Crossover Basics - The Zobel

Introduction to the Zobel

A Zobel network is used to flatten a driver's impedance (usually a woofer or mid-range), therefore making the filter (usually low-pass) more effective. There are many kinds of Zobel networks, but for speakers the simplest and most common Zobel circuit consists of a capacitor and resistor which are wired in parallel to a driver.

C1,R1 on the left are an example of the Zobel network. Their values are chosen to minimize the impedance rise of a driver above the resonant frequency.

Speaker Impedance

If you aren't quite sure what this means, please visit my blog post on Crossover Basics - Impedance before reading more.

Introduction to Series Circuits

Take a look at a very simple series circuit. It consists of two resistors in series (one after another) with the amplifier. The circuit is closed by the ground points (the downward facing triangles).

As before, this may be something you like to play with, so I encourage you to grab a copy of XSim and try this out along with the Peak Voltage chart.

What's important to understanding here is that the voltage across the resistors will be proportional to the resistance offered.  So

Vr1 = Vin * R1/(R1 + R2)
Vr2 = Vin * R2/(R1 + R2)

And of course:

Vr1 + Vr2 = Vin

That is, the voltage across R1 and R2 must add up to the input voltage.

Think of Ohms as elephants that eat volts. The more elephants, the larger portion of the incoming voltage they eat.  Yes, this is a very silly analogy.  Still, we can do some quick math. The total resistance is 100 Elephants (hah!) or 100 Ohms. R1 has only 10 elephants, so it gets 10% of the incoming voltage, whatever that voltage may be. R2 has 90% of the Elephants, so it takes 90% of the incoming voltage. There's a lot more to circuit analysis, but this is the bare minimum to understanding Zobel networks. Hopefully you'll be intrigued and learn more on your own.

Woofer Impedance

So with a little background under your belt you are now ready to look at a typical woofer. We'll use the LM-1 woofer, the Peerless 830991. You should know that impedance graphs will change once a driver is in the cabinet, especially if the cabinet is ported, so the data I present here will be different than a specification sheet which measures the driver in free-air.

You have heard the term "coil" used interchangeably with "inductor." Which is correct, but you may not have thought about the term "voice coil" in the same context. The voice coil is the part of the speaker that will electrically connect to your amplifier and produces the magnetic force which moves it against the magnetic field of the permanent magnet. We won't get too much into this, but suffice it to say that above the resonant peak the voice coil behaves like that of any other inductor, specifically it has both a resistive element (DC Resistance, or Re) and an inductive element (Le). These combine to give us the woofer's electrical impedance (Z) at any given frequency.

In the chart below we will compare the impedance of the woofer in a sealed cabinet (red) with a woofer that has a Zobel network applied (blue).

Let's ignore what happens below 200 Hz. That's a topic beyond this posting, and it's also far below our likely filter points. From about 200 Hz to 400 Hz the woofer is purely resistant, and we have the minimum around 6.6 Ohms, but what happens to the right? That's correct, suddenly more voltage-eating elephants arrive! The inductive qualities of the voice coil become more and more important and overwhelm the well behaved resistance. Compare the peak difference of the two impedance charts, all the way at the right. The blue line represents a woofer compensated with a Zobel network. The impedance never goes above 7 Ohms, while the normal un-compensated woofer goes to over 30! That's more than 4:1 difference. This increase impedance is going to compete with the low pass filter and make it behave in ways we probably don't want.

For clarity we'll leave behind the LM-1 schematic and create a new one, with two identical woofers and 2nd-order low-pass filters set to 4 kHz. Of course, this is not how a real speaker would be designed, we are just using this to see exactly how a Zobel circuit works. The Zobel consists of C1 and R1. 

The first thing we should do is examine the transfer function of the two filter sections:

As you can see, S2 is behaving like we expect a low pass filter to work. S1 however is having a very difficult time getting to the right slope. At 5kHz the output is almost 5 dB higher than we want it to be. That's a big deal. Eventually the impedance (elephants) on the low pas filter take over, but they don't reach our desired behavior until past 20 kHz, definitely not good enough for us. Let's take a look at the final acoustic results, below:

How Does a Zobel Work? 

