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#1
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Simple Test Circuit for Tube Noise
Can anyone suggest a simple circuit to measure vacuum tube noise? How
about negative (emission) grid current? Also, what are some of the quieter tubes out there? Thanks! Sean Broderick |
#2
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Simple Test Circuit for Tube Noise
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#3
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Simple Test Circuit for Tube Noise
On Sep 10, 5:02*pm, (Scott Dorsey) wrote:
wrote: Can anyone suggest a simple circuit to measure vacuum tube noise? * if you want low noise, why are you using tubes... just asking... Mark |
#4
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Simple Test Circuit for Tube Noise
On Sep 10, 4:16*pm, Mark wrote:
On Sep 10, 5:02*pm, (Scott Dorsey) wrote: wrote: Can anyone suggest a simple circuit to measure vacuum tube noise? * if you want low noise, why are you using tubes... It's entirely possible to get low noise from tubes, IF you do the design right and if the source impedance is appropriate (high). As a rule, the quietest performance will be found in tubes with high gm (mutual conductance); of course, since gm varies with operating conditions, you need to be cognizant of them. There are typically four noise sources in a triode circuit: the source resistance at the input, the cathode resistor (if unbypassed), the plate resistor, and the tube itself, which has an equivalent noise resistance of 2.5 / gm, where gm is measured in mhos -- er, Siemenses. The contribution of the plate resistance is Rp divided by the square of the tube's gain. Add that, the tube's equivalent input noise resistance, and the cathode resistance, again if unbypassed. If the total is 1/4 or less of the source resistance, the circuit will add 1dB or less to the inherent noise of the source resistance. In theory. In practice, there are other issues, some of which Scott has discussed above. But, if the source resistance is appropriate (either inherently high, or made so with a transformer), tube circuits can be as quiet as, say, a 5534 opamp. Peace, Paul |
#5
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Simple Test Circuit for Tube Noise
Mark wrote:
On Sep 10, 5:02=A0pm, (Scott Dorsey) wrote: wrote: Can anyone suggest a simple circuit to measure vacuum tube noise? =A0 if you want low noise, why are you using tubes... just asking... Freedom from overload? --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#6
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Simple Test Circuit for Tube Noise
PStamler wrote:
In theory. In practice, there are other issues, some of which Scott has discussed above. But, if the source resistance is appropriate (either inherently high, or made so with a transformer), tube circuits can be as quiet as, say, a 5534 opamp. Unfortunately, the thermal noise in the transformers is sometimes a bigger worry than the noise of the tubes themselves.... and certainly the interwinding capacitance of the transformer is always a bigger destroyer of bandwidth than the tube stage itself. I'm a big fan of tubes, it's the transformers I don't like... --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#7
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Simple Test Circuit for Tube Noise
On Sep 10, 8:56*pm, (Scott Dorsey) wrote:
PStamler wrote: In theory. In practice, there are other issues, some of which Scott has discussed above. But, if the source resistance is appropriate (either inherently high, or made so with a transformer), tube circuits can be as quiet as, say, a 5534 opamp. Unfortunately, the thermal noise in the transformers is sometimes a bigger worry than the noise of the tubes themselves.... *and certainly the interwinding capacitance of the transformer is always a bigger destroyer of bandwidth than the tube stage itself. I'm a big fan of tubes, it's the transformers I don't like... Use the right transformer, and it works fine. Try the Jensen JT-13k7; the thermal noise only adds 1 dB, and the bandwidth is good. It's the tube used in Dan Kennedy's original preamp design, which is very clean and neutral. It turns out the same transformer does a fine job with a vacuum tube. Peace, Paul |
#8
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Simple Test Circuit for Tube Noise
PStamler wrote:
Use the right transformer, and it works fine. Try the Jensen JT-13k7; the thermal noise only adds 1 dB, and the bandwidth is good. It's the tube used in Dan Kennedy's original preamp design, which is very clean and neutral. It turns out the same transformer does a fine job with a vacuum tube. That's about as good as you can get, yeah! --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#9
