Thank you all for your replies. I forgot to write the correct value of the coupling caps. Yeah I wasn't sure with the output imoedance but I read an article where was showed that with this values of plate and cathode resistors and with feedback resistor of 100k the output impedance should be 10 ohms which I know isn't still low enough for 32 ohm headphones. I was also wondering (maybe it is a stupid idea) if i could't increase the impedance by putting a proper resistor in series with the headphones?
Yeah I know that it would waste power but in this case I guess that the only other effect would be lower volume?
The Dynaco PAT5 (opamp based) preamp included a circuit where a series resistor (or perhaps voltage divider) was placed before the preamp's headphone output jack. The resistor values were chosen to present a suitable load to the power amp when you switched out the speakers for headphones-only listening.
If you had connected the power amp outputs to a set of binding posts on the back of the preamp, the headphone jack sent a suitably attenuated output signal to the headphones.
I guess I'm saying about the series resistor idea that 'Yes, that's been done'.
I'm not saying it's a good idea, though.
If you had connected the power amp outputs to a set of binding posts on the back of the preamp, the headphone jack sent a suitably attenuated output signal to the headphones.
I guess I'm saying about the series resistor idea that 'Yes, that's been done'.
I'm not saying it's a good idea, though.
Tom it would be worth giving ltspice a go to see what works and what doesn't. Although it may not give you a correct design it will show you where things go wrong and give a better understanding. I find it a useful tool. The tube library is on diyaudio.
It's interesting to compare with a transformer OPT. The later would give you the same drive at half the HT and heater current and better DC isolation. However they are bulky can be difficult to source for headphones/expensive and for headphones couple hum from the mains transformers very easily.
The low RP and high enough current rating is why everyone and they dog use 6AS7s 😀 .. next it will be a 6SN7 on the front and the move to the darkside is complete.
There are alternatives - especially when running high impedance headphones. The other alternative is to bring forth the silicon. Using a cathode follower to handle the current or simply make a solid state buffer on the back of the amp as an impedance matcher. Or you go down the ZOTL route.
No free lunches unfortunately.
My power amps draw 200W to 300W and output less than 1W most of the time... What's the difference? 😀
I get over 1W into the ATH-R70x (470R) from 6N6P though.
I get over 1W into the ATH-R70x (470R) from 6N6P though.
