The Chambers online dictionary defines ‘power+supply+rejection+ratio’ as “an activity that someone does for pleasure when they are not working”. And of course pleasurable it should be!

I used to find my hobby very pleasurable, with almost nothing negative to say about it. But while the arrival of an Internet connection brought me a wealth of information, and some good friends, recently I have found life on some of the forums shall we say less than pleasurable. Well, the argument goes that when the power from the external supply passes into the SB3, it goes through another switching supply that increases the voltage to supply the display, and then the rest of the unit is supplied after the ‘switcher’. My own experience is that in practice the external PSU does make a difference.

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DIY linear PSU, I could clearly hear a difference in the sound, despite only using the SB3 as a digital transport. It would seem that I am far from being the only one to hear a difference and there is now quite a choice of third-party SB3 PSU’s on offer. The first thing that I noticed about the SR1-5 is that it is well finished with lots of attention to detail. IEC socket and two connections for the 5 volt supply. For the technically minded amongst you, here are a few details taken from the PHD web site.

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These power supplies were designed to exceed the performance of all other supplies currently available for powering digital and analogue audio products including the Optima Red Top battery. The power supplies have an output impedance three times lower than the Red Top battery and this is maintained over a wider bandwidth. This allows three times less reaction on the power supply output, during load current changes. 10 nanoseconds and wideband noise 2 nanovolts root HZ. Regulator output impedance is less than 0.

001 ohm and the power supply line rejection ratio is 110 dB from DC to 200 KHz. Mains input is a standard IEC inlet. After a short session listening to my system using my own DIY linear PSU, I installed the SR1-5. At first, I felt that the sound with the SR1-5 was a bit leaner than I am used to with my own linear PSU.

I wasn’t sure if this was due to the bass being tightened up but in any case, after a couple of days, this ‘leanness’ had gone, so either I had got accustomed to it, or the SR1-5 had changed slightly as it burned in with use. I guess it was the latter because the SR1-5 continued to get better each day. I should point out that my SB3 has a modified digital output and I don’t listen via the analogue outputs. Fortunately I was able to borrow a stock SB3 and try that with its SMPS and the SR1-5 . So is that an enthusiastic thumbs up for the SR1-5? Well, as I said, it’s considerably better than my own linear supply. After reviewing the Transporter, I was left a little disappointed with the SB3 even with the modified digital output.

I can’t say for sure as I no longer have a Transporter here for direct comparison. I know that many of you will look at the cost of the SR1-5 and realize that it costs slightly more than the SB3. OK, let’s look at it this way. My own DIY effort using a lot of recycled bits like the case from an old PC PSU cost around 25 pounds. But if I used the quality of case and fittings that the SR1-5 uses, I could easily treble that, and still not have the same quality of sound because I wouldn’t have the quality of regulators that are in the SR1-5. One of the best known sources for SB3 modifications is Bolder Cables who will carry out internal modifications on the SB3 as well as supplying an up-rated PSU. At the time of writing, I have not heard the Bolder SB3 PSU which they call the The ULTIMATE Power Supply MK IV so I am not making a comparison on performance.

As it comes, the SB3 is not an audiophile device. It’s good, remarkably good for the cost but at the price point, compromises had to be made in areas like the PSU. For the improvement it gives at the price, I reckon the SR1-5 is a sound investment that raises the standard of the SB3 to audiophile levels, more so when used with a good external DAC. If you are contemplating one of these supplies, Paul Hynes make things a bit easier.

That’s because he offers a fantastic money-back offer if you don’t think that any of his products is worth keeping. And you get a very generous 56 days to take up the offer so that you can wait for the items to burn in, and have plenty of time to audition them at your leisure. Having heard the SR1-5 in my system, I doubt whether anybody is likely to want to return one anyway. For the DIYers amongst you, Paul Hynes also offer modules to enable you to construct your own PSU. What you will need to find is a suitable case and connectors etc.

