Super Triode Cascode SRPP Hybrid Amplifier
Such mouthful blog title.. The idea is nothing new: to get the so-called "tube sound" from a hybrid amp. Tube sound for me comes from these two factors:
1. Triode-like linearity
2. Low to moderate damping factor
Personally, i believe number two is the main reason for tube sound. Any solid state amplifier manipulated to get a damping factor of around 1-2, instead of the typical SS amp DF that falls into the tens to hundreds, will sound like a tube amp. This is because with such low damping factor, the amp ceases to ignore the speaker impedance curve and start to become less of a voltage source and more of a current source where the bumps of speaker impedance curve will affect the amp's output.
Without further ado.. here it is:
Some notes:
1. Ignore the transistor and MOSFET type. I chose them because that's what i have in LTSpice. The small signal BJT can use BC547 for NPN and BC557 for PNP. The MOSFET should use IRFP240 instead for lower Miller capacitance. I bet someone can suggest better option.
2. Paralleled triode is mandatory to have enough cathode current flowing to combat the MOSFET high Miller capacitance. 6N6P can be substituted with ECC99.
3. 6N6P is chosen to get enough cathode current with such high Vgk (+7V). This is done to keep the grid at ground level to avoid using input coupling cap. 6DJ8/ECC88/6N23P can be used with lower Vht2 of 130V just by moving the 1Meg grid leak resistor from ground to Q1 collector. Unfortunately, this means we can no longer DC-couple the input signal to the grid. See attached pic.
4. Worry more about getting less PS ripple on Vht2 (200V) than on Vht (48V). Such cascode connection yields poor PSRR and it's an issue that has to be tackled.
5. As shown, the output stage is working in push-pull or SRPP to be exact. We can turn it into single ended simply by moving R9 end that connects to M1 source to the lower part of R1 (M2 drain). We can use a relay with a switch on the front panel to change between push-pull and single ended. Of course single ended yields lower output power. Sounds like a neat idea though!
7. Notice how Q2 base is connected to Vheater. Vheater is a 6.3V DC supply for the tube heater. This will save us some hassle of the power supply section. Overall, the amp needs only three power supply voltages: 6.3VDC, 48V and 200V (Personally, i prefer to put the heaters in series and connect the base to the middle point.. 12.6VDC@750mA is easier for the connecting wires than 6.3VDC@1.5A).
8. There is no number 6 on this list
9. R9 is the key to get lowered damping factor and get us closer to tube sound. We can increase it to 1-2 ohm to get even lower damping factor but we are losing more output power.
10. Output Power before clipping is around 18W using SRPP configuration. Not bad for a Class A amplifier!
Here's some more reading related to the amp:
SuperTriode
Single-Ended Amplifiers & Super-Triodes
The Inverted Cascode
Variable Amplifier Impedance
Tube CAD Journal Page 24 good info on slew rate, Miller effect and driving current
1. Triode-like linearity
2. Low to moderate damping factor
Personally, i believe number two is the main reason for tube sound. Any solid state amplifier manipulated to get a damping factor of around 1-2, instead of the typical SS amp DF that falls into the tens to hundreds, will sound like a tube amp. This is because with such low damping factor, the amp ceases to ignore the speaker impedance curve and start to become less of a voltage source and more of a current source where the bumps of speaker impedance curve will affect the amp's output.
Without further ado.. here it is:
Some notes:
1. Ignore the transistor and MOSFET type. I chose them because that's what i have in LTSpice. The small signal BJT can use BC547 for NPN and BC557 for PNP. The MOSFET should use IRFP240 instead for lower Miller capacitance. I bet someone can suggest better option.
2. Paralleled triode is mandatory to have enough cathode current flowing to combat the MOSFET high Miller capacitance. 6N6P can be substituted with ECC99.
3. 6N6P is chosen to get enough cathode current with such high Vgk (+7V). This is done to keep the grid at ground level to avoid using input coupling cap. 6DJ8/ECC88/6N23P can be used with lower Vht2 of 130V just by moving the 1Meg grid leak resistor from ground to Q1 collector. Unfortunately, this means we can no longer DC-couple the input signal to the grid. See attached pic.
4. Worry more about getting less PS ripple on Vht2 (200V) than on Vht (48V). Such cascode connection yields poor PSRR and it's an issue that has to be tackled.
5. As shown, the output stage is working in push-pull or SRPP to be exact. We can turn it into single ended simply by moving R9 end that connects to M1 source to the lower part of R1 (M2 drain). We can use a relay with a switch on the front panel to change between push-pull and single ended. Of course single ended yields lower output power. Sounds like a neat idea though!
7. Notice how Q2 base is connected to Vheater. Vheater is a 6.3V DC supply for the tube heater. This will save us some hassle of the power supply section. Overall, the amp needs only three power supply voltages: 6.3VDC, 48V and 200V (Personally, i prefer to put the heaters in series and connect the base to the middle point.. 12.6VDC@750mA is easier for the connecting wires than 6.3VDC@1.5A).
8. There is no number 6 on this list
9. R9 is the key to get lowered damping factor and get us closer to tube sound. We can increase it to 1-2 ohm to get even lower damping factor but we are losing more output power.
10. Output Power before clipping is around 18W using SRPP configuration. Not bad for a Class A amplifier!
Here's some more reading related to the amp:
SuperTriode
Single-Ended Amplifiers & Super-Triodes
The Inverted Cascode
Variable Amplifier Impedance
Tube CAD Journal Page 24 good info on slew rate, Miller effect and driving current
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