Professor Dr. Bill Avery

Dr. Bill is an adjunct Professor of Electrical Engineering at the University of Nevada, Reno and a registered Professional Electrical Engineer.  He has been designing electronic circuits since 1958.  In his free time, when he is not water skiing or working on his fire truck collection, he plays the piano and his 9 rank pipe organ.  But for just pure relaxation, he settles into his home theater to enjoy the Soaring Audio amps and speakers that he has spent so many years perfecting.

What Makes an Amp Fun to Listen to?

Great Dynamic Range

An amplifier should be able to be completely quiet and in a blink, be at full power with no time lag. Lag can come from switching power supply's or protection circuits that alter the music's dynamics. The amplifier should not have a fan or overheat with heavy usage. Overheating can cause the amp to go into protection mode which degrades the dynamic performance. During listening you do not want to have to make changes to the level of sound.

Wide Sweet Spot

There is a point somewhere in the listening area that is formed by an equilateral triangle, the two speakers being two corners and the listener being the third corner of the triangle. At this location, the listener hears each speaker at an equal level and if the system has very low phase shift, a powerful illusion occurs that the sound instead of coming from two points is coming from a continuous wall of sound.

If an amplifier does not have low phase shift, there may be no sweet spot at all. Some sweet spots may only focus the mid-range, but the bass and highs will be blurred. This happens when there is a great deal more phase shift at the extremes. All Soaring Audio amps have miniscle phase shift, which makes the listening stage seem large and spacious. There are currently technologies used in the SLC-A300 that can increase the sweet spot to many times its normal size. This is especially advantageous in home theater listening environments where there may be low bit rate in the surround sound channels or MP3 files.

Dead Quiet Background Noise

It is critically important that an amplifier not add any electronic noise to the signal from the original source.  This ensures that the sound of the hall and the reverb of all music is clear and can be enjoyed by the listener.  The signal to noise ratio tells you how small the electronic noise a piece of equipment is compared to its maximum output.  It must be greater than the original source to avoid reducing the dynamic range.  A typical CD is 96 dB down.  An SACD is even quieter than that.  Soaring audio amps have less hum harmonics more than 120dB below peak output.  See Fanatics Only What does hum 120dB below the peak output mean?

There are two types of noise in amplifiers. One is the white noise which sounds like air escaping from a tire valve.  The other is hum.  Hum consists of the power line frequency and harmonics.  For example, if the power line is 60Hz, then the harmonics are 120, 180, 240 and so on.  When both types of noise are measured together with a voltmeter, white noise often dominates and the hum contribution being only a small part of the total is ignored

Problem.  The human ear can hear the third harmonic of the line frequency (180Hz) that is only 1/200th the power of the white noise.  By specifying the signal to noise by use of a broadband voltmeter, it is possible to have hum that can be easily heard even though it is much below the "noise floor" of the amplifier.

Soaring amps have hum components that are more than 120dB below peak output.  120dB is a millionth of a millionth of the peak power out.  The peak power out is 300 watts so the hum harmonics are less than 3/10 of one billionth of a watt.

These line harmonics can blur the tinier details of the sound stage.  Put your head into the woofer and you hear nothing at all on a Soaring amp with the source muted.

Removal of Digital Grit

Digital to Analog converters send out a voltage that is read from a CD or DVD at 44,100 or 48,000 voltage samples each second. A gritty distortion is generated by this process. MP3s and some surround channel with low bit rates simply do not have the bits necessary to accurately reproduce the sound. The SLC-A300 contains a circuit that smooths across these discontinuities and gives the impression of having a higher sampling rate.

Wide Frequency Response

An amplifier must have a frequency response much greater than the notes it will play. Just like a car with excellent handling can stay exactly in the center of it's lane at high speed, an amplifier with extremely wide response can more accurately follow a complex waveform.

Normally the range of human hearing is assumed to be 20Hz-20KHz. Each component in a system must have at least this range because the loss when the music passes through the source, preamplifier, amplifier, and speaker are additive. The combined losses must be small enough to not make the bass softer or the highs less crisp.

Normally the definition of frequency response is taken to be the frequency where the output has dropped to one half the power the same input produces in the midrange. This is unacceptable for audio because the loss in highs and lows would be grossly noticeable. Because of this, audio specifications state how much loss occurs at the published frequency limits. Typical is +/- 1 dB or about a 20% decrease in power compared to the flat response of the midrange.

Speaker specs often use frequency where the acoustic power drops to 1/2. If the amp only goes down to 20Hz, then there will be an additional loss from the amp at low frequencies. With Soaring audio amps the half power frequency is at least 0.1Hz or 200 times below the typical low frequency limit of speakers. Some amplifiers extend the low frequency response down to DC which can cause a small DC current to flow in the speakers which is not optimum. In addition, certain types of failure in components could damage speakers by continuously putting out a high DC current.