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I will cover building up such a DIY PSU using a module in a future article. For now, if you own an SB3 or Duet, and you have a decent system, you owe it to yourself to get an SR1-5. My final thought after this review is that if the SR1-5 makes such a difference even going through the ‘switcher’ in the SB3, how much improvement will the Paul Hynes replacement regulators make when connected directly to a circuit? As it turned out, I was pleasantly surprised by just how easily I did hear the improvement that the latest incarnation of this power supply made. Physically, the SR3 looks almost exactly like the SR1, using the same case. The improvements are internal where the regulated power supply has been refined.

The SR3-05 is for use a high performance master power supply for any item of equipment that requires 5 volts at up to 3 amps continuous. It will also deliver 20 amp short duration transients. The regulator circuit is a proprietary discrete component design that has been optimised for very low noise wide bandwidth regulation. 5 nanovolts root Hz with a noise corner of 1 Hz. 50 times wider than the LM317. It took around two days of leaving the SR3 powered up for it to sound its best.

Initially the sound was a bit thin. However, even from the start, the improvement in the definition was quite clear. Bass was tighter, and everything just a bit more focussed. The music was coming out of a ‘blacker’ background. A low-dropout or LDO regulator is a DC linear voltage regulator that can regulate the output voltage even when the supply voltage is very close to the output voltage. The adjustable low-dropout regulator debuted on April 12, 1977 in an Electronic Design article entitled “Break Loose from Fixed IC Regulators”. One input of the differential amplifier monitors the fraction of the output determined by the resistor ratio of R1 and R2.


The main difference between LDO and non-LDO regulators is their schematic topology. If a bipolar transistor is used, as opposed to a field-effect transistor or JFET, significant additional power may be lost to control it, whereas non-LDO regulators take that power from voltage drop itself. Because the power control element functions as an inverter, another inverting amplifier is required to control it, which increases schematic complexity compared to simple linear regulator. Power FETs may be preferable to reduce power consumption, but this poses problems when the regulator is used for low input voltage, as FETs usually require 5 to 10 V to close completely. Power FETs may also increase the cost. It is important to keep thermal considerations in mind when using a low drop-out linear regulator.

Additionally, efficiency will suffer as the differential widens. Quiescent current is current drawn by the LDO in order to control its internal circuitry for proper operation. The series pass element, topologies, and ambient temperature are the primary contributors to quiescent current. In this idle state the LDO still draws a small amount of quiescent current in order to keep the internal circuitry ready in case a load presented. In addition to regulating voltage, LDOs can also be used as filters.

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This is especially useful when a system is using switchers, which introduce a ripple in the output voltage occurring at the switching frequency. PSRR refers to the LDO’s ability to reject ripple it sees at its input. As an example, an LDO that has a PSRR of 55 dB at 1 MHz attenuates a 1 mV input ripple at this frequency to just 1. A 6 dB increase in PSRR roughly equates to an increase in attenuation by a factor of 2. Having high PSRR over a wide band allows the LDO to reject high-frequency noise like that arising from a switcher. Similar to other specifications, PSRR fluctuates over frequency, temperature, current, output voltage, and the voltage differential.

The noise from the LDO itself must also be considered in filter design. Like other electronic devices, LDOs are affected by thermal noise, bipolar shot noise, and flicker noise. Load regulation is a measure of the circuit’s ability to maintain the specified output voltage under varying load conditions. The worst case of the output voltage variations occurs as the load current transitions from zero to its maximum rated value or vice versa. Line regulation is a measure of the circuit’s ability to maintain the specified output voltage with varying input voltage. Like load regulation, line regulation is a steady state parameter—all frequency components are neglected.

Increasing DC open-loop current gain improves the line regulation. The transient response is the maximum allowable output voltage variation for a load current step change. The application determines how low this value should be. Inventor Updates A Classic 30 Years Later”. SCH files for this page, use this . PCB link below also links to the same .

Power-One has built open frame linear power supplies since the 70’s. These workhorses have been built nearly the same way for so many years, and there are many available on the new and surplus markets. The design is simple enough, the technology is understandable to someone with basic electronics background. In other words they are ideal for hacking. What can be done with them?

