All electronic components have a certain amount of noise associated with them that is additive through out the sytem. Look at the specifications of any device, solid state or tube and there are specs for the noise associated with that component. Therefore, signal or no signal, the circuit in questions will have some noise. Whether the gain in the circuit is there or not, there is noise, regardless of whether you can here it. It is measurable. The noise floor is there period. gain is added if gain devices are present and more noise is added if grounding is not good or ground loops are present. So, start off with a specific amount of noise (noise floor) of the device under test. It is there regardless. Connect this device to others and the other device's noise is also added. now you have system noise. For example. CD transport has noise A, DAC has noise B, Pre-amp has noise C, amplifiers have noise D. The total noise will be A+B+C+D. And if you have bad grounds or power line noise, then that adds also. Also, regardless of tube or solid state, a competent Engineer/Designer will design based on the noise, gain, input impedance, output impedance, load, etc. They pick electronic devices based on the load/gain characteristics and also based on the specified noise of the device. But, you get what you pay for. You want terrific gain/impedance devices? you want really low noise devices? you want devices that operate from DC to light? well, you will pay for it. This is also why some electronics costs soooo much.
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