6AS7 OTL headphone amplifier

[merlin el mago]

It's enough 100mA for B+?

 

[v4lve lover]

I would refrain from using a shunt regulator, if the output tube is removed, the shunt sees a lot of dissipation, and might just die. So a thermal cut out might be in order..

 

[merlin el mago]

I'm sorry I can't follow about thermal cut?

 

[v4lve lover]

I meant a thermal switch, you'r either going to have to protect the circuit against too much heat, which you can do with a temperature sensor circuit. There are however switches that are used in industry that go open at a specified temperature.

The Other option would be to use a heatsink sufficiently large to be able to cool the worst case power dissipation of the circuit, with some margin because you live in a hot climate.

I suspect you want around 180-200V from a 250V supply (Rectified 230V) so your normal dissipation is going to be : ((VIN-VOUT)*Current out)+ (Vout*(Shunt CCS current-output current)

For Line input variations you have to take into account that your mains might be 10% high.

 

[merlin el mago]

Now I understand you, the SSHV2 is well protected have a big heat sink.

 

[audiowize]

I would use one 0D3 per channel. The 0D3 can take 100mA at startup and run at about 40mA. I'd set it to run 35mA while the 6080 and driver tube are running.

 

[merlin el mago]

I used OD3 or VR150 in my #26 DHT preamp and the SQ is far better the SSHV2.

 

[merlin el mago]

What's the total power consumption for both channels?

 

[merlin el mago]

How to measure or how to calculate?

TIA
Felipe

 

[audiowize]

About 35mA per channel.

 

[merlin el mago]

Thank you, in total 70mA?

 

[audiowize]

Yes.

 

[merlin el mago]

Thanks again.

 

[stokessd]

I'm late to the party as-usual. About two decades ago, I built a very similar headphone amp. It uses a triode voltage amp based on a 12AU7, and a 6080 as a cathod follower. But I put a bit of feedback around the whole gain stage, which reduces distortion and more importantly lowers the output impedance. Plus it gets the circuit gain in a more comfortable region.

Here's the schematic and a couple pictures:

http://quadesl.com/diyaudio/tube_hp.pdf

http://quadesl.com/diyaudio/IMG_8033.jpg
http://quadesl.com/diyaudio/IMG_8034.jpg


The astute will notice that the power supply is super sleazy in that I took a dual primary toroid power transformer and used one of the primaries as a b+ winding. Don't try this at home, kids...

I'm not really adding much to the discussion, other than to suggest that you try some feedback around the circuits described above.

Sheldon

 

[merlin el mago]

Thanks to share your experience.

Attached one pic just for fun. Works like a charm without hum.

It's OK each 7N7 only draws 1.33mA, I measured 133 volts across 100K plate load resistor?

You can check the datasheet for the 6SN7 for reccomended anode grid leak and cathode resistances. @ 150V B+ The anode will then hopefully bias up at +80V with respect to ground. Then you can choose whether to have the tube directly coupled to the 6080 or to include a RC bias circuit.

Edit i just checked for you, 2K2 cathode resistor, 100K anode resistor, 100K grid leak (Or the direct coupled pot like you show in your drawing. Gain will be about 24-28. You can and should bypass the cathode resistor

What cap value to bypass the cathode resistor of 7N7, or better how to calculate?

It's OK each 7N7 only draws 1.33mA, I measured 133 volts across 100K plate load resistor?

It's OK?

What cap value to bypass the cathode resistor of 7N7, or better how to calculate?

On line bypass capacitor calculator gives me min. 13uF for 2K2 of cathode resistors with a corner frecuency of 60Hz, it's OK?

 

[PRR]

OK.

 

[merlin el mago]

Both?

Finally ended with attached schematic kind provided by artosalo diyaudio member.

It's possible to mod for cathode LED bias or cathode battery bias to improve the SQ?

 

[hpeter]

i do not understand the reason behind r3,r31
if ou want lower gain, just use small mu tube

 

[v4lve lover]

The 6AS7= 6080 needs quite a bit of negative grid voltage. the divider biases the tube up differently.

 

[goldenbeer]

i do not understand the reason behind r3,r31

This way you can dc couple both tubes and stay within a reasonable op for the 6sn7..