# Can an audio circuit be powered by a switched-mode power supply?

Most audio circuits are powered with large, heavy transformers and a small ripple after smoothing. SMPS are smaller and more efficient. EMI can be shielded by a metal enclosure and the output filtered for noise suppression.

Especially where the power is going to be further regulated. Why aren't switched-mode power supplies used in audio circuits, eg. power amplifiers, and what improvements can be done to make a SMPS suit an audio circuit?

• It is totally wrong that most audio circuits are powered with large and heavy transformers. Some old ones and some audiophool ones, but those are a small minority. Cell phones and portable music players are probably "most" audio circuits today, and I'd be surprised if more than 1% (being generous) of those use linear supplies since there is limited battery power that must be used efficiently. – Olin Lathrop Jul 27 '12 at 18:45

Let me give you a little background on myself... I've been working professionally in the audio industry for more than 14 years. I've designed circuits for most of the major pro-audio companies, one audiophile company, and several consumer audio companies. The point is, I've been around and know a lot about how audio is done!

SMPS can and are used for audio circuits! I've used them from sensitive microphone preamps to huge power amplifiers. In fact, for the larger power amplifiers they are mandatory. Once an amplifier gets over a couple of hundred watts then the power supply needs to be super efficient. Imagine the heat produced by a 1000 watt amp if it's power supply was only 50% efficient!

But even on a smaller scale, the efficiency of a SMPS often makes a lot of sense. If the analog circuitry is properly designed then the noise from the power supply gets rejected by the analog circuitry and doesn't impact the audio noise (very much).

For those super-noise-sensitive applications you can do a hybrid approach. Let's say that you have an ADC that requires +5v. You can use a SMPS to generate +6v, then a super-low-noise linear regulator to bring that down to +5v. You get most of the benefit of the SMPS, but the low-noise of the linear regulator. It is not as efficient as just a SMPS, but those are the trade-offs.

But one thing to keep in mind... A SMPS for audio applications needs to be designed with audio in mind. Of course you'll need better filtering on the output. But you will also need to keep other details in mind. For example, at very low current the SMPS might go into something called "burst mode" or "discontinuous mode". Normally a SMPS will switch at a fixed frequency, but in one of these modes the switching will become somewhat erratic. That erratic behavior might push the output noise into the audio frequency band where it becomes more difficult to filter out. Even if the SMPS is normally switching at 1 MHz, when in one of these modes you could get 10 KHz noise. Controlling how this happens depends on the design of the chip that the power supply uses. In some cases, you can't control it. In that case you have no choice but to use a different chip or use a hybrid approach.

Some people advocate using only linear power supplies for audio. While linear supplies are less noisy, they have lots of other issues. Heat, efficiency, and weight being the biggest ones. In my opinion, most of the people who preach linear supplies only are either misinformed or lazy. Misinformed because they don't know how to handle switching supplies or lazy because they don't care to learn how to design robust circuits. I've designed enough audio gear with SMPS to prove that it can be done without too much pain.

• +1 - This is one of the best answers on this site! You addressed most of the issues and approaches in a broad fashion, while still providing enough detail to allow the OP Google more where needed. This is like decades of experience summed up in a few paragraphs. – Joel B Jul 27 '12 at 18:05
• +1. I've "restored" many vintage audio amplifiers, and I've always replaced the original iron core transformer power supply with SMPS, because original or equivalent transformers were impossible to find. Never had a single problem. – Axeman Jul 27 '12 at 18:50
• You can build extremely large, high-power amplifiers without switch-mode power supplies, by employing a many-tap mains transformer, and doing rail-switching. You can get similar efficiencies to SMPS designs, albeit at the cost of a massive 50/60Hz power transformer. – Connor Wolf Sep 6 '14 at 8:43

A class D amplifier is a switching power supply. Those are more common these days and can have quite good specs. Audiophools may wrinkle their noses when they are told a amplifier is class D or has a switching power supply inside, but such a thing is harder to detect in a proper double blind test. In the audio world, it can be difficult to separate the science and measurable results from religious belief.

• Audiophools :D ... I've teased a couple of them to do a double blind test between their unobtanium/iridium/platinum signal cables and my "cable" made with... ahemm... i don't know how it's called in english: it's the UNI-CIG steel pipe we use in Italy to connect the kitchen ovens to gas sockets. My "cable" won about 80% of tests... I'll always remember their faces when they've seen how my "cable" was made:-) They never talked about "esoteric" audio again :-) – Axeman Jul 27 '12 at 18:57
• +1 - Absolutely - being a musician also, it drives me crazy to listen to the myths and legends floating around about audio gear. It's like a completely separate set of physical laws apply just for this area :-) I can understand perfectly why engineers design with valves, big heavy transformers, gold wire, etc - because they sell at stupidly inflated prices. Why build an efficient and high performance amp that sells for peanuts when you can design some inefficient monster with 2% THD and sell it for £5k? – Oli Glaser Jul 28 '12 at 17:13
• Having said all this though, some of it makes some sense, especially when the distortion/instability is desired - all the guitar amps I have ever owned have been valve, simply because I like the distortion better than anything I have managed to get from solid state or effects pedals. There are all sorts of digitally simulated amp sounds, but it's probably pretty hard to recreate exactly (e.g. digitally or with transistors) the subtle harmonic effects that valve amps can create under different conditions. – Oli Glaser Jul 28 '12 at 17:20
• @Axeman - wow, that's pure marketing genius. I love reading the product descriptions for this type of stuff, and the reviews - here's a snippet from a review of the "blackbody": I have found ORANGE PAPER (unwaxed) sticker dots (the ones you find at stationery stores to color code documents, folder, files, etc.) to be extrememly effective in enhancing sound quality. Other colors just did not cut it, the worse being blue and green. Orange stickers for £25 a pop anyone? – Oli Glaser Jul 28 '12 at 20:16
• "class D or has a switching power supply inside, but such a thing is harder to detect in a proper double blind test" ... if it's designed correctly. A poorly-designed switcher will have worse problems than a poorly-designed linear, which is why they have a bad reputation. – endolith Mar 26 '15 at 16:40

Short:

• SMPS are much used in many audio systems.

