Back to Back ADC and DAC

[sleddriver]

Try this: You'll need a stereo receiver or portable radio with a LINE-IN jack. Connect the LINE-OUT of your HU to this external device and see if you hear any noise. You don't need speakers if it has a headphone jack. If you STILL hear the alternator whine, then it means either the HU is poorly grounded OR connected to a noisy ground or the PS inside the HU is doing a poor job of filtering a noisy power line...or it could be all three.

If you have a quality AC voltmeter or access to one, measure the AC content of the power feeding the HU. An RMS voltmeter would be even better. An oscilloscope is preferred but if you don't have access to one, use an AC voltmeter.

Maybe a clamp-on ferrite core would eliminate this. Put it on the +12V line. Or take a ring, and wrap the line through it several times. Time to experiment. You can get these from a dead desktop computer PS or buy them from Mouser or Digi-key.

 

[Colt45ws]

So, I built the test connectors and it failed test 2. Basically where the test connector is plugged into the amplifier and the audio cables plugged into the test connector. This means the amp has an internal common impedance coupling aka "Pin 1" problem.

 

[sleddriver]

Quote:
I have been tempted to try another amp. This is an old one I got off somebody for $20 so it could very well be bad.


So far, so good. If possible, open it up and examine the PCB for burned traces, hot spots and blown fuses. I've read some of the gnds are fused to protect the amp. If so, that might be your problem. Previous owner(s) could have really abused it.

Perhaps this is why it only cost $20?

Congratulations on going the distance and building yourself some test jigs!

 

[Colt45ws]

So, really easy to see what is going on after I put my DVOM on the amp. From case metal of the amp to the ground block on the amp was 1/2 ohm. From RCA ground (sleeve) to the ground block was 100ohm on the nose for the rear input only. The Front input and pass-thru out were 1/2 ohm.
So I opened it up and the 100ohm is coming from a resistor on the board between the ground for the rear input and the ground for the other two. Obviously this is intentional, not sure why, maybe to separate the two grounds until the signal had been processed. My gut feeling is normal operation is for the ground to pass through at another location and right now that is open so it is being forced across this 100ohm resistor. I also have a feeling it has to do with the 4/2 channel switch.
My main problem is I haven't been able to find the clean ground for the front input. It goes all over the place hitting multiple resistors and capacitors along with an IC or two. IF I could figure out where it hits true ground then that would help in figuring out the rear side.
On the rear side Im having trouble tracing everything out due to the plugs themselves blocking the view of the traces. This is why I think it may have something to do with the 4/2 switch as it also heads that direction and disappears under a minifit jr plug so it may connect to one of the pins there too, not sure.
But yeah, thats what Ive got.
What I might do, to check if the 100ohm is the problem is run it bridged off the front input only and see what it sounds like.

 

[sleddriver]

Mobile audio power amps work differently. Both the + and - outputs are driven by an output stage. As such, never gnd the - audio output. Not sure if you realized this or not.

Metal case to gnd block should be lower than 0.5 Ohms. It ought to be the lowest reading on your meter, more like 0.1. Most DVM's don't read true-low resistances accurately.

I don't understand the differences between front/pass through & rear RCA inputs. Which are you using? In a mobile amp, I'm surprised there is any connection between input signal gnd and case/power-supply ground, especially given the use of switching-type PS's. I'd need a schematic to really see what's going on inside.

 

[Colt45ws]

Made some progress. I came across a post on another forum about this specific amp talking about a diode and resistor pair that was blown up on his 500X. D3 and R41 are there numbers. They provide a floating ground for the pre-amp section to prevent ground loop noise. So Im thinking gotta check those out since thats my exact problem.
So D3 is a diode (obviously) and it is in parallel with a 1k ohm SMD resistor R41. So I checked for continuity both directions across the diode and its Im gonna pull the diode and check it out of circuit as soon as I can locate some solder. My supply seems to have gotten lost.
What Ive read is that applying external voltage to the RCA ground shield will blow these two components. In my case I think the diode failed shorted instead of open which if it had done that the resistor wouldve blown right after it.

 

[Colt45ws]

Yeah, the diode is failed shorted. Its a 1N4003. I asked RF and they sent over a schematic of the amplifier. Im gonna look around and see if I can scav one out of some other electronic.

 

[Brad_C]

Any 1N400x will do the job. In fact, pretty much any power diode will do the job.

 

[Colt45ws]

I ended up ordering one for $0.14 from DigiKey. Along with all new electrolytic capacitors for it.
Got Panasonic 1200uf 35V for the input to the amp (Replaced Surge 330uf 50V)
Cornell Dubilier 3300uf 50V snap-in to replace Surge brand of the same.
A single Rubycon 100uf 25v to replace a Nichicon of same
And the rest of the electrolytics were 10uf 35v all over the input and amplifier sections which I replaced with Elna.
I cleaned all the old dried up (literally powder) thermal grease between the heatsink and the casing and replaced it with Arctic Silver Ceramique. Used a whole tube too.

Hooked it back up, fired up the car and..
No Whine!!!!!
Barely even any hiss. Like, if you put your ear to the speakers you can hear it but, quiet enough for me!
Im so happy Im gonna explode. Been listening to that whine for 3 years. Never liked having people ride with me because of it.
Such a stupid simple fix.

 

[Brad_C]

Good job and great learning experience.

 

[sleddriver]

Way to go for hanging in there and getting nitty-gritty with the tiny details!! Feels good, doesn't it? Time for champagne and a nice dinner! You deserve it!

ATTA WAY TO GO !!!

 

[Colt45ws]

Yeah, I was thinking I was gonna have to buy a new amp. If I hadn't come across that post on the diyaudio forum I would've had to. Somebody had just happened to blow the same components I needed to look at and was asking about repairing it.
But, yeah! New diode, new capacitors, fresh thermal grease...good for another 15 years? We shall see I guess.