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Simple Test Circuit for Tube Noise
On Sep 10, 5:45*pm, PStamler wrote:
On Sep 10, 4:16*pm, Mark wrote: On Sep 10, 5:02*pm, (Scott Dorsey) wrote: wrote: Can anyone suggest a simple circuit to measure vacuum tube noise? * if you want low noise, why are you using tubes... It's entirely possible to get low noise from tubes, IF you do the design right and if the source impedance is appropriate (high). true enough, but if you put an equal amount of effort into the design and impedance using a low noise FET, it will beat the tube hands down... Mark |
#10
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Simple Test Circuit for Tube Noise
Mark wrote:
true enough, but if you put an equal amount of effort into the design and impedance using a low noise FET, it will beat the tube hands down... Not really. There's a really nice paper from Marshall Leach on the subject, too. It's probably referenced on his website. Tubes and FETs come out about neck and neck, with similar transformers. Transformerless bipolar input stage comes out hands down, though, assuming 600 ohm source impedance. That said, Fred Forssell makes a transformerless tube preamp that really is shockingly quiet. It's not as quiet as the Great River or the Millennia Media, but it's way more quiet than it has any right to be. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#11
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Simple Test Circuit for Tube Noise
On Sep 11, 8:54*pm, (Scott Dorsey) wrote:
Mark wrote: true enough, but if you put an equal amount of effort into the design and impedance using a low noise FET, it will beat the tube hands down... Not really. *There's a really nice paper from Marshall Leach on the subject, too. *It's probably referenced on his website. Tubes and FETs come out about neck and neck, with similar transformers. Transformerless bipolar input stage comes out hands down, though, assuming 600 ohm source impedance. When you're looking at attainable noise figures of 2 dB over the inherent noise of the microphone, which you can do with a tube or a FET and the right transformer, there's not a lot of hands-down improvement possible. In fact, there's only 2dB of improvement possible, max, unless you dip the mic into liquid helium. Realistically, more like 1dB, which is barely noticeable. Modern designs, with any of the three technologies, are scraping pretty close to the theoretical limits. Peace, Paul |
#12
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Simple Test Circuit for Tube Noise
On Sep 12, 2:30*am, PStamler wrote:
On Sep 11, 8:54*pm, (Scott Dorsey) wrote: Mark wrote: true enough, but if you put an equal amount of effort into the design and impedance using a low noise FET, it will beat the tube hands down... Not really. *There's a really nice paper from Marshall Leach on the subject, too. *It's probably referenced on his website. Tubes and FETs come out about neck and neck, with similar transformers. Transformerless bipolar input stage comes out hands down, though, assuming 600 ohm source impedance. When you're looking at attainable noise figures of 2 dB over the inherent noise of the microphone, which you can do with a tube or a FET and the right transformer, there's not a lot of hands-down improvement possible. In fact, there's only 2dB of improvement possible, max, unless you dip the mic into liquid helium. Realistically, more like 1dB, which is barely noticeable. Modern designs, with any of the three technologies, are scraping pretty close to the theoretical limits. Peace, Paul I agree that an FET can achieve a 2 dB noise figure and that means an FET can be 2 dB away from as good as it gets at room temperature... Can you achieve a 2 dB noise figure with a 12AX7? Or are you talking about some special tube? Mark |
#13
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Simple Test Circuit for Tube Noise
On Sep 12, 5:27*pm, Mark wrote:
