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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
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." |
#6
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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 |
#7
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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 |
#8
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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." |
#9
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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." |
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