Good Example Balanced Input Circuit? (Ground Loop Isolation) - diyAudio
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Old 14th August 2008, 03:21 PM   #1
MartyM is offline MartyM  United States
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Question Good Example Balanced Input Circuit? (Ground Loop Isolation)

Hi guys I am hoping to finally resolve ground loop noise on my DIY 8-ch. LM3886-based car amplifier. After much debugging and modifications, it's a great sounding amp with blue LED illumination.

However even after isolating all signal grounds from power ground(s) & vehicle, I somehow manage to pick up ground loop noise when an RCA connector from the next pair of outputs from my source (Alpine PXA-H701 DSP/EQ) is used. Until then, (one pair only) no noise.

Looks like I'll have to finally compromise and add a balanced input circuit as Perry Babin & others recommend. Ground loop isolators aren't working as they cause my SMPS fuses to pop when used (amp oscillating?). The amps are PA100 design examples from National Semiconductor.

So my question:
Does anyone have a good balanced input circuit example they know will work for this?

I have been searching quite a bit without so much luck. Also tried Rod Elliot's Project 51, figure 2, balanced input but still noisy.

I guess basically I need to confirm the right approach, and right understanding of how the balanced input circuit can work for noise cancellation even if my input is not balanced.

Thanks as I would really appreciate the help. You can imagine how I feel after working on the amp for months, and thought it was resolved, only to find out when used fully....NOISE!
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Old 14th August 2008, 04:30 PM   #2
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When you used ground loop isolators, did you connect the secondary center tap of of the power supply to the chassis ground?
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Old 14th August 2008, 05:10 PM   #3
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Thanks for replying, no I did not. Why would that be done?

I should mention I am hoping to use solid-state inputs due to space and audio quality. I can't really fit that many isolators in the amp anyway. I was just testing the ground loop isolator method when I used it.
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Old 14th August 2008, 05:25 PM   #4
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Connecting the secondary center tap to chassis ground may stop the oscillation. The secondary is only left floating in amps that use the isolation of the secondary as a means of preventing ground loops.

Do you have buffers at the input of the amp or do you have the input feeding the amplifier ICs directly?
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Old 14th August 2008, 06:54 PM   #5
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Afternoon Perry,
wouldn't he benefit from the old Rockford Fosgate TOPAZ input design that used on most all their old stuff ? RF had a input design that almost totally isolate the input buffer and crossover section of the amp from the main amp circuits.
Seems like this might help the fella out, and just thinking out loud..
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Old 14th August 2008, 07:15 PM   #6
MartyM is offline MartyM  United States
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Hi Perry & Just1moreamp:

Yes I am driving the ICs directly; you may remember my original thread (direct path to amp modules).

It sounds like if I use ground loop isolators, and connect the grounds as described, I'll introduce a ground loop on all others and thus will need isolators on all 8 RCA inputs. Not very efficient vs. resolving using solid state parts.

The current set-up works quite well until I plug in the next pair additional RCA inputs (The source, the Alpine PXA-H701 is an EQ/Crossover/5.1 surround sound decoder all in one-no other units used). This is true for each of three SMPSs in my amplifier. No noise until next pair is plugged in.

I never had this issue with my older commercial amplifiers.

If anyone can recommend an amp schematic source, or the input circuit schematic, that would be appreciated. Not familiar with TOPAZ, but would love to learn more.

Thanks!

PS: That's my amp in the picture, sounds really good and glows bright blue when powered up. It's been my first "dream project."
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Old 14th August 2008, 08:06 PM   #7
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The rockford circuit uses the shields as a reference for the op-amp power supply which I would prefer to avoid.

The circuit below works. An RC network would probably be useful at the input of both buffers to shunt high frequency noise to ground. It's quiet when using good op-amps. You don't have to use anything expensive. NE5532s will work well. 1% components should be used between the buffers and the next stage of the circuit.
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File Type: gif balancedcircuit01.gif (7.2 KB, 456 views)
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Old 14th August 2008, 08:57 PM   #8
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Thank you very much, sir.

Would you mind noting the following, please?:

1. "which I would prefer to avoid" - Why do you say that? What is preferrable?

2. I assume the ground connection shown will go the op-amp power supply source, which is in my case the output of my SMPS-correct? (I use voltage regulators of course)


Thanks!!
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Old 14th August 2008, 09:09 PM   #9
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1. because it passes DC current into the shields.

2. The secondary center tap of the power transformer and the ground on the schematic will be connected to chassis ground.
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Old 15th August 2008, 02:49 PM   #10
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Thanks Perry-unfortunately that circuit did not work. I tried it after going home from work last night. Still had ground loop noise. Aye....
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