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How to Set Up a Home Studio (Part 3)

Soldering Techniques & Wiring Looms

Part 3: If you are a home studio builder, then sooner or later you are going to have to solder. And unless you like tangled cables, you will need to be able to make up your own wiring looms. David Mellor offers a man-in-the-street’s guide to simple and reliable wiring techniques.


There's no getting away from it. If you are home studio builder, then sooner or later you are going to have to solder. And unless you like tangled cables, you will need to be able to make up your own wiring looms. David Mellor offers a man-in-the-street's guide to simple and reliable wiring techniques.

To be an expert solderer you need four basic ingredients: the right tools, a quick lesson on how to do it, a bit of practice, and enough patience to make a good job of it every time. Perhaps you are pretty handy with a soldering iron already, or perhaps you have tried once or twice and have given up in frustration. Either way, I hope in a couple of pages to put you on the right track towards being able to handle any soldering situation you are likely to encounter in the home studio. What's more, I'm not talking about halfway decent amateur solder jobs - I mean solder joints that will never let you down, up to the limits of the cables and connectors you are joining. Does that sound like a tall order? Well, given the ingredients I have mentioned, with an extra large helping of patience, that is the level of skill anyone can achieve. Let's go to work...

WHY SOLDER?



It never does any harm to go right back to basics. What is the point behind soldering? Why is it an appropriate method of making electrical connections?

Touch two bare wires together. You have just made an electrical connection, but is it an adequate connection for audio purposes? Electric current will flow quite happily across a touching connection between two metal surfaces. It will not flow like this for long, however. Most metals do not particularly like to exist in their pure form, they have an intrinsic urge to combine with oxygen in the air. Iron, for instance, combines with oxygen to form rust. Oxides of metals are not such good conductors of electricity, so as soon as air gets into a touching connection, conduction will be impaired. Sometimes this can happen in such a way that the joint becomes a crude radio receiver - not a desirable situation in the studio!

What I described above is known as a dry connection. The opposite - a wet connection - happens when a bigger current, large enough actually to weld microscopic areas of the metal surfaces together, flows through the joint. This has more resistance against oxidation, but the currents found in audio circuitry are not really sufficiently large to 'wet' a joint to any useful extent. We need something to hold the two conducting wires together, allow current to flow, and keep the air out - solder.

Figure 1. Multicore solder.

The type of solder used for electrical connections is an alloy of two metals, tin and lead. As you probably know, the solder is melted and made to flow around the joint. Pretty basic technology, but effective. For our particular needs, the correct type of solder is called Multicore (I believe 'Multicore' is a trade name). There are several varieties available but the most appropriate is 60/40 tin/lead, 18 SWG (Standard Wire Gauge). Figure 1 shows a reel of solder with an enlarged view of the cross section.

Left to its own devices, plain tin/lead alloy is not good for joining metal. It melts easily enough (at about 185 degrees Celsius for a 60/40 alloy) but the resulting liquid does not easily wet the metal surfaces. 'Wet', in this instance, is like water is wet - not as in wet electrical joints. Molten solder will stand in a spherical blob on top of the metal it is meant to adhere to unless it is mixed with a substance known as 'flux'. The flux helps the wetting process and makes the solder flow easily. In Multicore solder, the flux is in five cores inside the solder wire. In the olden days, flux and solder had to be applied separately. Thank goodness, and technology, we don't have to do that now.

There are other types of solder which are not so good for our purposes. 22 SWG Multicore, for example, is great for soldering printed circuit boards, but it is so fine you end up using yards of it for each XLR connector. Another sort, known as 'Savbit' (trade name), extends the life of the soldering iron tip, but dries with a dull finish, making it more difficult to tell whether or not you have made a good joint. Other solders may have different melting points. Some are suitable for metals such as aluminium, which are difficult to solder, but which are not used in audio circuitry.

CHOOSE YOUR TOOLS CAREFULLY



You may have guessed that the first requirement is for a soldering iron. But what type? If you just want to solder the odd connector every now and then, a 25 watt iron with a tip around 3mm across is suitable, but for heavy duty work a temperature-controlled 50 watt iron is a must. There are some tiny irons on the market which are not really suitable for soldering audio connectors. They get up to the right temperature, but the tip cools rapidly in use, slowing down the work rate. Sometimes with a small iron, the heat is dissipated in the joint faster than the iron can supply it, resulting in solder that never melts. This generally causes an unwanted rise in the temperature of the operator, through frustration! With a good iron, soldering is easy. It may cost as much as £20 to £30, but the job is made so much more straightforward that it can be considered money well spent.

