How about attaching a screenshot with how far you got?
Nova/Groove Tubes/Sterling mod questions
- Thread starter Amled87
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How about attaching a screenshot with how far you got?
I'm attempting, but every time I take a screenshot it's a blank screen, haha. I'll see if I can get it worked out. I've switched over to another sim and it to doesn't work (or show rather) the HF change when adding the 1000pF cap, yet i can clearly hear/see it with a spectrum analyzer in my daw. Possible you could do math, I understand the "benefit" of learning/doing it myself though, but it would be much appreciated and I'm sure others in the thread would benefit from the information.
Worst case, take a photo with your phone, jeez
"Where there's a will, there's a way"..?And what signal / what circuit node are you probing, that no HF change is visible?
Probing the input/out of the fet. I'm new to splice and had to have help previous experimentation I had done. I'd use my phone, but it wouldn't do no good with a bad camera. I do believe I've got it now though. But, still simulating it isn't helping me understand the math.
In a 12ax7 gain stage (100k plate and 2k7/.68 cathode followed by a 1m pot) I understand that adding say a 500pf cap to the plate adds to the millier capacitance of the stage and that has a 3db point of 5-6k with further drop-off above that. But, how do I translate that here using the fet gain stage (effectively what I'm doing with the 1000pf cap is adding to the miller capacitance of the fet, no?)
In a 12ax7 gain stage (100k plate and 2k7/.68 cathode followed by a 1m pot) I understand that adding say a 500pf cap to the plate adds to the millier capacitance of the stage and that has a 3db point of 5-6k with further drop-off above that. But, how do I translate that here using the fet gain stage (effectively what I'm doing with the 1000pf cap is adding to the miller capacitance of the fet, no?)
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Not sure why you'd necessarily need "the math" - that sort of assumes that placing one single component in a spot in a circuit affects one parameter and one parameter only. Or if it doesn't, the math behind the complete effect might be convoluted, and likely arguably irrelevant.
Me, i couldn't give two hoots about the math - if 1nF gets me where i want/need, great. If not, what does 2nF do? Or 470pF?
Just saying that practice beats theory any day of the week
Oh I definitely agree about practice always winning out! The math comes from me wanting to understand exactly how what was chosen. Sometimes, it's not practical to continously solder on fragile traces or around sensitive components. If I understood the math better, in times when spice or another sim isn't available i could make decisions to based in the math. For example, this mic has a huge peak at 12-15k and "nasty" high-end doesn't even begin to describe it. If my model is right, the 1000pf has a 3db point at around 11k and significantly more above that. Out of curiosity does that seem right to you using the 1000pf cap?Not sure why you'd necessarily need "the math" - that sort of assumes that placing one single component in a spot in a circuit affects one parameter and one parameter only. Or if it doesn't, the math behind the complete effect might be convoluted, and likely arguably irrelevant.
Me, i couldn't give two hoots about the math - if 1nF gets me where i want/need, great. If not, what does 2nF do? Or 470pF?
Just saying that practice beats theory any day of the week
That's why we're happy to have simulators
It was just an arbitrary value I pulled out of my a.. hat
A starting value to work up from until you got the amount of HF attenuation your ears are pleased with.Ha! I wish it was much easier to figure like tube circuits. But, my question still is, by adding the 1000pf cap I'm essentially adding to the miller capacitance of the jfet thus moving the cutoff of the lpf created by the load resistor and miller capacitance down to a lower frequency, do I have that correct?That's why we're happy to have simulators
It was just an arbitrary value I pulled out of my a.. hat
Pretty sure it's not. I thought Miller capacitance occurs between plate and grid, or drain and gate. A capacitor in parallel with the plate / drain resistor only looks like a lower resistance with increasing frequency, thus lowering the gain coming out the plate / drain.
Ah, you're correct. Miller capacitance is the capacitance between the gate and drain multiplied by the gain stage of the amplifier in a jfet circuit. I wasn't thinking lol. Too early and not enough coffee! Adding the cap across the drain is just shunting highs to ground. We could also add a cap between the gate and drain to increase the capacitance (which is around 8pF for 2sk30A in the mic) and the lpf it creates when the capacitance is multiplied by the gain of the stage instead of adding a cap to just the drain. More testing to do to see which I like best!
Now, to figure out how to calculate the gain of the jfet (not familiar with transistor circuits being honest) it's effect on the Miller capacitance and what the roll-off is for that. I know the 2sk30a has an capacitance of roughly 8pf, so that multiplied by the gain stage must not be to high seeing as the mic itself has a huge 12-15kHz peak (thus the lpf formed by the Miller capacitance isn't in that range and must be higher) I'm assuming.
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Why ground? Neither end of that capacitor is connected to ground. In case i haven't beel clear enough, my suggested solution is adding a capacitor in parallel with the drain resistor. Doodle it on some paper in case it eases understanding, perhaps? Some people are more "visual" - myself included, in some circumstances.
No, that's just gonna reduce the overall gain. See KM84 schematic and U87 ("vintage", non-A) schematic.
Just forget about the whole Miller capacitance thing, it's not relevant in these cases.
The drain resistor is connected to the DC supply which would be no different than a plate bypass capacitor shunting highs to ground in a tube circuit, no? The capacitor across the drain resistor has to shunt the highs somewhere or it wouldn't be doing anything? Or am i putting to much thought into it?
Reducing gain at certain frequencies is exactly what a lpf does, is it not? I'm confused here. The 3db point of a lpf is the point at which gain is reduced by 3db at the given frequency. I'm not saying changing miller capacitance and bypassing the drain resistor offer the same effect, but both would cut high end, no? Cutting gain at 10kHz is still reducing high-end, no? Miller capacitance wouldn't matter so much because the input is such high impedance here though.
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