Audio - Balanced Interconnects

Installed

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n an earlier blog entry I described my bi-amping adventures. A key part of getting the whole setup put together was building balanced interconnects. I made two 10m runs of Belden 89207 to connect the pre-amp (at the back of the room) to the crossover (up front) and four ¹/₂m silver interconnects to link the crossover to the amps.

Since the Belden cables aren't very interesting, I'm going to focus this blog entry on the silver ones. The materials I used are:

• 24AWG dead soft .9999 silver wire
• Teflon insulation for 24AWG wire
• Three-pin XLR3 connectors
• ¹/₄" O.D. (outside diameter) sleeving
• WBT solder
• Heat shrink

As an aside, Rane provides all kinds of other great audio documentation in their RaneNotes.
(or open directory).

Most balanced cables use XLR connectors. The ones I made are no exception. When making any audio cable, Rane provides a great document on Sound System Interconnection^[PDF].

There are a lot of exhaustive explainations on the Internet about how balanced cabling works. But the Coles-Notes of it is: a balanced cable, or more correctly, a balanced system, works to eliminate any noise being introduced along the length of the cable.

XLR3 connector

To achieve this, it all starts at the source device. Three conductors are used:

	Pin 1	Ground
	Pin 2	Hot +	Carries the signal
	Pin 3	Cold -	Carries an exact copy of the signal, but inverted phase

Ground - Hot - Cold

Now consider if some kind of noise is introduced along the wire. For the most part, the noise will affect the Hot and Cold in the same way.

Once at its destination (the load), effectively the Cold can be re-inverted to the same phase as the Hot. Notice, by doing this, we’ve also inverted the noise that was introduced to the Cold signal. This inverted noise, coupled with the noise on the Hot will cancel, leaving you with the combined original signal. Clever, eh?

So technically, the "balanced" part of a balanced cable has more to do with the source and load devices than the actual cable. However there are a few features of the wire itself which helps reduce the chances of noise being introduced. The twisting of the two signal wires help reduce interference from electromagnetic induction. The ground, usually a shielding mesh which surrounds the twisted pair, provides a dedicated ground, not part of the signal like an unbalanced cable, and prevents possible ground loops.

When making such short interconnects (in my case from the crossover to the amps), the benefits gained with balanced cables is likely very little. Since there is more opportunity for external noise to be added to the signal in a longer length, my 10m interconnects from the pre-amp to the crossover are a more typical use of balanced cables. However, even with a very short cable there is still a chance of external noise, especially in an unfriendly environment. Also, regardless of length, the XLR connector provides a good, secure locking connection.

Teflon (polytetrafluoroethylene, a.k.a. PTFE) is often used as a wire insulator because it's such a good dielectric (it has a resistivity of 10²² to 10²⁴ Ω·m – that’s about 10,000,000,000 times that of glass! ...more about resistivity below). Its slippery nature and high heat resistance is great for making omelets, however, in this case, these properties aren’t of much interest.

I decided to simply braid the three wires. There are a lot of articles and theories about the optimal relative placement of the three conductors. I’ve read too many different and opposing opinions – so I decided to start simple with an ordinary braid and make my own judgment as to its performance.

Finished cables

For sleeving, I put the braided trio in a semi-transparent ¹/₄" sleeve - actually the sleeve is plastic tubing typically used as the waterline to the ice maker in your refrigerator. There are a ton of other cheap sleeving options including using shoelaces or no sleeving at all. Some folks will claim there are acoustic properties to different insulators and sleeves – maybe - but my considerations were primarily protection and secondarily aesthetics.

So why use silver wire instead of copper? The long and short of it is that silver conducts electricity better than copper. How much better? Well only an audiophile is unbalanced enough to justify the cost-benefit (yah, I know, bad joke).

Conductivity, actually on the flip-side it's called resistivity, is measured in ohm meters (Ω·m). The resistivity of silver (Ag) is 15.87nΩ·m (at 20°C). The resistivity of copper (Cu) is a poky 16.78nΩ·m (again, at 20°C).