Above we discussed how serial components work in a circuit. You may feel a little tricked because while we learned enough to understand why coil inductance needs to be compensated for, we never talked about how a parallel circuit works, which is what a Zobel is. A parallel circuit has one unique property:

The apparent impedance of a parallel section is never more than the smallest impedance.

If we imagine C1 as a short, then no matter what S2 rises to, the impedance will never go above R1, or 8.2 Ohms. It can be less than that, but never more.  In a parallel circuit you calculate the apparent impedance like so:

Rtotal = 1 / (  (1/R1) + (1/R2) + (and so on and so forth)  )

Things are more complicated because we are actually calculating impedance, but you get the picture.

Let's do some quick, Dr. Leach style of analysis on the components in a Zobel. At very low frequencies, C1 behaves like an open circuit, essentially removing R1 and C1 from meaningful contributions to the system impedance. Remember we mentioned that impedance cannot rise more than the smallest value? So at low frequencies C1 is so large that S2 becomes the limit on impedance. You can see the impedance below 500 Hz or so is barely affected. At high frequencies, C1 rapidly decreases until it evectively becomes a short, putting R1 in parallel with the driver, S2 and limiting the absolute maximum impedance to 8.2 Ohms. In this case we don't reach 8.2 until well-past 20kHz but it would eventually reach that point once the woofer's impedance was high enough. Also past our point of concern. If we can limit the impedance from 6.6 Ohms to 7  Ohms then we have a much more stable impedance curve than before, and that's good enough.

Do I Need a Zobel?

That's a tricky question! So let's examine this woofer and it's output. If you wanted to cross it over at 4kHz I would think the Zobel was mandatory, however if you were going to set your crossover frequency at 2kHz or lower I would say not really. The LM-1 uses it, but the effect of the Zobel is small, and benefits the phase response so I leave it in. It is possible a very similar sounding LM-1 could be built without a Zobel and with different choices in the filters left behind. The best chart to look at to see if a Zobel matters in your circuit is the transfer function chart.

It is very rare, but not unheard of, that a tweeter needs a Zobel because their voice coils are relatively tiny and therefore don't have a lot of inductance. The most common exception to this rule is with ribbon tweeters. The ribbon itself is not inductive but the entire assembly often include matching transformers. Transformers are coils .... and coils are inductive... see where this goes? :)

The real point to the Zobel is to make things better in the area you need the filter to behave at it's best. That's usually up to about -20 to -30 dB. Beyond that if your slope isn't perfect we no longer really care. There's no audible difference between -60 and -67 dB for example.

An important consideration in choosing a Zobel or not is that they are not free. The more parts in a system, the more expensive, the more chances of failures or parts being out of specification. If this is a personal project, no problem, it's all experience. However if you are building for mass production eliminating unnecessary components is the final stage before committing a design to the factory. 

Placement

In most cases, you want to place a Zobel closest to the driver. Put anything else such as padding resistors, filters, etc. before it. While the order of serial components does not matter, the order of parallel components does. Leaving the Zobel last prevents unexpected consequences.

There is a rare exception, when you must equalize a driver (usually a tweeter) by adding inductance. In which case you want the equalizing circuit closest to the tweeter, then the Zobel, so the Zobel can also control the EQ's impedance. I'll write more about this later in a section on handling difficult tweeters. 

The Secret Uses of the Zobel

Many will rely on on-line calculators to determine the right values for a Zobel network, and that's fine, but be aware that the absolute values can be tweaked. The main benefit of this tweaking is to gently nudge the phase charts one way or the other, helping you get near-perfect matching between two drivers you otherwise might not have. This is where having a tool like Xsim to simulate your tweaks comes in super handy.

Exercises

The data for the Peerless 830991 is contained in the LM-1 XSim files here.  Feel free to take it and modify it to help you complete these exercises.

