Need help designing a Pre Amp

[Elliot Woodman]

Hi, I am currently trying to upgrade the stock sound in my 1999 Volvo v70. However since I like the look of the factory Head unit, and would rather keep things looking stock as much as possible, I have decided to design and build my own car pre amp which will be nice and small so it can be mounted behind the original unit, and be sent to an external amp in the boot.

I am sure that it would be be much more efficient to just buy parts, but having a fair amount of electrical design experience, I am going to enjoy designing a circuit myself, even though I am well aware that it wont be anywhere near as good as off the shelf parts.

However I am not really sure what voltage my pre amp needs to output to pair well with a power amplifier, do I want as much voltage output as possible using the 14v available, giving me an output swing of around 12v peak to peak, or will the power amplifier also have some gain?

I have started designing a switching circuit that will allow for the pre amp to be triggered via the ACC feed from the ignition, and have done a sort of passive input stage, High Z, and with a high pass and low pass filter. however am not sure where to proceed, I will want some eq control even if its just treble and bass. and was planning on using NE5532 op amps for this and the voltage gain. hoping that they will be enough to provide an input to the power amp? possibly having a small class AB output stage if The op amps are being pushed too hard? does this sound suitable? I have attached my current progress.

I know this could be sorted in a couple hours if i just buy one, but again I am enjoying the process.

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[CarAudio.com - Hal AI Bots]

Hi, I am currently trying to upgrade the stock sound in my 1999 Volvo v70. However since I like the look of the factory Head unit, and would rather keep things looking stock as much as possible, I have decided to design and build my own car pre amp which will be nice and small so it can be mounted behind the original unit, and be sent to an external amp in the boot.

I am sure that it would be be much more efficient to just buy parts, but having a fair amount of electrical design experience, I am going to enjoy designing a circuit myself, even though I am well aware that it wont be anywhere near as good as off the shelf parts.

However I am not really sure what voltage my pre amp needs to output to pair well with a power amplifier, do I want as much voltage output as possible using the 14v available, giving me an output swing of around 12v peak to peak, or will the power amplifier also have some gain?

I have started designing a switching circuit that will allow for the pre amp to be triggered via the ACC feed from the ignition, and have done a sort of passive input stage, High Z, and with a high pass and low pass filter. however am not sure where to proceed, I will want some eq control even if its just treble and bass. and was planning on using NE5532 op amps for this and the voltage gain. hoping that they will be enough to provide an input to the power amp? possibly having a small class AB output stage if The op amps are being pushed too hard? does this sound suitable? I have attached my current progress.

I know this could be sorted in a couple hours if i just buy one, but again I am enjoying the process.

Elliot,

It's great to see your enthusiasm for designing your own car preamp! Keeping the factory head unit while upgrading the sound is an excellent way to maintain the original look of your Volvo V70.

Regarding your questions about the preamp output voltage and amplifier compatibility:

1. Output Voltage: Ideally, you'll want your preamp to output a signal voltage that matches what most amplifiers expect as a line-level input. Typically, this is around 1V RMS, but your gain structure will depend on the amplifier you're pairing with. Many car amplifiers will expect input signals anywhere from 0.5V to 1.5V RMS to function optimally. Regarding your mention of a 12V peak-to-peak output, remember that this will translate roughly to around 5V RMS, which might be a bit too high for most amplifiers unless you are planning to include a way to attenuate the signal at the input.

2. Amplifier Gain: Yes, most power amplifiers have some gain, which means they can amplify a low-level input signal to drive the speakers adequately. It's usually good practice to aim for a preamp output that is compatible with the input of your amplifier without overloading it. Check the specifications of the amplifier you plan to use to ensure the output from your preamp will not clip at high volumes.

3. Design Considerations:
- The design of your switching circuit to be triggered by the ACC feed is a smart move, ensuring that your preamp powers on and off with the vehicle.
- Your approach to having a passive input stage with high-impedance (high Z) input and implementing high-pass and low-pass filters will help to eliminate unwanted noise and provide good sound quality.
- The NE5532 op-amps are a solid choice for audio applications due to their low noise and good slew rate. They should suffice for your intended purpose of providing a line-level output to the amplifier.
- If you’re considering a small Class AB output stage, it adds complexity but can be beneficial if you notice that your op-amps are struggling under load or if you want to drive the speakers without needing a separate power amp in some scenarios.

4. EQ Control: For equalization, you can implement simple tone control circuits based on active filters using op-amps. Just remember that with EQ controls, the design will add complexity.

5. Testing Assembly: Once you have your circuit laid out, make sure to build it on a breadboard first to test different components and configurations. This allows you to try various EQ setups and filter designs without committing to a PCB layout right away.

Overall, your approach sounds promising, and it's great to see your passion shine through in the design process. Don’t hesitate to iterate your design, and make adjustments based on testing.

Feel free to share your progress or any schematics if you’d like more specific feedback. I encourage other members to chime in with their thoughts or suggestions too!