Back to Back ADC and DAC

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Colt45ws

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Basically Im wondering what people think about doing the above, going from RCA cables from my head unit to an ADC and then straight into a DAC to go back to RCA cables to my amp.
What Im trying to do is lift a ground loop that I cannot seem to fix. Ive tried everything I could think of and have found on the internet to fix it and haven't fixed it. My fix right now is just to run it really loud and it disappears but If I have people in the car obviously then you cant converse with them or each other so I actually usually just shut the system off in that case as its embarrasing for me to run around with the alternator coming through my speakers. The injectors like to show up to at idle rpms as well in there too.
So far Ive (in no particular order since I dont remember, only that its been done)
Moved the ground for the amp closer
Sanded the amp ground location on the body for better contact
Pulled the amp off the steel wall its screwed to and set it on the carpet to isolate the metal body from the car body.
Moved the radio ground from the harness to a dedicated ground on the transmission tunnel, that I also sanded.
Unplugged the amp from the head unit to confirm it was only occurring through the audio connection. Amp on and no signal is dead silent.
I also unplugged at the HU end to ensure it wasn't being induced through the cabling.
This is the second HU this amp has been behind so I doubt its that. I went from a Clarion CD player to a double din Kenwood touch screen.
Basically I think it s ridiculous we are still fiddling with an unbalanced analog connection in cars in 2015. I read all kinds of bad about those cheap ground loop fixers destroying the audio and I don't want to do that, make the whole thing a waste of time. So I was actually hoping I could find almost like and optocoupler (which is where the idea came from) where the ADC and DAC are right in the same unit you have RCA in and RCA out but I haven't found anything like that, but I still have been looking at the idea even if I have to run a short cable between the units.
 
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Nothing you've suggested or done indicates a ground loop issue. It sounds like you want some noise suppression on either the power to the head unit, the power to the amp or both.
 
If I have the amp on with the audio cables unplugged, its silent. Sooo, I pretty much figured it was a ground loop through the cables. I used a DVOM to measure the ground voltage differential and I dont remember the exact number but it was fairly high and it would drop as the volume increased.
 
If you can hear the difference in audio quality with decent transformers (quality does vary) in a car, then your hearing is much better than mine. BTW, I have no idea whether or not the ones I linked above are any good.

Are you an EE by chance? Some of the stuff you wrote is relatively detailed. Basically, you have voltage ripple from the alternator. You will have a capacitor at the power rail input of the amplifier. The capacitance causes a lot of ripple current to flow when there is voltage ripple. The ripple current causes new voltage ripple drops on your grounds (current times resistance), and this is what gets amplified.

You should try running real thick wires for your grounds - not tying to the body. And you should tie the head unit ground and the power amp ground to the same point, which will be tough due to multiple grounds on the head unit (there's also the antenna). Good luck.

PS. Former EE here, but doing other things now.
 
I guess Ive just seen a lot of negative reviews of them. I haven't tried one myself but I have seen quite a few and they all seem to be built pretty cheap to me on the outside, which if the outside is cheap the inside is probably cheaper.
Im not an EE, just well read.
I am running as thick of a ground wire that will fit in the amp, a 4ga. I did try tying the grounds together, though all I had was a 10ga that would reach that far; it made no difference. I did something else to tie them together with that wire that actually made the problem worse, but now I forget what it was I did.
I do have a custom capacitor on the input to the amp. I have 4 panasonic 3300uf 50v caps paralleled on some 10ga wire and duct taped together, but it did this before I added that and Ive lifted that out of the circuit as well before to make sure it wasn't doing anything.
Your comment about the antenna has my attention. I may try disconnecting that and see what happens. My car does not have a whip antenna as it uses the rear window defroster grid as the antenna and it has a box under the rear window that separates the signal out of the grid even when its powered up.
 
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Originally Posted By: Colt45ws
I used a DVOM to measure the ground voltage differential and I dont remember the exact number but it was fairly high and it would drop as the volume increased.


If that is actually the case, whereby you have a significant differential between the shield on the RCA from the head unit and the shield on the amp then you have a much bigger and far different problem than an earth loop. Go back to basics because something is wired badly wrong.
 
Originally Posted By: Brad_C
Nothing you've suggested or done indicates a ground loop issue. It sounds like you want some noise suppression on either the power to the head unit, the power to the amp or both.
+1 KISS
 
Power the amp with an external source for a test, battery or 12V regulated supply. Then, if there is still ripply, try another amp.
Have you looked at this ripple with an O Scope, what does it look like. What frequency? Switcher supplies do strange things when they get flaky.
 
Have you tried adding some resistance to one or both ground conductors, to prevent/minimize current flowing through? If the two conductors are at slightly different potentials, current can flow but if you add some resistance, I suppose it would limit it.

