6N6P preamp

[croccodillo]

Hi all,

I'm building a new stereo class-D amplifier, using Tripath chips (TA2024).
I'd like to add a line buffer to it, with a gain of 2-3.
It really need it, it seems my CD source has some problem driving the inverting input of such a chip.
My idea was to add a tube buffer (preamp), in manner to mix the oldest electronic available with the newest... it sound really interesting to me.
I have a bunch of 6N6P tubes, so why not to use one of those wonderful tubes as the line buffer?
I want to build a "classical" cathode grounded pre, maybe with a CCS as an active load, maybe with a low-impedance output, like Gary Pimm made.
Being the 6N6P tube a duoble triode, I could use a single tube for both the channels (I hope).
The input impedance of the Tripath amplifier is of about 20KOhm.
I have a lot of 6N6P available, so I ca use more than one for this project.

So, my doubts:

1. Can I use a single tube for both channels, or will I have problems with noise or mutual influence?

2. I was thinking to use the tube at about 110V, 20mA each triode: is this a good operating point, in your opinion?
I tried before this configuration, and i liked it very much; any other suggestion?

3. The strangest request: can I tie the grid to the anode? I mean, can I connect the heating voltage directly to the anode or will I have any problem with noise?
Of course in this case I will need a 6N6P tube for each channel...

4. Any other topologies you like for the buffer? Any schematic?

5. I'm also developing an SMPS to step up the low voltage used to power the Tripath chip (12V) to the high voltage needed by the tubes; any suggestion, schematic available?

Thank you in advance for your help.

Ciao,
Giovanni

[zigzagflux]

Wonderful little workhorse, the 6n6p. Mu is 22, so I'm not sure what circuit you plan on with a gain of 2-3. Could always step down with an output transformer, I suppose. Others will have better advice as far as circuits go.

1. Pin 9 is a shield, so you can absolutely use one tube for L and R channels.

2. Those are the exact operating points I use, tube is very content and happy there. You could experiment with running even a little hotter, it can handle it.

3. I would do the standard "elevate the heaters above the cathode by 30V" recipe. Tying to the anode would run you right at the maximum 100V Vh-k rating.

4. Will you be employing a volume control right after the CDP? I'm not familiar with the TA2024. There's a lot of possibilities, I suppose.

5. Sorry, nothing that immediately comes to mind. I don't really do SMPS.

[croccodillo]

Ciao,

Thank for the answer.

1. I already knew it, but I needed a confirmation... Thank you very much.

2. Perfect!

3. I completely missed such a parameter from the datasheet, I was sure the Vhk was much higher (300V)... but I was wrong, maybe confused the value from another tube.

4. As you can see from page 4 of the datasheet, HERE, the TA2024 chip input is an inverting one, with an heavy feedback on it, so you have a so-called virtual ground at its input pin (I hope this is clear, English is not my language...).
That means TA2024 does not like very much potentiometers at its input, cause they tend to change the feedback network impedance.
This is a common "problem" with a lot of amplifier; try to search in the Solid State forum, mainly in thread regarding Gainclone amps.
This is the main reason I need a buffer: I can place a potentiometer BEFORE the preamp, and then drive the TA2024 inverting input with a low, constant impedance source (preamp output).

5. Don't worry, I already have a schematic into my mind, I will try it.

Thank you again