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PStamler PStamler is offline
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Default 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