On Sep 12, 2:30*am, PStamler wrote: On Sep 11, 8:54*pm, (Scott Dorsey) wrote: Mark wrote: true enough, but if you put an equal amount of effort into the design and impedance using a low noise FET, it will beat the tube hands down... Not really. *There's a really nice paper from Marshall Leach on the subject, too. *It's probably referenced on his website. Tubes and FETs come out about neck and neck, with similar transformers. Transformerless bipolar input stage comes out hands down, though, assuming 600 ohm source impedance. When you're looking at attainable noise figures of 2 dB over the inherent noise of the microphone, which you can do with a tube or a FET and the right transformer, there's not a lot of hands-down improvement possible. In fact, there's only 2dB of improvement possible, max, unless you dip the mic into liquid helium. Realistically, more like 1dB, which is barely noticeable. Modern designs, with any of the three technologies, are scraping pretty close to the theoretical limits. Peace, Paul I agree that an FET can achieve a 2 dB noise figure and that means an FET can be 2 dB away from as good as it gets at room temperature... Can you achieve a 2 dB noise figure with a 12AX7? *Or are you talking about some special tube? It's a little hard with a 12AX7, but it can be done with a 12AU7 or a 6SN7, neither of which is exotic. The main thing is to use a transformer with low noise, like the Jensen 13k7 I mentioned earlier. Oh, and run the tube at a highish current to maximize gm (lower noise) and minimize the value of the cathode resistor. Peace, Paul |
#14
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Simple Test Circuit for Tube Noise
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#15
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Simple Test Circuit for Tube Noise
On Sep 13, 3:36*pm, (Scott Dorsey) wrote:
wrote: The 33 Meg resistor is being bootstrapped to at least 330 Meg. *The bootstrap is direct coupled. *The output Z of the capsules I'm using is ~50 Meg at 40 Hz. How did you get that number? *What is the capacitance of the capsule when idle? Capsule capacitance is about 80 pF. So its output Z it just its capacitive reactance, right? As long as the leak resistance is low enough to permit all the spurious charge appearing on the grid to be grounded out, you're fine. *The amount of spurious charge that appears there is dependant on the kind of tube you use and it's one of the more important things to select when you pick a tube for microphone applications. What do you mean by "spurious charge"? Do you mean input current? Sean B |
#16
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Simple Test Circuit for Tube Noise
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#17
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Simple Test Circuit for Tube Noise
On Sep 14, 8:48*am, (Scott Dorsey) wrote:
wrote: On Sep 13, 3:36=A0pm, (Scott Dorsey) wrote: wrote: The 33 Meg resistor is being bootstrapped to at least 330 Meg. =A0The bootstrap is direct coupled. =A0The output Z of the capsules I'm using is ~50 Meg at 40 Hz. How did you get that number? =A0What is the capacitance of the capsule wh= en idle? Capsule capacitance is about 80 pF. *So its output Z it just its capacitive reactance, right? I'm not sure if you can assume that, but it would make sense. *80 pF is a pretty high capacitance capsule and you can probably get away with a higher Z than most with it. As long as the leak resistance is low enough to permit all the spurious charge appearing on the grid to be grounded out, you're fine. =A0The amou= nt of spurious charge that appears there is dependant on the kind of tube yo= u use and it's one of the more important things to select when you pick a tube for microphone applications. What do you mean by "spurious charge"? *Do you mean input current? No, the whole point of the leak resistor is to drain off the charge that appears spuriously on the grid..... as electron flow occurs inside the tube, the grid slowly picks up a charge. *As long as there is a path to ground for that charge, everything is fine... but if the grid is completely disconnected it will gradually charge up toward the plate value. The value of the leak resistor required is determined by the degree to which this happens. *There is a good discussion in the Radiotron Handbook. Don't think about input current... it's really not a useful concept here anyway. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." Apparently you're right about enlarging the grid resistor, but for a different reason. A bigger grid/charge resistor results in LOWER noise. How's that for counter-intuitive? My beef with my prototype is noise, so hopefully this is the answer. I'll let y'all know. Check out the paper below, page 2: http://www.etymotic.com/publications/erl-0053-1967.pdf Sean B |
#19
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Simple Test Circuit for Tube Noise
On Sep 14, 1:08*pm, (Scott Dorsey) wrote:
wrote: Apparently you're right about enlarging the grid resistor, but for a different reason. *A bigger grid/charge resistor results in LOWER noise. * It's not lower noise, it's more signal. I'm surprised by this. I thought once the input Z is about 4 times the source Z, there is little to be gained in level by raising the input Z further. Sean B |
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