• In very top-end enthusiast targeted systems an iron cored transformer based supply may be preferred because of nuances in effect which are so fine that they can only be detected or claimed to be detected by true aficionados.

SMPS are regularly used to power audio circuits in many applications. Most domestic audio equipment probably uses SMPS.

Top end systems for audiophiles may use "iron transformers" because of actual and or perceived benefits. Noise elimination for 50 Hz transformer based supplies is well understood, most of the noise energy is at low frequencies which are a multiple of the main frequency which makes it able to be rejected by notch filter techniques if astoundingly high levels of rejection are desired. The main exception is probably diode switching noise caused by current peaks when the diodes conduct at the peak of the AC waveform, and this can be greatly reduced by spreading resistors and generally good design.

SMPS typically use switching frequencies in the 50 kHz to about 2 MHz range and usually in the few hundred kiloHertz range. These SHOULD be even more readily filtered out that than low frequency noise, but rejection levels of amplifier circuitry decrease with increasing frequency and will often be far worse at above 100 kHz compared with at say 10 kHz.

Whether a well designed SMPS supply is any more liable to significantly affect the quality of a top end audio system is open to debate - and much debate has been opened on thsi subject. BUT if users THINK that SMPS MAY be worse than a traditional supply and/or if suppliers state that they are or may be or that listening tests have confirmed that they are, then "modern stuff" is liable to be the loser when compared to iron cored supplies - regardless of what the reality may be.

• Funny how uour answer does remind me a bit about tube vs. transistor sound discussions. – jippie Jul 27 '12 at 20:14
• @Jippie - :-). I consciously avoided saying "Oxygen free copper" or "crystal structure" or talking about winding layer configurations of strands in cables. – Russell McMahon Jul 27 '12 at 20:55

Switching power supplies are being used to an increasing extent in many applications. Certainly wall-wart-sized audio applications are using switchers as often as not. I think a major factor limiting the adoption of switching supplies historically has been the fact that while most audio systems do not pass through very high (e.g. over 100KHz) frequencies in anything resembling a useful fashion, the presence of such frequencies at the input to an audio stage may cause distortion in the output. Especially in feedback-amplifier configurations, power-supply noise rejection is better at low frequencies than high frequencies. Consequently, it's easy for high-frequency noise on the supply of one audio stage to cause distortion in a following audio stage. Although 60Hz noise by itself would be far more audible than 100KHz, the effects of 60Hz power-supply ripple may be less severe than the effects of 100KHz power-supply ripple.

I'm sure that with time switchers will become more prevalent in audio gear, though marketing inertia may prevent it from happening as quickly as would be ideal from a purely-technical standpoint. If customers associate big clunky transformers with quality audio gear, and see the more-cost-sensitive manufacturers using switchers, they may perceive switchers as "cheap", especially given that some devices which sound fine with 60Hz-transformer-based wall-warts sound crummy when powered by cheap switched-mode wall warts that have the same nominal specs.

Cheap mass-produced Switch Mode Power Supplies (SMPS) with poor filtering and bad EMI/RFI rejection have tarnished the reputation of SMPS in the Hi-Fi audio world. It will take some top-quality SMPS in high-end equipment to overcome the damage that has been done. But there is no good reason why SMPS cannot be used to power audio circuits, large and small.

Many high band audio companies now use SMPS for various reasons, not all but mostly because of

• (A) Weight of iron core/copper winding transformers
• (B) Efficiency of coupling between windings {ie power loss}
• (C) Actual cost of copper these days

Anyone who has ever worked with high power PA systems will know that the bigger the amplifier (600W to 1Kw and above now are common) are heavy and large in size to fit into your standard rack mount road cases.

Standard linear power supplies deliver anything from plus and minus rail voltages of 75 and over 'Fixed". Any 'power' from the supply that is not being used is "dropped" into the heatsink.

For example, a 1 kilowatt amplifier running at only 10% will lose more energy as heat than the same amplifier running at 90%.

Some few audio manufacturers have taken advantage of this and use input detecting circuitry to vary the output voltage of the power supply to only provide the necessary level of supply rails as required. Switching at between 4 and 10 times the audio frequency, (any HF artifacts can be easily filtered filtered out of the DC supply)

This fast switching varies the output voltage from, say, plus and minus 30V for low level signals, to plus and minus 90V (or above, depending on the FET/Transistor design). Because of the efficiency of SMPS, this greatly reduces the cost and weight of the amplifier as there are no longer any great lumps of steel and copper to lug around, but also no huge aluminum heatsinks to dissipate the great power losses, or those large fans required to shift enough air around them.

Unless poorly filtered, no "power supply" should impact on the audio quality of any amplifier be it linear or digital. A voltage is a voltage; flat and ripple free of any sort: after that it is the design of the amplifier that determines noise and distortion