Frequently Asked Questions
What is a good power supply design?
As much as we'd like to tell you exactly what components to use, you need to choose them to meet your needs. Small size, low current? High current? High voltage? There are so many variables that we can't begin to tell you how to design your ultimate power supply. We do, however, provide some help on our transformer calculator page and in our data sheet, please consult those, and our tips page for information on keeping the regulator cool and preventing problems before they happen.
- T1: Power transformer
- D1: Bridge rectifier
- C1: Filter capacitor
- Superpower: Voltage regulator
- D2: Protection diode (optional)
- R1: Pull down resistor (optional)
- C2: Regulator output capacitor
- Z1: Load circuit (your DAC or mixer or whatever)
- T1 converts 120V or 240V mains voltage to lower voltage.
- D1 converts AC to pulsating DC.
- C1 filters the pulsating DC into much more stable "raw" DC, although under load it still contains substantial ripple voltage. If C1 is too small with respect to the load current and raw DC voltage, the ripple can dip below a value required by the regulator, causing it to momentarily stop regulating. This is called "drop out" and the voltage at which it occurs is called the "drop out voltage." There is no maximum value for this capacitor.
- Superpower is the regulator that turns the DC+ripple into real DC. (Ripple rejection is a measure of how much ripple gets through the regulator.)
- D2 is a protection diode. In normal operation it is reverse biased and does nothing. When input mains power is switched off, it gives a path for charge stored in the output capacitors to dissipate around the regulators rather than through them.
- R1 is an optional fixed resistor that gives transient charge a place to go. Think of it as a "pull down" resistor (a negative regulator will have a "pull up" resistor.) An unipolar regulator is great for sourcing current but not good at sinking it, and the pull down resistor compensates for that. For example, when load current changes from large to small, a transient charge appears at the regulator output and the pull down allows that transient to dissipate quickly rather than slowly. This greatly improves transient response.
Our measurements show, for a 100mA current step, a value that sinks about 20mA works well. Determining the optimum resistor is difficult because it depends not on the maximum average load current, but on the maximum change from average. However, a rule of thumb is a fixed load in the range of 10% to 20% of the average load.
- C2 is the output capacitance. Electrically this capacitor improves loop stability of a Superpower and, when optimized for the load, allows the fastest possible transient response. Superpower has a 4.7uF tantalum already on the PCB and you can add whatever you wish. The optimum value depends on the regulator (SPM, SPJ, SPHP, etc.) and the load. Our recommendation is a minimum of 10uF at the load, preferably a low inductance capacitor such as tantalum, and we've also seen good results with 100µF Nichicon UVR1H101MPD electrolytic. Again there is no maximum and you can freely experiment with various brands and technologies. One word of caution—always power the circuit down before changing output capacitor, we've damaged SPJ in our lab by connecting a large (e.g. 10 000µF) capacitor while power was applied.
Capacitors—where, how much, what kind?
We get a lot of questions about capacitors. One goal at Belleson is to have the optimum transient response, that is fast recovery from transients and minimum glitch energy, overshoot and ringing. We recently did an evaluation of output capacitors for our SPJ 2 Amp Superpower and determined that neither total capacitance nor ESR have a dominant effect on transient performance. It's not clear yet but it seems the big difference is in the self-inductance of the capacitor. Tantalum caps have low internal inductance and give great transient response. Also a capacitor we found to give great performance is 100µF Nichicon UVR1H101MPD.
Input: Generally, we recommend as much filter capacitance as will fit, although there is a point of diminishing returns. A local bypass cap near the Superpower, from input to ground, helps insulate the input from wire and trace inductance between the filter caps and the regulator.
One Superpower user has made these specific observations about an input capacitor:
- Anyway, I've now soldered a cap directly on the input of superpower. It didn't really improve the output noise, but the interesting thing to note is that it does alter the sound. I wasn't expecting this to happen, given superpower's very high noise rejection ratio. So far, I found that it really does take a good cap there, an audio cap preferably. I've tried Panasonic FM low ESR, Sanyo Oscon SP low ESR. The Sanyo sounded really bad. The Panasonic FM sounded almost as good as the Panasonic FC caps I've already got there for the filter caps. A cheap 330µF Rubycon cap also sounded bad (less ambient, more flat and dry). I ended up with a 1000µF Elna Silmic II cap which I have in my parts bin. It gives better detail, imaging, and it does make it sound special. So you may want to put in the application note that if user wants to further improve the sonic quality, an audio cap such as Elna Silmic / Silmic II is recommended.
Output: Given that Superpower SPJ and SPL have a 4.7µF tantalum cap on board, the best response is obtained by adding additional capacitance at the load rather than at the regulator.
