The best DIY resistor in the world. “Probably” ;)

A high quality DIY resistor for mission critical applications …

Do you need a high quality, low noise, low parasitic inductance DIY resistor for your mission critical audio applications ? When was the last time that you visited your local office supplies and stationary store ? And what has this to do with the best DIY resistor in the world ? (Probably) ?

You may have heard about some of those prestigious brands of resistors, that are so much adored by Hiend-Audio audiophiles. The ones for which they are willing to splash out like 15 USD a piece, or even more ( not to mention transport costs, customs, etc.) ?

I suggest that you look around in an office supplies or stationary store, and maybe purchase a set of those graphite inserts, those of a 2mm diameter, exactly the same as those used by children, as inserts for their mechanical, automatic PENCILS. To name just two possible suppliers of such stuff, you may be aware of such companies as the Czech KOH-I-NOOR, or the Faber-Castell brand (and many others), who produce them, in a whole possible range of available hardness, as follows:

9B, 8B, 7B, 6B, 5B, 4B, 3B, 2B, B, HB, H, 2H, 3H, 4H, 5H, 6H, 7H, 8H, 9H, 10H.

Get a pack of each of them. The total investment – about 5 USD (?)

A high quality DIY resistor for mission critical applications …

It just so happens, that graphite is a current conductor, albeit it has its resistance. The pencil inserts that we are talking about – are actually a mixture of fine graphite powder, some other ceramics material, also a fine powder, with probably some additional binding resin, all this combined together into the final product. Obviously, the more graphite within the mixture, the better it conducts.

In order to achieve a whole range of harnesses, the graphite rods are produced with a varying mix of the individual component materials that constitute them. The very “soft” graphite rods have a relatively high share of pure graphite. The “harder” rods have less graphite, but more of the ceramic material, and this results that they are “harder”. And it just so happens, that these “harder” rods have a higher resistance (less graphite within, less opportunity for the current to flow).
Now, back to DIY. According to the resistance that we actually need, we shall choose a graphite rod of appropriate hardness. The soft ones are suitable for resulting low resistance (like single ohms). The hardest ones – for resistances that even reach up to a values such as 280 ~ 300 ohms.

Take a small hobby vice and fix a drilling bit within it, one of a diameter of 2mm, clasping it “back to front”, i.e. so that the smooth cylindrical end protrudes to the “outside”. Take a copper, or silver coated copper, solid core wire, of a diameter of 0.8~0.9 mm and wrap it tightly around the drill bit. Make “a spiral” – with something like 5~6~7 turns. You may wish to use small hobby pliers for this.

Note: the “short” end to wire, the one where the spiral ends – do not press it totally down onto the surface area of the drill bit cylinder. Let it protrude slightly, so that it slightly “sticks out”, about a quarter of a millimeter above the surface of the cylindrical drill-bit. Between the individual turns of the spiral, please leave a small gap, somewhere around a quarter of a millimeter or so, between the successive turns / windings, so that there are small clearances, through which you can see the underlying surface of the drill bit cylinder.

Used upon the output of a Current DAC (AD1865)
A high quality DIY resistor for mission critical applications …

Take this spiral off the drill bit now. This spiral will constitute and ending of our resistor. Test how it fits the graphite rod. If it is loose, reinsert it on you 2mm drill bit, and then slightly tighten up the spiral, with the pliers, by applying some more “twist”, so as to reduce the internal diameter of the spiral.

You just made first screw-on “nut” ending, a resistor terminal, that shall constitute a connector for our resistor. Now please “screw” this nut on the graphite rod. preferably from the end which is blunt. The leading edge of our spiral, as we let it protrude slightly, will now constitute screw-on-leading edge, and will make the process of screwing the spiral onto the graphite rod slightly easier. It is just slightly raised, the short leading end, which we deliberately left protruding by about a 1/4 mm, so now it is elevated and makes life easier in terms of “screwing” the spiral onto the graphite rod.

This leading edge facilitates the “grabbing” of the cylindrical graphite surface of the rod and enables us to mount the connector on to the rod with a tight fit.

Winding the connector onto the rod should be fairly easy, an without too much trouble. We are winding the connector on the graphite rod, whilst holding the “long” end of the connector, so this action has a natural tendency to “loosen” the twist of the spirals. The “Screw-On” process involves a minimal amount of wear imposed upon the even surface of the graphite rod. We do this on purpose, so as to obtain a reasonably tight fit, a fit that will give us a good contact between the connector and the graphite rod – even prior to soldering. If the screw-on connector is excessively tight, so that it is difficult to screw it on – this obviously implies a risk of breaking the rod itself. If the “screw” is too tight – abandon the screw-on process, reinsert it on the 2mm drill bit, and loosen up the coil with the pliers a bit. Then try again. On the other hand, if you see that the winding is loose and moves freely along the graphite rod – this is also not good. Take it off. Reinsert it on the 2mm diameter drill bit. Use the pliers to tighten up the coil a bit. Try again.