Just a guess though.
 
Having an oscilloscope or access to one would really help here. Understand there is a difference between a DC ground and an AC ground. The later requires a low impedance to ground at the frequency of interest.

Do a search for "Interference Handbook" By William Nelson. You might also find any issue of The Radio Amateur's Handbook of use. There's lots of good info in there re: mobile mounting of Tx/Rx radios and minimizing noise. Your local library might even have it on the shelf. Further, you may find some books on marine electronics installation useful.
  • How old are your spark plug wires?
  • Do you notice the buzzing is worse when driving under a concrete bridge?
  • So it is possible to turn the vol. up enough to drown out the noise? So it doesn't get louder with increased volume?
  • Try a dedicated amp gnd. Also be sure the power & gnd cables are twisted together to minimize the loop area. Else they'll become antennas, picking up radiated noise.

Quote:
Moved the radio ground from the harness to a dedicated ground on the transmission tunnel, that I also sanded.
Unplugged the amp from the head unit to confirm it was only occurring through the audio connection. Amp on and no signal is dead silent.
I also unplugged at the HU end to ensure it wasn't being induced through the cabling.

Your HU & amp need to be grounded at a single point. You may also need to take this point all the way back to the battery, independent of the chassis. Grnd'd as stated above indicates you have a gnd loop. Understand that a vehicle chassis gnd can be full of noise.

Quote:
Basically I think it s ridiculous we are still fiddling with an unbalanced analog connection in cars in 2015. I read all kinds of bad about those cheap ground loop fixers destroying the audio and I don't want to do that, make the whole thing a waste of time. So I was actually hoping I could find almost like and optocoupler (which is where the idea came from) where the ADC and DAC are right in the same unit you have RCA in and RCA out but I haven't found anything like that, but I still have been looking at the idea even if I have to run a short cable between the units.

Balanced interfaces cost more. Jensen makes a high-quality RCA audio isolater, but it's a bit expensive. You may find an alternative in a broadcast catalog or pro-audio catalog like MarkerTek.

Another way to go after this is to remove the HU and amp, wire them up on the driveway or (better) workbench, power from the car with the engine running and hook up amp to spkrs in car. Still hear any noise? Now you've eliminated all potential gnd loops due to mounting screws/bolts/etc.

As you well know, it's a slow process of elimination.
 
Originally Posted By: HerrStig
Power the amp with an external source for a test, battery or 12V regulated supply. Then, if there is still ripply, try another amp.
Have you looked at this ripple with an O Scope, what does it look like. What frequency? Switcher supplies do strange things when they get flaky.


Well, I haven't done that but I can tell you on battery power (Key in ACC) its pretty quiet. Its basically only when the engine is running. The frequency is dependant on alternator speed but at some ranges you can detect the injectors clicking.
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.
I don't have an o-scope. Its something Ive been meaning to get but havent. I don't want to get a POS but at the same time I don't need $1000 unit.

Originally Posted By: Brad_C
Originally Posted By: Colt45ws
I used a DVOM to measure the ground voltage differential and I dont remember the exact number but it was fairly high and it would drop as the volume increased.


If that is actually the case, whereby you have a significant differential between the shield on the RCA from the head unit and the shield on the amp then you have a much bigger and far different problem than an earth loop. Go back to basics because something is wired badly wrong.

I think that was the issue but at the moment I can't simply pull the radio out to recheck as the dash has to be disassembled to remove it. I need to do that anyways in order to fix a HVAC issue so I may start that this weekend sometime.
I don't know what could be wired wrong. I even used an extra wire for the HU supply that was for the accessory lighter plug (I ran a new wire from a switched source for the lighter plug so the original cont. power wire was sitting unused.)
 
Originally Posted By: sleddriver
Having an oscilloscope or access to one would really help here. Understand there is a difference between a DC ground and an AC ground. The later requires a low impedance to ground at the frequency of interest.

Do a search for "Interference Handbook" By William Nelson. You might also find any issue of The Radio Amateur's Handbook of use. There's lots of good info in there re: mobile mounting of Tx/Rx radios and minimizing noise. Your local library might even have it on the shelf. Further, you may find some books on marine electronics installation useful.
  • How old are your spark plug wires?
  • Do you notice the buzzing is worse when driving under a concrete bridge?
  • So it is possible to turn the vol. up enough to drown out the noise? So it doesn't get louder with increased volume?
  • Try a dedicated amp gnd. Also be sure the power & gnd cables are twisted together to minimize the loop area. Else they'll become antennas, picking up radiated noise.