The Peerless 830991 has an Re of around 6.6 and Le of around 0.330mH. Try simulating this in Xsim using a resistor and coil in series. Compare your impedance curve with the red impedance curve, above. What's the biggest difference you see?

Try using an online-calculator to create a Zobel for this driver. Tweak the capacitor and resistor values. Can you do better than the on-line calculator?

Using the complete LM-1 schematics, compare the woofer response with and without the Zobel. Is it a big difference? Can you fix the LM-1 so it no longer needs a Zobel? What difficulties did you encounter?Pay attention to the phase matching as well as the frequency response.

Use XSim to calculate the power through the Zobel resistor if the amplifier is set to 100 Watts. This is wasted power. Is it worth it?

Stereophile - Data Part II

This is a follow up to a previous article on what makes a speaker great to the editors at Stereophile.

I thought that was the end of that discussion, but thanks to an article written in 2008 but recently republished by the good Dr. Joseph D'Appolito there is more. Those who don't follow speaker design and measurement will not know D'Appolito literally wrote one of the most cited books on speaker measurements, in addition to having a configuration named after him.

In the article published by audioXpress D'Appolito shares an interaction with Stereophile head honch John Atkinson (JA). JA did something I though was pretty interesting mathematically, but I call bullshit on his message. He claims he analyzed a number of speakers and compared them to those which would make the recommended components according to frequency response and that most were perfectly neutral. D'Appolito states:

[John Atkinson] defined the standard deviation (SD) from flat response over the frequency range of 170Hz to 17kHz as a criterion for judging flatness of frequency response.

Further:

Of the 15 speakers with an SD of 1dB or less, 14 were added to the list by Stereophile reviewers.

But take a look at two speakers Stereophile raves about in my previous post. FAR from neutral as defined above. Then take a look at the hatchet job they did to the Crystal Cable Minissimo Diamond here.

So, bunk. I personally don't care what John Atkinson likes. If he likes the B&W diamonds above all others that's fine with me. But to call them neutral, or try to sell them as the reference against which other speakers are too dull or bright is shilling.

Crossover Basics - Driver Response

The Decibel or dB

Decibels (dBs) are a curious way to measure electrical and acoustic energy. Curious, and terribly convenient! For us, we use relative electrical dBs to discuss how filters work, and absolute acoustic dBSPL to measure speaker output.

When discussing the effects of a filter on a signal, we'll use relative dBs. That is, there's no set standard, but we talk about something being +4dB or -18 dB. This is useful because we can map this to speaker outputs no matter the volume settings. It is how we will discuss how a filter works, without worrying about the absolute output levels.

On the other hand, when we discuss the acoustical outputs we'll use dBSPLs which are in absolute terms, but using a set input level. Don't worry too much if this is confusing, we'll make it more clear as we go along.

The LM-1 Crossover Revisited

As mentioned in my first installment on Crossover Basics, the effects of a crossover filter are additive to the speaker driver.

We are going to use the LM-1 crossover and focus on the tweeter response in detail. Let's refresh your memory about the crossover, here it is on the left.


We'll focus on the tweeter filter section. This includes C1, L1, R1, and R2. The woofer section will seem neglected by comparison, but we cover it in more detail in other blog posts, including the Zobel.

Let's go over the transfer function. That is, how the voltage at the tweeter is different from the amplifier output because of the crossover. 0 dB means there was no change, the input and output are the same. The woofer response (in red, below) is almost exactly 0 dB until around 700 Hz when the low-pass filter kicks in. The tweeter on the other hand is more complicated. Let's discuss.

Anytime you see a chart this clean, you can be sure you are NOT looking at acoustical measurements. The blue line is tne tweeter filter's response. Except for the level shifting, this seems like something straight from my previous post on Crossover Basics. First, notice the level of the tweeter. It has been "padded" or "lowered" 6 dB below input. This is accomplished by the 4.2 Ohm R1. R1 is effective at all frequencies. Everything gets shifted down about 6 dB because of it. It's not exactly always constant, but let's pretend it is for right now, which is very close to true.

