Thanks Thomas, I'm exploring other OTL alternatives.
You will not be able to drive 80 ohms phones to reasonable levels using a single tube like the 6SN7 or ECC88 in an OTL configuration. To output 10mW into an 80 ohm load requires a peak current of nearly 16mA which means the idle current of an OTL stage has to be at least this value. The thing is you only need a voltage of 900mV so you are asking a tube to produce a high current at a low voltage, exactly the opposite of what it does well. The answer is a transformer and with a suitable turns ration an ECC88 will happily drive a pair of 80 ohms phones to high levels at low distortion and with plenty of headroom.
Cheers
Ian
What about two 6N7 / 6N7C (Soviet) on top of each other? Or 6H6P-I ?
I don't have these tubes.
Phones are 300 ohms and I want to avoid OTL due to expensive prices. If necessary I can use more tubes.
TIA
Felipe
Oops, must have been thinking of another thread when I used 80 ohms in my calculation. For 10mW into 300 ohms you need a peak current of just over 8mA. You might just about get this from an ECC88 SRPP.
If you use 100 ohm cathode bias resistors and a 200V supply you can run at an idle current of 15mA and keep within the allowed plate dissipation and output 2.66 volts rms into a 300 ohm load with about 1.6% distortion according to LTspice which is just under 25mW. For 10mW you need about 1.75V rms which this circuit will do into 300 ohms with a distortion of just over 1%.
In practice I have found that LTspice overestimates the second harmonic distortion and underestimates the third.
You could reduce the distortion by an order of magnitude with suitable negative feedback.
Cheers
Ian
Ian, do you refer schematic post 5?
No, I was referring to an SRPP.
Cheers
Ian
Like this?
Yes, but ECC88, 200V supply and 100 ohm cathode resistors.
Cheers
Ian
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