Introduction: Bluetooth Amp + Isolation Switch (Two Amps Share a Pair of Speakers)

About: My name's Rob. I haven't made a lot yet, but I'm hoarding both bits and ideas, so something's bound to happen.

I have a Rega P1 record player. It's plugged into a little 90's Hitachi midi system (MiniDisc, no less), which is plugged into a pair of TEAC speakers I bought for a few quid from Gumtree, because I ruined one of the original speakers on a dodgy Technics amp I was given.

I love my record collection, but I listen to music most of the time and being able to stream just about anything on Spotify is incredible. I couldn't play my phone through my stereo without unplugging the record player though.

Well that's easy enough to sort, isn't it?

Step 1: What Is It?

This thing only makes sense if it's actually what you need. I spent ages searching for tutorials that covered my specific requirement and couldn't find the right thing, so I combined a couple of other things to make what I wanted.

So, do you:

  • have an amplifier or stereo with only one input, and
  • have a record player plugged into it via a phono amp, and
  • want to be able to play Bluetooth music through your speakers?

If so, then this is the build for you!

This is a Bluetooth amp in a box with a DPDT (double pole, double throw) switch. The wires that lead out of your stereo and normally end in speakers, instead are fed into the back of the box and link to one side of the switch. The Bluetooth amp inside is wired up to the other side of the switch. Then there's cables coming out of the back of the box into the speakers.

This means I can switch between my stereo, with record player plugged in, or my Bluetooth amp, and whichever one is selected will play out of the one set of speakers.

Step 2: Explanation

(You can skip this bit if you don't care. I quite like writing and this thing will probably end up being really long, so I'll try to point out when you can skip stuff.)

The issue is that it's easy enough to add Bluetooth to an old stereo, and you'll find a million and one tutorials telling you how, but they all rely on a spare input, or sharing one.

You're really unlikely to be able to share an input with a record player though. If your amp has a phono stage built in, you don't want to be plugging anything but a record player into it (and if it has a phono stage it's probably got another input anyway), and if it doesn't have a phono stage then you'll be using a phono pre-amp, and you don't want to share an input with another amp because it'll cause interference and could damage one or both of your amps. There's also a chance of it causing damage to the sensitive electronics on the stylus, but at the very least you'll get feedback.

So once I'd ruled out using splitter cables, I realised that what I was most bothered about wasn't really using one stereo, it was just not having two sets of speakers. I wondered whether I could just set up a Bluetooth amp and plug that and my other amp into the same set of speakers. When I read up on it I found that the problem there is basically the same, in that you can't have two amps joined together by their outputs, because you'll very likely end up with two dead amps. You could try being careful, and always make sure you switch one off before switching the other on, but chances are at some point you'll forget and BANG! Wrecked it.

I started searching for a switching box and found a few, but they were pretty expensive for what they were. Ludicrous! So I figured I'd make one. I found this tutorial, which did what I wanted but assumed having two external amps. I only had the one, so I knew I'd need to make a Bluetooth amp as well, and I figured I might as well put the amp and switch in the same unit.

As it happens, I went and put both inputs on my box anyway. Bit of future-proofing never hurt anyone. (Sarah Connor aside, obviously.)

Step 3: Parts

I chose the stuff I used after fairly careful consideration, so I'll give you the list but then try to explain my thinking.

So, my thoughts (you can skip this bit if you just want to get on with it)...

The switch: I liked the one they used in the tutorial I linked before, so I found one similar. I'd already decided I was going to use the cigar box (because I didn't want to buy a new enclosure, and it being wood would make it easy to use) and I thought the heavy mechanical switch would look good against it.

Speaker cable: Mine was from B&Q but they didn't have the 5m length on their website. If I was choosing again, I might choose something slightly thinner (or a bigger enclosure) because it was a bit of a pain to manipulate in the space, but since it's carrying the audio signal go as thick as you can manage.

Banana plugs: I just went with cheap ones because the point of this whole thing was to do it cheap. They're not great, but they allow me to unplug it all without messing around threading wires through posts. You could just omit them though if you wanted.

Binding posts: Cheap ones. Same reason as above. The amp I used wasn't great so I didn't really see the point in going all out on the stuff connected to it.

Amp: More thought went into this. I wanted to be able to use a laptop power supply because I didn't feel confident playing with power supplies and figured using a barrel jack would be tidy and low risk. I also wanted to be able to just plug the Bluetooth adapter in with a 3.5mm jack, in case I wanted to upgrade or plug something else in instead. So I trawled AliExpress for an amp that took a barrel jack for power and also had a 3.5mm input socket. I also didn't want something too powerful. My speakers are 90W max input so it needed to be less than that, so I went with a 50W per channel stereo board. Ideally I would have chosen a better chip (I spent a while reading about different amplifier chips) but the one I went with ticked my main boxes and was cheap, which meant if I messed it up I wouldn't have spent a load of money. (It's worth mentioning that this is my first real attempt at making anything, so I was being cautious.)

