Andy's Blog : buzz

Hum and Buzz – Part V

Andy W — Mon, 21 Jan 2008 14:03:00 GMT

Balanced Connections.

In pro audio balanced connections are used almost exclusively, and with good reason. Using balanced connections is very effective in reducing and eliminating pickup and ground loops in the audio.

The most common balanced connections are in the form of XLR cables and ¼” TRS (tip-ring-sleeve) cables, and spade lugs are sometimes used.

Let’s take a look what a balanced connection is and at the physics behind why it works so well. (Balanced cables are used in other areas besides audio, so it applies equally to those situations too.).

A balanced connection will have two signal cables, an audio ground, and a shield. The shield in an XLR cable does not carry any audio information. The shield is connected at each end to the equipment or chassis ground (which is generally connected to the earth ground; many studios, especially ones in a building structure have a special grounding system, and other building codes to follow, but that is beyond our scope here).

The shield is effective against electric fields (and I won’t go very deep into any of these explanations, the physics involved can be found with Google), since the electric field inside the shield is zero (or approaches zero). The shield is made of a foil or braided wire or both.

Sometimes inexpensive cables will have a shield that is not braided, but only a layer of wire wrapped around the core. This can lead to an opening in the shield, especially at a bending point. The result is generally not severe for short runs, but on long runs, well, sometimes you get what you pay for, and at the very least you pay for the amount of copper in your cables.

Another thing to watch out for is a balanced cable with a drain wire running under a foil shield. The drain wire reduces the cable resistance somewhat, but since it will tend to be closer to one of the signal wires, it can cause an unbalanced pickup on the cable, which spoils the magic of the balanced cable.

So there, I said it… Sometimes there is a real, tangible, scientifically valid reason to choose one cable over another… Now back to the magic… er, physics of balanced cables.

The two signal cables are also twisted together. This is a common noise reduction technique called a “twisted pair.” Cables can pick up currents from outside electric and magnetic fields, and field strength is reduced as distance from the source is increased. If one wire is closer to the source than another wire, then the closer wire will be affected more. A twisted pair causes each wire to “take turns” as to which is closer to the source. In the end it tends to average out and both wires will pick up the same current/voltage. It is a physical fact that wires will pick up noise, and so we use this to our advantage as you will see…

The two signal cables in an XLR connection carry the same signal, but 180° out of phase. There are a few ways that the source equipment can do this but that is not important right now… what is important is what happens on the other end of the cable, at the receiving end. For simplicity, let’s say that the voltage one of the signal wires is 1Vrms and the other signal wire is -1Vrms (same magnitude, opposite direction, compared to the signal ground).

The equipment at the receiving end will have a differential amplifier (isolation transformers work too). For simplicity let’s say that it is a unity gain differential amplifier. That means it multiplies the difference of the incoming signal by one. So, (1Vrms - (-1Vrms)) * 1 = 2Vrms.

Now any interference that gets injected into the signal wires, as we saw a few paragraphs back, will be the same on both wires, both in magnitude and direction. Engineers have a fancy term for this. We call it “common mode” noise. Let’s say that each wire picked up 1Vrms of random noise on it way. Now watch what happens when we put it into our differential amplifier. (1Vrms – 1Vrms)*1 = 0V. The noise is gone!

Now if we mix signal and noise on the same cable let’s see what happens. ((1Vsignal + 1Vnoise) – (-1Vsignal + 1Vnoise)) * 1= 2Vsignal. In the real world the difference may not be exactly zero but a noise reduction of 40-60dB is not uncommon, so if the noise floor was halfway decent to begin with in the unbalanced case, the pickup with a balanced system will be vanishingly low.

Side note: instead of getting 1Vrms from an unbalanced signal, there is a 2Vrms signal at the receiving end, an apparent 6dB “gain” without the noise penalty. Some equipment employs this method some does not.

The other part of the magic of an XLR connection is this… separate signal ground and cable shield. Any currents due to pickup or ground loops are routed harmlessly in the shield, and stay out of the audio signal ground! Not so in a balanced ¼” TRS.

