Preamplifier with NE5532 for CAR Audio - diyAudio
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Old 16th May 2011, 04:56 PM   #1
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Arrow Preamplifier with NE5532 for CAR Audio

Hi!
I want to connect my MP3 player to my Car Audio AUX input, but the signal is a little bit weak, so I would like to amplify it by a factor of 2-6.

I think an NE5532 opamp would have enough good quality for this purpose (Do you agree? ). I don't need bass, treble or balance control (can be controlled already on both the MP3 and the car audio), just gain/volume control (some MP3 players are louder and doesn't need so much pre-amplification).

My questions:
- Should I use a dual power supply or a single with virtual ground?
- Would a simple non-inverting amplifier (like on the picture) be enough good? (with capacitors on input and output )
Click the image to open in full size.
- Replacing R1 and R2 with a linear potentiometer for gain/volume control would be a good idea?
- Could you post me a simple schematic or a link?
- I already looked at a design ( Simple High Quality Preamp For Hi-Fi ) but I do not have much experience with analog circuits. Which parts of the circuit should be kept if I only need simple amplification with gain control?

Thanks in advance!
Aaron Szabados
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Old 16th May 2011, 08:51 PM   #2
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that is intresting, it should not be weak actualy, headphone output should drive it properly.
But anyways, single supply would be okay, or You can use a simple resistive divider to get dual supply. Makes no difference if You ask me (in this application at least).

If You ask me, the simples solution would be a set gain somewhere around a factor of 3, and add a simple potmeter to the input of the opamp. So the gain is fixed, You just trim the input with the pot till it is suitable for Your amplifier.
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Old 16th May 2011, 10:48 PM   #3
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MP3 players seem to have notoriously weak output. Your non-inverting op amp is probably the easiest solution. Do you really need variable gain, or a volume control? Volume can be controlled via the head unit. What I would suggest is to set the volume on the MP3 player where you want it, then determine the amount of gain needed to give an adequate signal to the aux input. You can do this by measuring the signal amplitude and comparing it to the head unit's input sensitivity or you can "play it by ear." 2x to 6x ought to be in the right neighborhood. Set the op amp for that fixed gain.
A single-supply virtual ground is what I would use, with a capacitor on the op amp output. A capacitor isn't needed on the input as the MP3 player likely has one on its output anyway.
I'd be wary of using a shared ground with the rest of the system. For me, these are the sort of things that cause me ugly ground loop issues. I'm thinking I'd use a separate small 12VDC SLA battery, using it create a virtual ground +-6V supply for the op amp and regulating it down to power the MP3 player . An accessory (cigarette lighter) plug could be added to recharge the battery via a dashboard socket.
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Old 16th May 2011, 11:15 PM   #4
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I don't want to use a separate battery, it should be powered from the car's +12V rail. The reason is, that this preamp will be part of a "factory CD-changer faker". My Volkswagen Beta radio has a CD-changer input (it has no real general purpose AUX), which can only be enabled by a factory CD-changer head unit. My circuit fakes this head unit, and I want to add audio signal amplification to it.

The cars supply is quiet noisy, but I think it could be "cleaned up". I think the +12V is also full of spikes. My car has Start-Stop system, so the engine is stopped every time you stop and it starts automatic as soon as you accelerate,. I think this will also affect the stability of the +12V supply.

What about using a 7809 voltage regulator? Wouldn't it be enough to get a stable supply for my preamp? Or maybe some sort of DC-DC converter? And of course bypass and filter caps.
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Old 17th May 2011, 01:02 AM   #5
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"I want to connect my MP3 player to my Car Audio AUX input" is what you said in your original post, so I went with that. If this is only part of a larger circuit that's fine; adjust accordingly.
I assume GND is part of the CD changer interface. Maybe that's a good ground point for the additional circuit. But it is a "faker" after all, so separate battery power isn't necessarily excluded. Electric and hybrid vehicles are beyond my experience though.
A 7809 should work, but a LDO regulator may be a better choice here.
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Old 17th May 2011, 01:52 AM   #6
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most car head units have gain controls for their auxiliary inputs. might save you from building anything.
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Old 17th May 2011, 12:34 PM   #7
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#1: I don't want to save myself from building this, I am a Computer Engineer student, I have some basic experience with developing embedded systems (digital circuits) and I would like to extend my knowledge with some analog circuits
#2: My head unit doesn't have a real AUX input, it's input is intended only for the factory CD-changer unit, so it doesn't have a gain control. This is actually a radio hack.

#3: Look at the attached file, this is what I came up with. It could be the worst preamp you have seen in your life, hope you can help making it better. I have not drawn the power supply yet. My idea is to use a buck/boost (SEPIC) converter to create a +10V rail from the +12V of the car (which variates between 9-14V) , and to use another buck converter to create a +5V rail for the virtual ground, uC and digital circuits.


Attached Files
File Type: pdf preamp.pdf (19.4 KB, 314 views)
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Old 17th May 2011, 04:55 PM   #8
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I attached another preamp schematic, i will use an NE5532 instead of the TL071C but there is no NE5532 in TINA

Please comment all two circuits. Where should I put the pot for volume control in this one, before the input cap?
Attached Files
File Type: pdf preamp2.pdf (47.3 KB, 386 views)
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Old 19th May 2011, 10:04 AM   #9
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no comments?
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Old 19th May 2011, 07:05 PM   #10
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Datasheet minimum supply voltage for the 5532 is +/-5V.
I would not use it in a circuit that has +4,-5V supplies. Note the output will also be limited to some value much lower than the supply voltage.

You could use an ~8V LM317 based regulator and the ever popular OPA132 series opamp, good for supplies down to +/- 2.5 V. Use a virtual ground on this: Two 5-10k resistors each with 220uf across them.

Put an RC filter before the regulator (~10 Ohm, 1000 uF).

Use R2 = 10k, R1 = 3.3k, nice 4x gain.
Use R3=100k for lower noise. Get rid of R4, C2. Use virtual ground.
Remove R5 or R8.
Add a 220pf cap across R3 for HF suppression. Make R6 3.9k (also move R6 before R3).
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