistance to a constant current source circuit (available Junction field effect tube, transistor or electron tube), the simpler method is to connect the cathode resistance to a negative power supply. The negative pressure rectification circuit of the power supply printed circuit board of the original kit can be used, about -70V, so that the cathode resistance can be increased to about 30kΩ. Because the first two stages of the circuit are directly coupled, this resistor should be adjusted so that the cathode voltage of the 6N8P is about 76V. At this time, the amplitude of the output signal is large and the distortion is also small.

After the installation of the machine, a simple test was carried out. The main technical indicators are as follows:
Closed loop voltage gain: 29.8dB; negative feedback: 5.5dB; open loop frequency response: 28Hz ~ 33kHz (-1dB, 1W); closed loop frequency response: 20Hz ~ 40kHz (-1dB, 1W); residual noise: left sound Channel 0.35mV, right channel 0.4mV; maximum output power: 15W (RMS).
During the audition, the Moray MW166 + MDT32 self-made speaker is used, with a sensitivity of about 86dB. Compared with the super linear connection method, the main feeling is that the mid-high frequency sound is very delicate and soft, and the high frequency extension is better. The low-frequency strength is slightly worse, but the low-frequency clarity is much better, especially the replay of musical instruments such as the cello is very good and very listening.
In order to adapt to everyone's listening preferences and the requirements of different software for sound playback, two toggle switches can be added to convert between three-pole and super-linear connection. These two switches can be installed in the position of the original gate negative pressure adjustment potentiometer. When listening to some more dynamic programs, use the super linear connection method, when listening to some light music, switch to the three-pole connection method, each has its own place.