After screwing the first connector onto the rod, we start warming up our soldering iron – the power of which should be at a minimum 100 watts. The Soldering must take place in a fairly high temperature, and the power of the soldering iron should be high enough so as to heat up the bulky spiral and the rod inside it. With regards the solder itself, I use a 50 / 50 mix of two types of solder. The first is an old style Tin-Lead (Sn + Pb) solder, of a composition 60% / 40%, and the second solder is a Lead-free solder, but with silver content, of about 4% silver. You of course you also need a rosin soldering flux, liquid or solid. It is best to moisten the entire end of the spiral.

Pre-heat the spiral along with the entire end of the graphite rod within, to a very high temperature (as in comparison to ordinary soldering temperature standards). Apply the tin solder to such heated-up spiral, taking special care that it PENETRATES into the GAPS between the windings, so that the tin it is “sucked” in underneath the surface of the windings. Lavishly apply rosin, flux or similar – this helps the tin to penetrate deep into all the cavities…

WARNING: Hold the Graphite Rod indirectly, via a protective cloth. Graphite is a good conductor of heat, so sooner than later the whole rod will exhibit a significant temperature and you can burn your fingers, if holding it without protection. Although you only heat up only one end of the rod at a time, the heat will tend to flow over the entire length of the rod. Allow the solder to penetrate directly into the gaps between the turns of the spirals. Apply the tin solder lavishly. Apply a lot of it. Just enough so as that id does not drop off from the “excess”. When the solder is still hot, highly liquid and floats around all over the spiral on the tip of the graphite rod, start to rotate the rod horizontally around its own axis, in such a way that the tin is uniformly spread around the circumference of the spiral. Whilst rotating, the whole thing gradually cools down, the tin solder gets dense, and in the process it evenly deposits around the whole circumference of the spiral.

OPTION: You may also try a trick called “shock cooling”, by suddenly bathing the rod in cold water. This should provide you with an even firmer grip of the coil to the rod, as the tin gets dense very quickly, the windings shrink and hence, the tin does not have a chance of “escaping” from within the coil. The shrinking winding imposes a murderous, strangling grip on the rod. There is no fear, not the slightest chance of the possibility even, that the electrical contact between the rod and the coil will be compromised. I would rather expect that the rod will be crushed by the murderous strangle as a much higher probability.

When it all cools down, prepare your Ohms meter, together with “alligator” clips so as to catch the one connector that is already mounted onto the resistor, as well as the other one, that we shall be dealing with now.

When the tin solder cools down, down to room temperature, the spiral of the first resistor connector shrinks and tightens upon the solder that penetrated into the insides of the windings. When the spiral was hot, it actually increased it’s diameter, due to thermal expansion of the copper, and allowed some tin to penetrate “inside”. When it cooled of, it tightened it’s “grip” on the graphite rod, but also applied enormous pressure on the dense tin, that got inside, but as it now is dense, cannot escape. The cooling down of the spiral is the paramount key do a successful, highly reliable connection between our connector and the graphite rod. The spiral shrunk and tightened with a “strangle-hold” on the tin and graphite. The contact is very reliable and of a very high quality, albeit it is only “kitchen-table-DIY”. I have not yet experienced a single problem pertaining to an uncertain contact between the connector that the graphite rod. And believe me, for my own purposes, I made quite a few of these resistors.

Before we start making the other connector, attach the crocodiles of your ohmmeter to both connectors. You shall be constantly monitoring the actual read-out DURING the process of screwing on of the second connector. What you shall now measure in terms of read-out, will tend to exhibit a slightly higher resistance than the one which you will obtain in the final product, after ultimately soldering on this second connector. This is probably due to a fairly poor contact area between connector’s spiral and the surface of the graphite rod, prior to soldering. But do not worry about that. The phenomenon is repeatable and predictable. There will not be too much of a problem of making close tolerance pairs of resistors, if need be. If we are producing, for example, a resistor of a target resistance of 25 ohms, I would “set” the screwed-on resistance to something like 26 ~ 27 Ohms.

Now, we solder this second connector, similarly as the first one. For the sake of curiosity, please measure the “HOT” resistance of your resistor. You shall observe that It will be different (higher) than that measured on the final “COLD” resistance. It is an interesting observation, as to how this positive temperature coefficient behaves in such a resistor. This is actually a very good and positive feature, which I shall explain later on.

