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Need help with a radio project!

django

May 26, 2014
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Hi guys. I'm most definitely an amateur at this kind of thing. I have a good basic hobbyist knowledge, but that's it. I was wondering if someone could help with a few things on a project I'm working on?

Project Overview

The plan was to buy an old FM radio shell from eBay, and hook a Raspberry Pi, with mpc/mpd installed, up to the amp. Then plug a portable drive into the Pi, and use it as a jukebox. The Pi setup has gone fine, and I've just about managed to locate the right place to solder my stereo inputs from the Pi to the amp board to get the whole thing working as a standalone device.

The IC's are:

KA22471: http://pdf1.alldatasheet.com/datasheet-pdf/view/97331/SAMSUNG/KA22471.html - for the FM receiver I think
KA2201: http://pdf1.alldatasheet.com/datasheet-pdf/view/37140/SAMSUNG/KA2201.html - for the audio amp part


Questions

Image here: http://i.imgur.com/qbPTXZz.jpg


1. I've connected a stereo input to points A and B. I'm not sure if this is the right place, but it gives me audio. It's noisy, and a bit distorted. Where would be a more sensible place to hook it up?
2. The FM aerial is still connected at point C. I'd like to isolate this somehow and I think it's where the extra noise is coming from. What's the best way to kill it, as I don't want to use it?
3. What are the things at D and E?
4. When I do connect to A and B, I have limited control over volume using the volume pot I've connected it to. Any ideas why?

Thanks for any help you guys can give. If you'd like more photos, please ask!!


 

KrisBlueNZ

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Nov 28, 2011
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That's a mono radio. The giveaways are (a) the volume control is mono (stereo pots have two sets of three terminals); (b) the audio amplifier IC is mono, and probably (c) there is only one speaker. You will have to merge the left and right channels together, using a resistor (e.g. 3k3) from each channel to a common point, or get the RPi to play the tracks in mono.

That is not a very good quality radio. I think you'll find some distortion is unavoidable. Personally I would remove that board and replace it with a good quality mono audio amplifier module - actually, I would get a bigger radio, with two reasonably sized speakers, and fit a stereo audio amplifier module into it.

The thing marked "D" is an IF (intermediate frequency) transformer. It's part of the radio circuitry. The thing marked "E" is the ferrite rod which is used for AM reception. If you want to disable all of the radio receiver, just remove the IC.

You are injecting your audio signal at the wrong point in the circuit. Points A and B are connected to the wiper of the volume potentiometer; you should inject your signal into the fully clockwise end terminal of the potentiometer. Also, you'll probably find that points A and B are connected together.

The fully clockwise end terminal of the volume potentiometer is the one closest to the IF transformer you've marked "D". Remove the wire that connects the potentiometer tag to the circuit board; this will carry the audio from the FM demodulator. Feed your mono signal into the potentiometer tag.

P.S. Welcome to Electronics Point :)
 

django

May 26, 2014
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May 26, 2014
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Hiya Kris

You can't even begin to imagine how useful this is!

I knew the radio was only mono, (yeah, one speaker!) but was hoping to find a way to merge L+R together from source, eg from the RPi.

Regardless, as this is only a project, you've been really helpful in improving my understanding. I just had one more question, (for now!)

You say I should inject my (mono) signal into the fully clockwise end terminal of the potentiometer? This is fine, however the red wire that's already connected to this joins up with the transformer that powers the board. Won't this cause problems?

Thanks again!
 

KrisBlueNZ

Sadly passed away in 2015
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No, those terminals are the on/off switch. I'm talking about the three terminals on the same side.
 

django

May 26, 2014
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May 26, 2014
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Got it. Thanks for the input - I'll update when I've done all of that!
 

django

May 26, 2014
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Right, wired it up as stated and it provides sound output as expected - sounds fine for what I want but it's only mono. I'm looking into ways to mix my stereo signal into mono before it leaves the RPi, but if I don't use this board, I do have an alternative - this one: http://atoomnet.net/tda2030a-amplifier-diy-kit-schematic-and-soldering-howto/

I'm correct in thinking it isn't safe to wire it up and test it using a regulated DC bench power supply?

Thanks folks!
 

Arouse1973

Adam
Dec 18, 2013
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Hi django
What do you mean mix my stereo into mono. I thought you wanted stereo? Or am I missing something.
Cheers
Adam
 

django

May 26, 2014
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May 26, 2014
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Hi Adam,

Sorry I've not been clear on that! I want one of the following:

1. Use the old board (pictured), and have the stereo audio mixed into a mono signal (somehow), before it reaches the amp stage, or..
2. Use the new board, (linked), mount a second speaker inside the case somewhere, and find a safe way of powering that board for testing.

If anyone is interested, here is a better picture of the the case I'm using!

http://www.imgur.com/nusx1Gg.jpg


Thanks folks
 

BobK

Jan 5, 2010
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Kris already told you how mix the two channels. Simply place a resistor between each output and the single input. He suggested 3K3 which I would concur with.

Bob
 

django

May 26, 2014
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Hi guys

Thanks for the input guys. Think I'm gonna use my stereo board, I rigged the mono one up with a pair of resistors like you said and it sounds....ungood (is that a word?) Just wondering if anyone could confirm if it's safe to test this board: http://atoomnet.net/wp-content/uploads/2012/12/tda2030a-finished-BTL-amplifier-s.jpg - using a DC power source? Will the bridge rectifier on this board help with that, or do they just convert AC into DC?

Cheers!
 

BobK

Jan 5, 2010
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If it has a bridge rectifier for the power input, you can place DC of about 1.4x the AC voltage. And you can put the pos and neg either way!

Bob
 

django

May 26, 2014
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May 26, 2014
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Awesome. Do I need to ground it to anything (eg from the ground terminal between the AC terminals?)

EDIT: NM, seems to power the amp OK - just need to sort the speaker balance out now. Thanks guys - will update this thread if I get stuck again!
 
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