Now look at the ground loop HACDBJ in this sketch with a balanced XLR connection. Any currents that are picked up in the loop have no part at all in the audio signal. And any noise injected into the twisted pair cable is cancelled at the differential amplifier in DUT 2. Additionally, if (or when, as is generally the case) the amplifier stages in DUT 1 have power supply noise injected (as in an op amp… assuming that the two amplifiers are part of the same package and injected equally) there’s a good chance that it will be common mode as well, or nearly common mode, and therefore rejected at DUT 2 (at least to the level of CMRR at the receiving end).

What is CMRR? It is the Common Mode Rejection Ratio, expressed in dB, and for a good audio quality op amp can be 100dB, which is probably better than the cable, and that’s the point. How much rejection do you need? Well that depends on how much noise is picked by the cable. In a professional installation when you’re running hundreds of feet of mic cable or line level signals, you need all you can get.

Hum and Buzz – Part IV

Andy W — Fri, 14 Dec 2007 19:28:00 GMT

Antenna/Cable TV Ground loops.

OK so you’ve narrowed your ground loop issue down to your cable connection or your rooftop antenna. You’re not alone; this is one of the more common ground loop issues, but I don’t necessarily want to want to get into the gory details, so here are a few links.

http://www.epanorama.net/documents/groundloop/antenna_isolator_building.html

http://ecmweb.com/nec/electric_article_radio_television/

http://www.hdtvprimer.com/ANTENNAS/basics.html

Look in this link above and scroll down to the “Grounding Outdoor Antennas” section and you can see why it’s so easy to have a ground loop here.

Ground isolation is really one of the only answers here due to the grounding requirements of aerial antennas and cable systems. Isolation transformers or capacitors are commonly used. Both types will break the ground loop and provide isolation.

Breaking the ground loop can either be done on the coax side of the TV tuner/cable box or on the audio side between TV tuner/cable box and the audio preamp/receiver. When breaking the loop between the TV tuner/Cable box and the audio preamplifier, a wideband audio transformer for the line level signal is probably my best recommendation.

BEWARE… many of the inexpensive audio isolation transformers are based on telephone transformers and don’t have the bandwidth required for hi-fi audio. Perhaps this made little difference in the past, but many shows are now broadcast with much better audio quality than before. Read the specs… if the audio bandwidth is not specified keep looking.

Also keep in mind that the ground loop can be connected to your system on the video cables if your receiver is doing the video switching. In addition, DC level is important in video signals so whatever you do may have an impact on video quality as well.

Ground loops on the video side of things can also cause “hum bars” to appear on your video screen (horizontal stripes that move slowly across the screen).

New cable standards, including HDTV and digital cable, along with widespread use of cable modems for high speed internet access have placed serious constraints on the use of ground isolation equipment if you choose to isolate your ground on the coax side of the cable box. So again if this affects you, read the specs and make sure the unit you are purchasing will not prevent you from using your cable or internet access the way you plan to.

My best recommendation, if your equipment has it, is to use the optical audio output on your cable box or HDTV receiver to connect to your audio equipment. Maybe our readers will have some more recommendations.

Other than this I can’t make a specific recommendation since I don’t have cable… you know, that whole tightwad thing… J

Hum and Buzz – Part III

Andy W — Fri, 30 Nov 2007 22:17:00 GMT

Ground Loops 101

Let’s start our discussion of ground loops with two pieces of chassis grounded equipment. Class II equipment can exhibit hum and buzz too, but the mechanism is a little different, but we’ll talk about that in another installment. We’ll also start with an “unbalanced” signal connection since it is more prevalent in commercial equipment (RCA-type connections and stereo mini-plug connections are examples of unbalance signal connections).

In the first example below a ground loop is created when the two pieces of equipment are plugged in and connected. DUT 1 might be a CD player and DUT 2 might be a Receiver. The signal from DUT 1 is passed through a signal cable to DUT 2. The “signal ground” connection is made between points “C” and “D”. Generally points “C” and “D” are also connected to the metal chassis. The “chassis ground” connection is made where the “earth ground” wire attached to the chassis. This happens at points “A” and “B”. The bare wire connection at points “H” and “J” on the AC outlets are earth grounded by the wire running back to the breaker panel.

The ground loop is formed, starting at point “H” to “A” to “C” to “D” to “B” to “J” and back to “H”. For simplicity we’ll just say “HACDBJ”. Any magnetic flux that passes through the loop will cause an electric current to flow in the loop. Any time current flows in a wire there will be a voltage impressed on that wire. Any voltage on the signal ground connection “CD” will become part of the audio signal and will be amplified. By virtue of the fact that your whole house is wired with AC wiring with currents flowing every which way, the ground loop will be bathed in magnetic flux. (Cable shielding is only effective for electric fields.)

