12ax7+12au7 for DAC i/v

[erez1012]

hi
i have ad1865N DAC with 1ma on 50ohm resistor >> 50mv

today i use clone m7 preamp with 3*12ax7 and it work fine.

in the past people in this forum offer me to work with more simple design with no feedback and 3 stage, i think of cathode follower design like this.

what do you think about the 12ax7 scheme? the gain is great for me.

what about 12au7: i have only one . can i mix the design?
first stage with 12ax7 and 2 stage with the 12au7?

thanks

 

[audiowize]

Are you able to implement a constant current source without too much struggle? I ask because you will no longer need 300V for that 12AX7. 150-200V with a CCS set to 1mA and any variety of devices under the cathode will give you maximum gain from the first stage with minimum distortion. You should also bypass the cathode resistor under the 12AX7, or use a low impedance diode to obtain your bias voltage.

Directly couple the plate of the 12AX7 to the grid of the 12AU7. Assuming you've reduced B+ to 200V and used the CCS, use a ~33K resistor under the cathode of the 12AU7.

The 12AU7 will make for a better cathode follower than the 12AX7, but the 12AU7 doesn't have the gain you need to be used in the first stage.

 

[erez1012]

thanks for the comment audiowize.
i just order the Kubota low noise HV regulator pcb and finish my 300vdc PS, so i prefer to stay old fashion.
Anyway, with my PS what scheme do you prefer?
thanks

 

[nigelwright7557]

The 12AU7 gets slated quite a bit.
However I have just used one in a mixer/preamp and it sounds very good to me.
Crisp and clear.
I have also used it in the past for a phase splitter, it is good with its low output impedance.

 

[wa2ise]

I have a number of i/v and preamp circuits at my page Modifying CD player DAC circuits

 

[erez1012]

hi
i think i go for the 2*12ax7 circuit for the DAC i\v with 100ohm i\v resistor.

the DAC output include DC component, ~2v

some designers use filter (HPF?) between the DAC output and the tube, some use nothing.
how the DC in the tube input can influence the tube stage?
thanks

 

[wa2ise]

The 12AU7 would be a better cathode follower, while the 12AX7 is a better voltage amp. I'd use the 12AX7 for both left and right channels, and a 12AU7 again for both channels.

As for the 2V bias on the DAC output, you could use a coupling cap to the 12AX7. Or direct couple it and make the 12AX7 cathode resistor bigger to get the desired plate current (but at reduced voltage gain).

 

[Koifarm]

Just use 6n6p in srpp, works perfect for 10 years in my shanling T100-se.

 

[erez1012]

wa2ise you are right, 12ax7+12au7 is the optimum for my tube stock.

for the 2v bias. i prefer not to add more cap to the system, how i include the 2v in the load line calculation, Perhaps it is negligible?
thanks

 

[ballpencil]

Are you sure about the 2V offset when using current output? I can't find any reference for it anywhere on the datasheet http://www.analog.com/media/en/technical-documentation/obsolete-data-sheets/AD1865.pdf

Also not found anywhere on the datasheet, but if AD1865 is anything like TDA1541, it prefers less than +/-25mV offset on the current output for lowest distortion.. so your I-V resistor should be less than 25ohm.

 

[erez1012]

i measure 30mV on the 50 ohm R i/v
I hope its negligent in the tube design
thanks