Essential accessories to the iron are a firm stand and a sponge. The stand will probably have a holder for the sponge, which is a special type - not like you would use in the bath! - used for cleaning the tip of the iron.

Soldering is one thing, desoldering is quite another, and there is a special tool for the purpose - known as a desoldering tool or solder sucker. This hand-held gadget has a spring plunger which, when operated, sucks molten solder from the joint. There always comes a point where solder has to be removed (old solder will have had most of its flux burned away and will therefore not flow properly). Having the correct tool is vital. Trying to shift old solder with just an iron is an unrewarding exercise.

Also, if you are not blessed with three hands, you will need a vice. A small one will do, or an acceptable alternative is a block of wood with holes drilled to fit the connectors you use. Glue on a male XLR insert so that the block will hold female XLRs, too. For good soldering, the workpiece has to be held firmly. Some people 'get along' without one, but why make life more difficult?

To recap, here is a list of the tools which are essential for good results:

• 50 watt temperature-controlled soldering iron with medium (around 3mm) bit
• Soldering iron stand
• Soldering iron sponge
• Vice
• Desoldering tool
• 60/40 tin/lead 18 SWG Multicore solder

Possible compromises are a smaller iron (but not too small) and a block of wood, as described, instead of the vice. Do without a proper stand and you'll risk burning something.

FIRST STEPS...



Once you have put a mains plug on your new soldering iron, the first step is to 'tin the bit'. The bit is the removable end-piece of the iron, made of copper or iron-coated copper. Tinning the bit simply means applying a coating of solder when the bit is first heated up. I find that the best way is to wrap a coil of solder around the end of the bit while it is cold, switch on and apply more solder as necessary. This protects the bit against oxidation and premature wear.

For your first soldering job, let's try something easy - an XLR connector.

Figure 2. XLR solder buckets.

Figure 2 shows the insert of a male XLR. Have a look at one of your own, and you'll see that the ends of the pins where the wire is connected are hollowed out. These hollows are known as 'buckets' or 'solder buckets'. Follow these steps for a perfect joint:

1. Place the connector shell on a length of mic cable.

2. Remove 20mm of the outer insulation of the cable.

3. Remove 5mm of the insulation on each of the conductor wires (obviously, the screen conductor is already bare).

4. Clamp the cable lightly in the vice. Twist the end of each conductor.

5. Touch the soldering iron to the bared end of one conductor. At the same time touch the solder to it. The solder will run into the strands of the conductor. Remove iron and solder simultaneously. This procedure is known as 'tinning'.

6. Repeat steps 4 and 5 for the other two conductors. For the screen conductor, tin half the length of the bared wire.

7. Place the connector insert in the vice.

8. Touch the soldering iron to one of the buckets, hold for around 10 seconds.

9. Keeping the iron on the bucket, melt solder into the bucket until it is nearly full. (Some cheaper XLRs don't have a proper bucket. Make sure there is plenty of solder at the point where the conductor will be attached).

10. Repeat steps 8 and 9 for the other two buckets.

11. Touch the soldering iron to the bucket of Pin 1 (pin numbers are marked on the plastic insert). Hold until the solder melts.

12. Insert the screen conductor into the bucket. Remove the iron and hold still until the solder solidifies.

13. Repeat steps 11 and 12 for the other two conductors (if the conductors are coloured red and black, connect the red one to Pin 2, the black to Pin 3).

14. Assemble the XLR connector.

It looks a lot more complicated when it is written down than it is to perform, but if you follow this procedure precisely, then success and a lasting connection will be the result. But what I have described above is the situation when all is going well. At several stages, you need to inspect the cable and connector to make sure that things are as they should be. Here are the possible problems:

- When the outer insulation is removed, the inner insulation may be damaged. If the inner insulation is cut even slightly or if any of the strands of the screen conductor are cut, start again.

- When tinning the ends, the insulation may melt. If this happens at any stage of the procedure, start again.