So consider:

ρ = R

A ℓ

Where:
  ρ is the resistivity in Ω·m
  R is the resistance in Ω
  A is the area of a cross-section in m²
  ℓ is the length in m

A 24AWG wire has a cross-section area of 0.205mm² or 2.05x10^-7m². Divide this by a length of 0.5m. Now solve for R when ρ is 1.587x10^-8Ω·m.

1.587x10-8Ω·m = R

2.05x10-7m2 0.5m

R = 0.0387Ω

So for an equivalent copper wire, ρ is 1.678x10^-8Ω·m.

R = 0.0409Ω

...a difference of a whopping 0.0022Ω! Okay, so not a huge difference but it is a 5.4% improvement over copper. So you've got two options: use slightly heavier gauge copper, or spend way more, use silver, swear you can hear the difference and do the math to try to prove it.

Automotive - Electrolytic Rust Removal

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t has been estimated that the cost of corrosion is over one percent of the world's economy. Ninety percent of all mining of metallic ores is for the extraction of iron. As much as twenty-five percent of the annual steel production in the United States goes towards replacement of material that has corroded. And probably most interestingly, over thirty-seven percent of statistics on the Internet do not have a valid reference.

Every car guy's arch enemy is corrosion; some days I swear I can hear it. To clean up some corroded parts from my 1973 Super Beetle, I built an electrolytic rust remover. I've affectionately called this apparatus "The Rust Bucket" (or TRB-3000).

I can't take credit for its ingenuity - I think I first saw it on instructables.com, and since then multiple postings across the Internet. With this blog entry I just want to add some additional information and describe my design and lessons-learned.

For my build, the materials were:

• Five-gallon bucket (an old beer brew bucket in my case)
• 5-10 feet of 10 AWG copper wire
• Three aligator clips
• 8 feet of ¾" reinforcement bar (a.k.a. re-bar)
• Solder
• Heat shrink (red and black)

Tools:

• Drill (with ³/₁₆" bit)
• Pliers, wire stripper, knife, etc.
• Soldering iron

Consumables:

• Water
• Sodium carbonate (I use pH booster)
• Electricity (via a 12Volt automotive battery charger at about 5-10Amps)

TRB-3000

The construction details really depend on preference, intended use and the size of parts to de-rust. The general concept is to surround the cathode (the rusty item) with electrolyte (a conductive solution) and evenly-spaced anodes.

To understand how it works requires some chemistry. The corrosion of iron to iron oxides (rust) is a series of chemical reactions and depends on the amount/presence of oxygen (O₂), water (H₂O) and available electrons, producing different types of rust (different combinations of Fe and O). Typically the outer layer is known as red rust or hematite (Fe₂O₃). Under it is a purple-grey material (Fe₃O₄) known as black rust or magnetite. FeO (wüstite) is another type of rust, but typically only a concern if you are dealing with meteorites or native iron.

Chemical reaction (simplified)

By adding electrons, the "rust" reaction can be partly reversed. The black rust (which conducts electricity) can be converted back to metallic iron with a good bond to the original metal. The red rust is converted to black rust and becomes detached from the surface.

Another thing to consider is that most car parts aren't pure iron - rather, likely some kind of iron alloy, the most common being some type of steel. For the most part, this process will still work although there will be a few more elements (e.g. carbon, nickel) involved and potentially some not-so-nice gases produced. Be careful; don't inhale. Any alloy containing chromium (e.g. stainless steel) can pollute the solution into a toxic swill - so don't be tempted to use stainless for your anodes.

There is no magic in the sodium carbonate (Na₂CO₃) solution. It acts as the electrolyte, simply carrying the electrons from the cathode to the anode. During the reaction there is no reduction in Na₂CO₃. There is however some loss in H₂O due to ordinary evaporation but mostly due to the break up of the hydrogen and oxygen as it passes the electrons.

