In another thread rongon posted a simulation he did of a hybrid headphone amp using an ECC88 and a IRF710. https://www.diyaudio.com/community/threads/5842-headphone-amplifier.370857/post-7215958
What caught my eye was using a single supply to power the mosfet as well. Most hybrid designs usually run the silicon at a lower voltage. I threw together a messy prototype on the bench with a couple of minor resistor differences. R8 and R9 are now both 220k, the 10k pot is not present, and R12 is 27 ohms because well, I had these value on hand. I also added a 10uf cap on the B+ because the power leads were a few feet long. I increased C2 to 470uF to see how it responded to lower impedance loads. The input is pretty sensitive and 0.2v input will just about drive it to full output. The following was all taken at 0.2V in at 1Khz. The current is pretty close to the simulation.
One strange effect was taking the measurement unloaded. My scope and distortion analyzer were the only devices connected. The current draw was much lower at 25mA and the gain was initially low but steadily increased over a few minutes before stabilizing. Still not sure what was happening there. Looks like a good performer for high impedance headphones.
What caught my eye was using a single supply to power the mosfet as well. Most hybrid designs usually run the silicon at a lower voltage. I threw together a messy prototype on the bench with a couple of minor resistor differences. R8 and R9 are now both 220k, the 10k pot is not present, and R12 is 27 ohms because well, I had these value on hand. I also added a 10uf cap on the B+ because the power leads were a few feet long. I increased C2 to 470uF to see how it responded to lower impedance loads. The input is pretty sensitive and 0.2v input will just about drive it to full output. The following was all taken at 0.2V in at 1Khz. The current is pretty close to the simulation.
| 0.73V Out | 0.02% THD | Unloaded |
| 0.72V Out | 0.024% THD | 249R Load |
| 0.68V Out | 0.4% THD | 32R Load |
One strange effect was taking the measurement unloaded. My scope and distortion analyzer were the only devices connected. The current draw was much lower at 25mA and the gain was initially low but steadily increased over a few minutes before stabilizing. Still not sure what was happening there. Looks like a good performer for high impedance headphones.