12ax7 srpp dac output

[Koifarm]

I did some calculation without knowing the requested output impedance.

One of my favourit tubes Telefunken ECC85 is a good tube for the job.

An externally hosted image should be here but it was not working when we last tested it.

Output voltage about 3 Vrms with 28 Ohm I/V resistor. Output impedance between 10 and 20K.

Ronny

 

[kevinkr]

Google brought me here: I'm looking for a new output stage for my TDA1541. I'm using a 28 Ohm I/v resistor with a 6211 SRPP. I am deeply in love with the sound but seriously lack gain. The 6211 is extremely microphonic btw. I was looking for tubes with a mu of around 100 and the 6N2P seemed a good candidate. After reading this the D3a seems a better choice, but I haven't made up my mind yet

Source impedance with an SRPP using tubes with high rp will be excessively high. Depending on implementation the impedance could approach almost half of the tube's rp, but typically will be at least 20% of it. A mu follower would give you both more gain (maybe up to 6dB more) and a much lower output impedance. High transconductance types like the D3A are your friend because of their low rp - crank lots of current through them..

 

[toufu]

I built the Lampizator using Oliver's board, and I did try 12AX7 as well as 6N2P, I have a pair of old Baldwin tubes and it sounds really good. I used Rk of 300 which is a bit low. The only thing is that it has a lot of gain.

 

[nigelwright7557]

I use a 12AU7 with 1k2 resistors.
It sounds very clean.

 

[fdegrove]

Hi,

Source impedance with an SRPP using tubes with high rp will be excessively high. Depending on implementation the impedance could approach almost half of the tube's rp, but typically will be at least 20% of it. A mu follower would give you both more gain (maybe up to 6dB more) and a much lower output impedance. High transconductance types like the D3A are your friend because of their low rp - crank lots of current through them..

I'd say forget the 12AX7 SRPP for this application, forget a mu-follower using the same valve as well.
A D3A based circuit is much, much better if you can source the valve.

Other than that, and I haven't really looked into it, an Aikido topology using a valve with a mu of 40 (a la E80CC, 6085) could be a tempting alternative perhaps.

Cheers, 😉

 

[Miniwatt]

@ koifarm: thanks for the circuit!

I can get some D3a's, and I wonder why fdegrove is so convinced this is the way to go.

 

[fdegrove]

Hi,

Very low noise, pentode with high perveance, low Zout.

Just about the ticket, no?

Cheers, 😉