In addition to the padding there is a high pass filter reducing the midrange and bass at about 8 dB/octave below 2kHz. We discuss pads by an absolute number, like "6 dB" because it's effect is constant at all frequencies but we talk about high and low-pass filters with rates. In this case, 8 dB/octave means every time you cut the frequency in half, you will loose 8 dB. This is the actual "high-pass" section at work. This is C1,L1,R2. Notice that after about 4 kHz the high pass filter effectively stops working. It's as if it wasn't there anymore.  Above this level the only parts still involved in the high frequency response are the tweeter and R1. 

Putting it All Together

The point of this post is that these changes are not in isolation, but rather in combination with the driver so let's take a look at how the padding resistor and thigh high pass filter combine withe the acoustical response of the driver to produce the final outcome.

Notice the scale is now different. We are now looking at dBSPL, or sound pressure dBs. It is most common to take the frequency response measurements of a driver at 2.83 Volts input with the microphone at 1 meter distance. As you can see, below, this particular tweeter outputs about 90 dB at 2.83 volts above 4kHz or so.  2.83V is a common reference standard because at 8 Ohms this is about 1 Watt.

The top black line represents the tweeter with no filter at all. The green line represents the tweeter with just R1 added. It's not exactly 6 dB down everywhere due to the tweeter's impedance curve, but it's close enough for us! You'll learn more about this in the next post which covers the Zobel. The red line represents the addition of the high pass filter section, C1, L1 and R2. You can see it pivots around 3 kHz.

By carefully selecting the filter knee (-6dB point) and it's Q, or steepness we can get a little bit of EQ thrown in for free. Take a look at the original response (black) at around 2 kHz. You see the broad bump centered there? The bump is pretty much gone thanks to the high pass filter. We have not only added the high pass filtering, but we also tamed a little over-activeness int he tweeter without increasing the part count.

Padding

In the chart below you can see the final LM-1 design in red, vs. the a redesign without R1:



It may not be obvious from this, especially since this author likes to use far-field as his reference, but the LM-1 without padding would shriek.

In designing a crossover, I find it easiest to start low and work my way up. The low pass filter will reduce the sensitivity of the woofer at the crossover point. After this, we must adjust the tweeter to match and then add the high pass filter.

The total amount of padding (dB loss) depends on a number of things, including:

• Innate woofer efficiency
• Woofer low pass filter and baffle step compensation
• Innate tweeter efficiency
• Tweeter high pass filter

Unfortunately there is no simple, accurate way to go from a manufacturer's sensitivity specs to appropriate filter design.

The crossover designer must balance all four of these issues at the same time which is why in-cabinet measurement and simulation are so important. I encourage you to grab the LM-1 simulation files and attempt this for yourself.

Also note, that doing the reverse, padding the woofer, is generally discouraged because the power dissipation needed to lower a woofer a few dB is pretty large and requires big resistors and will waste a larger amount of amplifier energy. If your tweeter is too insensitive you probably need to change tweeter or woofer. It is pretty rare to find any design that does not require any tweeter padding.

Summary

With this posting, you now have learned:

• How crossover filter's add to driver output to create the combined effect of both. 
• How you can use leverage a high pass filter to also work as an EQ for you. 
• Why tweeters usually have resistors to pad them down. 

In my next post, Crossover Basics - The Zobel,  we'll go over the LM-1 woofer response but spend particular attention on the often misused or misunderstood circuit, the Zobel.

Cheers! 

LM-1 2.1 Subwoofer System

Work is under construction!

I've come across a couple of incredible deals over at Parts Express I hope to have the time and money to take advantage of. As you may know, I've been listening to the LM-1 as desktop speakers. They do a fantastic job with just 20 watts, but.... they really have no bass. It shows up more on games than movies.

The LM-1's have amazing bass in a bookshelf with very good rear wall reinforcement though! 

