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Switching between phono and Bluetooth inputs

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myleslewis View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote myleslewis Quote  Post ReplyReply Direct Link To This Post Topic: Switching between phono and Bluetooth inputs
    Posted: 17 May 2015 at 9:56pm
So I hope I've posted this in the correct forum. First a little background into my project(s).

Not too long ago I obtained a QTX12 12" 2-way monitor, cheap chipboard hobby, for £10 of a friend of mine. Another friend of mine introduced me to the tda7297 chip amps you pick up for a couple of quid on eBay, so I decided that is turn this into a boom box, powered by a 12v cctv battery.

So I switched out the existing bare wire terminals and jack input from the backplate for a phono plate, power switch and 5.5mm x 2.1mm barrel socket, the same size they use on the cctv batteries and are commonly used on 12v dc mains adapters. Wired the phono plate into a 3.5mm jack plug, this goes to the amp, existing crossover in the cab is fed from the amp. Job done, or so I thought...

Whilst the phono input gives me nice flexibility at home, for use with phones I can also use the box with my dj controller so long as I don't drive it it too hard, the tda amps only really like line level input. This is great, although means I have to keep reaching round the back of the box to change the cables between phone/dj controller. So I thought it would be great if I could connect my phone wirelessly. A bit of eBay searching and I found a Bluetooth receiver module, that has 3.5mm audio output... Brilliant ! However how would I switch between them...

After posting on Facebook (cue groans from other users :p) I confirmed my suspicions that it would not be a good idea to just tie the phono 3.5mm and BT receiver module 3.5mm together and run straight to the amp. I was pointed in the direction of a 3PDT switch, excellent ! However one thing that was highlighted was the potential of a pop when switching inputs. I was then told that a 50k resistor bypassed by a 100pf cap between the grounds (tied together so as to enable switching between the l/r channels of each input) and both the outputs should eliminate pops without a detrimental effect on sound quality. However my electronics knowledge isn't immense and I don't quite know how I'd implement this. Which leads into my question.

Anyone shed any light on this ? Or have any other ideas ? This is a learning curve for me, and will prep me for my next boom box of sorts, reloading an empty intimidation aka112 cabinet (inset groans once more :p) with some better quality drivers, same tda amp, own crossover, and hopefully another Bluetooth receiver module. I've tried to figure it out myself, I will upload two wiring sketches tomorrow, posting this from my iPhone and unable to inset images to post, but not sure which is right, if either.

Thanks in advance,

Myles

P.s apologies if this is in the wrong area, but it is 12v related... Somewhat. And will help for future projects :)


Edited by myleslewis - 17 May 2015 at 9:57pm
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amlu View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amlu Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2015 at 10:55pm
connect all signal grounds at one place,  (both blutooth and external input), choose betwen sources with a 2 pole DT switch... if pops bother you turn the volume down on the amp or power it off when you operate the switch , swapping the inputs on a running amp is not a good practice anyway.

or get a small £10 mixer from ebay, it will have proper knobs and ability to properly drive the input of your amp.
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myleslewis View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote myleslewis Quote  Post ReplyReply Direct Link To This Post Posted: 18 May 2015 at 8:58am

Turning the amp down isn't really an option as it's inside the cab itself, also for this project the idea is to have everything inside the cab so no external controls as such. Just phono in, volume is controlled via phone/controller volume control, same when connection will be via bluetooth.

 
Have thought this morning, if I use a break before make switch (I actually have a 4PDT kicking around at the mo I can use) and put the amp on the spare poles it should shut the amp off momentarily between switching ? Don't know why I didn't think of that before lol.
 
However if anyone can advise on how to implement the caps/resistors, it would be appreciated. I am interested to learn :)
 
I will add that when switching between sources, no signal will be going to the amp. And I'll be using only one input at a time, either bluetooth or phono, so mixing/summing the signals isn't nessecary.
 
Thanks,
 
Myles
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myleslewis View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote myleslewis Quote  Post ReplyReply Direct Link To This Post Posted: 18 May 2015 at 3:37pm

Here are the two wiring diagrams I drew, in the second it doesn't actually connect to pole 4, just how it was drawn. 

Moderator's if you feel this thread would belong better in the electrical forum, feel free to move it over :) 

Thanks,

Myles
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