Modified, they can be used as lab supplies, battery chargers, or any place a lab supply is needed. They can even be scavenged for parts or better still, subsections, they can be re-packaged into a different chassis. Here is a 24V 2A supply that I hacked years ago and have used as a lab supply ever since. I set the voltage range to 4V to 25V, removed a second board and drilled the chassis to make room for the AC line cord, switch and fuse, binding posts, and a Digital panel meter and 10 turn pot. I mounted rubber feet on the bottom to keep it from scratching up the bench and also to isolate its heat from the bench. Each Power-One supply has everything that a linear power supply needs: An unregulated DC power source, an output stage capable of dissipating some power, a regulator containing a reference voltage, a difference amplifier and a current limiter. DIP version of the venerable uA723 Voltage Regulator.

What else do they all have in common? Condor and International Power also build nearly identical supplies. When you are searching for a supply on Ebay, use all the vendor names in your searches. The differences are subtle, if any.


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Needless to say this page is not sanctioned by any power supply manufacturer. By disassembling and modifying a supply you will of course void the warranty, and you may never be able to get your power supply back to its original splendor. You may even blow something up. These things operate from raw AC line voltage which can be LETHAL.

Regarding efficiency, linear supplies are quite inefficient, and therefore burn holes in the ozone layer, cause more coal-burning power plants to be built, and speed the decline of civilization. They are fine for low power applications or occasional use such as a lab supply. But if you’re going to keep any power supply heavily loaded for long periods of time, please do the environment a favor and use a nice efficient switcher. Your electric bill will also appreciate it.

The Supplies Here is a simplified circuit of a Power-One supply. 15V, 3A supply similar to the HC15-3. Here is the full schematic of the HC15-3 in ExpressPCB format as well as in . Here’s a photo of a HC15-3 below.

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You can get ExpressPCB’s excellent software for free at www. Most units have current sense resistors to enable current limiting. These are typically 2-3Watt power resistors, less than 1 ohm, wired from the output transistor’s emitter to the supply output. Some supplies use 2 or more resistors in parallel to achieve low resistance and higher power ratings. The resistors are connected to the current sense pins on the uA723. These inputs are simply the base and emitter of an internal transistor. When the voltage from pin 3 to pin 2 exceeds about 0.

Higher current supplies use multiple output transistors in parallel. To achieve proper current sharing, each transistor has its own emitter resistor. Without the resistor, the hottest transistor would have the lowest Vbe which would cause even higher collector current, causing it to get even hotter. This effect is called thermal runaway and ultimately can cause failure of an output transistor as it hogs all the supply current. Emitter resistor help to balance these currents. Most supplies have an extra ‘boost’ supply which is typically a half-wave rectifier and cap that provides a bit more voltage than the main supply. This is to provide the few extra volts that the regulation circuit and output transistor requires without increasing the main supply.

If the main supply were simply increased, the output transistor would need to burn the power from that extra voltage drop. Most are built around single-sided phenolic PC boards. The usual mounting method is via the TO-3 transistor mounting screws. Removing the screws that mount the T0-3s.

Carefully seeing where all the insulation hardware goes so you can replace it later. Some newer units use socket pins for the TO-3s, so you don’t need to unsolder the transistor to remove the board. Remote voltage sense is provided on some supplies. Look for an extra pair terminals near the voltage output terminals. Remote sense is a “Kelvin” or 4-wire connection used to compensate for voltage drop in the power wiring by sensing the voltage at the load instead of at the power supply. A second pair of terminals is provided, and these are wired to the remote load.

Over-Voltage Protect is provided on some supplies. It is also called a “Crowbar” because it shorts the output of the power supply in the event of a fault that increases the output voltage too much. This is implemented with an SCR and voltage sense circuit consisting of a Zener diode and resistors. When the voltage increases, the SCR turns on and shorts the output of the supply. Some supplies use circuitry on the board for this.

