testing head unit output voltage
 

Added an Arc Audio KS 900.6 amp to my boat this past weekend. already have a sony 4 channel running the tower, in boats were running off the head unit so it was time to get more power to those

when I ran the RCAs I noticed that whoever installed the tower speaker amp was using the left front and right rear outputs on the head unit. I reasoned that they may have had better voltage if pulling one side per channel instead of running both on the same channel. I get everything hooked (now running a set of RCAs on both output channels) and the amp starts throwing a short code. will eventually go into permanent protection where I have to use a 12v bulb to drain the amp completely then reinstall positive power lead.

Cleaned up the speaker wires to make sure there weren't any stray copper strands going anywhere causing the code. Then started researching possible issues with this amp. Arc Audio KS 900.6. Seems its very finicky with RCA voltage. It's a microprocessor amp, whatever that means. been out of the mobile audio game for a while....

HU is a Sony cheapo, wouldn't mind replacing it. But I don't want to start throwing $$ at things without a better idea of whats going on. Is there a way to test output voltage of the HU? Got a DMM just not sure how to use it on RCAs.

Also tried splitting channels of the head unit to get to 4 RCAs. no luck there. it will play just fine until I start turning things up, then the short code. any ideas?

I don't have any solid theories but I can throw out some ideas, some thinner than others.
Since the KS900.6 has been disco for some time I assume its used. So it might be a good idea for Arc to check out the amplifier. Could they do a software reboot on the microprocessor that governs the amplifier? Maybe worth asking about.
If either the HU or amplifier isn't grounded properly or are not grounded to the same location, this could cause DC voltage to travel down the RCA ground shield. And perhaps the amplifier is sensitive to any DC offset. In any case, grounding is critical.
I really dislike the use of a ground loop isolator but maybe until you fix the amplifier or replace the HU, a GLI could provide the RCA shield ground isolation to make this problem go away temporarily.

yea I bought it used. With Arcs support, I took a chance. I'm realizing now that 5/6 channel amps aren't what I thought they were. Seem very inefficient, but again I'm trying to get caught up with the technology. With AB/D classes, does it make any sense at all to push components and a sub with the same amp?

GLI: so basically a noise filter? my search shows a couple Boss and PAC models. anything else I'm looking for? For 8 bucks, I might as well try it. Anything I can do to test the HU?

Do I need a full HU? Running 3.5mm input or BT 99% of the time

Nacho,
The KS900.5 is all Class AB. Not H or G like the other KS models. But it is a beast.
A GLI is not really a noise filter as much as an isolation device that blocks DC flow in places where it should not be. It is a Band-Aid for an installation flaw. PAC would be fine.
If you have a source other than the HU (like a phone or BT module) that you can plug directly into the amplifier as a test, this could eliminate or confirm the amplifier as the culprit. Also, in this scenario, a phone that is battery powered versus a phone that uses a power supply deriving power from the boat, could make a difference in the test.
Back to the grounds. If the grounds aren't right or the voltage isn't right then all the other diagnostics won't be as telling.

David, thanks for sharing your expertise. Got a PAC GLI ordered
HU ground? I haven't messed with the HU at all, except for plugging RCAs in. Should I run a separate ground to HU?
amp is grounded to the battery. batt is grounded to block.

will do some testing before I take it to a shop

RCA shield grounds of amplifiers and processors are usually 'above ground' and reference back to the HU where they are chassis grounded...providing a single reference to true ground. So the ground of the HU is especially important. The HU ground should be grounded at the same place as the amplifier, or as close to the amplifier primary terminals as possible.
Really inspect all grounds. Every connection and termination. They may look good or even measure good under static conditions. But that doesn't mean you have a quality ground once current starts flowing.