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MP3 player with old radio : one solution .

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nali View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote nali Quote  Post ReplyReply Direct Link To This Post Topic: MP3 player with old radio : one solution .
    Posted: Jan/29/2010 at 10:08pm
Here s the goal ..
Using a modern mp3 player on a vintage radio without paying 100 $ .
We can for 5 $ , in fact 2  $ if you get a free box for this , a soldering iron and a few wires . Notice the commercial
adaptor also need soldering , if they are not FM transmitters . What if you just have mono AM ? :P
If you have to fix something , you d better know how it works ...
So a few basics : It s really basic knowledge , easy to understand for people not familar with electronics , so be indulgents if I explain it quite easily ...

Any car radio , home amplifier or whatever basicelly works the same way . You need a source , to provide a low signal  from CD player , radio, 8 track tape or whatever .
Then , you need a power stage , to move the loudspeakers  . It s the amplifier .
Mostly all radios have  an ampli inside . Not always really powerfull , but there s an amplifier . Usually between 4 or 11 watt in an old stock radio .

Between the source and the amplifier , there s a way to choose how much output ( sound  ) you want from the radio . It s the volume button .

This is were we can play ....

A volume button works quite like a rheostat , the same we have to have more or less light in the dash .
The source comes from the top , and bellow is the ground . The middle cursor move between both and is connected to the amplifier .
The shape is just round :P
So if you turn left , you move the cursor to the ground , less sound , if you turn right , you go to full source and have more sound .
Just as easy ....




So  ... What now ?  Instead of sending to the amplifier the sound comng from the radio, we can send the sound from
anything .  
The ugly way would be to cut the wires coming from the source , and solder wires connected to a mp3 player .
For just 2 $ more , we ll give the beast 2 lifes .

Remember the FM receiver you had . Usually the sound comes from the loudspeakers , but when we plug ears , the speakers
stop and there s sound only in the ears .
It s just a mecanical connector . That s what we have to use .

Go to any electronics store parts , and ask for a stereo jack connector ( 1/8 I think ), with a mecanical switch , to  choose from head / speakers .
Cut wire on the volume button , inside the radio ,
Test it with your favorite Beckman , Fluke , ohmetre , solder it so that without anything plugged inside the sound comes from the radio , and when you plug a mp3 player , the sound comes from it . It s stupid and easy but it s enough .
In french , but quite easy to understand :


Oh .... Your mp3 player is stereo and your radio is mono ....  Connect the right and left throught 120 ohms resistors ,
that s all  ( 80 to 220 ohm in fact , to be safe ) .

PRO :
easy.
cheap.
it works .

CONS :
You have to solder inside the radio, this may kill it .

The ground from your volume button may not be the same ground as the radio ground . I STRONGLY recommend not to plug a
USB mp3 player to a USB charger in the lighter , unless you are SURE the ground is the same .
In my Ambassador , it s not the same ground . This could results to dead for the radio, electric fire , or ugly noise
in the loudspeakers .


More : you can plug 2 RCA output after the cursor and add an external amplifier , so you can enjoy classic music on
mono AM with 500 Watt :P

I know it s quite simple , it works for me , but take care to what you do . I m not responsible for anything if you
kill your cat , neighboor , wife , etc ..
I m french , so my english may not be perfect  . If anyone wants to correct me , I ll be glad of it .
Phil



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danahyatt View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote danahyatt Quote  Post ReplyReply Direct Link To This Post Posted: Dec/13/2010 at 11:36pm
This seems too simple, and most certainly one will have a voltage at the rheostat post you described (Noted: you do use a resistor).  The voltage at the rheostat post will damage the MP3. My understanding is to use the amplifier; one will need to connect where the tuner input is to the amplifier. Yet disconnect the tuner signal wave from the input to the amplifier. By inputting the MP3 signal to the amplifier one will have a full amplified signal strength to the speakers.  I don't read or write French but I do read schematics and these drawings are probably useless to many people who read this blog.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote danahyatt Quote  Post ReplyReply Direct Link To This Post Posted: Dec/13/2010 at 11:39pm
Additionally, I do have vintage cars but would like to keep them original so I will not mess with my radio to install an MP3.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote gryzynx Quote  Post ReplyReply Direct Link To This Post Posted: Dec/14/2010 at 1:46am
Well, that looks easy enough and is fundamentally sound, but you need to be careful. I actually have done this many years ago.
 