  • It has no wires, COPs. I don't think its the spark noise Im picking up. The chatter noise I hear along with the alternator whine sounds more like injectors to me as Fords multi-strike the spark plugs at low rpm and the chatter is engine speed. But I'd have to check with a scope to verify. Im trying not to spend any money right now though due to work slowing down so it may be awhile before I can do that.
  • I don't recall anything like that.
  • Correct, the volume of the noise is always the same so I can run loud enough that I don't hear it. When I was checking the voltage differentials I could watch the differential drop as I turned up the volume.
  • Ill try that on the HU end when I pull the dash. Right now its inaccessible.

Originally Posted By: sleddriver

Quote:
Moved the radio ground from the harness to a dedicated ground on the transmission tunnel, that I also sanded.
Unplugged the amp from the head unit to confirm it was only occurring through the audio connection. Amp on and no signal is dead silent.
I also unplugged at the HU end to ensure it wasn't being induced through the cabling.

Your HU & amp need to be grounded at a single point. You may also need to take this point all the way back to the battery, independent of the chassis. Grnd'd as stated above indicates you have a gnd loop. Understand that a vehicle chassis gnd can be full of noise.

From what I understand, even though the resistance of steel is high compared to copper there is so much of it in a car body that in order for me to match grounding through the body with a cable I would have to use something thicker than 4/0.

Originally Posted By: sleddriver

Quote:
Basically I think it s ridiculous we are still fiddling with an unbalanced analog connection in cars in 2015. I read all kinds of bad about those cheap ground loop fixers destroying the audio and I don't want to do that, make the whole thing a waste of time. So I was actually hoping I could find almost like and optocoupler (which is where the idea came from) where the ADC and DAC are right in the same unit you have RCA in and RCA out but I haven't found anything like that, but I still have been looking at the idea even if I have to run a short cable between the units.

Balanced interfaces cost more. Jensen makes a high-quality RCA audio isolater, but it's a bit expensive. You may find an alternative in a broadcast catalog or pro-audio catalog like MarkerTek.

Another way to go after this is to remove the HU and amp, wire them up on the driveway or (better) workbench, power from the car with the engine running and hook up amp to spkrs in car. Still hear any noise? Now you've eliminated all potential gnd loops due to mounting screws/bolts/etc.

As you well know, it's a slow process of elimination.

Jensen! That's what I was looking at earlier. Yeah, I was going to get one of those at one point but haven't and was trying to figure out an alternate.
 
Im going to make their RCA test adapter which is a male and female RCA plug with the shield connected together and to both of the signal pins with a pair of 1k ohm resistors.
 
Originally Posted By: Colt45ws
If I have the amp on with the audio cables unplugged, its silent. Sooo, I pretty much figured it was a ground loop through the cables. I used a DVOM to measure the ground voltage differential and I dont remember the exact number but it was fairly high and it would drop as the volume increased.


Shielded cables, grounded at each end, can ( but should not in well designed and constructed equipment ) cause ground loops between individual components.

Have you tried modifying the cables, so that only the shield at one end ( the source ) is grounded?

If it is a loop, that will break it, provided you have not created another loop with your improvised grounding.

This assumes that this is a ground loop. I haven't heard one in a car to know what they sound like. That cheap $20 amp may be oscillating.
 
Originally Posted By: Win
Originally Posted By: Colt45ws
If I have the amp on with the audio cables unplugged, its silent. Sooo, I pretty much figured it was a ground loop through the cables. I used a DVOM to measure the ground voltage differential and I dont remember the exact number but it was fairly high and it would drop as the volume increased.


Shielded cables, grounded at each end, can ( but should not in well designed and constructed equipment ) cause ground loops between individual components.

Have you tried modifying the cables, so that only the shield at one end ( the source ) is grounded?

If it is a loop, that will break it, provided you have not created another loop with your improvised grounding.

This assumes that this is a ground loop. I haven't heard one in a car to know what they sound like. That cheap $20 amp may be oscillating.

I have read about using 10nf capacitors on the amplifier end of the shield to break the signal. The RCA plugs I bought to do the test adapter are screw terminal on the other side, so I can do anything with them I want.
Its a good amp, but its used. I think its from 2001 or 2?
 
I honestly couldn't tell much from your video. The alternator noise came through clearly.

Maybe you should go back to the basics - make sure the charging system is in fact working correctly, everything wired directly to the battery, cabled grounded at one end, etc.

If your amp is oscillating, it could sound hissy if it's in the audible range, otherwise you probably need a scope or other instrumentation to check it.

With my mobile ham radio stuff, I never rely on the chassis for a ground connection. A ground plane for an antenna, sure, but never for a dc ground return path. You have no way of knowing how the panels are connected together, and what the resistance in the joints are.

Edit: Someone mentioned the ARRL handbook. If you look at the older handbooks, you'll see they recommend bonding body panels together for noise reduction. I expect modern vehicles have better build quality, but I still wouldn't be comfortable using the chassis as a ground path.
 
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