Recieved a new edition to my setup yesterday with the automatic 6 disc changer!
Pretty cool how it still works and only needs a sony bus cable and RCA output to the radio (which has a dedicated external changer input, so you can still connect to aux as well!)
Hey everyone,
This will be quite a long post as I want to try and explain the issue and what I already tried as good as I possible can.
I never had anything other than some upgraded speakers in my cars before this, so it is all a bit new to me. I am a mechanic, and love to learn as I go, and want to know where exactly the hiss is being added to the system (I have a feeling it has to do with my older headunit that id love to keep)
System specs
Car: 1992 Nissan 240sx s13 hatch
Headunit: sony mdx-m870x (5.5V pre out)
Amp: JL Audio RD400/4
Speakers (F): morel maximo ultra he602 mk2 component (woofers in doors, tweeters on the mirror trims)
Speakers (R): same as front, but coaxials
Subwoofer: kenwood sw201x bassreactor
HU-AMP connection: seperate front and rear RCAs
HU-SUB connection: RCA sub out to internal sub amp
Replaced all speaker wires with twisted pair OFC wires.
RCA not near big power cables
Amp mounted on wood (not sure if that matters, but its not touching metal)
Amp ground: rear seatbelt bolt (well sanded)
HU ground: well sanded chassis point under dash
The issue
I have a very noticeable hiss in the system even at idle (no phone connected, or connected but not playing while on aux). Its so loud that you can hear it in the music. (Minidiscs mask it a lot better as their volume is always a lot higher than my phone, so headunit volume is lower for the same loudness adding less hiss)
My previous solution was to lower the amps gain to about 25% so it wasnt as noticeable, but I feel like im leaving a lot of performance on the table if i do that. (Is that true?)
If i disconnect the RCA cables to the amp it goes away. It stays if i disconnect the aux cable.
I added a extra ground cable between the headunit and amp grounding spots to see if it was a grounding loop or something, but that didn't change anything either.
I do really love my headunit and enjoy listening through my minidiscs on it, so id love to keep it if I can, even if that means spending heaps of time modding it to work better!
So yeah, Im pretty confident it has something to do with either the age/degration of the headunits internals, or just the very limitation of the units components from back then.
Let me know what else it could be and what i could try to diagnose it better. Maybe I'm just blind and mis something crusial.
If it IS however the headunit, I read that it is probably the op-amps inside? (This is where my knowledge truly stops, but would love to dive into!). I also read that it could be possible to tap the pre op-amp audio signal and route it to an external dsp or linedriver to send a clean unmolested signal to the amp while retaining the functionality of the headunit that id love to keep! If you have any info or experience with something similar, please let me know!
Thank you all in advance, and sorry for any grammatical mistakes. English is not my native language