Do math.
The cathode follower has 62V across 33k. We ask to drive a 68k load. The question is, HOW BIG??
At an extreme, the CF will pull-down to 62V * (68k/(68k+33k)) or 41V peak.
Would we ever ask a Line Amp to deliver 41V peak? No, maybe not even 1/10th of that.
The CF's distortion at MAX output will approach 5%/Mu or about 0.3%. At 1/10th that level it will be about 0.03%.
Then there is overall NFB. The gain stage has gain near 16. The closed-loop gain can be 2 or 4.7. The noise gain is then 3 or 5.7. NFB factor is 16/3=5 or 16/5.7=2.8. CF added distortion will be effectively 0.01%. "Dead clean" by tube standards.
I have ignored the THD of the gain stage. This will clip near 28V peak. Just below that say 5% THD. Near 2.8V peak more like 0.5%. Under NFB, about 0.2%.
I've never felt that sub-1% 2nd Harmonic was a big deal. But if you are sensitive to it, you may disagree.
Putting 230V into a 200V primary may lead to saturation, unless the mains transformer is very good quality. Even if it works OK, 15% more voltage will make little difference to the circuit. You need 50-80% more voltage.georgehifi said:
12AU7 is a poor choice for a cathode follower. It has low gm; CF's need high gm. 6SN7 is also a poor CF, for the same reason.TonyTecson said:
12AU7 and 6SN7 were aimed at the same applications, such as multivibrators in TV circuits. It just happens that the 6SN7 is more linear in audio circuits too.
Deliberately increasing distortion in one stage in order to cancel distortion in another stage is rarely a good way to design audio circuits. I have no idea whether that was the aim here.
Did you regulate the power for the circuit? How are you certain it was running properly?
It was a one off Allen insisted I make to graft onto the Lightspeed
Yes power supply also came from Allen, all I had to do was add the linear tranny he recommended.
And yes working perfect, as Allen was a very good acquaintance and with emails, got me to check all voltages and currents when I told him it wasn't as transparent or dynamic as it was without it.
Funny we're talking about it as it's just now been sold for the 4th time since I built it more than 10 years ago, at least it's reliable, yes it's no oil painting.
FS: Lightspeed Attenuator - But with valves ! - Classifieds - Audio - StereoNET
Cheers George
Yes power supply also came from Allen, all I had to do was add the linear tranny he recommended.
And yes working perfect, as Allen was a very good acquaintance and with emails, got me to check all voltages and currents when I told him it wasn't as transparent or dynamic as it was without it.
Funny we're talking about it as it's just now been sold for the 4th time since I built it more than 10 years ago, at least it's reliable, yes it's no oil painting.
FS: Lightspeed Attenuator - But with valves ! - Classifieds - Audio - StereoNET
Cheers George
Did you and he have any conversations about Miller Effect? What was the effective value of the shunt to ground of the Lightspeed control? I assume 0 ohms to -??
Nice!
Was that at all positions of the control? Usually the trouble spots are mid-band.
We're going back to a one off over 10 years ago, but probably still at the worst would have been around 2'oclock like they all are, unless I did some thing different to it which I doubt very much.
Cheers George
No just a little blame shifting trying to be applied to the Lightspeed that's the volume control for the SLCF buffer, which didn't sound as good in an A/B when the SLCF was in the signal path.
Cheers George
No blame intended. Its a simple fact though that when splicing a control into an active circuit that often Miller effects are ignored and don't show up unless a bit of testing is done. So I was asking to see if that base had been covered.
There are a lot of variables involved with an active line stage as have been seen just on this thread alone!
We had several customers modify our MP-1 preamp with SuperReg regulators. In all the cases, they were not able to get the preamp to sound as good as it did with our stock regulator. I worked with two of these preamps and in both cases the solution was to remove the SuperRegs and replace them with our own regulation. On paper the SuperReg should have been a better regulation scheme! We played with the shunt current and the like on the SuperRegs, they were set to the correct voltage and so on.
I'm mentioning this as an example of how something that for all intents and purposes should have been the cat's meow turned out to not be the case in an unexpected way.
Help needed, playing around with this again, one more question if you tube gurus can take it.
Out of the tweo (feedback) gain settings 1.15x (1.2db and 3.1x (9.8db) which one of these will drive 47kohm better, even if both are below 12o'clock on the volume control, I'm torn between the two.
Cheers George
Out of the tweo (feedback) gain settings 1.15x (1.2db and 3.1x (9.8db) which one of these will drive 47kohm better, even if both are below 12o'clock on the volume control, I'm torn between the two.
Cheers George
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What will that do? gain is not the issue with either.
Sorry didn't make myself clear enough. I want to know out of the two gains (feedbacks) I mentioned, which is going to sound better, into >47kohm even though both are below 12o'clock so gain is not the issue.
Drive characteristics are what I'm after.
Cheers George
Sorry didn't make myself clear enough. I want to know out of the two gains (feedbacks) I mentioned, which is going to sound better, into >47kohm even though both are below 12o'clock so gain is not the issue.
Drive characteristics are what I'm after.
Cheers George
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Change the circuit.
If you need to drive a lower external load then it is possible to parallel the triode sections of the 12AU7 to make it a cathode follower only. The value of the cathode resistor is halved. You will need to alter the biasing arrangement to the grids and there is ample information on how to do this on the net. The gain is basically unity.
If you need to drive a lower external load then it is possible to parallel the triode sections of the 12AU7 to make it a cathode follower only. The value of the cathode resistor is halved. You will need to alter the biasing arrangement to the grids and there is ample information on how to do this on the net. The gain is basically unity.
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