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Author Topic: Anyone good with designing circuits?  (Read 1128 times)

Offline massarosareloud

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Anyone good with designing circuits?
« on: July 01, 2013, 11:09:38 AM »
So here is my dilemma, I just recently got surgery on my hip and I'm out of commission for quite awhile, so in the downtime I'm trying to come up with easy things to do that don't require much moving around.

I came up with installing an auxiliary port in my Jeep's radio. I have a 1996 Jeep Grand Cherokee Limited with the Infinity sound system, which has a head unit and a separately located amplifier. So the headunit has a CD player, tape deck, and AM/FM radio; up until now I've been using a tape deck adapter which provides adequate sound quality. The head unit sends out individual line level outputs, one for each of the 4 speakers, to the amplifier which then obviously amplifies that line level signal into a useable amount of power for the speakers.

What I'd like to do is wire a 3.5mm headphone jack in between the headunit and the amplifier, and therefore be able to flip a switch and hear my ipod through the car speakers, thereby having the best possible sound quality. I tried this already, by wiring directly into the amplifier, and even with the iPod at full volume the sound coming out of the speakers isn't louder than the engine. The cause of this is because the iPod runs at millivolts while the headunit outputs on a 2 or 4 volt system (I believe).

I've found line output converters, which are designed to provide a line level signal from the speakers - the opposite of what I'm trying to accomplish. So is it possible to "step up" the voltage of the signal coming from my iPod so my amplifier can "hear" it better? I am very capable of wiring and installing switches and things like that, I just don't know where to turn to make my iPod louder.

For more information on line output converters: http://www.bcae1.com/loc.htm
Amazon: http://www.amazon.com/Scosche-2-Channel-Adjustable-Lineout-Converter/dp/B00009UHRE/ref=sr_1_2?ie=UTF8&qid=1372697887&sr=8-2&keywords=line+out+converter
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Offline Adondo

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Re: Anyone good with designing circuits?
« Reply #1 on: July 01, 2013, 12:28:16 PM »
What you need is a preamp. Velleman kits are the way to go, they're cheap, easy to build, and have nice circuit boards. (Better than breadboards or a mess of wiring on perf-board)

This should be what you need:
http://www.kitsusa.net/phpstore/html/UNIVERSAL-STEREO-PRE-AMPLIFIER-29.html\

By the way, amp line levels are usually about 600 mVAC, and the iPod must be too low to drive it hard enough. Line level converters reduce speaker levels from may tens of volts AC to the 0.6VAC needed.

Offline AndyCivil

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Re: Anyone good with designing circuits?
« Reply #2 on: July 01, 2013, 01:37:48 PM »
If it's really an iPod then your dock connector actually has "line out" levels. All you need is an adapter that can access it. There's one here:
http://www.amazon.com/Ziotek-ZT1900665HC1-3-Feet-30Pin-Stereo/dp/B0051MQJ7A/
And there's a discussion of the issue here:
http://beavishifi.com/articles/headphonejack/

Offline massarosareloud

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Re: Anyone good with designing circuits?
« Reply #3 on: July 01, 2013, 02:35:24 PM »
Ok thanks for the links! That set me on the right path.

One more thing, I'm wondering if I could have some help determining how something like this http://www.ebay.com/itm/Chrysler-Jeep-Dodge-1984-2002-AM-FM-CD-Cassette-Car-Truck-Radio-w-Aux-iPod-Input-/151062011033?pt=Motors_Car_Truck_Parts_Accessories&fits=Year%3A1996|Make%3AJeep|Model%3AGrand+Cherokee&hash=item232bff5c99&vxp=mtr works? That is the exact same radio as the radio in my car and it is modified with an auxiliary input, which according to the listing is tied into the amplifier. Anyone have any insight as to how that is possible?
« Last Edit: July 01, 2013, 02:52:58 PM by massarosareloud »
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Offline AndyCivil

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Re: Anyone good with designing circuits?
« Reply #4 on: July 01, 2013, 02:50:02 PM »
By the way, not that there's anything wrong with either using an amplifier or accessing the signal from your docking connector, but it does kinda suck that your headphone output has plenty of power to drive the line-in, it's just that it's at the wrong impedance. It would be a more intellectually satisfying solution to use an impedance matching transformer to turn the headphone signal into a higher voltage.

