Head-Fi.org › Forums › Misc.-Category Forums › DIY (Do-It-Yourself) Discussions › How to calculate value for Rk for DAC's tube output stage
New Posts  All Forums:Forum Nav:

How to calculate value for Rk for DAC's tube output stage

post #1 of 4
3/25/11 at 2:18am
Thread Starter 

So far there's been no reply in the main thread in diyaudio so I bring the question over here. Hope someone can enlighten this for me.

http://www.diyaudio.com/forums/group-buys/156149-tube-i-zator-professional-pcb.html

In short I'm sourcing parts and I'm not sure which value for the Rk I should pick. Lampizator said he chose 39R, while someone I know who built the DAC-end with similar output stage told me to go with 100R. But what I really want know is based on what formula or equation can you get the correct value?

 

My DAC is the Buffalo es9008, in the manual it says that

 

Quote:
The differential output of the buffalo is 0.924*1.65V (3.1746 Peak) + 1.65 DC bias and the series output resistance is nominally 195 Ohms at each output.

I'm thinking about picking the 6n2p or the 12ax7.

Thanks for reading.

Reply
post #2 of 4
3/25/11 at 9:11am

You pick the resistor based on the operating point of the tube you want to use, and the voltage offset from the DAC chip. 

 

I could not find a schematic with a few clicks through that thread, do you have one?

 

I'd lean away from the 12ax7 unless there is a SS Buffer after it. 

 

The output impedance of a 12ax7 SRPP (I think the lampizator is SRPP, schematic?) is relatively low which is generally good, but the relatively low currents the tube is capable of make driving anything lower than a 50Kohm load iffy which IMO outweighs any advantage from output impedance. When you consider cable capacitance its not hard to push a 12ax7 over the edge.

 

I should note: I'm not such a fan of tube DACs, or the 12ax7, but maybe the goal is to run them ragged to sound more tubey? 

Reply
post #3 of 4
3/26/11 at 4:58am
Thread Starter 

Below is the schematic for the pcb. 

Why is there DC offset here? I thought it would have been filtered off before reaching the output stage?

The signal will be fed into the B22 with a 50k attenuator so I'm not worried too much about the impedance mismatch. Perhaps not in the near future.

Tube-I-zator V2.0.jpg

 


Edited by tranhieu - 3/26/11 at 5:00am
Reply
post #4 of 4
3/26/11 at 5:55am
Thread Starter 

Opps, just realized I misread the diyaudio posts. It should be 200R for the Rk, but for the Ri/v Lampizator said 39R while Brian from TPA said 'as close to 0 as possible'. Is there anything I'm missing here?

 

Reply
New Posts  All Forums:Forum Nav:
  Return Home
Head-Fi.org › Forums › Misc.-Category Forums › DIY (Do-It-Yourself) Discussions › How to calculate value for Rk for DAC's tube output stage