Amazing!!!, tHANK YOU very much, i like the switchable 120k ohm resistor. that will be the best way to try and hear the difference in sound. i just want to experiment with different levels of NFB to find out what sounds better in my system.
Don't use those switches while the system is powered on.
sure, thank you.
even when volume turned all way down ? or switched to different channel ?
You need to have an objective in mind. What kind of signal will your preamp be used with? There are many preamp designs both new & old already published. No need at all to reinvent the wheel.
Forgive me if I’m saying something that you may consider obvious, but the amount of feedback is the difference between the open loop gain and the closed loop gain. Thus, there are two ways to adjust the amount of feedback, either vary the open loop gain, or the closed loop gain.
You first need to decide if you’re happy with the overall gain of the stage. If so, then you want to leave the feedback network alone. If not then you need to set the closed loop gain to whatever it needs to be to provide the correct gain. Then, adjust the amount of feedback by varying the open loop gain in a way that doesn’t touch the feedback network.
Let me also add that most proponents of reduced feedback do so by reducing the open loop gain, which can improve open loop bandwidth. Reducing feedback by increasing the closed loop gain does not deliver that benefit. I am personally of the opposite persuasion, but you asked how, not whether, and I’ll respect that.
You first need to decide if you’re happy with the overall gain of the stage. If so, then you want to leave the feedback network alone. If not then you need to set the closed loop gain to whatever it needs to be to provide the correct gain. Then, adjust the amount of feedback by varying the open loop gain in a way that doesn’t touch the feedback network.
Let me also add that most proponents of reduced feedback do so by reducing the open loop gain, which can improve open loop bandwidth. Reducing feedback by increasing the closed loop gain does not deliver that benefit. I am personally of the opposite persuasion, but you asked how, not whether, and I’ll respect that.
...classic answer of someone who knows all better !
i do understand your help, ..many thanks for being part of it all.
why?
You need to have an objective in mind. What kind of signal will your preamp be used with? There are many preamp designs both new & old already published. No need at all to reinvent the wheel.
true story, many thanks for your valuable comments.
best regards.
You could have large noises, a burst of oscillation, a large increase of volume, etc. These would not be good for the speakers, or even the amplifier.
To change the switch, turn down the volume control, turn off the system, change the switch, and turn on the system. Now set the volume control carefully, since the loudness at a given position will change if the gain changes.
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that's nailing it to the essence of NFB or not NFB,.....i will have few days off now and i will experiment and i will report my findings.
, we all know that more negative feedback is the way to go because that's how it should be by since/knowledge, but not always by audiophile/ours ears! proper design is not always equivalent of good sound.thank you for your comment.
thank you.
thank you,..objective mind is the only way to go.You need to have an objective in mind. What kind of signal will your preamp be used with? There are many preamp designs both new & old already published. No need at all to reinvent the wheel.
can you tell me/us ...in fact, what makes you think its a poor design???
and why do you think its loaded with NFB because of it?
no one does!, that's the point
,...have some life instead.
, ..be happy. No. Classic answer of someone who has other things to do.kropka235 said:
People who don't design circuits might not realise that spotting errors is often much easier and quicker than designing a new circuit, especially if the errors are obvious ones. Sadly, obvious errors are quite common.
You may be confusing faithful low distortion amplification of a signal with the sound which you prefer. Correctly applied NFB will enable the former, but not necessarily the latter as tastes vary.kropka235 said:
I don't think it's a poor design, sound very good in fact.This is a poor preamp design, patched up to a limited extent by adding feedback.
Depends what it is used for. Why is it a poor design & state your reason(s) why.
What design topology do you prefer? No new design required on your part, just a simple explanation for your preferences
I just want to experiment with nfb to find out what's hidden on the other side.
Perhaps to add a swith and etc.
I just didn't know the formula to calculate it.
Many thanx 4 your answers.
Pls let me know what your signal/program source is, approx voltage level.
Did you trace the rest of your circuit yet into a schematic?
Pls let me know what your signal/program source is, approx voltage level.
Did you trace the rest of your circuit yet into a schematic?
I have made a mistake in the drawing, cathode resistor of first tube where nfb is connected to is 4.7k
I marked it on the drawing as 820 ohm.
I don't have the rest of the schematicsame drawn yet.
I will post as soon as it's done.
To answer the question you asked:
Change the 120K to what-EVER you want. 240K will be "about half the NFB".
Gain will change along with change of NFB. Up to gain of over 400. I think before you get any effect of "reduced NFB" you will have FAR too much gain to use in a hi-fi.
You can reduce the 120K to 50K or somewhat less. While this is heavy loading on the tube the working levels are so low, and there is so much NFB, that it will be nearly colorless.
Change the 120K to what-EVER you want. 240K will be "about half the NFB".
Gain will change along with change of NFB. Up to gain of over 400. I think before you get any effect of "reduced NFB" you will have FAR too much gain to use in a hi-fi.
You can reduce the 120K to 50K or somewhat less. While this is heavy loading on the tube the working levels are so low, and there is so much NFB, that it will be nearly colorless.
In my opinion, the 12AX7 is not cut out for usage in a line stage. My choice would be either an ECC88, 12BH7 or 6SN7.
Like many people have said, it would be very helpful to know what you are using as a source. Your typical DAC or CD player does not need much gain to drive a typical power amp- in many cases, none at all.
Like many people have said, it would be very helpful to know what you are using as a source. Your typical DAC or CD player does not need much gain to drive a typical power amp- in many cases, none at all.
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Here is the preamp results with the corrected cathode resistor. The simulation shewed a 4.5 db peak at 0.833 Hz. Can be fixed by increasing the coupling cap between stages to 0.5 microF.
The end result is a gain of 2.5, leads me to believe this cct is meant for nominally a volt in. This cct should have quite a low internal Z at the output. But if it simply drives a following power amp, not needed. Perhaps the pwr amp itself did not have enough gain.
Also checked the nfb at the cathodes, that which biases & linearizes the stage. Otherwise all looks OK. Interesting to see the rest of the cct.
Don't need to know much to be an engineer, just where to look it up. And how to the use tools available. Used to be a slide rule, a soldering iron & a steady hand. Very different today!
The end result is a gain of 2.5, leads me to believe this cct is meant for nominally a volt in. This cct should have quite a low internal Z at the output. But if it simply drives a following power amp, not needed. Perhaps the pwr amp itself did not have enough gain.
Also checked the nfb at the cathodes, that which biases & linearizes the stage. Otherwise all looks OK. Interesting to see the rest of the cct.
Don't need to know much to be an engineer, just where to look it up. And how to the use tools available. Used to be a slide rule, a soldering iron & a steady hand. Very different today!
Attachments
Looks like 25, not 2.5, FWIW.
(Still using a slide-rule? I just got a new old one.)
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