Some instructions I've read called for washing soda (a.k.a. soda crystals or soda ash) to be used in the solution. However when I tried it, it foamed up like a latte - I'm guessing because of an additive to make it a better soap (I would consider this hydrogen/oxygen foam a bit dangerous). Since then, I've been using pH booster (the stuff used in swimming pools). I use roughly 1 tbsp per gallon (about 3^grams/_litre).

Oxygen and hydrogen bubbles

Because the reaction produces hydrogen (and oxygen, and potentially some other gases), I've read a lot about people being fearful of this rig. Granted their visions of the Hindenburg are somewhat justified, but considering how slowly the hydrogen is produced and how quickly it dissipates (when using pH booster, not washing soda), you are not likely to set up an explosion if done outside. Having said that, sparks caused by the power source and smoking should be avoided. As well, when dealing with electricity and a conductive solution, always use your common sense.

Another potential negative side effect is hydrogen embrittlement where hydrogen gets diffused through the metal causing it to weaken. This should only be a concern if you are dealing with high-strength steels, which you may not want to try anyway because of the additional metals involved in the alloy.

The reaction is most effective with a line-of-sight from the cathode to the anode. Seeing how lazy electrons are, they always try to take the shortest path. Because of this, it helps to periodically reposition the cathode to get multiple angles of exposure to the anode(s).

As a bonus, this electrolytic process is effective at removing paint. I found it best to clean the part well, scratch the paint a bit with a wire brush (down to the metal) and after a few hours in the TRB-3000, the paint falls off like a skin.

The time required in the bucket differs. Some parts are more intricate and required more repositioning. But on average, anywhere from 2-12 hours does a good job. For safety reasons, I only run it when I can keep an eye on it.

Sorry I don't have any before and after pictures of the car parts. Just imagine a rusty part - and now imagine that same part not rusty...

Automotive - Beetle Seat Release Mechanism

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bout fifteen years ago I bought a 1973 Volkswagen Super Beetle with the intent of restoring it. I never really drove it since almost immediately I had it torn apart and sorted in many (many) bags/boxes and spread across two provinces. Now I am tasked with the challenge of reassembly.

When I purchased the car it didn't have any front seats, another reason why I never really drove it. Over the years I was able to find a pair of 1974 seats quite cheaply. Granted they need some restoration and reupholstering but I had all the parts I needed - or so I thought. Recently, when it came time to fitting the seats in the car, I realized that I was missing the mechanism that holds/releases the seat so it can be adjusted forward and backward. I searched my stack of various VW manuals as well as the Internet to try to identify what these pieces looked like or even what they where called - no luck.

Parts (one side)


VW 113 867 232


Installed

Finally by getting little bits of information from various sources and people, I managed to piece it together, so to speak. Turns out that the mechanism is more a part of the floor than the seat. Like most things on a vee-dub, the mechanism is elegantly simple, yet clever, and consists of only a few parts:

• lever (attached to the floor/tunnel)
• rod
• spring
• pin

Unfortunately I still don't know the official names and part numbers of these pieces.

There is also a cosmetic piece (113 867 231 driver side, 113 867 232 passenger side) that covers the lever box. This black plastic piece, one per side, snaps over the ugly metal cage that is attached to the tunnel and houses the lever.

So that's it. Hopefully this blog helps someone else who is staring at their Bug floor and scratching their head. This mechanism should be the same for all 1973-and-newer Super Beetles.

Oh, and when putting the spring on, be careful not to let it slip and hook your finger. It will likely hurt and bleed a lot. Just trust me on this.

Audio - Bi-amping

System hookup

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challenge I recently took on was to bi-amp my Magnepan IIBs. I managed to get hold of two beautiful Bryston 4BSST amps and a matching Bryston 10B-LR active crossover. The active crossover splits the line level frequencies and feeds the amps, one per side, and bypasses the Magnepan factory crossovers going direct to the high and low panels. I don’t have any issues with the factory passive crossover, I just wanted to be able to dedicate a channel to the highs and probably more importantly, have a dedicated channel for the lows.