You would have to choose your transformer(s) carefully - you'd need two of course for stereo. They would have to be of audio quality, otherwise the frequency response would suck, and they would have to be shielded so as not to pick up crap from the ether.

A suitable transformer would have a ratio of around 10:1 to turn your 100mV (ish) output into more like 1V. It would have to have a primary impedance of at least 16 ohms (as headphones would be) and a secondary impedance of not more than 10kohms (to be able to drive the line signal).


Offline massarosareloud

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Re: Anyone good with designing circuits?
« Reply #5 on: July 01, 2013, 02:55:57 PM »
AndyCivil I like the idea of using transformers. I'll look into that, I guess a site like Digikey is a good place to start?

EDIT: Looks like the quality transformers, ones specifically for audio uses that also fall within the ranges you specified for ohms would cost about $30/transformer. Seems kinda unrealistic, when replacing the stereo wouldn't cost much more than that.
« Last Edit: July 01, 2013, 03:04:11 PM by massarosareloud »
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Offline AndyCivil

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Re: Anyone good with designing circuits?
« Reply #6 on: July 01, 2013, 04:40:40 PM »
Bummer. I looked at the Digikey site, and the affordable ones are dropping 2dB below 300Hz and introduce distortion. It totally sucks to use a pre-amp, but sadly, I think that's the only alternative to fetching the signal out of the docking station connector. If you go that route by the way, make sure that your iteration of the iPod actually has that signal. Sorry for the distraction.

Offline Adondo

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Re: Anyone good with designing circuits?
« Reply #7 on: July 01, 2013, 05:38:14 PM »
We use a lot of these in the two-way radio biz. (I order them 20 at a time) The specs lack above 3.5 Khz, which is about where the fidelity of cop radios end anyway.  :P

http://www.mouser.com/ProductDetail/Triad-Magnetics/TY-304P/?qs=%2fha2pyFaduhgWkK4A6L%2fR867CkDsf%2fc9PIGrt4hO4uI%3d

The Mouser link should give you a starting point for a search and/or cat page though. They're hard to beat price wise.

Offline AndyCivil

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Re: Anyone good with designing circuits?
« Reply #8 on: July 01, 2013, 06:01:38 PM »
The mouser website looks like a clone of digikey! Anyway, that one you referenced is a 1:1 turns ratio.

Offline massarosareloud

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Re: Anyone good with designing circuits?
« Reply #9 on: July 02, 2013, 06:54:32 PM »
Well I made some interesting progress today. I disassembled the head unit and found this, which I thought was interesting:



From what I could surmise, that is the connector which houses the low level signal which is yet to be preamped to the level necessary for the amplifier. Or in other words, it is the input into the preamp within the head unit. Unsure of its use I decided to further disassemble the head unit and remove that circuit board, as it turns out there are multiple circuit boards all connected to each other either with flat ribbon cables or long pins connected to another circuit board.





This picture above shows the circuit board after being removed from the head unit. The yellowish-white connector on the far left is the connector pictured in picture number one, of which I am unsure of its use. The frontmost yellowish-white connector in the center of the picture, the one with pins, houses a wire which connects to the external connector for the speakers. In other words, it is the last place that any signal travels in the head unit before leaving the head unit and being sent to the amplifier. The rearmost yellowish-white connector in the center of the picture accepts the pins from another circuit board once installed.





As it turns out this circuit board is called the "PCB-AMP", which makes me optimistic.






Here are some more pictures of the circuit board. The first picture shows some IC's that I believe do the "preamping" within the head unit, but I would like some clarification, or a second opinion on what they are. The second picture shows the bottom and the large black rectangles on the top of the circuit board are the IC's shown in the picture above it.



So it is my belief that the PCB in the pictures above is responsible for taking the low level signal generated by the AM/FM radio, tape deck, or CD player, and pre-amplifying it to the level necessary for the amplifier. I don't know for sure what it does, and I'd love some second opinions. As of right now, I am scrapping the idea of buying a separate pre-amp or transformer and increasing the power of the output of my iPod so the amplifier can "hear" it. AndyCivil, you had a good idea using the line level output provided through the iPod connector, but I reread that article and it said that the line level output of the iPod is still only about 2 volts, compared to the line level output of a CD player which was about 6 volts. So accessing the line level output through the iPod connector would not be beneficial.

