G'day Guys,

One of my many current projects (they seem to multiply when I'm not looking) is to build a small integrated amplifier for the bedroom.

This project is an offshoot of the gainclone boombox project. Circumstances changed and now the boombox is no longer an appropriate solution for the bedroom.

It was also ugly and was never really supposed to a permanent solution.

Now it is time to make a permanent solution. The speakers we will deal with later, right now the problem is relays and grounding like normal.

The design brief is as follows:

-LM1875 big cap boards for the output stage, this time set for a gain of 12x since I have plenty of gain.

-I will reuse the original Guanzo 6n3 preamp board, after all the work spent designing new boards, they are too bulky for this application. Also the original works just fine with plenty of lovely distortion for that tube sound.....

-Bluetooth 

-FM radio 

-A regular RCA auxiliary input at the back

-3.5mm auxiliary input at the front.

Some of this is my preference, some of it mandated by SWMBO.

WAF will also be very important for the final result. I anticipate a plywood front panel, finished to match the speakers.

So now we come to the questions.

One of the design hurdles is that I would like the bluetooth and radio stages to not be powered when not selected as the source.

So far what I have come up with is to switch a pair relays at the output of the 5vdc PSU that powers both boards using essentially a variant of a source selector board and an unused pole on the source selector switch.

I think I would want a dummy load at the output of the 5vdc PSU so that it has a load when neither of the bluetooth or radio is in use. Say something like a 1k resistor across the output.

I have also considered simply using an extra 2 relays with dummy loads for the 2 sources that don't use the 5vdc PSU

The other option would be to switch the AC going into the 5vdc however this would require 2 power supplies and therefore more space....

So here is what I have come up with so far. This design would require a dummy load at the output of the 5vdc PSU.

For simplicity's sake the outputs of the radio and bluetooth boards were drawn as connected but these would of course be separate connections.

It looks pretty good to me. The only issue I can see is that since the bluetooth and the radio boards share a PSU, they also share a ground and therefore I can see a ground loop between the 2 boards and the preamp stage.

Any thoughts, suggestions or critiques?

Any reason I am going in the bad direction?

No need to use a dummy load on the unconnected 5v power supply. Connect both Bluetooth and radio grounds together for the power supply ground, since only one is in use at any time.

AMR-iFi R&D

G'day Colin,

Thanks for that.

There is no issue with leaving something like a 7805 regulator without any load?

No, just ensure it has the minimum capacitance on the output to keep it stable, 100n. During switch-off, if the input voltage drops faster than the output and draws a lot of current, you can add a diode connected k-a from input to output.

AMR-iFi R&D