It isn’t outside the feedback loop. It is part of the input impedance. There is nothing that magically separates it from R23.
Thanks. I had a feeling I didn't know all of what's going on there, which was why I was questioning myself in that post. I'm not very experienced with solid state stuff. NPN transistors are easy for me to conceptualize, but PNP transistors still confuse me a bit. Taught myself electronics by working on tube amps (guitar amps first). A little Dunning-Kruger effect at work here, I'm afraid -- 😳
This was hashed out years ago in a thread about anode followers. In practice the effect of putting a volume control on the input is pretty much negligible as long as the volume control has a much lower output impedance than the series resistance. In this case, I was thinking the series resistor would be 150k ohms (shunt feedback resistor 470k ohms). The 50k volume control in front of it shouldn't change the feedback ratio by very much at any point in its rotation. Changing the series resistor to 100k degrades that a bit.
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25k in relation to 150k isn't negligible. In any case it is still totally incorrect to say it is 'outside the feedback loop'. and 'won't affect V1B.' It isn't, and it will. So will the output impedance of the source.
In fact you contradicted yourself. If R23 is part of the input impedance seen by the source, which it is, the volume control has to be part of the input impedance seen by the valve. You're having a bob each way here.
I don't know what a "bob" is, but you're right, the volume control is part of the source impedance, which does affect the feedback loop, but you'd need to pick a really big resistance value for that to make any difference at all.
What would you suggest the OP do to solve his problems?
What would you suggest the OP do to solve his problems?
What do you think about this circuit? 6SN7 based.
I am still waiting for the transistors so I can fix the psu. Been listening passively for the last few days. I just don't think the dac is able to drive the power amp properly. Sound is flat and lifeless with less of a sound stage and no impact. I will continue to pursue the tube path 🙏
I am still waiting for the transistors so I can fix the psu. Been listening passively for the last few days. I just don't think the dac is able to drive the power amp properly. Sound is flat and lifeless with less of a sound stage and no impact. I will continue to pursue the tube path 🙏
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Again the question becomes, "How much gain do you need?"
Answer that question and you'll be able to pick a circuit that will work well in your particular situation. Don't answer that question and there will be some trial-and-error ahead.
Answer that question and you'll be able to pick a circuit that will work well in your particular situation. Don't answer that question and there will be some trial-and-error ahead.
Gain 5 to 20dB. But minimal noise. I think that thr gain was not really the biggest problem, but the noise.I could have lived with the original circuit, but the hiss was annoying.
The problem with too much gain is that it will almost always introduce too much noise.
It's best to choose the right amount of gain so that you don't invite extra noise from employing too much gain and just burning it off with a resistive voltage divider of some kind.
2x gain is 6dB.
3.2x gain is 10dB
5x gain is 14dB.
10x gain is 20dB.
20x gain is 26dB,
A 12AU7 has mu of about 16, which means typical gain of 8x to 12x, which is actually a lot of gain. Far more than needed.
A 6SN7 has mu of 20, which means typical gain of 12x to 15x, which is even more gain. Far more than needed.
You could use negative feedback to reduce the gain, decrease distortion and lower output impedance. All good things, yet many people feel negative feedback is the devil's work. Oh well.
A 6V6 wired triode has mu of about 10, which means typical gain of 6x to 8x. See https://www.diyaudio.com/community/threads/6v6-line-preamp.102352/
A 12B4A triode has mu of about 6.5. which means typical gain of 4x to 5x. See https://www.diyaudio.com/community/...the-12b4-better-than-the-grounded-grid.68267/
It's best to choose the right amount of gain so that you don't invite extra noise from employing too much gain and just burning it off with a resistive voltage divider of some kind.
2x gain is 6dB.
3.2x gain is 10dB
5x gain is 14dB.
10x gain is 20dB.
20x gain is 26dB,
A 12AU7 has mu of about 16, which means typical gain of 8x to 12x, which is actually a lot of gain. Far more than needed.
A 6SN7 has mu of 20, which means typical gain of 12x to 15x, which is even more gain. Far more than needed.
You could use negative feedback to reduce the gain, decrease distortion and lower output impedance. All good things, yet many people feel negative feedback is the devil's work. Oh well.
A 6V6 wired triode has mu of about 10, which means typical gain of 6x to 8x. See https://www.diyaudio.com/community/threads/6v6-line-preamp.102352/
A 12B4A triode has mu of about 6.5. which means typical gain of 4x to 5x. See https://www.diyaudio.com/community/...the-12b4-better-than-the-grounded-grid.68267/
Sounds like will have to explore other circuits and other tubes yes.
I recieved the 2N5401 transistors today and got it up and running again. But for sure I must have blown something else too, because now I measure 170V B+ instead of 320V.
Put in a couple 6SN7 tubes with adapters. They sound miles better than the 12AU7's. Seems like the gain and hiss is similar. I will try adding a bit more feedback and see if it helps.
And I will explore the other circuits you suggested.
I recieved the 2N5401 transistors today and got it up and running again. But for sure I must have blown something else too, because now I measure 170V B+ instead of 320V.
Put in a couple 6SN7 tubes with adapters. They sound miles better than the 12AU7's. Seems like the gain and hiss is similar. I will try adding a bit more feedback and see if it helps.
And I will explore the other circuits you suggested.