Clearly this is possible to do. I posted an eBay auction in a previous post that contains a modified radio that claims to have an iPod connector tied directly into the head unit. So I just need to find the right place to tie it in. I'm thinking of experimenting with that connector pictured in the first picture.

*FIXED* I am still trying to get the radio to think that it is installed in the car, even though I have it powered on my workbench with a power supply. For whatever reason it thinks only the battery is connected, and therefore won't power on but only will give me the time. *FIXED*

So any comments at all are greatly appreciated!
« Last Edit: August 05, 2013, 09:07:12 AM by massarosareloud »
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Offline massarosareloud

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Re: Anyone good with designing circuits?
« Reply #10 on: July 02, 2013, 08:15:49 PM »
WOO HOO!!! The connector pictured above is exactly what I thought it was and after splicing in my iPod everything worked perfectly! Thanks Adondo and AndyCivil. I'll report back later with more info, but as for now I need to get some rest.
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Offline Adondo

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Re: Anyone good with designing circuits?
« Reply #11 on: July 03, 2013, 06:00:12 PM »
Those large IC's are heat-sinked to the metal piece. They're definitely audio amp IC's, probably with output of many watts. They look to me like stereo amp chips, probably one for each set of speakers. (front/rear) Those type of IC's put out "floating" audio, and the mid sized capacitors are DC blockers. (They prevent DC from the amps from getting to the speaker coils) Floating audio is NOT ground reference. The two big capacitors are probably DC filters to supply the amp IC's to give them clean DC power.

On the back of the board, I see several SMD transistors, which could be pre-amps to the big chips. The rest would be resistor networks to get the levels right to the amp IC's with various inputs.

Sounds like everything worked out.

By the way, I posted those 1:1 transformers to just be a starting point on Mouser's site. We use them often to convert floating audio to ground ref among other things.

Offline AndyCivil

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Re: Anyone good with designing circuits?
« Reply #12 on: July 03, 2013, 11:10:44 PM »
Those chippy looking things stuck to the heatsink look like stereo pre-amp modules to me. What I don't understand is why they would get so hot as to need the heatsink. I'm guessing that they were designed to be stereo, but this unit is using two to cover four channels.

The reason I say they're only pre-amps is that the tracks simply aren't thick enough to have insignificant resistance compared to a speaker. I agree that the two big caps are reservoir capacitors, I'm thinking they use two, one per chip, to minimise crosstalk.

I disagree that the four mid-sized caps are DC blocking for a speaker coil. They're not big enough. The impedance of the capacitor would be too high compared to 8Ω at audio frequencies. They could be DC blocking if the output impedance is higher.

What I find very interesting is that they've labelled the components in different series depending on which channel it's part of. Meaning that R352, R452, R552 and R652 are the same resistor in four copies of the same circuit. The power components are numbered in the 800's. Therefore Q551 must be one of a set of four signal transistors, whereas Q835 is unique.

Offline massarosareloud

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Re: Anyone good with designing circuits?
« Reply #13 on: July 04, 2013, 08:25:38 AM »
While I understood little of what either of you said, I can say that the stereo is a 2 ohm system, not an 8 ohm AndyCivil. Weird, I know, but that's how it was made.

The only downside I have seen so far is that I am patched in past the volume control, so it has 2 settings, all on or all off. But I'm willing to pay that price since I've already figured out how I'm going to wire the relays and such, and also because I can control the volume on my iPod. I'm planning on using 2x 4PDT relays that normally close to the regular radio, and are normally open for the iPod so when I turn on the switch, obviously the input changes to the iPod. I was planning on permanently mounting the auxiliary port in the dashboard, but I think I'm going to try to fabricate a little metal or wooden box, that I will paint black to match the rest of the car that will go where the ashtray is. I don't smoke, and I don't know anyone that smokes, so I won't miss it (:
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Offline AndyCivil

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Re: Anyone good with designing circuits?
« Reply #14 on: July 04, 2013, 11:27:39 AM »
... I can say that the stereo is a 2 ohm system, not an 8 ohm AndyCivil. Weird, I know, but that's how it was made.
Actually the moment I read that I realised why it would be. An 8Ω system running of 12V wouldn't be loud enough; a home stereo would run at a higher voltage to increase the power, but for a car system it's probably easier to use a lower resistance and stick with 12V rather than have to add an inverter to boost the voltage higher than the battery.