Parts Express is selling a powerful but small 10" subwoofer/cabinet bundle for under $200, plus, they have a 2.1 channel plate amplifier for another $100. The combination makes the foundation for a perfect multimedia system that would still be small, and couple very well with the LM-1 kit.

I've been using the LM-1 with a full-range 20 Watt digital amplifier and they really sound great. I expect the 2.1 amplifier to sound even better. The plate amplifier adds a high-pass to the amp. So the speakers will still have 20 Watts, but dedicated to 80 Hz on up. A separate 50 Watt amp drives the subwoofer itself. See where this is going? Of course, everything depends, but you could end up with the same volume and power of a s150 Watt/channel system. Bi-amplified systems are more power efficient than single amp systems, generally speaking and with music so this idea makes mathematical sense.

The combination is the perfect size for a desk or dorm-room. I just hope the cash magically appears so I can build it before the sale ends.

The Dayton iMM-6 Calibrated Microphone

When designing and building speakers I use OmniMic. It's perfect for that but when it comes to basic Blu-ray or Home Theater setup I rely on a much less expensive and much more convenient device, the $30 Dayton iMM-6 calibrated microphone. 

Dayton provides calibration files via their website which make this almost a laboratory piece of equipment. It works in your cellphone or tablet. I personally use it on an Android along with Audio Tool which will read the calibration file and adjust the levels accordingly.

One of the main benefits of this device is how deep and flat it goes and is therefore more accurate than the old SPL meter method. It's much easier to set your speaker levels, including your subwoofer using it and your tablet than any other way I know.  It also has outputs for a mini-jack so you can use test signals from your iPhone/Android device simultaneously. You can use a cable like this one to connect it to your stereo inputs.

If you are a DIY hobbyist and want to build your own speakers or want to do detailed acoustical analysis of your listening environment this is also the perfect front end for Room EQ Wizard but you will need an adapter cable from it to your PC or laptop.

The only real downside of this is that it's tiny and I'm constantly misplacing it after I use it.

Fix Ground Loops Quickly, Safely and Easily

What is a Ground Loop?

It is video noise or audible hum that plays through your speakers that occurs when multiple ground points with slightly different potentials are connected together through your equipment. More on this is available from Jensen Transformer's web site. It's called a loop because it's actually that, a closed circuit.  All we have to do is either connect everything to the same ground, or break the loop in the safest and most convenient location.

A clue to a ground loop is that the hum stays constant whether you play music or video or not. Sometimes equipment such as a TV doesn't even have to be on to help cause in the loop.

Note that this is not mechanical hum. If you hear the chassis or transformers vibrating and making more of a buzzing sound your problem is more likely DC on the line. I'll cover that in another post, but this is often caused by PCs, light dimmers, and modern compact florescent bulbs.

Ground Loops and Digital Signals

The usual "scientific" belief is that digital signals are immune or at least very resistant to noise. While it is true that digital circuits are resistant to many types of noise these circuits, including digital video, can participate in a ground loop which can cause enough jitter to be audible or visible. This includes HDMI, coaxial and USB circuits. Optical digital connections however are completely immune to all external noise, including ground loops.

Ground loops will not occur in Ethernet cabling unless there was a fault in the switches / routers. In other words, almost never since preventing ground loops was part of the design of the entire Ethernet eco-system.

Diagnosing the Cause

There are a few common culprits:

• Cable TV or Satellite Dishes
• External antennas like FM or television
• Audio or video cables from a Personal Computer
• Laptops! (Problem goes away when you disconnect USB or charger) See USB fixes, below.

The best way to find the root cause is to disconnect each suspect and listen for the problem to go away. Sometimes the problem is related to two devices interacting, which gives you a choice of where to break the loop. This process also works for finding noise sources in general. Turning lights off and disconnecting wall-wart supplies may solve other symptoms.

Lethal Fixes and Myths

One type of fix can be lethal to you and your neighbors. That's a "cheater plug like this one. Any attempt to defeat the ground pins in equipment that has them may be lethal. Do not do it. Do not rely on signal grounds to work the same way. They don't.

