RCA to XLR cable off USB DAC -- reduce noise? - Page 2 - AVS Forum
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post #31 of 43 Old 08-07-2013, 08:51 AM
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Originally Posted by arnyk View Post

Usually the worst thing about PC's from a noise viewpoint is the third pin on the power cord, but no way will I recommend doing away with it.

Yeah. I have often used ground lifters (3- to 2-wire adapters) in troubleshooting, rarely if ever as a solution, and never in any system but my own where I controlled the safety ground (perhaps by rerouting to a different ground point).

In this case we are in complete agreement; I think it is noise, if not a loop, around the USB connection. Galvanic isolation should cure it. As you've said. Several times. smile.gif

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post #32 of 43 Old 12-18-2013, 10:53 PM
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Originally Posted by TheGSRGuy View Post


Computer --> USB DAC --> stereo audio cables --> subwoofer --> XLR cables --> speakers


If I plug the USB DAC into my laptop, there is zero RFI/feedback/hiss. Here's what I've tried without success:

When I switch to the motherboard's on-board Realtek "HD audio", the problem is almost entirely gone, leading me to believe the motherboard is introducing a ton of noise onto the USB bus somehow. eVGA is allowing me to RMA it, so hopefully that will be the end of it.


Can't say I know anything about solutions for your problem - but I just wanted to ask if you are sending decoded/uncompressed digital signals to the USB (bypassing the motherboard/OS's DAC, Audio resampling etc)
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post #33 of 43 Old 12-19-2013, 11:21 AM
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Originally Posted by JD in NJ View Post

I could be wrong about this, but I believe that 'balanced' cables are only less subject to interference if the equipment at each end is also balanced.

Not true. Most of the benefits of a balanced connection depend on just the receiving end being balanced. A balanced cable must also be used. Some so-called balanced line transmitters are passive, and either simply ground one signal output (usually the minus) or connect it to ground via a certain value of resistor. The latter is called impedance-balanced output, and it still yields a link with common mode interference rejection which is the major general benefit of balanced I/O.

This is the way to wire a cable for an unbalanced to balanced interconnect that takes advantage of the interference rejection possible with a balanced input:



The above is taken from Rane Note 110:

http://www.rane.com/note110.html
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post #34 of 43 Old 12-19-2013, 11:24 AM
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Originally Posted by arnyk View Post

No, the presence of a balanced input is far more signficant than a balanced output.

There are three ways to drive (output to) a balanced line.

(1) Most ideal is a true active balanced output. One of its benefits is that it can apply twice the signal voltage to the cable as the other alternatives.

(2) Less ideal but surprisingly effective is the impedance balanced source. One wire of the balanced cable has a signal voltage applied to it with a given source impedance. The other wire is connected to a resistor with the same impedance, but the resistor just goes to ground, no signal.

(3) Least ideal but still more effective than an unbalanced input is connecting one wire to a signal source, and simply hooking the other wire to ground on the same piece of equipment.

In case 3 the balanced input still provides an advantage because it responds to the difference between the signal voltage and the source component. If there were no wire connected to ground at the source, the input of the equipment receiving the signal would respond to the difference between the signal voltage and ground at the receiving end. There can be a voltage drop across the ground side of the interconnect due to strong ground currents flowing in the wire of the ground side of the interconnect and it will add to the signal received at the receiving end.

This document sheds more light on this topic:

http://www.jensen-transformers.com/an/an003.pdf . Item 2.1 describes the approximately 30 dB (minimal) benefit of connecting a balanced input through an unbalanced cable - minimal, but beats nothing!

For those of us who just want simple answers, do commonly available RCA -> XLR adapters or cables provide either option 2 or 3 to us, or would we have to cobble something together on our own?

Depends on the adaptor/cable but an RCA to XLR cable adaptor on the end of an XLR can come close. You can check the actual wiring with an ohm meter, This is the pin connections that you are hoping for:



the picture is from Rane Note 110:

http://www.rane.com/note110.html
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post #35 of 43 Old 12-19-2013, 11:33 AM
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This is the way to wire a cable for an unbalanced to balanced interconnect that takes advantage of the interference rejection possible with a balanced input:

Of course, this only works if there are no other connections between the equipment, like video connections, and that the chassis and signal ground aren't tied together, as they are in 99% of equipment.
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post #36 of 43 Old 12-19-2013, 01:39 PM
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Originally Posted by SAM64 View Post

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This is the way to wire a cable for an unbalanced to balanced interconnect that takes advantage of the interference rejection possible with a balanced input:

Of course, this only works if there are no other connections between the equipment, like video connections, and that the chassis and signal ground aren't tied together, as they are in 99% of equipment.