Now, wait a few minutes, or bathe the thing in cool water, so that it **completely** cools down to room temperature (I stress: ROOM temperature, and not “almost” room temperature. If it is Luke-warm, there will also be a difference ! ).

Now, in such a cold state, measure with the ohms meter how much resistance you came up with. Is it OK? Is it exactly what you wanted ? If yes - great.

But if, on the other hand, the resulting resistance is not what you are willing to accept, simply heat up the spiral again, from the sharpened side of the graphite rod and now, in such a “hot, liquid solder” condition, screw or unscrew the spiral so as to correct the error, by a length that will be suitable to compensate the difference. Easy.

If you wish to produce a “matched pair” of resistors, .. simply do so. There is no problem with matching the resistors – due to the possibility of “correcting” the resistance by screwing or unscrewing the second resistor connector, whilst in a hot condition. When the “second” resistor of you matched pair is ready and cooled down, measure the resistances and compare the difference in results between the two resistors. If necessary, reheat the second connector of the second resistor to a state of liquid solder and then adjust it, in a hot state – accordingly, so as to correct the discrepancy. Apply the correction by means of rotational movements of the spiral (screwing / unscrewing) according to the needs and the magnitude of the required adjustment.

That’s it. The only other thing now that you may wish to worry about is to come up with an idea as to a "housing" for this fragile resistor. Possibly a tube section of an aluminum tube, or even better, of a non-metal material (to cut down parasitic capacitance!).

For me, within my DAC, there is a matched pair of 280 ohm resistors, serving the purpose of a current-to-voltage passive conversion, applied directly behind the DAC chip. I am sure that I will NOT be exchanging those resistors for anything else. For a long time. The only thing that I would probably now consider doing differently, is to make two “halves”: 140 + 140 ohms, so that both connector ends come out from the same side, and so that the whole “resistor” is …. shorter. That way, I could get rid of that extremely large loop (“antenna”) that is a side effect of applying a single long graphite rod, instead of two halves. Anyways, as they are now – they work just fine.

For speaker crossovers, you would need needed resistances in the range of 1~10ohms (for the tweeters and other cross over sections), therefore I would rather go after the softer graphite rods, 2mm in diameter, with a hardness grade in the range of 5B ~ HB, depending on your needs.

Please note: excessive current flowing through your tweeter .. may actually damage it. But then again – excessive current flowing through your resistor – might just heat it up a little bit, increasing it’s resistance, therefore this phenomenon serves the purpose of even “protecting” the tweeter, to a small extent. In the case of tweeter resistors - you may even want to produce them in "multi-tap" versions, so as to regulate the amount of high frequency damping, via a selector switch.

For solid state power amplifiers, where you need high quality power resistors for the emitter circuits of the power transistors (degenerated feedback), with resistance values of as low as 0.22 Ohms, I definitely would recommend the 6,5mm standard graphite rods, as used by children, those with a hardness grade of “HB”. As they are BIG an Bulky, with 6.5 mm of diameter, believe me, their high current handling capability is unprecedented. If need be, you can even equip them with RADIATORS (easy enough !)

A high quality DIY resistor for mission critical applications …

Recently I saw that there are also 2mm rods available with an even higher hardness, mainly a grade of 10H … these I do not posses and did not measure, because when I bought graphite rods for my personal purposes, the 10H were not available at that time. So if some of you face a 10H rod, and make an “end to end” resistor out of it, please let me know via Email, what is the maximum possible resistance that can be achieved on a 10H rod. With the 9H rods that I used, the maximum limit is about 280 ~ 290 ohms.

But why should you actually be doing all this ? Because they SOUND GOOD. Time and again confirmed by various audiophiles – be it in the context of their tweeters with their serial padding resistance, be it the voltage-to-current conversion on the output of current-driven DAC’s, be it transistor power stages with the emitter degenerated feedback for the paralleled power transistors.

How come ? WHY do they sound good ?

Think about the physical properties of these resistors:

· They are of an extremely low parasitic inductance.

· They are of an extremely low parasitic capacitance.

· They have a slightly positive temperature coefficient, which can be used for a good purpose.

· At the same time, depending on the type of rod used, they can be of a fairly high mass, meaning that they are very inert – temperature-wise. They will not heat up and cool down to the rhythm of your bass guitar or drums beat.

They also have a very big cross section area …. (So what ?) … Well, that implies that:

· They are inherently low noise resistors ( due to the fact that there is a very small density of current flowing through the cross section ).

· They have extremely high current handling capability.

Happy Soldering !

Best Regards,
Matej Isak. Mono and Stereo ultra high end audio magazine. All rights reserved. 2006-2013. ..:: None of the original text, pictures, that were taken by me, links or my original files can be re-printed or used in any way without prior permission! ::..