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Pre-Amp for Mp3 Player to Head Unit

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markbognor South East Posts: 9,970 Club RR Member Number: 56	Pre-Amp for Mp3 Player to Head Unit markbognor @markbognor Club Retro Rides Member 56
	Jan 19, 2011 12:22:31 GMT Select Post Deselect Post Link to Post Back to Top
	I've been frustrated for a while that my head unit's output seems suppressed when playing from the line-in/aux input. It's noticably quieter than the radio. I thought I might have a go at solving this by making an inline pre-amp for the input signal. I've no idea if this will work, or possibly lead to the premature death of my head. But lets give it a go anyway. My starting point was a circuit that my BTEC engineers make as part of one of their assignments: These are a mono amplifier, using an LM380 op-amp chip. They are a bit brutal, they just connect the stereo inputs to form a common input and mono output, but they are simple to make, don't have too many components and pretty reliable. I wanted stereo so I've built up two boards side by side with a common ground and power supply to use one board for the right channel and one for the left. I've also bodged in a power indicator in the hope that I might remember to turn it off. I didn't have a stereo 3.5mm socket to hand, so for now I have improvised using a 3.5 to (cant remeber the size) adaptor. Initial tests show it works well, with the Mp3 player volume set at its lowest level I can still get a very loud output from the head. Which is great because with the volume set low the battery in the player will last longer. But will I kill my head - what do the techies out there think?
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lewist123 Posted a lot Posts: 2,006	Pre-Amp for Mp3 Player to Head Unit lewist123 @lewist123
	Jan 19, 2011 13:10:37 GMT Select Post Deselect Post Link to Post Back to Top
	If the head unit isn't clipping, I can't really see any issue! Lewis


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jrevillug Posted a lot Posts: 3,520	Pre-Amp for Mp3 Player to Head Unit jrevillug @jrevillug
	Jan 19, 2011 14:15:14 GMT Select Post Deselect Post Link to Post Back to Top
	I agree - as long as you aren't clipping either the pre-amp or power amp stages of the head unit, you won't kill it. However, I might suggest a small improvement - set up the amplifier for less gain, with a very high input impedance, and using the MP3 player with the volume turned right up. My reasoning is that the volume control on the MP3 player is digital, so turning it down looses resolution. Ok, so losing a little resolution is not the end of the world - it's playing MP3s in a noisy environment - but to me it seems silly to throw away something that could so easily be there. Of course, if it sounds good to you then there is little point changing something. To reduce the drain on the batteries in the MP3 player, you need to reduce the current flowing in the output stage - despite having a higher voltage. Increasing the input impedance of the pre-amp would make this possible. Is the amplifier wired as inverting or non-inverting? What are the resistor values? I'd recommend a non-inverting circuit, purely because it provides a higher input impedance. Finally, it might be worth switching to something like a TL082 (general-purpose dual op amp) or a TL072 (low-noise version) - you don't need the 2.5W output capability of the LM380 to drive the input circuits of the head unit, you just want some gain. This would make the 9V cell last much longer - especially if you cut down the current through the LED as well. The Jfet input of the 072 further increases the input impedance over the bipolar input of the LM380. Just a few thoughts - but I entirely understand getting it good enough with bits that are just laying around. EDIT - does the amplifier board have an output capacitor? I doubt that the headunit's input stage is DC-coupled, but it would be sensible to have one just in case - Op Amps often have a small DC offset on the output which could upset the bias of an input stage.
	Last Edit: Jan 19, 2011 14:17:10 GMT by jrevillug Caution: As the buoys marking the shoals are often out of position, mariners are cautioned to be on their guard when navigating these shores 75 Imp 72 Clan - What? An update? Never!

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Lawsy Posted a lot Posts: 2,615	Pre-Amp for Mp3 Player to Head Unit Lawsy @lawsy
	Jan 19, 2011 14:29:56 GMT Select Post Deselect Post Link to Post Back to Top
	a lot of that is a bit to techy for me, but i noticed the same thing I have an old iphone (now just use it as an i-touch) that when plugged in to the headphone jack and then into the aux of the headunit it was quieter than the radio I also noticed a power drain on the ipod, so plugged ina cigarette lighter charger and when the ipod is plugged in and on charge the volume seems to match that of the radio migth just be my ears playing tricks on me, but thats how it seems however in the current vehicle the stereo is all over the place so for me now its better to have i quiter as I have more control of the stereo (but thats another issue for another day) lol


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Soopahfly Posted a lot Posts: 2,352	Pre-Amp for Mp3 Player to Head Unit Soopahfly @soopahfly
	Jan 19, 2011 22:46:22 GMT Select Post Deselect Post Link to Post Back to Top
	I had this problem, but I just bought one of these for a fiver anythingbutipod.com/2008/07/fiio-e3-headphone-amplifier-mini-review/


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Paul H Posted hell of a lot Posts: 7,208	Pre-Amp for Mp3 Player to Head Unit Paul H @amazonsprite
	Jan 20, 2011 12:10:22 GMT Select Post Deselect Post Link to Post Back to Top
	Jan 19, 2011 12:22:31 GMT markbognor said: I've been frustrated for a while that my head unit's output seems suppressed when playing from the line-in/aux input. It's noticably quieter than the radio. Is this though a front aux-in, or rear RCAs?
	1967 Volvo Amazon 122S o/d estate, 1961 Austin Healey Sprite Mk2 & 1961 Catamaran

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