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Old 10th July 2013, 09:02 AM   #1
thommy is offline thommy  United Kingdom
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Default 2ch passive stereo summing circuit?

I'm building a class d amp, mini pc, dac & possibly tube pre in one box (see plan below).
The main intention is connecting it between a nas drive and speakers for a one box hifi solution with ipad remote control. My expected spend is about 350 and for that I think it should include an analogue aux input to make it useful for other purposes.

I want to keep the project very simple and elegant with just a power switch and volume control so I am thinking that the most appropriate solution would be to passively mix the stereo signal from the dac with that from the aux input, before making up the gain and offering it to the tube pre or volume attenuator.

Perhaps adjusting the input impedance of the pre would be more appropriate to make up the loss, but i'd also like to consider forgetting the tube pre altogether and just using a stereo stepped attenuator before the amps.

This is my first project and knowledge is limited, i've searched this forum and elsewhere but have yet to find anything that's not overkill. Maybe I'm using the wrong search terms, and someone here can tell me what it is i really need?
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Old 11th July 2013, 09:48 AM   #2
AndrewT is offline AndrewT  Scotland
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Build this on the bench ONE stage at a time.
Get each stage working properly. Then get the stages to work together.

When you have the WHOLE thing working properly, only then look at how you can package it into a one box solution.
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Old 11th July 2013, 02:47 PM   #3
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It seems odd that you would consider a tube pre between a digital source and a class D amp, but to each his own.

When you isolate the two sources with resistors it will lower the signal level. OK, you may still have a signal high enough to drive your amp. Test before committing to a preamp of any kind if you want to keep it as simple as possible. Better solutions are adding a good quality double pole double throw switch as a source selector or placing a buffer on each source and then mixing the signals.

Take Andrew's sage advice, go step by step.

I've been toying with the idea of a raspberry-Pi as a music server for my bedroom, it will be interesting to follow your project.
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Old 11th July 2013, 03:38 PM   #4
thommy is offline thommy  United Kingdom
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Thanks guys. Putting a tube pre between the two just takes some of the brittleness out of the sound and makes it a bit more homogenous. Both the dac and the amps offer incredible separation which can be a step too far for some tastes. Plus valves are cool, aren't they?

That's the only non essential part and may be left out hence my reluctance to modify the pre.

It would also be set at a comfortable max volume and then remain out of sight with the output of the pi controlled by the ipad hence my reluctance to avoid a switch.

So it's back my original question; how to passively mix the two signals without losing gain. Apparently my chosen amps have an impedance of 2x10kr and others have suggested that this might be too low.
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Old 11th July 2013, 04:07 PM   #5
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Does R-Pi have an analog in? Let it do the source selection...
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