Duration of Performance

With some amps, the performance specs only last for a fraction of a second.  People can hear the deterioration of quality at normal listening levels in a few minutes.  The other common failure of amps is that they overheat and shut-off or worse, burn out.  Soaring Audio amps are designed to operate at full power indefinitely.  There is no limitation in its design to prevent it from performing just as well several hours later as it did in the first few minutes.

Stable Image

In order to maintain a solid sound stage an amplifier should have it's power supply voltage filtered and regulated.  Any change in gain or phase or a shift in the bias will break the illusion and the enjoyment of listening will be diminished. Soaring amps all have a power supply voltage that is filtered and regulated. Thus they have a rock solid sound stage image.

Consistancy between Amps - Parts Quality

With many amps, if you play three amps of the same model, they all sound different.  It took years of exhaustive study to be able to maintain sound consistency among all Soaring Audio amps.  The first goal was to make one amp that sounded incredible.  Just as great sugar cookies require real butter and real vanilla, so great sound comes from using only the best parts.  We had to use precision matched parts of impeccable quality.  Each part contributed to the sound of the amp, so each part had to be listened to for character of sound.


The Soaring Audio SLC-A300 and Phoenix amplifiers are class AB. Class D amps weigh less and cost less, so they are often chosen to keep costs down.  However, though both Class D and the Soaring amps may say they put out the same Watts, the listening experience will be vastly different.  Just like having a V8 engine does not necessarily give you race car performance, all 300 Watt amplifiers do not give the same performance. 

The most obvious difference is weight - 3 pounds versus the Phoenix 38 pounds. In a 7.1 installation you are looking at 30 pounds versus over 300 pounds.  The additional weight comes mainly from the transformers and heatsinks.  These, along with the 28 extra storage capacitors in each amp, give the ability to provide great dynamics and stability of sound over an entire movie. 

Class D amps do not have that temperature and power stability. Thus their performance will not be consistant over time.  This explains why amps with similar specifications can sound so different once they have warmed up.

Without the large transformer, it is unlikely for the sound to have a convincing image.  Class D amps are notorious for being two dimensional and having no feeling of space or depth.  If you want totally immersive sound effects, always choose an amp with a large transformer.

Class D amps use a digital process and thus have all the problems of digital.  Yes, blu ray is digital, but it has no power.  It only has a couple of volts with almost no current.  When a digital power amp tries to change 9 amps of current at high frequency, it has the ability to change a well recorded Blu-ray concert into the quality of an 128 kbps downloadable song.

Class D amps also are not isolated from noise. Ideally you want no noise in the speakers when the source is turned off but the amplifier is still on.  See "Perform the hum test yourself" on Fanatics Only.

Amp Spec Secrets

A Weighted Measurements

Weighting is a specific frequency response curve that makes a meter respond to the loudness of different frequencies in the the same way the human ear would judge the loudness. The A weight curve mirrors the response of the human ear at low volume levels. The secret is that using an "A weighted" response for measuring noise instead of the normal flat response, reduces the measured noise. It yields a lower, more impressive figure to publish in the list of specifications. It is rarely used by credible amplifier manufactures. If you see it, keep both eyes open and hang on to your wallet.

C Weighted Measurements

C Weighted Measurements, like A weighting, C weighting, again reduces the measured noise by attenuating the high and low frequencies before going to the meter. It is much closer to the actual noise level than A weighting but can still improve the published spec by several dB.

Frequency Response

What does a frequency response of 20 to 20 KHz mean? The definition of bandwidth is the difference between the low frequency where the signal is attenuated (or down) by 3dB and the high frequency where the signal is attenuated (or down) by 3dB. (See Fanatics Only for a discussion of dB.)

At the -3dB or half power frequency, the phase shift is 45 degrees. +45 for the low frequency and -45 for the high frequency. Thus if you want very small phase shift at 20 Hz or 20 KHz, your amp must have a -3dB frequency far past the lowest or highest frequency where you want the phase distortion to be much less than 45 degrees. If the -3dB figure is a factor of 10 past 20 Hz or 2 Hz, then the phase shift will be +45 degrees at 2 Hz, but only about 6 degree at 20 Hz.

With Soaring Audio amps the low frequency -3dB point is 0.1 Hz, or a factor of 200 farther than the lowest frequency we want to be faithful at. This means the phase shift at 20Hz is only about 1 degree. See Flat Phase Response in What makes an amp fun?

Power Output

The first secret is that there are two ways to measure power:  instantaneous and average.  The peak is much higher than the average.  For different types of music there are typical ratios for peak to average power.  They vary from as close as 6 to 1 to as far as 40 to 1.

Sometimes the power rating is given as the peak instantaneous power.  More often, the average power rating is given (or RMS - root mean square which just means average power).  In this case, the waveform assumed is a sine wave which looks like the edge of a Spanish tile roof.  For a sine wave, the ratio of peak to average (or RMS) is two.  In other words, an amp rated at 150 watts RMS can produce 300 watts peak.