Buck converter: Ten a penny really, but I went with a USB one so I could just plug it all together when I was ready.

Ground loop noise isolator: You'll need one of these if you want to power the Bluetooth receiver off the amp board. When I first put it together I had two power supplies - one for the amp and one for the Bluetooth - which is too much when you've got other stuff plugged in too. If your output and input share a power supply though, you'll get a load of horrible hissing and popping noise, but one of these little things filters the signal to remove that. I also bought another one to put in my car because I use a Bluetooth receiver in there and can hear all the electric noise my turbo and indicators produce otherwise.

Wire: Wire's wire. Or is it? 26AWG is easy to work with and I bought a bundle of little lengths because I figured they'd be handy for other stuff.

Bluetooth adapter: I wanted one that would be always on, so I didn't have to listen to it talk to me whenever I connected. I also wanted Bluetooth 4.0 to get the best sound I could for my money. Actually, it took a while to find the right one, but the one I got worked a treat. The only issue with it really is that it has a bright flashing green light on it, but once it's inside the box you can't see that so it doesn't matter.

Enclosure: I really wanted to use a sleek aluminium case, or order a 3D printed one, but it was going to more than double the cost, so in the end I found a cigar box in the garage and went with that. I think it was a great choice actually. It was easy to work with, looks pretty cool, and if I'd messed it up it wouldn't have cost me anything.

Step 4: Tools

I used:

  • A drill with various sized wood-boring bits (10mm, 7mm, 6mm and 2mm, though that all depends on the exact parts you use)
  • A rotary tool with a small sanding bit (which I'd only just bought and I made a bit of a mess, but it was on the back so it's not the end of the world)
  • A soldering iron
  • Solder
  • Wire clippers/strippers
  • A pair of long-nose pliers
  • A small adjustable spanner
  • A ruler
  • A pencil
  • A small flat-head screwdriver
  • You might want to use a helping hand for the soldering

Step 5: Decide on a Lay-out

Once I'd got all my parts, and found the cigar box, I had to figure out how best to lay it all out. I made a couple of mistakes, so take note of those!

I decided on putting the switch smack in the middle of the top of the box. That was basically to avoid tipping the whole thing up every time I switched it over. The switch is fairly stiff so I thought if I put it on the front it'd be awkward, and if I put it towards the front or back on top I'd risk tipping it over.

I put the amp at the front so that the volume dial and power LED would be visible and accessible. It could have gone in the lid with the dial and light on top, which might have made it a little easier to get to the stuff inside, but stereo stuff normally has the volume dial on the front, so on the front it went.

The binding posts (inputs and outputs) needed to go on the back to keep them out the way.

Step 6: Preparing the Box

On the back, I figured I'd put the outputs at the top and the inputs at the bottom. If you look at the first picture on this step you'll see how I laid them out. I measured the width of the posts and the distance between the two on each set and drilled holes for them all, measuring out and marking where they'd go beforehand. The mistake I made was in putting them too close to the bottom of the box, which made screwing them together inside a right pain.

The laptop charger had a chunky bit that would need a decent-sized hole to get it through the back of the box, and I didn't have a drill bit big enough, so I drilled a 10mm hole and then used the rotary tool to widen it. I didn't have a small enough sanding bit so used a sharpening bit instead and basically burnt the wood away until the hole was big enough. What a mess! But I only have limited tools, so I had to improvise. (Plus, as I said, this was on the back so I wasn't too worried.) It would have been neater just to use a needle file but I didn't have long to do all this while the little'un was napping inside, so I was in a rush.

On the top, I used my ruler from corner to corner in both directions to mark a small diagonal cross in the middle of the top of the box. I then measured the diameter of the screw thread on the top of the switch and drilled a hole where my cross was. Luckily it was 10mm, which meant I had a drill bit big enough and wouldn't need to widen it. (We know how that went last time.)

I measured across the front of the box for the volume dial and marked a spot that would mean the amp would be about a centimeter from the bottom of the box inside (knowing the nut holding the volume dial in would hold it steady) so air could circulate. (I don't imagine it'll get particularly warm, but better safe than sorry.) I measured the distance from the centre of the dial to the centre of the LED and marked a spot to drill for the LED.