Also any current on the earth ground between points “H” and “J”, regardless of where it comes from (like a light dimmer on the same circuit) will create a voltage in proportion to the current on “HJ”. Since the connection “HACDBJ” is in parallel with “HJ” part of the current will flow in “HACDBJ” and part of the current will flow in “HJ”. In electronics terms a parallel path is known as a “current divider”. The current flowing in “CD” will have an associated voltage, and this will be added to the signal from DUT 1.

The second example above is essentially the same as the first. The difference is that the two pieces of audio equipment are powered by two different circuits on the breaker panel. DUT 3 sends a signal to DUT 4 along an unbalanced cable with a signal ground “CD”. The chassis are grounded at “A” and “B” and the outlets are grounded at “H” and “J” just like before. The big difference here is that now the ground loop goes all the way back to the breaker panel. The loop is now a very large loop and can have even greater potential to pick up hum from magnetic fields. Also, now you have ground and neutral currents on the neutral bus bar coming into play. Ground current and neutral current from all the other circuits in the house can now put voltage on your signal ground “CD”.

So what can be done?

1. Make a block drawing of the components in your system, showing the signal connections (a single connection is fine for stereo connections) and the power cables. Also show in your sketch which power cables have an earth ground. Identify the house circuits that you are using, and show any other equipment that is sharing the same circuit (light dimmers, motors – like appliances, things with switch mode power supplies – computers and such, and things that might draw a lot of current or be injecting noise on the ground).

2. Study the diagram to see where your trouble might be coming from. Then try some trouble shooting and make notes of what happens. Only make one change at a time; do and undo the change so that you can have an A/B comparison.

3. Make loops smaller. Smaller loops have less magnetic flux inside them. Try running the AC cables right next to each other; use twist ties to hold them together temporarily. Shorten signal cables and twist tie them up.

4. Does your AC cable or signal cable hang in a “half turn” around a power transformer. (If this needs a sketch please let me know.) The cable will become part of the transformer. If your equipment cables hang in loops; tie them up. Reroute cables to eliminate or shorten loops.

5. Move or reroute cables away from cables that carry large currents or are known to have currents with high harmonic distortion (almost every appliance/piece of equipment that is not resistive will have varying levels of high harmonics on the AC power line/cord). Distance is your friend in these cases. Try a different cable orientation or a stack your components in a different order. The sensitive part of a PCB in one component may be right on top of a noisy transformer or rectifier in another. Maybe it sounds crazy but a sheet of steel between components can help, and in extreme cases it may be necessary to wrap a steel sheet around the offending transformer (a magnetic “short” circuit). I have measured hum reduction on the order of 25-30dB by doing this. But it’s better and easier just to move the noisy transformer away from the sensitive equipment.

6. Reduce the ground voltage between components. Plug all your components into the same AC outlet. Use a power strip if you have to. If there is a noisy light dimmer on the same circuit and you plug the system into a single outlet you can reduce the ground voltage difference (the voltage on “HJ”) to zero or nearly zero. Make sure your power strip is rated for 15-20 Amps.

7. Plug the offending equipment into a different circuit outlet, so that the noisy ground will not be on the same line as your audio equipment. Or if there is a permanent fixture/appliance (e.g. a ceiling light on a dimmer), plug your audio equipment into a different circuit.

8. If you are building a house, it can’t hurt to run a dedicated AC line to your equipment. This will generally cost $200-$300. If you can talk them into it, run a 20A circuit with 12AWG with a corresponding breaker and outlet instead of a 15A circuit. Keep the run as short as possible and don’t run it parallel with any other runs. And for a truly esoteric touch (I’m not claiming this will help, but it might) run the line in flexible steel conduit. If you are in an existing construction, it is not impossible to run a dedicated line, and sometimes there are creative solutions to getting an AC line where you want it, using an existing line that is more noise free.

9. Transformer coupling. Sometimes there is no other way… perhaps that can be its own installment.

10. The “pin 1 problem” (this is in reference to pro or “prosumer” products using balanced connections). This may require internal modifications... the manufacturer may already know about if if they have a product with this issue... but that's for balanced connections, and that's another discussion for another time.