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Some use an external board wired to the output terminals. When hacking a supply, you typically don’t want it to ever crowbar, so I recommend disabling the overvoltage circuit. This can be done by simply disconnecting an external overvoltage, or by removing the SCR on the on-board types. The SCR is typically a TO-220 device located near the outputs.

Unsolder and unscrew it, or clip its leads. It is the core, the brain, the technological center, if you will of each Power-One supply. It contains all of the building blocks of a voltage regulator: voltage reference, difference amplifier, power stage, and current limiter. Below is a block diagram, courtesy of TI.

Here is the uA723 full data sheet. This IC is very flexible and low cost. The voltage reference is nominally 7. So keep these signals between 2V and the 7V reference.

The ‘723 output voltage can be about 2. Internally, the ‘723 uses a current source followed by two emitter followers and can output a maximum of 150mA. In Power-One supplies this is used to drive external TO-3 output transistors that boost this to a couple of amps. 15V 3A supply that I have uses another transistor, a TO-220 TIP29 between the ‘723 and the 2N3055. Figure the TIP29 has an additional beta of 25. Another supply I own uses a darlington output transistor type 2N6059 to provide the higher current gain required.

About 2 or 3 amps is the maximum you want to draw from a single TO-3 without forced air cooling. A 2N3055 is rated for 60V, 10A and 115W, but 115W is only under ideal conditions: with an infinite heat sink and at 25C. Lower current, up to 5A, Power-one supplies have only the aluminum case as the heat sink. Larger supplies up to 50A often use additional finned heat sinks and encourage forced air cooling. Watt without a finned heat sink and without forced air cooling. If you modify a supply and want to operate at lower voltages, you will drop much more voltage across the transistor and burn more power.

20V or so at 3A or 60W. The unregulated parts don’t care if most of the 60W is being dissipated on the heat sink or in an external load. Well, they do care a bit since they also are mounted to the same heat sink, and when it gets hot, so do they. The device can be removed and simply wired via the traces on the board to the transistor located within 6-12″ away. Once you do this, the Power-One case is no longer needed: remove the board and transformer and mount them wherever you want. Increase the Voltage Range Before you do the voltage range mod or the constant current mod, realize that both of these will cause a larger voltage drop across the power transistor at high currents.


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This is the main source of power and therefore heat in the supply. Get the transistor too hot at too high a power dissipation, and it will fail, usually by shorting its collector-to-emitter. This causes the supply to output its full unregulated voltage. The voltage output of a Power-one is set with resistors and a trimpot. Older units use a big metal trimpot and newer ones use smaller plastic ones. A resistive divider with the trimpot reduces the output voltage to 7V to be applied to the – input.

The trimpot allows the output to be accurately set and allows for some adjustment range. Vout is wired directly to the – input and the reference is attenuated to the output voltage by a divider and trimpot. 7V supplies are easier to hack. One thing you typically want is a wide output voltage range.

I like to use the 15V or 24V supplies at 2 or 3 Amps to build lab supplies. I want the output to go all the way to 0, but the ‘723 won’t allow this easily. 00K to ground and and 1. 4 is 1852 ohms Use a 1.


Depending on your unsoldering skills you may remove the trimpot too. Replace the low divider resistor with the 1. 82K, and the pot plus the other resistor with 2 wires. Then run wires to the external 10K pot. 2 digit meter at full resolution. 9V range and only have .

1V of resolution, or add a range switch to the meter so at lower than 20V you can get 0. This is what I did on my first supply. Current Limiting In order to accurately limit current, it is generally required to measure the voltage across a low value resistor in series with the load. The ‘723 can be outfitted for foldback by simply adding a resistor divider to the current sense input such that as the output voltage drops, the current threshold also drops. This basic circuit is shown above. And also without varying the low-value current sense resistor, there is no easy way to reduce the current limit value. However, for a lab supply, a precision constant current mode is more desirable than current foldback.

Power supply rejection ratio

The Chambers online dictionary defines ‘hobby’ as “an activity that someone does for pleasure when they are not power supply rejection ratio”. And of course pleasurable it should be!

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