- When connecting the conductor to the bucket, the solder may not flow freely around the conductor. Unless you see the solder flowing properly at this stage, the joint will not be good. So it must be done again.

- If the conductor is moved while the solder is solidifying, the joint will not be good. Often, re applying the iron and melting the solder already in the bucket will be OK.

The picture is probably becoming clear. You have to see the solder flow like water around the conductor. If you don't, the joint will be bad and will eventually fail. Also, the joint has to be kept rock solid while it cools, or a 'dry joint' will result. A dry joint is one which lets air in, carrying corrosive oxygen. You can often spot a dry joint by its dull finish, but the only sure way to know that a joint is good is to see that the solder flows and solidifies properly as you are making it.

And now for a few 'do nots':
- Don't use the soldering iron to carry blobs of solder to the joint. This technique had its value before Multicore solder was invented, but if you do it now, the flux will burn away before it has time to do its work, ie. making the solder flow properly so that it wets the metal surfaces.

- Don't dab the soldering iron at the joint to smooth away any irregularities in the surface of the solder. These irregularities are caused by not heating the joint enough to allow the solder to flow properly. Either melt the solder completely or not at all.

- Don't try and turn a bad joint into a good one. It can't be done. Use the solder sucker and start again instead.

Musiflex cable soldered to a male XLR connector.


Now is the time to look at the photo showing what it should look like. This is one of mine, and you can see it is fairly tidy. It wasn't achieved with any great amount of skill or dexterity, just practice and patience. And in case you are wondering, I did have to remake one of the joints because it didn't turn out right first time. I removed the old solder, refilled the bucket, retinned the conductor, and did it again. A professional wireman would probably regard this example as a bit on the scruffy side. Wiremen can solder connectors and make them look like jewellery. We ordinary mortals can't expect that level of expertise, but we can make joints that will fulfill their function and, what is most important, will not fail in normal use.

MAKING A LOOM



As I explained in last month's article, a 'loom' is simply a handy method of making connections in bulk. Between mixer and multitrack tape recorder, for example. But first, the materials required:

• Connectors
• Lapped screen cable
• Expanding braid sleeve
• Heatshrink sleeve
• Masking tape
• Cable numbers
• Heat gun

The type of connector you use is dictated by the equipment you intend to hook up. Choice of cable is up to you (see last month), but it needs to be fairly thin and fairly flexible. Lapped screen is ideal. The first step is to cut it into lengths and make a bundle of all the ends so that you can measure the approximate diameter of the loom. You'll need to know this to know what diameter sleeving to buy.

The expanding braid sleeve is what's going to hold all the cables together throughout the length of the loom. You can see it, and the other items, in the photo. Several diameters are available, but since the material expands there will be one to fit your loom exactly. Let's assume that your bundle of cables measured 25mm in diameter. Looking in the Electromail catalogue, I see one that expands from 12mm to 30mm. That sounds ideal.

The heatshrink sleeve is to hold the expanding sleeve firmly in position at either end. This stuff shrinks to about half its original diameter when heated, so the 38mm size (chosen from the catalogue) seems most suitable to give a good grip. Here are the steps to make a wiring loom:

1. Cut the cable into lengths.

2. Solder connectors to one end of each length.

3. Number both ends of each length.

4. Gather together the cables at the connector end into a neat bundle. Secure the bundle with masking tape.

5. Arrange the cables into a neat bundle, along a length of about 30cm from the last strip of masking tape, and secure with more tape (the tape is used to hold the loom together until you can get the sleeve on).

6. Repeat step 5 until the entire length is neatly taped up.

7. At the unconnected end, wrap masking tape in a spiral until you have made a point. Don't worry about covering the numbers, the tape will be removed later.

8. Pass the cables through the expanding sleeve until it reaches the connector end (leaving enough length of cable uncovered so that the connectors can be plugged into the equipment). It's a bit like trying to put a shed skin back on a snake, but you'll get the hang of it.

9. Pass a 10cm length of heatshrink sleeve over the expanding braid, towards the connector end, until its midpoint is over the end of the braid.

10. Blow hot air over the heatshrink sleeve until it grips firmly.

11. Tighten the braid once more and apply heatshrink sleeve to the other end. Leave enough cable uncovered to suit the connector arrangement on your equipment.

