I have a DAC with both SS and tube output stage. The tube output tube, 5670, was configured as a cathode follower as attached. Since my amplifier has low gain so I need more gain from the DAC. I plan to change the tube circuit to a single stage amplifier with feedback. Without feedback, the output impedance and distortion will be high. I also do not need a lot more gain.
I found detailed description on the design of such an amp from both Aikenamps and Vale Wizard site. I attached the example design from Aikenamps. My question is that the original design has an output capacitor with a resistor to ground. As the feedback amp contains a capacitor already, I don't think I would need that output capacitor. However, do I really need the Rl in the design? I assume Rl is the load impedance from the next stage.
The original design has a rather low resistor of 10K after the cathode follower. I can replace it with the a 100K resistor as in the example if necessary.
Finally, is there any downside with this kind of feedback single stage amplifier? Should I go without the feedback and deal with the high gain with a resistor divider? The THD will be higher. However, will the no feedback amplifier preserve the sound stage and details of the music?
I found detailed description on the design of such an amp from both Aikenamps and Vale Wizard site. I attached the example design from Aikenamps. My question is that the original design has an output capacitor with a resistor to ground. As the feedback amp contains a capacitor already, I don't think I would need that output capacitor. However, do I really need the Rl in the design? I assume Rl is the load impedance from the next stage.
The original design has a rather low resistor of 10K after the cathode follower. I can replace it with the a 100K resistor as in the example if necessary.
Finally, is there any downside with this kind of feedback single stage amplifier? Should I go without the feedback and deal with the high gain with a resistor divider? The THD will be higher. However, will the no feedback amplifier preserve the sound stage and details of the music?
The top schematic is a CF. The DC voltages are not correct for a 70V supply.
The bottom schema is an Anode Follower. The gain, input impedance & output impedance depends on the values of the FB resistors connected to the tube grid.
With a proper design the Anode Follower would work for you.
Thanks for the reply. So do I need a load resistor (Rl) for the Anode Follower design? I found another design which had a Rl of 470K.
You need a load resistor, as in that anode follower schematic it provides a path to ground for the grid. It also keeps the output at 0V (except during startup) when you don't have it plugged into anything.
A Simple Anode Follower like this will give a gain of about 4X the CF,
The circuit is not optimized. More gain is possible if required.
Don't ask how the 12AT7 'sounds'. Ideally it would add nor take away from what the musicians intended.
If they intended 2nd harmonics to be added to their performance they would have done that while recording & mixing.
The circuit is not optimized. More gain is possible if required.
Don't ask how the 12AT7 'sounds'. Ideally it would add nor take away from what the musicians intended.
If they intended 2nd harmonics to be added to their performance they would have done that while recording & mixing.
Attachments
For a summary discussion on local feedback (circuit n°2) you can see here:
http://www.valvewizard.co.uk/localfeedback.html
as jhstewart9 says you have more gain than CF and finally you have to choose the right resistor. As far I know it's not so commonly used circuit but in the past I had circuit like that and worked fine.
http://www.valvewizard.co.uk/localfeedback.html
as jhstewart9 says you have more gain than CF and finally you have to choose the right resistor. As far I know it's not so commonly used circuit but in the past I had circuit like that and worked fine.