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Old 9th Jul 2019, 1:26 pm   #105
Hartley118
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Join Date: Aug 2015
Location: Cambridge, Cambs. UK.
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Default Re: Mullard 5-10 amplifier

Quote:
Originally Posted by vampyretim View Post
Hi all,

With the preamp connected or disconnected from the BNC input the 5-10 hisses but with the preamp disconnected and the 5-10 BNC input shorted to deck the hiss practically disappears. I have tried replacing the 2.2M resistor but it made no difference. I'm a bit confused, does this make sense to anyone?
This finding suggests that the power amp is blameless and you're just hearing the thermal noise of the 2 megohm input resistor.

Any resistor generates a thermal noise voltage equal to 4kTRB, where:

k is Boltzmann's constant
T is absolute temperature (degK)
R is resistance (ohms)
B is bandwidth.

Being a trifle lazy, I used the thermal noise calculator on this website
https://www.daycounter.com/Calculato...lculator.phtml
which indicates you'll be getting around 25 uV from 2 megohms. That's probably about 85 dB below the max output of your preamp, so will be audible.

So....sorry, you're hearing thermodynamics at work.

Once the low output impedance of the pre-amp is connected, that will shunt the 2 meg resistor and reduce the power amp's thermal noise a lot. However, your noise level will now be dominated by the pre-amp circuitry, which is probably noisier than the power amp.

Incidentally, one of the reasons that solid state amplifiers tend to exhibit lower noise levels than valve amplifiers is that the resistor values are generally lower.

Martin
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