Again not so. It works fine because any hum picked up by the chassis is common mode with respect to the 2 signals on the XLR connector.
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post #37 of 43 Old 12-19-2013, 03:38 PM
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Originally Posted by arnyk View Post

Again not so. It works fine because any hum picked up by the chassis is common mode with respect to the 2 signals on the XLR connector.

No arny, the two signal carrying conductors must have the same impedance with respect to ground. If one is at ground potential, they won't be the same. The signal induced into each conductor will be at a different amplitude with respect to the other. The common mode rejection ratio is reduced because of this.
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post #38 of 43 Old 12-19-2013, 04:23 PM
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Originally Posted by SAM64 View Post

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Originally Posted by arnyk View Post

Again not so. It works fine because any hum picked up by the chassis is common mode with respect to the 2 signals on the XLR connector.

No arny, the two signal carrying conductors must have the same impedance with respect to ground. If one is at ground potential, they won't be the same.

That is completely true if you desire the best possible performance, but back in the real world where even 20 dB suppression of hum and noise can be a big help, that requirement can be well, relaxed..
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The signal induced into each conductor will be at a different amplitude with respect to the other. The common mode rejection ratio is reduced because of this.

All true, but if you study the operation of non-optimal situations, you can find that the balance input is still a significant help.

I've done a lot of work measuring the performance of equipment with unbalanced (RCA) outputs. The equipment I test with generally has balanced inputs, so I use test cables that are built like this:



I find that unbalanced outputs have SNR of nn better than 90 to 100 dB, which I can imprrove by 20-40 dB using this cable, which is not impedance balanced. If it were impedance balanced I would get something like 20 dB more supression of hum and noise, but the 20-40 dB improvement makes a significant difference and generally puts the hum and noise well below the residual noise and distortion in the UUT.
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post #39 of 43 Old 12-19-2013, 05:59 PM
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Once again , this only works if there's no connection between chassis and signal ground at either end of the cable, not true 99% of the time.
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post #40 of 43 Old 12-20-2013, 04:47 AM
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Originally Posted by SAM64 View Post

Once again , this only works if there's no connection between chassis and signal ground at either end of the cable, not true 99% of the time.

Once again, I totally disagree, partially because I've done the circuit analysis and partially because I've used the method incessantly with good measured and audible results.

What really happens is that the better the chassis are tied together, the less hum voltage there is for the differential input to cancel out.

Here is the circuit with the chassis floating:



The chassis ground voltage at the ends of the balanced line are indeterminate because nothing ties them together except the balanced line and any casual return path though input resistors, etc.. It is possible to exceed the common mode dynamic range of the balanced input stage and still get hum. Been there, done that.

Here is what happens if there is a connection between the two chassis (inter-chassis bonding)



The first question that pops into my mind is "What's wrong with that?"

The chassis ground voltages at each end of the balanced line are now determinate and known to be roughly equal. Common mode cancellation is not interfered with.

Comments?
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post #41 of 43 Old 12-20-2013, 07:41 AM
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The first question that pops into my mind is "What's wrong with that?"

The chassis ground voltages at each end of the balanced line are now determinate and known to be roughly equal. Common mode cancellation is not interfered with.

Comments?

We'll have to agree to disagree, once again.

I'm a little surprised that you can't see how the inverting input on the right hand side is now at chassis ground.

The 'balanced line' is no longer balanced.
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post #42 of 43 Old 12-20-2013, 08:12 AM
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Originally Posted by SAM64 View Post

We'll have to agree to disagree, once again.

I'm a little surprised that you can't see how the inverting input on the right hand side is now at chassis ground.

The 'balanced line' is no longer balanced.

I'll agree with Arny. There is a benefit to what he outlined. Broadcast analog video gear was never balanced. 1vpp into 75ohm coax. A common trick was to isolate the ground on differential amplifier inputs. This did provide a level of hum and noise rejection, about 20db, mostly 60hz hum. It was not nearly as good as a true balanced system but was better than straight unbalanced.

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post #43 of 43 Old 12-20-2013, 08:39 AM
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There is a benefit to what he outlined. Broadcast analog video gear was never balanced. 1vpp into 75ohm coax. A common trick was to isolate the ground on differential amplifier inputs.

I agree with you, but as mentioned, we're not talking about broadcast gear where signal and chassis ground are isolated. This is a consumer level forum, where, 99% of the time, signal ground is tied directly to the chassis, and there are multiple connections between equipment.
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