Good Example Balanced Input Circuit? (Ground Loop Isolation) - Page 2 - diyAudio
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Old 15th August 2008, 03:40 PM   #11
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I know the circuit works. I've used it many times. Did you ground the sec. CT to the chassis ground?

You can test the circuit by driving the signal into both the center conductor AND to the shield. If the circuit is working properly, there will be no output from it.
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Old 15th August 2008, 04:08 PM   #12
MartyM is offline MartyM  United States
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Hi, yes I did reconnect the CT of the SMPS output to chassis ground, which only reintroduced more noise into it.

I'll take a break later (I'm at work) and repeat the process, ensuring I didn't overlook something.

Thank you.
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Old 15th August 2008, 04:21 PM   #13
ppia600 is offline ppia600  United States
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So using one 4.7k resistor from the rca shield at the input jack goint to the common center rail of the power supply wouldn't work? (for all channels) Sorry if its already been mentioned.
Also, are you sure some of the ground shields at the alpine processor aren't broken?
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Old 15th August 2008, 04:27 PM   #14
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Why not just connect the shield direct to the inverting leg of the diff amp, and the signal to the non-inverting leg? i.e. ditch the two front end buffers.

I did this when I had a bad alt whine many years ago and it worked perfect.

Or does your way have better CMRR or something Perry?
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Old 15th August 2008, 04:35 PM   #15
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Rockford has patent #5751823, which it's critical point is choosing to which point the opamp's regulator reference (317 and 337 in fig.5) connected to. It is connected to head unit's ground, separated by R//D to power amp's 12V ground.
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Old 15th August 2008, 06:37 PM   #16
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richie00boy:
I've done this but for some reason there was more noise (noise floor slightly higher) than when I used the buffers.

MartyM:
I think there may be another problem. Having all grounds (except the RCA shield ground) connected to chassis ground has worked for me. I've used the circuit with speaker level and RCA level inputs and it's always been quiet.

Can you post a schematic of the circuit you're using for the audio IC?
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Old 15th August 2008, 10:21 PM   #17
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Balanced inputs are not a solution to solving ground loop problems.
I know of many situations where commercial amps are used, that have balanced inputs, and they still have ground loop hum.

Balanced lines are simply designed to eliminate (reduce) common mode noise that is picked up by (usually long) cables that feed an amplifier or mixing device.

If you are experiencing hum or earth loop issues, then it's time to look at the design of your power supply, input stages, grounding, isolation and decoupling.
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Old 15th August 2008, 11:13 PM   #18
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In mobile amplifiers, you have to provide some way to ensure that the reference for the audio is the shield of the head unit (or whatever signal source is driving the inputs of the amplifier).

The most common method (used for virtually all budget amplifiers) is to float the secondary of the power supply. This works well as long as all inputs are being driven from the same signal source.

The input circuit that Rockford uses floats the ground for the 15v regulators. This works well but can produce noise unless all signals are from the same source.

Other amps use balanced inputs (or some form of active input circuit). These work best when the inputs of the amp come from different sources (i.e. sub output from head unit directly to the amp, high end signals passed through an EQ or other signal processor before going to the amplifier). For these systems, a fully balanced input (or isolation transformers) on all channels is the most likely to be able to prevent ground loop noise. In this case, they are used to ensure that each input uses only the shield ground from the signal source as a reference. With other input circuits, the common shield grounds can cause noise from ground loops.
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Old 16th August 2008, 03:05 AM   #19
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Quote:
Originally posted by lumanauw
Rockford has patent #5751823, which it's critical point is choosing to which point the opamp's regulator reference (317 and 337 in fig.5) connected to. It is connected to head unit's ground, separated by R//D to power amp's 12V ground.

lol lol lol I wasn't worried about RF trying to prosecute an end user adapting there design to suit his one time use and need lol lol catch me if you can in this case lol lol lol
RF amps have a long and very good reputation for noise free installs, Hence my reason for suggesting their approach.

Perry has pretty much covered all methods and there relative disadvantages, and advantages

I myself have found that I have never really needed a noise canceling input on any of my gear.

But in a pinch there are always the high end Jensen studio grade transformers that list at over $110.00 a stereo set.

But using ANY transformer can also create issues as the coils act like magnetic pickups and any placement near a AC generating noise source will possible lead to hum and noise pickup worse then the original problem.
I use these on my test bench to isolate my music and signal source from any device I am testing < I have seen 60 volts DC standing on the RCA shield of some amps like the older Kicker ZX and Zr products. That much juice will surely damage any source you might care to hook up.

The circuit Perry posted should give good results, But nothing is perfect as Perry has warned....
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Old 16th August 2008, 04:04 AM   #20
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What do you guys think about this page http://www.dself.dsl.pipex.com/ampin...balanced.htm#1
At that page it is different between balanced line and balanced impedance (fig1).
To get balanced impedance, the RCA out ground of the head unit should be given Rs too, but I never saw a car head unit that do this.
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