Part Selection Guide

NOTE: I am no longer offering pre-made circuit boards for this project. The reasons for this are many, and intertwined. Please do not email me asking me to change my mind on this. I assume that if you are reading this page, you do not have one of these boards, as you should have selected and purchased your parts already. I am therefore leaving this page up only for those who want my advice on how to select parts for similar circuit designs. This advice is being offered for informational purposes only. The TREAD was, and remains, a DIY project, which means it is up to you, the builder, to apply this advice correctly.

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Some of the parts below are optional. For most of these parts, you simply leave them out if you don’t want them. If a jumper is required, it will be mentioned.

Resistors

5% carbon film resistors are the default choice. Resistor accuracy isn’t a big deal in this circuit since the output voltage is adjustable. You might nevertheless choose to use metal film resistors because of their lower inherent drift and noise.

All of the resistors should be ¼ W types, except for R2 which should be ½ W in most situations. When R1 is 120 Ω, R2 must be under 2.5 kΩ to have less than ¼ W dissipated through this path. R2 has to be even smaller if you use a 100 Ω R1, no more than 1.6 kΩ to avoid burning it up. Even when ¼ W is sufficient, you may want to use a ½ W unit anyway to keep heat in this resistor down, improving long-term accuracy. (Drift is a function of operating temperature.)

If you want to use the Vishay Dale RN/CMF series resistors, the RN55s are ¼ W and the RN60s ½ W at the temperatures you should see in this power supply. They’re specified for lower wattage because their temperature range is so high.

R1

This resistor sets the current used by the regulator to operate. There is always 1.25 V across this resistor, so you pick its value to put about 10 mA through this path. While some LM317 type regulators will operate with as little as 5 mA, they are spec’d to require 10 mA in the worst case situation. Better safe than sorry...

R2

This resistor sets the regulator’s output voltage in conjunction with VSET.

The output voltage formula is:

There is a table on the schematic with several useful R1 and R2 values, assuming that you use a 500 Ω VSET trimmer.

If you want to calculate your own values, then assuming R1 is 120 Ω:

This will put your desired voltage right in the middle of the adjustment range.

R2 is normally a ½ W resistor, for reasons stated above.

VSET

This trim pot allows you to adjust the output voltage.

There are four rough categories of parts you can use here.

The first is to just jumper the position, to turn the TREAD into a fixed-voltage power supply. This will save you a buck or two, the cost of a good trim pot. (Avoid cheap trim pots! I find that they break easily.)

The next option is to make it a small-value precision trimmer. Say, 100 Ω to 500 Ω. The smaller the value of the pot, the more precise you can be in dialing in a particular voltage, and the less likely vibration will cause the value to drift appreciably. If precision is important, consider putting a dab of nail polish on the trim shaft after it’s adjusted to keep it from moving.

The third option is what I include in the TREAD kits, a middling value trim pot, currently 1 kΩ. This, along with a middling value for R2 gives a usefully high base output voltage, with the trim pot adding resistance to get up to some higher value which is also desirable. In the current TREAD kits, I’ve chosen the values to cover the popular 24 V and 30 V values, with some adjustment slack beyond each.

Finally, you can use a relatively large value for this pot, and use a really low value for R2, or even jumper it. A 2.5 kΩ pot with R2 jumpered gives an adjument range of approximately 1.5 V to 32 V.

You might now be having visions of an adjustable lab supply dancing in your head. Beware, there are downsides to this plan. The first is, the more volts of adjustment per degree of pot turn, the less accurate the adjustment will be. A proper lab supply has a more complicated adjustment scheme to cope with this. The second is, you’ll want to switch to a full-size pot instead of a little trimmer if you’re going to adjust it frequently, as trim pots aren’t designed to tolerate frequent adjustments. Finally, beware that as you adjust the output voltage downward, the voltage drop across the regulator increases, thus increasing the heat it throws off. You need a very large heat sink to allow for relatively high currents at low voltages when the supply voltage is high enough to allow for such a wide adjustment range. All of this and more goes into the cost of a proper lab supply. They are worth the cost.

R3

This sets the current through the LED. The LED is more than just a power indicator. It also keeps a small, steady draw on the power supply, which can improve performance in some cases. It also helps discharge the filter cap when the power is removed. If you don’t want a power LED, add R3 anyway, and just jumper across the LED.

You should consider setting the current through this path to be 5 to 20 mA, even though that will make the LED rather bright. This will ensure quick discharge of the filter caps.

Capacitors

C1, C2, C3, C4 (ceramic)

These caps suppress any noise generated by the bridge as the diodes in it switch on and off. The exact value to use here is not terribly critical. 100 pF is a reasonable value. If you choose a different value, I’d say smaller is better.

