Line Level Comp and Preamp


2016-02-05 1:57 am
Hello audio DIYers.

I am trying to solve a problem with my phone's audio and I think the best way to do that is with a compressor and preamp. When I switched from an iPhone 4 to a GS4, 3 years ago, I noticed that the headphone output was a good bit lower. This was never a problem with headphones, but it was a problem with my car "stereo". With the road noise and reduced signal strength of the output, I had my volume maxed on my car stereo. This was never a problem with any of my iPhones (3G, 4, or my wife's 5C).

Now I have a Nexus 6P and the headphone out seems to be even lower than the GS4! With the car's amp at 100%, I have trouble hearing audio books at highway speeds.

Audio books and podcasts make the situation worse because sometimes (almost always with audio books) they are not leveled out to a web appropriate LUFS. I find that leveling recordings to -16 LUFS is just about right for mobile devices. If voice navigation interrupts, it is way hotter than the audio book or podcast and my face get's melted.

I think a compressor and preamp/booster would work well to make things listenable in the car. I want to compress the signal but I also need to increase the level a fair bit too. If I could get a fixed comp ratio of 1.5:1 - 3:1, variable threshold, and output boost of 6-10dB, that would be perfect.

I do music production and video post-production for a living (my office) and have a lot of experience with pro-audio gear for live sound. I also know a bit about electronics. This is a DMX arduino LED controller I built a few days ago.

I have looked around on this forum and found some references to various ICs but I don't know enough about electronics to know if that will work. I have a few rackmount compressors and would use one of those, but it's a bit big for the minivan with 4 offspring in the back. I also am aware of the RNC, but I don't really want to drop $170 on this. I am looking for something more like $100 and fairly small.

If you could point me in the direction of a similar project (I know it's unlikely) or a simple circuit that is close to this idea, that'd be great!

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I fear you are discovering the limits of the old plug style. I suspect lower output headphone jacks are result of potential ear damage from excessive volumes.

Have you considered upgrading the head unit? Bluetooth capable head units might work better. Apparently USB connectivity isn't fully supported for some dumb reason when connecting a phone, as that would be ideal. My Pioneer treats the usb port as if it's a CD, so all the sound enhancements like SLA for the CD player are also available to the USB device, which w/o the feature the usb device is quite a bit quieter than the CD player, but with the feature everything can match the tuner output to avoid abrupt volume changes between input sources.

You might want to try going to a stereo vendor (store, car audio shop, etc.) that has display model head units both with bluetooth and your old style connector, and see if their line gain is better, or the bluetooth makes it louder.

Are you saying your phone's navigation voice is loud, but audio isn't? That would suggest the actual phone is capable of louder output, but something possibly in your settings is messing with playback.

You can probably find a compressor app, that'll be less fuss than a physical unit.

If your van is like a panel van or has excessive road or exhaust noise bleeding into the cabin area, I suggest getting an ultra quiet muffler, touring tires, and even install something like dynamat under the headliner on the metal panels, on wall panels (under any trim, etc.) as that will greatly reduce road noises and make the ride quieter, hence making it easier to hear.

Other option, though it's legality needs checked, is using earbud style headphones. Could also install a better amplifier with higher SPL speakers, closer to you.


2016-02-05 1:57 am
Thanks for the reply. The larger issue is that the content I listen to varies. Voice nav on a lot of devices is processed in such a way that it is very loud. Some podcasts are lower but are the podcasts I listen to seem to be usually leveled to an appropriate target loudness. Audio books, for reasons that escape me, have a much lower loudness. This problem is compounded by the Nexus 6P's headphone output, which is a good bit lower than other devices (notably my collection of iPhones).

My van is a 08 Toyota Sienna (mini van). I am not looking for earsplitting levels, I just want audiobooks audible (no pun intended) at highway speeds. A louder "head unit" would work, but if I use voice nav my face would get melted whenever that popped up. The stock radio/amp is perfectly capable of producing a loud signal, just not with my phone.

This is why I am looking for a bit of preamp boost for the signal, and a little compression of the signal. I would think that the cMoy headphone amp would probably be able to get a bigger signal to the car stereo, I would just like a simple limiting/compression added on top.