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I've been having this issue for a while now, and I've been doing many things to troubleshoot it. At first I was just throwing things at the problem, big 3, more batteries in the trunk, etc. After those things were addressed and I was still having issues(although not as bad as before I did those things...) I've been digging deeper.

My setup was 2014 Taurus SHO w/ sony 12 speaker system, took output from factory amplified signal into an audiosource lc8i. Out of there to helix DSP, out of there to a zapco c2k 9.0xd for subs. I have an amp for highs I didnt even hook it in yet but its mounted. Unhooked factory front and rear center channels, and the factory "subwoofers" on rear deck.

The main issue here was the input to the amp was being over driven and there was some noise introduced. I think something funny was going on with the audiosource lc8i and I finally had time this weekend to work on it. I bypassed that and now highlevel right into the Helix DSP, big improvement! Still cutting off though. I ran a 50hz sinewave through my factory deck at full volume and on the helix I had to adjust the sub out gain -5.25 minimum. I currently have it at -6.5db. The signal fed to the zapco is clean. The SLDIN is set to 0db gain. I hooked an oscope upto the zapco outputs and measured the sine wave and adjusted the gain knob to just before it breaks up.

When I have my subs hooked up I can still trigger the amps protect mode when I crank it up too high. So my subs are 2 platform 12HD subs from psi. I purchased these in august of 2013 when I took delivery of my vehicle. They're "1ohm" DVC versions and I have them wired to a mono "4ohm" load. They're in a 4.3cu/ft ported box tuned to ~29hz.

The reason I give the date there, is at the time I didn't realize these subs aren't REALLY 1ohm, they're slightly less. And now when I visit the psi site, they actually give the option of a .7ohm or true 1ohm coil. The .7 seems like what I have as my measured resistance of the mono load is coming to around 3ohms not 4.

The million dollar question I have to anyone familiar with zapco amps, does the "pre-clip" function on these amps work as a sort of way to adjust for the impedance? How should I measure the output with regards to adjusting this knob if that will do it. The manual states this knob is a way to protect your speakers from being overdriven by the amp, by artificially limiting the output of the amp, I'm thinking since they are a lower impedance its probably causing the amp to be over driven at the top. Am I getting less than the amp is rated for by doing this? Should I look into having my coils swapped out to the true 1ohm coil version?

When you oscope an amp output, are you supposed to have a load on there? I don't have my speakers hooked up because that would be annoying to say the least, but I probably could... my oscope is pretty damn old but it works, its by no means portable either, so anytime I have to scope the amp in the trunk its a big of a production :laugh:
 
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