Magnepan IIB back panel

The Magnepan MG-IIB model is just begging to be bi-amped. The friendly folks at Magnepan were thoughtful enough to build the IIB with removable jumpers making bi-amping a simple plug-and-press-PLAY►. To use the internal passive crossover, three jumpers are located across the posts as labeled (3-4, 5-6, and 7-8) and the single pair of speaker cables connect across the posts labeled "AMPLIFIER" (1 and 2). In order to bypass the internal crossover, all three jumpers are pulled, the low is connected to posts 3 and 5, and the high is connected to posts 4 and 8. The jumpers can then kept in a location you will soon forget.

Here is an excerpt from page 6 of the Magneplanar MG II B Instruction Manual:

CROSSOVER POINT--DUE TO THE FREQUENCY RESPONSE OF THE INDIVIDUAL DRIVER SECTIONS IN THE MG-IIB, IT IS NECESSARY TO SPREAD THE CROSSOVER POINTS. THE LOW PASS (BASS) CROSSOVER POINT IS SET (-3DB) ELECTRICALLY AT 400HZ. THE HIGH PASS (TWEETER) CROSSOVER POINT IS SET (-3DB) ELECTRICALLY AT 1600HZ. BOTH SLOPES ARE 6DB PER OCTAVE. THE ARRANGEMENT RESULTS IN A FLAT ACOUSTICAL RESPONSE. TO OBTAIN THE IDEAL ACOUSTICAL RESPONSE AS PROVIDED BY THE CROSSOVER BUILT INTO THE SPEAKER, YOUR CROSSOVER MUST BE SET AT:
     LOW PASS--400HZ, -3DB WITH 6DB PER OCTAVE SLOPE
     HIGH PASS--1600HZ, -3DB WITH 6DB PER OCTAVE SLOPE

Apparently in the days of Air Supply and acid-washed jeans, audio manuals were written in all-caps. Perhaps there is a joke in there about audiophiles being deaf and needing to yell - but I'm not going to touch it. Interestingly though, Magnepan chose to "spread" the low and high pass - I'm guessing this is to flatten the lobing error produced by the lower order Butterworth crossover and create what they refer to as a "flat acoustical response".

Magnepan IIB factory crossover

Remembering that the frequency doubles for every octave, there is a gap of two octaves between 400Hz and 1600Hz. The midway point is 800Hz and is reached at -9dB with the 6^dB/_oct. slope.

So to summarize, these are the factory specs:

Low-Pass (LP):	(400Hz, -3dB) at 6dB/oct.
High-Pass (HP):	(1600Hz, -3dB) at 6dB/oct.

This gives an effective crossover point of 800Hz at -9dB.

Without getting into a “what’s-the-optimal-crossover-point” discussion, for now I trust the brains at Magnepan and I will keep the same crossover frequency of 800Hz as designed. But since the 10B-LR’s Linkwitz-Riley filters are formed by cascading two second order filters, it has a much steeper slope of 24^dB/_oct.. The question then became, what should the new low-pass (LP') and high-pass (HP') be?

Using the same low-pass and high-pass as the factory specs would create a huge gap. I knew my new LP' and HP' would need to be much closer. I decided to work under the assumption that Magnepan's primary motivation for the "spread" was compensation for the slow roll-off of the Butterworth filters and avoidance of a lobing error. Fortunately with a Linkwitz-Riley filter I don't need to worry about a spread at all since it produces moderate roll-off and a flat sum, resulting in zero lobing error. Without a spread, both my LP' and HP' can be 800Hz at a slightly louder -6dB.

Proposed crossover

(Un)fortunately I learned all this because of some hindsight. Initially I thought I needed a ½-octave gap between the LP' and HP'. I was thinking the low-pass and high-pass frequencies where defined at 0dB, not the -6dB for Linkwitz-Riley filters. I didn't discover this mistake until I got it all together and heard an audible gap around the crossover point. I reviewed my logic, dug deeper, learned some more and found my error. (Note, the pictures below are for incorrect LP and HP.)

