The Broskie Cathode Follower for a DAC

bequerel · 19th September 2008, 09:57 AM

John Broskie over at www.tubecad.com has a lot of interesting ideas.
One of them involves using a special version of the cathode follower as a balanced to single ended unity gain stage for a dac (like eg. the Wolfson Vout dacs).

I am trying to figure out the correct values for the Rk and B+ in circuit below when using the Russian 6N6P tubes.

Typical values for this tube are:

Ua = 120V
Ia = 30 mA
Ug = -2V

Is the Rk calculation as simple as:
2V/30ma = 66.67 ohm, and are they identical in value?

And the B+:
120V+2V *2 = 244V ?

I suppose that it is not this simple!

And will I have to lift the heaters of the upper tube (as you normally would in a high voltage SRPP stage)?

Ciu · 19th September 2008, 10:50 PM

Hello !

As I can remember a discussion with a dyer, about these tubes on Aikido, his advice was 200-240v, with Rp=Rk, about 390 to 420 Ohms (never tried by myself)
I built something similar, but with a low B+, and 6gm8 tubes : B+= 24v and Rp=Rk=180 Ohms. The advantage is that you manage heater and B+ with 2 lm317 (2 slow turn-on PSU on a 5x4" board)
My Dac is a cheap board, with CS8416/cs4397 , with Dale resistors and blue Elna Caps, (added some Oscon and solid tantalum), bought on Ebay ~80$...Now sounds terriffic !!!(not yet boxed, too much pleasure to listen to )
Hope this help !
R.C.

bequerel · 20th September 2008, 02:25 PM

Thanks a lot, CIU!
That gives me some ballpark values to start with.
Actually I have some 6gm8 tubes I could try out as well.

pftrvlr · 21st September 2008, 05:49 PM

I too have one of the eBay CS8416/cs4397 board lying around, and thinking to make a integrated headphone amp.

I have been thinking along the line of a 6n6p differential input. Does the CF stage have any advantage over a differential one?

bequerel · 21st September 2008, 06:42 PM

I suppose that the only advantage CF holds over a diff input stage is simplicity. (?)

Ciu · 25th September 2008, 10:34 PM

Hello !

The Broskie Cathode follower, schematic as above, is a buffer, used in an output balanced voltage DAC (ie : cs4397,cs4398, opus 874x, etc...) .
The gain is near 1, the output Z depends on tube you would use
The main advantage is to cancel DAC noise, and give an unbalanced output from balanced DAC...
Don't mismatch with an Aikido !!!

R.C.

19th September 2008, 09:57 AM	#1
bequerel diyAudio Member Join Date: Sep 2003	The Broskie Cathode Follower for a DAC John Broskie over at www.tubecad.com has a lot of interesting ideas. One of them involves using a special version of the cathode follower as a balanced to single ended unity gain stage for a dac (like eg. the Wolfson Vout dacs). I am trying to figure out the correct values for the Rk and B+ in circuit below when using the Russian 6N6P tubes. Typical values for this tube are: Ua = 120V Ia = 30 mA Ug = -2V Is the Rk calculation as simple as: 2V/30ma = 66.67 ohm, and are they identical in value? And the B+: 120V+2V *2 = 244V ? I suppose that it is not this simple! And will I have to lift the heaters of the upper tube (as you normally would in a high voltage SRPP stage)? __________________ Thanx!

20th September 2008, 02:25 PM	#3
bequerel diyAudio Member Join Date: Sep 2003	Thanks a lot, CIU! That gives me some ballpark values to start with. Actually I have some 6gm8 tubes I could try out as well. __________________ Thanx!

21st September 2008, 06:42 PM	#5
bequerel diyAudio Member Join Date: Sep 2003	I suppose that the only advantage CF holds over a diff input stage is simplicity. (?) __________________ Thanx!

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19th September 2008, 10:50 PM	#2
Ciu diyAudio Member Join Date: Feb 2005 Location: Northern France	Hello ! As I can remember a discussion with a dyer, about these tubes on Aikido, his advice was 200-240v, with Rp=Rk, about 390 to 420 Ohms (never tried by myself) I built something similar, but with a low B+, and 6gm8 tubes : B+= 24v and Rp=Rk=180 Ohms. The advantage is that you manage heater and B+ with 2 lm317 (2 slow turn-on PSU on a 5x4" board) My Dac is a cheap board, with CS8416/cs4397 , with Dale resistors and blue Elna Caps, (added some Oscon and solid tantalum), bought on Ebay ~80$...Now sounds terriffic !!!(not yet boxed, too much pleasure to listen to ) Hope this help ! R.C.

21st September 2008, 05:49 PM	#4
pftrvlr diyAudio Member Join Date: Feb 2007 Location: SoCal	I too have one of the eBay CS8416/cs4397 board lying around, and thinking to make a integrated headphone amp. I have been thinking along the line of a 6n6p differential input. Does the CF stage have any advantage over a differential one?

25th September 2008, 10:34 PM	#6
Ciu diyAudio Member Join Date: Feb 2005 Location: Northern France	Hello ! The Broskie Cathode follower, schematic as above, is a buffer, used in an output balanced voltage DAC (ie : cs4397,cs4398, opus 874x, etc...) . The gain is near 1, the output Z depends on tube you would use The main advantage is to cancel DAC noise, and give an unbalanced output from balanced DAC... Don't mismatch with an Aikido !!! R.C.

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