These caps are optional.

Largest Part Size: 4 mm × 2.5 mm

C5 (electrolytic)

This is the main filter capacitor.

If you’re using an AC transformer directly with the TREAD board, this cap’s voltage tolerance should be at least twice the rated voltage output of your transformer. For instance, if you have a ±15 V transformer, you should use 63 V caps. The reason is, when the transformer is lightly loaded, its voltage will go up by as much as 40%. Also, the peak voltages put out by the rectifier bridge will be about 1.4× the RMS voltage. Taken together, 2× the voltage tolerance is required for safety.

If, instead, you’re using an unregulated AC-DC wall wart with the board, this cap’s voltage tolerance can be much lower. Simply measure the wall wart’s output voltage without any load on it and make the cap a bit higher in voltage than that. Or, leave it out entirely: the unregulated supply will have an output capacitor already, and its size may be sufficient.

Largest Part Size: 18 mm diameter.

C6 (film)

This is a small, fast film cap to lower the ESR of the main filter cap. Its value is not critical, and it can be left out without a big penalty.

Largest Part Size: 5 mm × 7.5 mm

C7 (tantalum)

This cap bypasses the regulator’s adjustment pin, increasing the regulator’s ripple rejection. You could put a 100 µF electrolytic here instead of the default tantalum, but the tant will work reliably over a greater temperature range and it will last longer.

This cap is optional.

Largest Part Size: 5 mm diameter

C8 (electrolytic or tantalum)

This cap does a bit of post-regulation filtering.

For the LM317 it can be as big as you’d like. A 1 µF or so tantalum or a 22 µF of so electrolytic is a good base value. Performance will improve a bit with larger values. You can leave it out if you want.

For LDO regulators (LM1086, LT1085...) a cap is required here and it must have an ESR of 1 to 2 ohms to guarantee the regulator’s stability. Since higher value caps have lower ESRs (all else being equal), stick to 1 µF for tantalums and 22 µF or so for electrolytics.

This cap is optional if you’re using the LM317.

Largest Part Size: 5 mm diameter

Diodes

B1

This is a monolithic rectifier bridge.

The standard part here is a W01G. Other names for this part are the NTE5304, RC202 and RB152. Some of these are simple round things, but I prefer the type with a flatted side, because it makes it easy to get the orientation right.

If you can’t find this part, a 1KAB has the same pin spacing and pin ordering. Due to the case shape, however, the snubber caps will prevent it from sitting all the way down on the board.

If the TREAD’s power source is a DC supply (for instance, an unregulated wall wart), this part is not necessary. If you leave it out, however, you will have to hook the power supply up to bypass the bridge position; the simplest way to do this is to hook the power source to the C5 pads, which is also not needed in this situation. You can still add the bridge if you want; it just drops the voltage a bit, which is harmless in most instances, but unnecessary.

D1, D2

These diodes protect the regulator when the capacitors around the regulator discharge. Generic 1N400x types will work. 1N4001s are probably sufficient, but make them 1N4002s or higher to be safe.

These diodes are required if you add C7 and C8. If this supply will drive a capacitive load (such as a headphone amplifier’s power section) you still need D1 at least. My view is, these are 4-cent parts...why leave them out if there’s even the possibility that they may help?

LED

This is the power indicator LED. See R3 for details.

Choosing a Regulator

There are many choices for regulators that will work in this design, ranging from a 40 cent off-brand LM317 to a $10 industrial grade LT1084IT. The standard audiophile reaction to these facts is that the LT1084 must be the best choice. The fact is, there are tradeoffs among these choices, so more expensive isn’t always better, even when money is not a concern.

The lowly LM317 has many advantages: it’s cheap, easy to find, docile, and its performance is very close to that of its more expensive family members. If you want to spend a little more on your regulator, get the National Semiconductor version instead of an off-brand clone, or get the high-spec LM317A variant instead of the plain LM317.

All the other regulators that work in this board are “low drop-out” types, meaning that the voltage drop across the regulator is lower than for the standard LM317. The downside of these LDO regulators is that they are sensitive to the impedance on their output. This means you carefully take into account the characteristics of the power supply’s output capacitor, the wire between the power supply and the load, and any power rail capacitors in the circuit being powered. For example, powering a PPA that has 2000 µF of rail capacitance (C1) with an LDO TREAD will most likely fail to work. For more details on this, read the datasheet for the regulators you’re considering. If you don’t want to read datasheets, you should stick with the LM317.

Another problem with LDOs is that they only provide a benefit when there is a low voltage drop across them. If your configuration puts a 3 V drop across the regulator, an LDO has absolutely no advantage over an equivalent standard regulator.