So the following are my desired specs:

Low-Pass (LP'):	(800Hz, -6dB) at 24dB/oct.
High-Pass (HP'):	(800Hz, -6dB) at 24dB/oct.

This gives an effective crossover point of 800Hz at -6dB.

The programming boards


Single channel Linkwitz-Riley board

As you might have noticed, there are no frequency selection knobs on the faceplate of the 10B-LR. Instead of presets, the low and high-pass are set using "programming boards" which plug into each channel’s pull-out circuit board. Each of these programming boards (a total of four per channel) is programmed using a pair of ±1% metal film fixed ¼W resistors. At a price, Bryston will build these little boards for your preferred frequency – but it’s really easy to reuse and modify existing boards with some simple de-soldering.

Doing some rounding and using the BRYSTON 10B-LR Crossover Additional Frequency/Resistor Tables^[PDF] document, the following resistors are used:

LP' at 806Hz			HP' at 806Hz
Ra:	6.98kΩ		R1:	29.4kΩ
Rb:	6.98kΩ		R2:	59.0kΩ

Note that there are multiple tables for different versions of the 10B-LR based on the capacitors used on the channel. In this case the low-pass is C₁ = 40nF and C₂ = 20nF and high-pass is C_a = C_b = 4.7nF.

Not only do Magnepans look unusual, these also run at a nonstandard 5Ω. Each channel of the 4BSST delivers a hearty 300W at 8Ω. So admittedly, it is a bit too much umph for these circa 1981 speakers, however I’ve found it quite safe up to just over half volume on my Bryston BP-25 DA.

The listening room

I’m very pleased with this active crossover configuration. The low end is exceptional – so much so, I now need to do a DIY on sound-deadening wall panels to make my listening room acoustically larger. The highs of course are also noticeably better. When the hungry Maggies demand more power to drive the bass panels, the channels dedicated to the highs remain unfazed. The Linkwitz-Riley filters are amazing - I can't believe it's not Butterworth... Sorry, uncalled for. (Actually I guess it is Butterworth, just two in series)

Besides all that, it looks pretty cool too.

Automotive – Porsche Turbo Badge Detailing

Removing the badge

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dmittedly this is the most lame DIY ever but I had to start by posting something. This morning I took about fifteen minutes to clean under the “turbo” badge script on my 1984 Porsche 930. Being such a focal point on the car, it was a bit of a dishonor with about twenty-five years of wax and crud built up.

Tools & product:

• 8mm socket or wrench
• Small brush (e.g. toothbrush)
• Non-harsh car wash
• Clean lint-free cloths
• Clay bar (or a liquid clay product)
• Wax remover
• Your favourate car wax or polish

The removal is simple – two 8mm speed nuts from the backside.

The crud underneath The paint on the car is original so I wanted to take some care in cleaning the surface. Having said that, the paint is no longer perfect (the previous owner touched up a chip above the “o”) so I wasn’t overly picky. I first soaked the area and got most of the dirt off with a bit of general car wash. Then I used some Turtle Wax ICE Liquid Clay to remove the hard stuff. After a rinse, the outline of the script was still a bit visible but not enough to be a concern.

The “t” was also bent a little askew – a careful tweak put it back in place. I noticed that the black painted aluminum had some corrosion on the back; a new OEM badge for this car (930-559-317-00-M260) is relatively cheap at only about $20-25. After scrubbing mine with an old toothbrush and wax remover, it looked almost new.

ResultsBefore putting it back on, this is a great time to put on a fresh coat of wax. Instead of wax I used some Turtle Wax ICE Liquid Polish. And no, I’m not being paid by Turtle Wax; I just like their products.

Admiring the results I know I will sleep better tonight. Porsche’s choice of black on black is subtle ...unlike the turbo itself.