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?
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.
I think I would use somewhat smaller resistor values in the pad. Maybe 12k and 2.2k? Not too important though.
With the volume set at maximum, the input attenuation
of your circuit before the tube is a factor of 0.13, or -17.7 dB.
The input impedance at Vin is around 64k. This is what the DAC sees.
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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
It seems ok as it is. I would use the values in your sketch.
The DAC won't have a problem with this.
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The DAC can easily drive a high impedance load like 64k.
A difficult load would be less than around 1k.
Right
55K series resistor and 10K tail resistor.
0.154 attenuation, 63.3K input impedance
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As long as the first series resistor is 10k or more (to ensure compatibility with various sources),
you can use a wide range of resistor values for the same attenuation.
I edited my post above
Will go with 55K/10K
I think this will be suitable for the casual listening I do with the DAC.
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