I hope everyone gets the point that your AM radio is mono, (1 channel) and you mp3 source is stereo, (2 channels). It will work, but you won't get the sound quality you may expect. The trick with the resistors will get the L+R signal to the single, mono input, but it will not give you a stereo output.
 
One thing that may be lost here is the fact that your volume control shaft actually serves 3 functions, power on/off, volume and tone. The three connections Nali refers to need to be for the volume control only. If you refer to the picture, you may see that the control is divided into 3 sections. The rear portion has two wires and  controls power. +12 V is switched on and off here when you turn the volume control. The front section uses 3 wires and performs the tone control function. The middle section is the volume control. The view you see is with the bottom plate of the radio removed. The middle section, (volume control), has 3 connections. The middle connection is the one Nali refers to as the cursor. The english name would be the wiper. The signal source and ground are exactly as described and can also be called the hi, (source) and lo,(ground) connections to the volume control. You need to determine the difference between these two connections and the easy way is to look at the volume shaft straight on and envision the arc travelled by the movable contact, (cursor, or wiper),as you increase/decrease the volume. If the connections are at the bottom of the control, (as I suspect they are in this example), the cursor will travel from the left terminal, (lo or ground) at minimum volume to the right terminal, (hi or source) at maximum volume. This would place the ground terminal closest to the outer frame, (white wire) and the source at the terminal towards the inside of the chassis, (white/black stripe).
 
The next thing you have to organize is the jack you purchased. This will be similar to the earphone plug already on your mp3 player. These plugs make 3 connections. They are called tip, ring and sleeve. If you examine the plug, you will easily identify which ones they are. The ground, or signal low is common to both left and right sources. It is formed by the sleeve, (circular contact band nearest the wire end of the plug). The tip and ring, (tip and middle band) make the L and R source connections. You may need our continuity checker, or ohmmeter to identify these connections on your jack.
 
The example Nali provides contains two extra connections called normalling connections. These are used to connect the radio source to the volume control when the plug is removed from the jack. Using your continuity checker, identify which plug connection mates with the jack connection when the plug is inserted. These are identified as sortie droite, (left source), sortie gauche, (right source), and masse, (ground) on the diagram. (Pardon my French Nali). The resistors mentioned to connect both sources to the mono input at the volume control will be connected in a Y configuration with one lead each channel towards the jack and one lead, (two resistors) to the source side of the volume control. It may be advisable to add a capacitor of about 0.05 microfarads between the jack and the volume control to provide DC blocking. between the mp3 player and the radio. 
 
The radio source, (white/black stripe) that was removed from the volume control should be connected to the normalling contacts of the jack to enable normal function of the radio when the mp3 source plug is removed. You can identify the normalling contacts with your continuity checker when the plug is removed from the jack. First, they are the two that are left over and one will connect with the tip and the other with the ring when the plug is removed. They will disconnect and isolate when the plug is inserted to the jack.
 
Still heed the warning about using the USB adapters to power the mp3 player as they may float the ground and the ground thru the radio via the masse/signal lo is really chassis ground and you could develop unwanted current of a significant magnitude.
 
One last consideration should be the nature of the input you have just provided. It is a low level input and you need to take care to not overdrive this input. The volume on you mp3 or other device should be set to a point where you cannot drive your speaker to distortion with the radio volume control. If you mp3 source is set too loud, the sound will be distorted at almost any setting of the radio volume control.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ronilevi Quote  Post ReplyReply Direct Link To This Post Posted: Dec/29/2010 at 4:05pm
Hi all!

I would really love to try out this hack on my '66 Ford radio but I am having a little trouble locating the stereo jack with the internal switch. From what I could find, the local Radioshack does not carry these so I have to resort to online retailers and I want to be certain that my purchase will work.

If I understand how this works correctly, the jack is normally shorted when there are no headphones (or mp3 player) connected to allow the signal from your antenna to stream through. When you plug in your mp3 player the connection cuts the antenna signal and instead feeds from your plugged in device. From what I understand, this is accomplished through the use of an electrical shunt and that is what I should be looking for in the jack that I purchase.

