I am working on an OTL headphone ampifier with 6SN7 for input tubes and 6AS7 in push pull for the DC coupled output stage. Since this also mean I have a risk of having b+ voltage send straight to my headphones I would like to keep the voltage as low as possible.
I have already fitted a diode/TVS clamp on the output for safety but would also like to keep the b+ voltage low, but how low can I go with the 6AS7 tubes?
I have seen designs from TubeCAD with +/-50V but can i go lower? And what would the consequences or effects of low voltage be? And can I trust my tube models at such low voltage?
The input tubes will still be allowed to have high voltage and will be AC coupled to the output stage.
I have already fitted a diode/TVS clamp on the output for safety but would also like to keep the b+ voltage low, but how low can I go with the 6AS7 tubes?
I have seen designs from TubeCAD with +/-50V but can i go lower? And what would the consequences or effects of low voltage be? And can I trust my tube models at such low voltage?
The input tubes will still be allowed to have high voltage and will be AC coupled to the output stage.
You have a output cap?
I suspect you’ll need a different output stage tube. The operating point for the tube would been to run low enough voltage but decent current.
Also depends on the headphones you’re attempting to drive. Not may tubes run with 5-10V whilst being in the best operating point and cope with high current. So your solution may need to specialise.
If you go hybrid then you could run low voltage but still have current. The only other way is old valves designed for cars at 12V. 12U7 for example maxes out at 30V unlike the 12AU7.
12 Volt Car Radio Tubes
Max seems about 20mA, so you’re back to higher voltage and high impedance phones I suspect..
I suspect you’ll need a different output stage tube. The operating point for the tube would been to run low enough voltage but decent current.
Also depends on the headphones you’re attempting to drive. Not may tubes run with 5-10V whilst being in the best operating point and cope with high current. So your solution may need to specialise.
If you go hybrid then you could run low voltage but still have current. The only other way is old valves designed for cars at 12V. 12U7 for example maxes out at 30V unlike the 12AU7.
12 Volt Car Radio Tubes
Max seems about 20mA, so you’re back to higher voltage and high impedance phones I suspect..
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The 6AU7 tube I intend to use is a tube made for line regulation and as such is a high current tube, which is able to handle 125mA average with a plate resistance of about 280ohm.
Unlike other designers then I do think I am being more realistic about the uoutput voltages I expect. My current aim is more or less 1-2Vrms which should be enough to drive most headphones to high volume or enough to drive a power amp if I decide to use it as a preamp.
My concern is more linearity than current capability if i choose to lower the supply voltage to something like +/-20-30V for the output stage while asking 2Vrms over a 40-300 ohm headphone.
Unlike other designers then I do think I am being more realistic about the uoutput voltages I expect. My current aim is more or less 1-2Vrms which should be enough to drive most headphones to high volume or enough to drive a power amp if I decide to use it as a preamp.
My concern is more linearity than current capability if i choose to lower the supply voltage to something like +/-20-30V for the output stage while asking 2Vrms over a 40-300 ohm headphone.
Recommended 6au7 Up is 100V for class A. However with near vertical load line, you could go below 50V.. but how linear the output is would perhaps need some eTracer testing etc.
Current compensation works but with headphones the impedances will vary over frequency (Fontana bridge for example). However you could use the 6au7 in a multiphase circlotron. However that would need enough isolation in the headphones.
Current compensation works but with headphones the impedances will vary over frequency (Fontana bridge for example). However you could use the 6au7 in a multiphase circlotron. However that would need enough isolation in the headphones.