lowish voltage (110V) tube HP amp/preamp idea...

[kodabmx]

It uses a 6N6P and a 6P43P loaded with a TL783. I think it will work as a headphone output for my 470R phones, as well as being a line stage.

Does anyone see any glaring errors?

 

[Osvaldo de Banfield]

It may work OK, but always it would better to use a White Cathode Follower because of its lower output impedance, and active "pull down" of the load.

 

[kodabmx]

Trying to keep it SE... But ya, that's an idea...

 

[Osvaldo de Banfield]

WCF is also a kind of SE. But it is only my suggestion.

 

[kodabmx]

"Active pull down"
It's actually push pull.

"It is a push-pull buffer design that allows the delivery of up to twice the idle current into the external load impedance, twice as much as a conventional cathode follower can deliver."
"The White cathode follower belongs to the family of push-pull, class-A circuits that includes the SRPP and SRCFPP, as all three circuits use a current-sense resistor to create the needed anti-phase signal for push-pull operation."
Variations on the Theme of the White Cathode Follower

 

[Mark Tillotson]

I'd worry about the current noise from the TL783 - it claims voltage noise is 0.003%, which through that 43 ohm resistor becomes about 1µA, ie 0.5mV across the 470R phones just between 10Hz and 10kHz...

 

[Osvaldo de Banfield]

Yes, OK, but this scheme lowers the output impedance, an important fact when loaded by nonlinear load like speakers. Also, a small NFB loop can enhance this point.

 

[kodabmx]

I'd worry about the current noise from the TL783 - it claims voltage noise is 0.003%, which through that 43 ohm resistor becomes about 1µA, ie 0.5mV across the 470R phones just between 10Hz and 10kHz...

What do you think the noise of the 6P43P above it is? Do you think another 6P43P in place of the TL783 would be lower noise?

 

[Mark Tillotson]

1µA current noise is way above anything else in the circuit I suspect. Remember its over a 10kHz BW, so its 10nA/√Hz, current noise is normally measure in pA or fA. A 10k resistor has 1.3pA/√Hz thermal current noise for instance, 60dB less.


Of course that noise spec in the TL783 datasheet is probably only a rough/ready thing, perhaps a worst case including bad flicker noise, but still its high - most voltage regulators are not low noise amplifiers. Worth trying to see how bad it is in practice with a real part - can always substitute a 1k5 resistor as a load to compare.