Superpower are compensated with enough capacitance for stability under any load condition. You may add as much or as little as you wish. Best dynamic response is achieved with a low inductance 100µF capacitor near the load. Capacitors distributed across a PCB, including decoupling caps near digital components are also good. If you are replacing regulators on a commercial PCB, this will likely be the case already, and Superpower works well in these applications.
Feel free to experiment with any brand or type of capacitor. As always, we do not recommend anything that will "sound better", we only measure the electrical performance and you decide what you like.
Should I use a pre-regulator?
Superpower works best without a pre-regulator. The best source of
energy for a Superpower is a full wave rectified and filtered power
supply. A pre-regulator increases source impedance and limits dynamic
current to the Superpower.
What is special about the high voltage Superpower?
Superpower uses a boot-strapped (self-powered) floating reference
and error amplifier that sets its own power supply voltage range. The
Superpower design has output voltage set by a constant current through
a resistor. Only this single resistor and one transistor have to
sustain a constant high voltage, making the circuit more reliable than
other designs. It also does not require a separate floating transformer
or other costly and complicated circuitry.
The low drop out voltage keeps power dissipation in the load where
it belongs, not in the regulator. For situations where low drop out is
not possible, the high voltage Superpower is designed with TO-220
devices to allow easy heat sinking.
Why is Superpower used in audio circuits?
Clean, clear audio needs reserve power for bass and low midrange frequencies, fast response for mid and high frequencies and low noise across the audio spectrum and beyond. These requirements are not independent but must be met simultaneously.
The step responses shown in the load regulation page illustrates how Superpower can provide a great deal of current in a very short time. The spectra in the line regulation page show great noise performance across the audio spectrum.
So no matter what your audio requirements may be, a Superpower will contribute to clean, clear dynamics in any audio subsection.
What is the Load Sense Connection?
Superpower is available with a load sense connection, allowing voltage to be better controlled at the load by including impedance between the regulator and the load in the regulator feedback loop. The load sense connection has a potential to introduce noise so careful routing and placement of both the regulator and its load are essential.
What can affect dynamic performance measurements?
Delivery of high current means a small amount of resistance can
cause relatively large voltage changes. One amp times 10 milliOhms is
10 mV, thousands of times more voltage than the
AC at the output of a Superpower. To achieve the measurements as seen
in the graphs and photos shown, extreme care must be taken to minimize
wire and PC trace lengths, maximize wire gauge and carefully place
How is dynamic performance measured?
Good question. A standard power supply has limitations on output current that affect dynamic performance of Superpower, so what can be used for a Vin source? The only solution we found so far is to use a Superpower regulated source or an unregulated tranny/rectifier/filter cap as Vin to test another Superpower. We use a Superpower regulated supply to test both dynamic performance and ripple rejection.
Dynamic performance is tested by switching a resistor from Vout to ground through a high speed transistor.
Ripple rejection testing is described on the ripple rejection page.
Does Superpower have an output current limit?
To provide minimum output impedance and maximum current, SP and SPJ
products have no current limit and no thermal protection. Careful
design and heat sinking must be done to prevent damage to the Superpower
and your circuit.
The SPL product is a Superpower with output short circuit current limit circuitry. See the description here for more information.
Can noise performance be improved?
As of September 2013, the answer is YES! We re–engineered the internal
design of Superpower and decreased noise by a factor of 3. See
our noise page for measurements.
Bootstrap powered by its own output, Superpower is inherently low
noise (see SPICE noise simulation results below). A high quality reference
and low error amplifier gain keep noise at a minimum.
However, good input bypassing as described above
decreases noise, as does a large electrolytic capacitor across the output.
Editorial on the noise about noise
Audiophiles are all over voltage regulator noise. However, it's way down the list of what's important for a voltage regulator in an audio circuit. Consider:
- Most audio systems are differential in the critical input stage, which nulls power supply noise.
- Noise matters most in the very first stage of a preamp or DAC
- Other power supply parameters such as ripple rejection and transient load response can result in milliVolt (or tens to hundreds of mV in some poor regulators) voltage change whereas noise is typically microVolts.
The most important power supply parameters are, in order,
- transient step response, so power supply does not change due to digital pulse load of the clock
- ripple rejection, to prevent input supply noise from affecting output voltage
Voltage regulator noise is important for
- Single ended preamp input stage such as SET triode
- Phono preamp input stage, even differential, because gain is so high
- Microphone preamp
- DAC current to voltage (I to V) converter
- Direct heated triode filament supply
Nonetheless, we have gone to considerable design effort to make our regulators very quiet! Here is a video of a noise comparison measurement of SPZ78@12V and LM7812: