Usually I just take the manufacturer's word for it on highs amps, but I just love my little PPi 900.4 and I recommend it all the time so I decided to test it and see if she is really doing the clean power.
This will be short and sweet. I didn't crack it open or take pics of the controls or inputs. You can see closeups of them here... Precision Power PPI P900.4 (p9004) 4-Channel Phantom Car Amplifier
The amp is active capable and claims 145w x 4 channels @ 4 ohms in a tiny footprint. For less than $200. That's an awful lot of amp for the money. Let's check it out.
From the manual...
RMS Power Rating (14.4V):
4 ohms: 145 watts x 4 chan.
2 ohms: 225 watts x 4 chan.
Bridged, 4 ohms: 450 watts x 2 chan.
RMS Power Rating (12V):
4 ohms: 100 watts x 4 chan.
2 ohms: 150 watts x 4 chan.
Bridged, 4 ohms: 300 watts x 2 chan.
Yes... we will be getting to that big bish, but let's test the little one first. That's a PDX-10k, in case you don't recognize it. I also think it's hilarious that this is likely to be someone's setup. That teeny thing on the fronts and that monster on the substage. All SQ and ****.
Setting unloaded. A lot of people feel like this is useless, but I like to see what an amp does unloaded so that's why I always include it. If you just used the DMM SQRT method here, you'd be at ~ 24 and leaving some power on the table. This is also why I advocate the use of a scope. Almost 31v here.
Sometimes you will see a highs amp tested and see them say, "all channels driven". This is why they say that. This is one channel driven at 5 ohms. This is almost 150w @ 5ohms but only a single channel driven.
Now we load down all channels and see what we get.
115w per channel @ 5 ohms and 13.4v. All channels driven. It's a power supply thang. 50A draw measured but not pictured.
Bridged @ 5 ohms front and rear. All channels driven.
340w bridged @ 5ohms and 13.2v. This is also basically what you'd see at 2.5ohms per channel. ~ 170w per channel @ 2.5ohms all channels driven. 70A draw measured but not pictured.
So, yep. She's doing work. Observed efficiency was ~ 75% at 5ohms and 2.5ohms. I'd have liked to see it a little higher at 5 ohms per channel, but at this price point, you really can't complain about much.
Subjectively... I think the little amp sounds great. And I know a lot of people who would agree with me. The amp has a "full" setting that lets you bypass all the filters and that's where I run mine so I can't comment on how the amp sounds using the onboard crossovers. I've had this amp next to several much more expensive amps and I honestly couldn't *hear* a difference. Maybe you can. But if your ears are that good, then maybe you need to spend $1,000 for this much power and make yourself feel better. For my money though... this amp is a fantastic buy.