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Craig Sawyers · 22nd Sep 2017, 6:15 pm

Tried to edit, but my 30 minute edit limit had expired.

Amended second paragraph:

But changing the transistors to BC550 has less clear benefit. The noise factor of a BC109 is, at 500 ohms source and 200uA collector current:

4dB at 120Hz
2dB at 1kHz
2dB at 10kHz

Since the resistance of a typical moving magnet cartridge is about 500 ohms, the critical input transistor is generating less than the noise as the cartridge, and hence is insignificant. Changing to the BC550 elsewhere has an even less than insignificant (!) effect on noise.

22nd Sep 2017, 6:15 pm	#3
Craig Sawyers Dekatron Join Date: Nov 2010 Location: Oxford, UK. Posts: 4,982	Re: Quad 303 Tried to edit, but my 30 minute edit limit had expired. Amended second paragraph: But changing the transistors to BC550 has less clear benefit. The noise factor of a BC109 is, at 500 ohms source and 200uA collector current: 4dB at 120Hz 2dB at 1kHz 2dB at 10kHz Since the resistance of a typical moving magnet cartridge is about 500 ohms, the critical input transistor is generating less than the noise as the cartridge, and hence is insignificant. Changing to the BC550 elsewhere has an even less than insignificant (!) effect on noise.