Source Selection: Bypassing/Padding Preamp

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I am setting out to build a 12B4 preamp for my vinyl setup. The 12B4 preamp provide Av = 6.5 (mu of the tube).

The preamp is going to be primarily used for my vinyl setup. Along with my turntable I also have an Apple Airport Express (DAC) that I use to for casual listening (read: when I’m lazy). This DAC does not need a preamp, it just needs a volume control between itself and my poweramp.

I want to incorporate an input source selector switch on my 12B4 preamp. Instead of trying to figure out complex switching arrangements to bypass the preamp while the selector switch is set to the DAC position I figured I could just pad the input of the DAC input so that my overall gain is 0 (i.e. pad the input to give Av = -6.5 then have the 12B4 preamp give Av = 6.5, net 0 gain).

Any flaws to my idea? Another suggestion perhaps?


Below is my sketch on the idea for padding the DAC input. I tried to keep resistor values low while simultaneously not affecting the input impedance too much. Even though the padding voltage diver takes the input impedance down from 50K to 8.33K the DAC has an output impedance of 152R so my “padding” network only attenuates the signal 2% more than if the padding network was not there. Anything I am missing here?



 
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At an overall gain of 0 you won't be able to hear the music. I suggest you design for a gain of 1 instead. This means that the attenuator pad has a 'gain' of around 0.16, not -6.5. I fear you are confusing ratios and decibels.

I think I would use somewhat smaller resistor values in the pad. Maybe 12k and 2.2k? Not too important though.
Yes I was getting my gain vs decibels mixed up. Gain multiplies, decibels add.

Ok so 0.16 x 6.5 ~ 1, good.

12K series resistor with a 2K2 going to ground. This provides 0.155 output, close enough to 0.16.

2K2||50K = 2.1K
2.1K + 12K = 14.1K this is the new input impedance
 
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