I realised just before drilling that the post for the volume dial wouldn't reach all the way through, so I measured the width of the nut that holds it on as well. I drilled into the front of the box first, but not all the way through. Then I drilled within that indent a hole the width of the post. That meant the post would go through, and the nut would tighten and be flush with the outside of the box. Hopefully you can see what I mean from the second picture. I then drilled the LED hole.

(Check out my great drawing if you need any extra help understanding any of this. Haha.)

The box was ready.

Step 7: Preparing the Cables

This part is a little more step-by-step, and I'm slightly conscious that it's been a bit of a wall of text so far, so here's some numbers (you'll be soldering on step 5 so switch that on now):

1. Cut some appropriate lengths of speaker wire for the internal connections. I needed two lengths to connect the amp to the switch, two to connect the external input to the switch (remember there're two inputs, but I'm only using one), and two to connect the switch to the outputs. So six in total, and I cut them to about 12cm each.

2. Speaker cable is dual wire, so before stripping some outer plastic off you'll need to split the two wires down the middle, enough to allow you to work with it. I found that about 4cm down each end worked.

3. Then strip about 15mm of the plastic outer sheath away from each wire on each end, so that's four wires stripped per length of cable. Twist up the ends so they're tidy. Stray strands of wire in here could result in ruined amplifiers, so make sure there're no strands floating about.

4. The two lengths that you're going to connect to the amp need to stay straight on one end, but on the other end of those, and on both ends of every other piece, bend the wire over a thin screwdriver to give you an open hooked shape on the exposed wire, like a shepherds crook.

5. Soldering time. You're going to cover the curved wire ends in solder. This will help you connect it all up (because the nuts on the switch and the binding posts will grip the single soldered chunk of wire better than a load of twisted strands) and it'll stop stray strands, which could cause shorts. Hold your soldering iron out and basically drape the hook over the top of it so the tip is heating the wire. Hold the solder on the top of the wire (so the wire's between the tip of the iron and the solder) and when the wire is hot enough it'll act like a wick and suck the solder into it as it melts it. You don't need loads; just enough so you can see it in and on the cable. (Careful not to melt the plastic sheath further down.)

That's the wires ready. You should have six lengths of cable (each consisting of two wires) with all of the ends exposed, two of which will be straight on one end and hooked on the other, and the other four will all be hooked on both ends.

(Later you'll need cables to connect your stereo to the box and the box to your speakers, but you'll have two of those already, currently connecting your stereo to the speakers, and you should have plenty of cable left over to make the other two with.)

Step 8: Inserting the Binding Posts

Now to start putting it together. Binding posts first.

Take your binding posts apart. Each pair of posts will slot through a pair of holes in the back of your box. You want the coloured bits on the outside.

Mine came with two black plastic spacers. I used one of the spacers on the outside of the box, but there wouldn't have been enough of the screw-threaded binding post available on the inside if I'd used the other one in there, so I stuck with one on the outside and put the spare one aside. Place the spacer on the outside over a pair of holes, then slot the pair of posts through from the outside.

From the inside, put a washer over the post, and then tighten a nut over it to secure the posts to the box.

Do that with all your posts.

Step 9: Wiring Up the Switch

Next attach your cables to your switch. A picture of this doesn't really help, as you can see, so you need a diagram. I could give you a diagram but truth be told I'd just be copying the one in the tutorial I mentioned earlier, so just use that one (here). It's just under the picture of the box with all the wires. (It's worth reading through that whole article as well. It's good and might further help to explain what we're making.)

I chose to have the top row of the switch as my Bluetooth amp (so I used the cables with one hooked and one straight end here) and the bottom row as the connection to my stereo, so that when I turned it over and set the switch into the lid of the box, what was the top when I was looking at the underside of the switch would become the front row in the box when the lid was closed. That way the wires weren't all going to be crossed over each other.

You're going to hook your hooked and soldered wires round each peg and tighten the nut onto them. It'll be a bit fiddly, but the soldering you did will make sure you don't squash the cable and end up with strands splayed out of the sides.

Once you've done that, you can slot your switch up through the hole in the lid (from the inside, so the switch is protruding out the top) and fasten it with the nut if you want, though it's probably easier to leave it till you've connected the wires to the rest of it inside.

(One thing to note here, is that I decided on the binding posts at the back being right and left when looked at from the front, so that my right-hand speaker would be connected to the output on the right from the front and wouldn't be crossed over. But that means when you look at the box from the back, it'll be the output on the left you're looking at. I wrote on the back of the box in pencil which was which so I wouldn't forget when it came to putting it together.)

