. Can please suggest someone to replace those part no's with interfet has a lot of jfets in mouser.in. In my country mouser.in has only website has lot of inventory.
Thank you, Now I am in right direction, Can we use LSJ689/LSK489 or LSK 170 & LSJ 74 ?
Hey, new user, been poking around to try and get my head around how this amp works. And I have a lot of questions that I'll just throw out there.
Can anyone explain why the positive feedback increases damping factor or decreases output impedance? If you consider it as simply cancelling the negative feedback, isn't it just counterproductive?
I am also confused as to what needs to be matched in the amp? Do you need to match just the JFET pair (2SK170 and 2SJ74) ? Or do you also need to match the JFETS with the lateral mosfets. I've read the PassDiy matching guide, which states that you should use a source resistor to drop some of the extra Vgs. What resistance are we adjusting in order to match the transistors? Is it the pot between the sources of the JFET's? If so wouldn't that also affect the negative feedback proportion?
Can anyone explain why the positive feedback increases damping factor or decreases output impedance? If you consider it as simply cancelling the negative feedback, isn't it just counterproductive?
I am also confused as to what needs to be matched in the amp? Do you need to match just the JFET pair (2SK170 and 2SJ74) ? Or do you also need to match the JFETS with the lateral mosfets. I've read the PassDiy matching guide, which states that you should use a source resistor to drop some of the extra Vgs. What resistance are we adjusting in order to match the transistors? Is it the pot between the sources of the JFET's? If so wouldn't that also affect the negative feedback proportion?
The JFETs should be matched for IDSS. The laterals don't need to be matched for/to anything, but it would improve symmetry if you could match the N and P parts for Vgs. This circuit, the F7, does not use source resistors anywhere - not on the JFETs or laterals.
I'll leave explaining the effect of positive feedback to someone with greater engineering background.
I'll leave explaining the effect of positive feedback to someone with greater engineering background.
Taking a part of output signal to the input is called feedback. If returned signal is in counter-phase with the input, the process is reductive and we call it negative feedback. It lowers the gain, the distortion and the output impedance (Zout) and does some other jobs that are not important right now.
But if we take a part of output signal to the input where both signals are in-phase, the process becomes additive and it makes circuit's gain and distortion higher. That's positive feedback.
In F7 we have two feedback loops - one negative (which is dominant) and one positive. Positive feedback is used here in very small amount (2-3 dB) and that's enough to sufficiently raise the circuit's gain so that we achieve stronger effect of negative feedback i.e. lower Zout. We get higher distortion too (few dB) but the Zout (i.e. DF) changes whole lot more - roughly, for an order of magnitude (and that's because with some PF we have more gain to invest into NF).
So, it's a trade - a bit more distortion in exchange for a lot more of damping factor.
But if we take a part of output signal to the input where both signals are in-phase, the process becomes additive and it makes circuit's gain and distortion higher. That's positive feedback.
In F7 we have two feedback loops - one negative (which is dominant) and one positive. Positive feedback is used here in very small amount (2-3 dB) and that's enough to sufficiently raise the circuit's gain so that we achieve stronger effect of negative feedback i.e. lower Zout. We get higher distortion too (few dB) but the Zout (i.e. DF) changes whole lot more - roughly, for an order of magnitude (and that's because with some PF we have more gain to invest into NF).
So, it's a trade - a bit more distortion in exchange for a lot more of damping factor.
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It's simpler than you might think. Start with a regular amplifier. Normal
amplifier with regular negative feedback and gain and so on which for simple
example is non-inverting and has the minus terminal of the speaker going to
ground.
We put a low value resistor (say .1 ohms) in series with that connection to
ground and feed the small voltage that is found there (when current flows
through the speaker) back to the input of the amp.
When positive current flows through the speaker, that connection sends
a little bit of positive voltage to the input, making the amp work a little
harder to deliver that output current. Same for negative.
The result is that the amplifier has less output voltage drop under load,
and effectively a lower output impedance and higher damping factor.
Also effectively, this action steals some of the negative feedback of
the amplifier.
Because the amplifier already has a decently low output impedance (say 1
ohm for example) it doesn't take much to make it 0, so typically we are
stealing a decibel or so, and we see a slight increase in THD figures as a
result of less negative feedback.