Pangea originally sold "high end" IEC cables with removable ground pins. Don't buy them, don't let your friends buy them. They appear to be discontinued, probably due to safety concerns. There is now a 2 conductor C7 version with a removable pin, but that's perfectly safe, and the feature is kind of useless.

I recently heard this argument:

I've been removing ground pins for years and never had a problem.

The problem is these pins are like safety belts. So imagine me telling you this:

I've not had a car accident in 20 years, so I no longer wear a seat belt.

That should sound dangerous to anyone who drives a car. That's how we electrically minded people think when we hear of audiophiles removing ground pins for that last bit of audio nirvana. There are better and safer ways. Another myth, spread by audiophiles who do not understand the safety ground or the life safety issues involved in UL certification and the National Electric Code:

Your system will be grounded by your RCA cables. No problem!

If no problem means dead and on fire, they're right. 

Free Fixes

If your problem is caused by a piece of audio/video gear, try connecting it all to the same power strip or conditioner. This ensures all the ground wires are at the same potential.

Another free fix may be to use XLR cables. XLR cables are not usually quieter in homes than balanced, BUT! there is a difference. XLR cables don't mix the ground and signal together. You avoid this contamination and XLR inputs often have a safe "Ground Lift" switch. It prevents the grounds loop from occurring at all.

Not A Ground Loop

Some issues have nothing to do with ground loops but are caused by induced noise from other sources. This noise can come through the power lines OR be induced by proximity to interconnects and electronics. Things to try turning off, disconnecting, or removing from the environment:

• Compact Flourescent bulbs - VERY noisy! 
• Wall warts - These tricky bastards stay on, and polluting even if the device they are feeding is off.
• Wall dimmer switches
• PC and laptop power supplies (yes, again!). Disconnect your PC or laptop cables to your stereo, TV, etc. If the problem comes and goes with the PC/laptop being plugged in, then you have a noise problem and will need to relocate it.
• WiFi devices, including routers, streamers, receivers, modems, etc. If your Wifi device is part of your stereo, try moving the antenna or putting it on a different power strip/conditioner. 

 

Noiseless Cables

Sometimes the problem is noise our cables pick up. Especially problematic in apartments with a heavy concentration of WiFI signals or near transmission or cellular towers. Some electronics will help pick this up more than others.

Make sure your interconnects are 100% shielded. Most cheap and a lot of expensive RCA cables use a braided ground, which is more of a pick-up antenna than anything else. Regardless of whether you use RCA or XLR cables, the best use 2 conductors plus a foil shield. In essence they are built of conductors:

• Positive conductor
• Negative conductor
• A super thin and delicate foil shield
• The drain wire which is used to attach the foil to a ground conductor on the RCA or XLR jack

On RCA cables the drain wire should be attached to the negative conductor at the source. The destination end does not use it but instead uses the positive and negative wires. With an XLR cable all 3 wires are attached at both ends, unless the destination does not have a ground lift pin in which case the ground may go unattached at the destination.

My favorite brands for non-esoteric cables:

• DH Labs
• Connex
• Mogami
• Belden

I personally use Connex solid silver cables for everything, but they are delicate. Get the more expensive DH Labs varieties if you need rugged.

Cable TV & Antenna Problems

This problem can also cause Internet access issues. I use a dedicated Cable TV isolator like this one. You put it immediately inside the RF plug, unless you have a satellite dish.  More on that, below.

Satellite Dishes

Make sure the satellite dish cable is grounded before entering your home. Even though it is required by code, installers often fail to do this, and ends up in having your receiver or antenna getting fried by heavy wind causing static as it blows across the dish.

Unlike Cable TV and overhead local antennas, satellite dishes require DC power to operate the RF amps built into the little head. For this reason isolating them is a little trickier. Normal isolators block DC in all forms.

The trick is to buy a separate DC power supply for your antenna. Place the ground loop eliminator closest to your receiver, and your antenna power supply closer to the antenna.