12. Remove excess masking tape.

13. Solder on all connectors.

Items used in making a cable loom: lapped screen cable, expanding braid sleeve, heatshrink sleeve, heat gun.


Figure 3. A cable loom.


Once again, the basic concept is simple, the writing down of it makes it seem more complicated than it is. Figure 3 should make the desired result a little more obvious. There are some details which don't fit neatly into a step-wise order. Such as:

- If you have to cut the expanding braid, it will tend to unravel. The solution is to fuse the loose ends with a soldering iron before it has chance.

- Although the heat gun looks a bit like a hair dryer, it is a lot hotter. Therefore, don't plan on using a hair dryer because it won't work. Also, don't use the heat gun to dry your hair! Something else to be aware of is the fact that the heat gun is, unfortunately, hot enough to melt the expanding braid, so waft it around a little to avoid this.

So that is how to make a loom. Aside from soldering the connectors, it should take around half an hour. If you take a look at the photo, you will see what the result could look like. Notice the push-on numbers, which I make a rule of fixing so that I can hold the end of the cable in my left hand and read them left-to-right. Without a rule like this, 6's and 9's could be mixed up.

DON'T FORGET...



The first rule of good soldering and good wiring is never to be satisfied if it's not quite right. Take it apart and do it again. The time spent at this stage will be amply rewarded later on. Problems during sessions due to faulty soldering are entirely unnecessary. Problems due to poor connector design are another matter, but I'm sure I can squeeze in a few hints on how to minimise these later on in this series.

FURTHER INFORMATION

Electromail, (Contact Details).
Canford Audio, (Contact Details).
Kelsey Acoustics, (Contact Details).

ERRATA: PART 3

• In last month's article, Figure 1 does not show lapped screen cable clearly enough. The strands of copper wire in the screen are not woven together as in braided screen cable, but simply wrapped around the central conductors. Also, under the 'Other Cable Types' heading, the diagram for speaker cable shows an advanced speaker cable which packs a lot of copper into a given diameter. For home studio use, bog standard 13 amp mains cable is quite adequate at about a third of the cost.

• Apologies to Kelsey Acoustics, who were inadvertently omitted from last month's list of cable and accessory suppliers.

Kelsey Acoustics, (Contact Details).



This DIY loom (for a Fostex E16) is not made to professional wireman standards of neatness, but is a practical example of a loom that any sound engineer should be able to put together quickly and easily.

This particular equipment has to be moved around occasionally, which puts stress on the cable clamps of the connectors, which may work loose. Once this happens, the solder joints themselves are stressed and may fail. By adjusting the lengths of the individual cables, it can be arranged that the connectors share any stress more or less evenly, preventing one or two failing early. There is a better (but more elaborate) way of achieving this, which will be described in a later article in this series.


TIPS ON SOLDERING PHONO CONNECTORS

Phono connectors are to the sound engineer what pot holes are to the motorist - sometimes there is just no way around them. Still, since we have to use them we might as well try and minimise the difficulties involved - namely that they are hard to solder, and they are prone to failure.

It's always best to buy good quality phonos, the cheapest sort really are the pits. I use the type shown in the photo and diagram.

Figure 4. Phono connector.


Between the diagram and the photo you should get a good idea of how to solder them. The first step is to prepare the cable as in the photo, with a long tinned inner conductor. This conductor should be threaded through the connector until it appears through the hole in the tip, where it is soldered. Also, the outer insulation should come about one millimetre inside the cable clamp, which should be pinched together firmly with a pair of pliers before soldering the screen.

Lapped screen cable prepared for and soldered to a phono connector.


There is a distinct risk, with most phono designs, that the screen could come in contact with the inner conductor - either as it is being soldered, or later in use. To avoid this, bend the screen backwards and away from the centre of the connector before soldering it to the tag. If it comes closer than a millimetre to the central pin, it will create a short circuit sooner or later in service.


Series

Read the next part in this series:
How to Set Up a Home Studio (Part 4)



Previous Article in this issue

Paradise Re-Found

Next article in this issue

Yello


Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.

 

Sound On Sound - Feb 1989

Donated & scanned by: Mike Gorman

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Topic:

Home Studio


Series:

Setting Up A Home Studio

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9


Feature by David Mellor

Previous article in this issue:

> Paradise Re-Found

Next article in this issue:

> Yello


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