Lampy cd player

[Richard]

Good thought, just checked and there's -171 and -173 mV on the grids, I had noticed it on those figs we did a while back but thought no more. I imagine it won't make much difference?

It's nice when the numbers work but I'm not sure it's a better sound, it is clearer but maybe it was mostly 2nd harmonic that's gone and I'm missing it, a little cool perhaps, I will listen some more.

To the future, its been good getting to know this a bit better but I agree it seems very tweaky and would like to try something not so load dependant.

Need about 20x gain but really would like to stick with this 6n2p valve for a while longer at least.

Could a dc coupled cathoed follower like this work and would it just need the gain stage optimising? Or would distortion and gain be too great? You'll recognise it

[Nick]

Yep, thats the sort of thing I suggested earlier. Though maybe ECC82/5687 would be better gain wise.

[Max N]

How about trying an Aikido - I've been planning to try that with my Audionote DAC, got all the parts, but haven't got round to it yet.

[Richard]

Hi Max, Nick, I hear very good things about the Aikido but couldn't fit it in here so want to explore something simpler first.

I also would like to stick with 6n2p for a bit, perhaps it needs to be heard to understand why.

So, the dc cathode follower,
I need 20x and think gain will fall short with ECC82. Perhaps better to start with too much. This stage has what, perhaps 60x? It could be done for 40x or 50x.

It could then be dropped at the output I suppose but what happens if it's dropped before? Off the top of your head, is it worth exploring do you think or will it just complicate things?

[Mike H]

Its a shame it can't be plugged into a sound card, http://audio.rightmark.org/news.shtml would possibly show some interesting things.

Oo interesting. That just needs an audio input right?


 

[Nick]

Its a shame it can't be plugged into a sound card, http://audio.rightmark.org/news.shtml would possibly show some interesting things.

Oo interesting. That just needs an audio input right?


 

Yep.

Output as well, or I think you can burn a CD.

[Nick]

Hi Max, Nick, I hear very good things about the Aikido but couldn't fit it in here so want to explore something simpler first.

I also would like to stick with 6n2p for a bit, perhaps it needs to be heard to understand why.

So, the dc cathode follower,
I need 20x and think gain will fall short with ECC82. Perhaps better to start with too much. This stage has what, perhaps 60x? It could be done for 40x or 50x.

It could then be dropped at the output I suppose but what happens if it's dropped before? Off the top of your head, is it worth exploring do you think or will it just complicate things?

You don;t meed the ? resistor, the second cathode will be at about as volt above its grid.

You can take the output back to the input and sum to make an anode follower with two stages to reduce gain.

[Mike H]

Question ~ R29/30 seems a mite on the large side at 330k


 

[Richard]

Hi Mike, 330k is there because it's borrowed from WD phono . We can make that stage whatever we want I think if it is to be a plate follower.

I like that idea Nick if I'm following right Seems better than just wasting the output but the parts count is rising and it'd need a cap on the input. Global is more elegant but would local fb be better for any reason?

Assuming global the 82R and input cap could go on the cdp pcb and the fb resistor could go anywhere in line. The rest would fit on the valve bases.

[Nick]

No, you can do it without needing the cap, the missing point is that the input to a anode follower becomes a virtual ground. I would also be tempted to make the cathode follower on the output a mosfet source follower, but thats another discussion.

[Mike H]

I concur, like that.

Or as you had it, the red line...

You can vary the ratios of the resistors to get what gain you want


 

[Nick]

I am tempted to suggest (though haven't tried it) that as the grid of the first valve is a ground point, the 1k resistor input could be the 100R or whatever you want to load your DAC with. make it the I/V resistor.

[Mike H]

That is true actually 'cause it's very like an inverting op-amp type scenario


 

[Nick]

Meanwhile, back to Steves posts about I.V converters.

[Richard]

Thanks Nick,
I've just been having a look at John Broskie but cannot get the formula to work.
Open loop gain "a =" will always be less than 1 the way he has it here, which will always make the "Gain =" less than 1 also.

http://www.tubecad.com/october99/page9.html

The first step is to determine the open-loop gain (a) of the Grounded Cathode amplifier and then the ratio of resistors R1 and R2. Thus,
R = Ra||RL||R2
a = muR/(muR + rp + (mu + 1)Rk),
if Rk is bypassed, then
a = muR/(muR + rp)
Ratio = R2/R1
and finally,
Gain = aRatio/(a + Ratio + 1).
Now that we have the gain, we can determine the output impedance.
Zo = (R||(rp + (mu +1)Rk))(Gain/a)
if Rk is bypassed, then
Zo = (R||rp)(Gain/a).

Has he got mu in there by mistake do you reckon?