In the specs game, bigger is better. Thus there is the desire to publish the largest number - which is the peak.  Of course, if the amp rating is peak, then you cannot assume it will produce twice that, since it is already the maximum possible output.

The other secret about power is how long it can be maintained.  Some amps cannot even finish a whole CD at maximum power without meltdown.  Soaring Audio amps have no limitations. They can play forever at maximum power.  That is part of the reason they weigh 38 pounds!

Power is volts times amperes.  Just like the area of a room is the length times the width, power (like area) is the product of two numbers.  If we assume that the speaker looks like a resistor (which is approximately correct), then the current that flows through the speaker is equal to the voltage divided by the speaker impedance.  The speaker impedance can be considered a pure resistance as a good approximation.

Let's take a numerical example.  At 20 volts of electrical pressure across a four ohm speaker, the current that flows is 20/4 = 5 amperes.  Power is 20V times 5 amperes or 100 watts.  Let's repeat this example with half the voltage which is  10 volts.  Now the current would be 10volts/4 ohms or 2.5 amperes.  The power is now 10 x 2.5 or 25 watts.  Notice that the voltage dropped to 1/2 and the power dropped to 1/4.  Power then, is proportional to voltage squared and is actually equal to voltage squared divided by resistance.

The loudest an amp can play is that point where either the voltage hits the maximum or the current hits the maximum.  Beyond that, clipping distortion occurs and the top of the waveform is sliced off horizontally.  Clipping on bass is most noticeable, but less noticeable at midrange and even less at high frequencies.  The ideal load for an amplifier is that impedance speaker that draws the maximum current just as the voltage reaches its maximum.

Peak amplifier voltage divided by peak amplifier current gives the impedance speaker that the amplifier will deliver the maximum power to.  At any other impedance, the peak power will be less.  If a lower impedance speaker than ideal is used, then the current maximum will be reached before the voltage maximum.  A higher impedance will result in the maximum voltage being reached before the maximum current occurs.

The SLC-A300 puts out about 38 volts maximum and 9 amperes maximum which implies that the maximum power is delivered to a 4.2 ohm speaker.  While it will happily drive a 2 ohm load, the peak power is limited to the maximum current (9A) squared times 2 ohms or 162 walls peak.  At 4 ohms the peak power is 38 x 9 or 342 watts.

Sometimes an amplifier can put out peak power for a limited time, but overheats if music is played continuously for a long time.  As part of final testing, every SLC-A300 is connected to wire wound enameled power resistors and is left running near the level that produces maximum heat over night.

The maximum temperature in the amplifier heatsinks occurs when the RMS output voltage is approximately .35 times the peak output voltage. The SLC-A300 is UL 6500 approved which means it had to pass a load test where besides operating continuously at the maximum temperature generating output voltage level, the load resistance was dropped to 1/2 the nameplate rating of 4 ohms. This doubles the heat. This is why UL requires that heat tests be done with the RMS output voltage adjusted to 1/3 or .33 of peak and with 1/2 normal load which doubles the power flowing into the heatsinks. The SLC-A300 can run at this maximum heat stress level indefinitely without a fan.

Noise Measurements

All circuits produce noise, some more than others.  In addition, hum from the power supply can be heard at less than 1/100th of the value of the measured noise. All Soaring audio amps have hum so low that you can't hear it with your head right next to the woofer. See Fanatics Only to learn how to perform the hum test yourself.

UL rating

Many amps cannot pass even the weakest UL rating.  The SLC-A300 is UL 6500 approved which is the more stringent international standard.  It had to pass a load test where besides operating continuously at the maximum temperature generating output voltage level, the load resistance was dropped to 1/2 the nameplate rating of 4 ohms. This doubles the heat. This is why UL requires that heat tests be done with the RMS output voltage adjusted to 1/3 or .33 of peak and with 1/2 normal load which doubles the power flowing into the heatsinks. The SLC-A300 can run at this maximum heat stress level indefinitely without a fan.

Under UL6500, to eliminate the need for a third wire ground, quality components must be used in the primary of the amplifier to increase the isolation between primary and secondary.

While the Phoenix amps were not submitted to UL, they make use of the underlying SLC-A300 design and can also run indefinitely without a fan.


Normally amplifiers are required to have a third wire ground to the chassis.  The "green" or "U ground" is a safety ground and provides a redundant ground path to the neutral conductor for additional safety.  It can cause small currents to flow on the shield of interconnects and cause hum.  All Soaring amps have been designed to UL 6500 specifications.  Under UL6500 if higher quality components are used in the primary of the amplifier that increase the isolation between primary and secondary, no third wire ground is needed.  This absolutely prevents any ground loops and completely minimizes current on the interconnect shields. This ia a contributing factor in the Soaring amplifiers' low hum. See Fanatics Only to learn how to perform the hum test yourself.


Soaring Audio, Inc.
2900 Fantasy Ln
Sparks, NV 89441-8593