If what I stated is correct, then this device should work right? If you have better suggestions or if I am mistaken in any way, please let me know.

Also in response to gryzynx:  using a 0.05 uF cap between the mp3 source and the volume control is a great idea! I have two questions about this:
1) Will this have any negative effects on the sound by filtering frequencies?
2) If it blocks the DC then wouldn't this negate the potential problems of charging the mp3 device while it is playing through the radio (due to the fact that the grounding *may* not be the same)?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sidewinder Quote  Post ReplyReply Direct Link To This Post Posted: Dec/29/2010 at 4:16pm
Just found this site recently...they will do an MP3 for your radio for under $100.00, also they restore vintage radios.....  Good to know....

http://www.wardsclassiccarradiorepair.com/index.html

Chuck Page
Chuck Page ,AKA Sidewinder

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Post Options Post Options   Thanks (0) Thanks(0)   Quote crazy4amc Quote  Post ReplyReply Direct Link To This Post Posted: Dec/29/2010 at 6:52pm
I;m an audio tech by trade however I don;t work on internals of old car stereos but I can tell you that ingeneral you have two types of pre-amplifier signals ( mic level, line level), you have to be very careful to not put a line level into a device that is looking a mic level because it can overdrive the amp circuit and blow up speakers and amps. Also you match the levels by using a transformer not resistors since audio signals are ac not dc and require step down through a transformer, direct boxes do this for you using switchable transformers.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote gryzynx Quote  Post ReplyReply Direct Link To This Post Posted: Dec/29/2010 at 8:35pm
Originally posted by ronilevi ronilevi wrote:

.

If what I stated is correct, then this device should work right? If you have better suggestions or if I am mistaken in any way, please let me know.

Also in response to gryzynx:  using a 0.05 uF cap between the mp3 source and the volume control is a great idea! I have two questions about this:
1) Will this have any negative effects on the sound by filtering frequencies?
2) If it blocks the DC then wouldn't this negate the potential problems of charging the mp3 device while it is playing through the radio (due to the fact that the grounding *may* not be the same)?
For the jack you sourced, yes, it will work.
 
Q # 1, this is a somewhat complicated issue. The value of the capacitor, in conjunction with the impedence of the input will affect low frequency response. Given the values expected in the radio volume control circuit, this should not be noticeable with 0.05 uF.
Q # 2, the capacitor will not change anything as far as grounding. The capacitor would only be on the "hot" side of the circuit and the danger is in different ground potentials. There could be an alternative in using a transformer as suggested, but that changes the design of Nali's original circuit. Transformers would have to be chosen carefully for impedence and ratio and would add considerable cost to the mod.
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote gryzynx Quote  Post ReplyReply Direct Link To This Post Posted: Dec/29/2010 at 8:49pm
Originally posted by crazy4amc crazy4amc wrote:

I;m an audio tech by trade however I don;t work on internals of old car stereos but I can tell you that ingeneral you have two types of pre-amplifier signals ( mic level, line level), you have to be very careful to not put a line level into a device that is looking a mic level because it can overdrive the amp circuit and blow up speakers and amps. Also you match the levels by using a transformer not resistors since audio signals are ac not dc and require step down through a transformer, direct boxes do this for you using switchable transformers.
 
The point described in the original post is typically in the -10 to -20 db(mv) range. They also are likely around 10K impedence. As you probably know, line levels are typically 0 dbm @ 600 ohm impedence. Point well taken about overdriving the input, but the resulting distortion would likely have you turning it off well before any speaker and/or amp damage is done given the typical car audio specs of the OEM radio. For everyone else, please forgive the techno mumbo jumbo.
 
I think you are missing the point about the purpose of those resistors. They are not to "match levels", rather, they are being used to sum the two low impedence outputs of the L and R audio source from the MP3 player into a single mono analogue signal while providing adequate isolation between the two outputs. The value chosen is probably a good compromise between isolation and minimal signal loss. Remember, this is offered up as a $5 fix. Sky is the limit if you want to reengineer the whole works.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RSX 401 Quote  Post ReplyReply Direct Link To This Post Posted: Dec/29/2010 at 11:36pm
Has anyone considered something like this?
 
Rick Sipole

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