If an LDO regulator will work in your situation and you want the small advantages it will provide, the cheapest option is the LM1086. This gets you the LDO feature plus a tiny performance boost over an LM317. The next step up from that is Linear Technology’s version of the same design, the LT1086. Beyond that, there’s the 3 A and 5 A versions of this family (LM1085, LT1084, etc.); you probably don’t need the higher current, but to provide that higher current they necessarily have lower output impedance, which improves things a bit further still.

Again, I must caution you about spending exorbitant amounts of money on the regulator IC. There comes a point where the extra money you can spend on a better regulator IC would be better spent on a different, inherently better power supply design. This circuit gives the best value with ICs from the low end of the cost range.

Choosing a Power Source

The easy way to go here is to use some kind of wall wart, rather than a traditional transformer. You can find AC-AC wall warts, which are just a transformer encapsulated in a box with a wall plug and a low-voltage power cord coming out. More common are unregulated AC-DC wall warts, which follow the transformer with a diode bridge and a filter cap, to give DC with a fairly high ripple component. The advantages of using a wall wart are several:

• It puts the transformer far away from the regulator circuit, avoiding induced AC line hum in the output.
• It greatly reduces your exposure to the dangers of AC line wiring.
• The casework is simpler: a low-voltage barrel jack is easier to add to a case than an AC power inlet module.
• It’s an easy way to make the powered device world-power compatible: just pick up an appropriate local wall wart when traveling, or use a world-power compatible wall wart.

If you choose the AC-DC type of wall wart, there are a few additional advantages over the AC-AC type:

• You can leave the diode bridge and filter cap out when building the TREAD.
• AC line hum is even lower, since much of it is taken care of within the wall wart.

Using a bare AC transformer isn’t without its advantages, however. The primary ones are:

• Putting the transformer in the same case as the regulator and using an AC power cord is neater.
• You can get “dual secondary” type transformers, which you can use to make a dual supply. See below.
• If you believe audiophile power cords have any benefit, this lets you use the IEC inlet you need to accept these cords.

If you’re going to use an AC-AC wall wart or a plain AC transformer, the current rating should be at least 120% of the current you will be drawing from it. The voltage question is a little trickier. Instead of explaining all the details here, I’ve wrapped all it all up into my Power Supply Parameter Estimator.

To make a dual supply, you need a dual-secondary transformer and two TREAD boards, configured like so:

It’s common for dual-secondary transformers to also have dual primaries, as shown in the schematic above. For ~240 V power systems, you connect AC line to 1, AC neutral to 4, and 2 and 3 together. The secondary side is the same for all power systems.

Choosing a Heat Sink

The size of heat sink you need is a function of the amount of heat that the regulator will be putting out. I won’t go into details here, as my Power Supply Parameter Estimator pretty much covers it. You put in your configuration details, and it will tell you how hot the regulator will get. If it says your configuration will get too hot, you have several choices: lower the power source’s voltage, raise the output voltage, lower the load current, or use a bigger heat sink. I’d try those in that order. “Bigger heat sink” is the lowest priority, because it attacks the symptom instead of the cause. Be certain that the cause can’t be treated before going after the symptom.

You may be tempted to bolt the regulator’s tab to the case of the circuit your TREAD is powering to get a “free” heat sink, but I recommend against it. The first problem is that the tab on an LM317 family regulator is tied to the output voltage, so you will need to use an insulator kit to make this work. This adds thermal resistance, so it reduces the effectiveness of the “heat sink.” The second problem is that most cases don’t have a high surface area to volume ratio, as a proper heat sink does. Many people make the mistake of conflating massiveness with heat sink effectiveness. The effectiveness of a heat sink is a function of surface area, not mass. A heavy case with flat sheet metal walls isn’t an especially good heat sink.

Miscellaneous Hardware

If you’re using an AC transformer, I recommend that you add an IEC power inlet of some sort, rather than a captive AC power cord. I recommend the Qualtek 723W or the Schurter 6200 series, since they have built-in fuse holders.

If you’re using a wall wart, you’ll probably need a power jack of some sort. Most wall warts use 2.1/5.5 mm or 2.5/5.5 mm barrel jacks.

You’ll notice three holes in the board labelled TP1 through TP3. These are test points, useful for testing the power supply. I like wire loop type test points, so I can grab onto them with my meter’s grabber leads. Keystone type 5005 through 5009 wire loop test points work well here. You can also use the pins cut from SIP pin strips, or make loops from resistor lead cuttings, or leave the holes empty to form a crude DMM probe “socket.” I include some of the Keystone test points with the TREAD kits, but not with the bare boards.

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