 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
96.227.67.63
In Reply to: RE: feedback applied to grid posted by Ralph on December 27, 2024 at 08:53:56
So why not use the two inputs with the same gain?
Cathode has the same gain as the g1. Apply signal to one, and FB to the other. Say, FB to g1 and signal to the cathode?
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Follow Ups:
its more than just gain. The tube isn't linear.
So when the feedback is mixed with the audio in the tube, it gets distorted. When that happens the feedback is generating harmonics of its own. So it would seem that what you want to do is mix the audio and the feedback in a more linear way.
That can be done by using a series resistance with the incoming audio signal and use a simple divider network to mix the feedback with it, before it enters the tube. Now the feedback is able to correct for the non-linearity of the tube.
Let's forget feedback for a moment.
Are you saying that the tube will respond to signal applied to the cathode in a fundamentally different way than signal applied to the control grid?
I have always looked at input signal as something that is simply modifying the bias. Bias being the instantaneous voltage potential between the grid and the cathode.
How does the tube know if that signal is being applied to the cathode vs. the grid?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Are you saying that the tube will respond to signal applied to the cathode in a fundamentally different way than signal applied to the control grid?
No.
It will be harder to drive the cathode because the impedance is lower. But the tube responds the same. And that's the problem: its not linear.
Resistors are a lot more linear. The correction Voltage is taken from the output; we're assuming that the circuit isn't linear so that Voltage has distortion which we are tying to reduce. If we mix it with the incoming signal using a divider network rather than the tube the feedback won't be distorted. Instead it will be the undistorted correction signal to compensate the non-linearity of the circuit.
Think of the tube as a house of mirrors. If you try to mix the audio (which is being distorted by the mirrors) in that space with the feedback (which is being distorted by the mirrors) then the result isn't the audio mixed with feedback. Its a distorted version of that.
The article I read by Crowhurst mentioned that in addition to harmonic distortion generation you also get IMD because the feedback node is non-linear. By mixing at the input (in a manner identical to how its done with opamps) the feedback node is far more linear (I'm assuming not perfect since resistors aren't perfectly linear either but they are a lot more linear than any tube).
If feedback is returned to the cathode, and feedback is instead applied to the control grid, is there a problem because the grid is not in phase with the cathode?
In order for this to work the phase has to be opposite. In a PP amp this isn't a problem; you can change it out the power tubes or the drive to the power tubes.
"In order for this to work the phase has to be opposite. In a PP amp this isn't a problem; you can change it out the power tubes or the drive to the power tubes."
This also works out correctly in a line stage with a cathode follower output. It's a mystery why designers haven't exploited this technique to a greater extent.
Reducing one or the other( FB or signal ), is not going to linearize anything. Resistance is by definition linear... :)
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I described the difference between mixing at a non-linear node as opposed to one that is linear (the resistor divider network).
I think you missed my point or else I misinterpreted your comment.
You said to 'linearize' the FB with a resistive network. Not needing to quote you( I think ). Resistors, being linear are not going to make any non-linear input, linear.
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
The context lost was the bit about how the feedback has to be mixed with the incoming audio. To do that it must be inverted with respect to the feedback signal that is used when feedback is applied to the cathode.
In a PP amp this is fairly easy since there are usually coupling caps involved with driving the grids of the output stage. They can be set up to drive the other power tube, thus inverting the output. Or the plate connections of the power transformer can be reversed.
Well...yes. Seemed you were going for something other than basic NFB application when you wrote of mixing NFB in a more linear way:
So it would seem that what you want to do is mix the audio and the feedback in a more linear way.
and then using resistors to accomplish this:
That can be done by using a series resistance with the incoming audio signal and use a simple divider network to mix the feedback with it, before it enters the tube.
Thanks for clarifying anyway... :)
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Well...yes. Seemed you were going for something other than basic NFB application when you wrote of mixing NFB in a more linear way:
I'm looking for the same loop gain and the same damping the feedback loop might have if applied to the cathode. The difference is because the grid has so much higher impedance the feedback values have to change to accommodate that.
Opamps use this exact technique to do their feedback. This does mean you need an inverting input. If the feedback is applied to the cathode the amp circuit is usually non-inverting. So some sort of accommodation needs to be made to allow the input to be inverting.
| FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
||||||||||||||
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2025, Audio Asylum, all rights reserved ]