Step 10: Connecting the Cables Up

Again, the picture doesn't help a great deal because of all the wires, but I took one anyway. Look at that.

Anyway, connect the two straight cable ends to the Bluetooth amp. Pay attention to your rights and lefts. The outputs on the amp should be marked left and right, but you'll need to make sure you're connecting the correct wire from your switch to each one. I stuck with the convention of right and left when looked at from the front.

Connect the other wires to the appropriate binding posts. (The ones on the middle row are going to be connected to the inside of the binding posts at the top (i.e. the output) which is where your speakers will be plugged in, and the ones on the bottom row will be connected to either the two pairs of binding posts on the right, or the two on the left (i.e. one of the two available inputs). It doesn't really matter which two, since the other two are unused anyway. Again, pay attention to your rights and lefts.

(Right is red, by the way. Rs go together. And chances are your speaker cable will have either a black and red wire, or at least a red wire. You also get some that have a square and a circle cross-section to the sheath, which isn't obvious unless you know it's like that. Check yours.)

Step 11: Connecting the Bluetooth Receiver to the Amplifier

My laptop power supply is 19V, which will be fine for my amp board, but the Bluetooth chip is USB, which means it's 5V. By connecting the buck converter to the input on the amp, you'll create a 5V power supply for your Bluetooth chip.

You're going to need to solder a couple of short lengths of wire to the underside of the amp board where you plug the power in, and then solder the other end of those wires to your buck converter. You need to go positive to positive, and negative to negative.

My amp board's power input, like most barrel connections, has the positive on the pin/tip and the negative on the sleeve. The jack (where the power plugs in) has three pins that are soldered to the board and protrude from the underside. The one at the side detects if there's a plug plugged in so you can ignore that. The one at the back is for the tip (so will be positive) and the one at the front (nearer the edge) is for the sleeve (so will be negative).

(This Sparkfun page has some info about barrel connectors if you want a visual of that.)

Solder your two wires to these pins on the underside of the board. (Ideally use two different colours. I only had black so used that for both.)

My buck converter didn't have the + or - printed on it, but the AliExpress listing had a picture and you can see which is which when you hold it whichever way round. So solder the other ends of the wire to the buck converter.

Once you've done that, you can just plug your Bluetooth receiver into the USB slot on the buck converter (or take your receiver apart and solder the chip to the buck converter if you didn't buy one with a USB slot).

You can then plug your short 3.5mm extension wire into the Bluetooth receiver, and plug the groundloop noise isolator into the other end. Then plug your other short 3.5mm extension wire (you'll have got one with the groundloop isolator) into the other side of the isolator and the other end of that into the input on the amp board. (It doesn't matter which way round you put the isolator by the way. They're bidirectional.)

So that's the buck converter attached to the power input on the amp board, the Bluetooth receiver plugged into the buck converter, the Bluetooth receiver output plugged into the groundloop isolator, and the groundloop isolator plugged into the amp board.

Step 12: Put It All in the Box

Now you can just put it all in the box.

  • If you didn't do it earlier, slot the switch up through the lid of the box and fasten it with the nut from the outside.
  • Take the volume knob and nut off the front of the amp board and slot the post and LED (you might need to bend the LED's legs) through the holes on the front of the box, from the inside, and fasten the nut back on. It should sit inside the indent you made with your bigger drill bit. Then put the knob back on.
  • Make sure the rest of the stuff sits nicely inside. There should be room for it all to sit in the box without anything being draped over anything else, so there's no risk of anything getting melted or shorted.
  • Close the lid.

Step 13: Banana Plugs

I added banana plugs to the ends of the cables that would be connected to my new contraption, basically because I wasn't confident it was going to work and I didn't want to mess around unscrewing and screwing wires in over and over again. As it turned out, it all worked perfectly so I needn't have been concerned, though I think it's tidier having them, so it was 70-odd pence well spent.

Step 14: Enjoy

Once you've got it all plugged in (you'll need to slot your laptop power supply in through the hole in the back and plug it into the amp board, and connect your stereo to the input and your speakers to the outputs) you can turn it on, connect your phone up to the Bluetooth, and play away.

The switch on the top will allow you to switch between the Bluetooth amp inside (in which case you need to switch it on with the dial on the front) and the stereo (in which case you need to switch that on).

When the Bluetooth amp is in use, your stereo isn't, so you only need either your stereo or your Bluetooth amp switched on at any one time.

And that's that. I know it was a bit long-winded, but I wanted to explain what I did and why, because it would have saved me a load of effort if someone else had done this first.

Then again, I wouldn't have learnt about barrel jacks if I hadn't done all that.


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