HDMI

Your best / cheapest way to eliminate issues from your television over HDMI are to fix any connections going to it such as cable tv, satellite, a PC, etc.

USB/DAC

Early in the history of external DACs ground loops could occur through coaxial cables. Most good DAC's today provide what is called "galvanic isolation" meaning that there is no DC or ground loop path between the input plugs and the rest of the circuits in a DAC. This can be done by purpose built transformers or modern monolythic IC's. Unfortunately no magazine or agency I know of tests for this so there is no way to 100% guarantee a DAC's isolation. Of course, the way to test this is to disconnect your USB input and see if your hum goes away. There are also cases where you have very little ground loop noise. To fix either use a USB isolator like this inexpensive model designed for medical professionals, but works just as well with USB 2.0 DACs.

Another fix is to use a purely optical cable between your source and DAC such as the Audioquest Forest or Monoprice both of which come in a variety of lengths.  Check the size of the plugs, some may interfere with other sockets on your equipment. Optical cables are also a very good choice for going from PC to a DAC or receiver if the PC supports it since PC's are such noisy environments.

Of course, since the entire point is to avoid a current path, gold plated optical cables are kind of silly.

Audio Signal Isolators

Audiophiles hate putting anything in the signal path, and some fussy recording engineers may also, but pro's also know that it's far better to put in a transformer than delaying a show, so here are a couple of isolation products from Ebtech that are reasonably inexpensive.

I'm currently working on Cable Mittens, a new concept to warm up the sound of an amplifier while breaking ground loops and reducing noise.  Until then, the choices below are the best available!

The Hum Eliminator takes 1/4" jacks, but adapters are easily found. For a little more you can get the XLR version shown here.

The EbTech models, especially at their prices, are very good, but audiophiles who only want the very best turn to Jensen Transformers for the gold standard in high quality audio isolation. If that's what you need, I present the RCA Jensen Iso-Max for your approval. It's usually the best solution for PC audio problems.

The XLR Iso-Max, below, is also available for around $250.

Last Ditch Efforts

If your problem is your electronics and the single power strip idea doesn't work, the only remaining almost safe way to prevent the problem I know of is the Ebtech Hum X. The Hum X is only rated for 6A which limits it to line  level electronics. You cannot use it on power amps, which is not really a problem because we can prevent the ground loop at either end. You will find it just as effective by putting this on a preamp, TV or source as on the amps. It should also be effective on PCs, but I'm not sure if it could cause other problems.

I say it's "almost safe" because it has not yet been UL approved. The 6A rating is probably why, as there's no way to guarantee users will only plug-in 6A devices.

Iffy Solutions

If you are an audiophile you might have gotten to the end of this article wondering why the real "power conditioners" weren't mentioned. The truth is that the solutions provided above are the most effective in solving ground-loops than almost any high-end power conditioner.   Balanced power conditioners, which are often touted as the best solution for this, may or may not fix actually fix a ground loop problem.  Bill Whitlock and Jamie Fox of Jensen wrote a great paper for the Audio Engineering Society on the matter. Balanced conditioners ARE completely effective at removing DC from an AC line however, and very effective at reducing other types of incoming AC line noise.

Other types of power conditioners will have no effect at all on ground loops but may reduce other types of noise or provide surge protection and, as mentioned, connecting all your electronics to a single strip or conditioner may also eliminate the problem but don't go spending big bucks on them trying to fix ground loop issues, or ignore the solutions above because they don't seem high-tech or expensive enough.

The LM-1 vs. Home Theater

As mentioned The LM-1 is most at home on a bookcase, with the back panel wall within 6" to 12" of the rear wall. For the all-analog audiophile that wants a purist system this is perfect.

However, if you are more progressive, and have access to room-correction then you can use the LM-1 pretty much anywhere.  Of course, this usually means a home theater.

If you are going to use them with a sub you may wish to plug the ports though, as that will give you the most dynamic range and make for the easiest crossover matching.

Good listening!