VELODYNE HGS SYSTEM
Velodyne's prime focus is in providing low distortion loudspeaker systems that impress the dedicated audio-phile as well as the casual listener Velodyne developed its patented High Gain Servo (HGS) system to bring nonlinear distortion down to unprecedented levels on its ULD-15 and ULD-18 high performance subwoofers Your new Servo F-Series Subwoofer uses this same HGS technology to reproduce the music you enjoy without the excessive nonlinear distoritons that plague conventional loudspeaker designs
The Problem of Distortion
Nonlinear distortion is defined as any form of distortion that moves energy from one portion of the audio frequency spectrum to another, When a single tone is applied to a nonlinear system, the result is harmonic distortion, in which some of the energy leaves the system at integer multiples of the original input frequency When multiple tones are applied to the same system, intermodulation tones are produced, usually as simple combinations of the input frequencies, i.e., sums and differences of integer multiples of the applied tones In general, 3% nonlinear distortion is considered a reasonably small amount.
This means that 3% of the total energy leaving a loudspeaker is at frequencies other than the intended pure tone applied to the input. Our tests, however, indicate that about 0.5% nonlinear distortion at normal listening levels must be achieved before the average listener is truly affected by an improvement in bass response.
In the reproduction of deep bass notes, loudspeakers suffer from a "double-whammy" when it comes to
nonlinear distortion. First, most loudspeakers have much more than even 3% THD (Total Harmonic Distortion) at
low frequencies.
Even at 1 watt drive levels, many loudspeakers have 8 to 10% THD at 40 Hz With the 50 watt drive levels needed to produce realistic bass intensity, these distortion figures rise to astonishing levels. Second, the distortion products produced by a nonlinear woofer are much easier to hear than is the fundamental fre¬quency Consider a speaker driven at 30 Hz with 5% 3rd harmonic distortion If the 30 Hz tone is 90 dB SPL (Sound Pressure Level), equating to about 5 watts into a typical woofer, the 3rd harmonic, at 90 Hz, would be down 26 dB, or at 64 dB SPL. However, humans are more than 10 times as sensitive to sound at 90 Hz than at 30 Hz. The nonlinear distortion produced by a woofer must be weighted by the sensitivity function of the human ear. When this is done, it becomes clear that even a small percentage of harmonic distortion is unacceptable in a bass driver. It is also clear that high-order (3rd, 4th, 5th, etc ) harmonic distortion products are more serious than are 2nd harmonic distortion products th
ese studies have led to the conclusion that 0.5% is the maximum toler¬able amount of nonlinear distortion for a bass driver, and the less distortion the driver produces, the more capable it is of accurately reproducing live sounds
Since most woofers today have at least 10% THD when 0 5% is the maximum tolerated, minor improvements in woofer technology are essentially ineffective. Servo F-Series Subwoofers have about 90 times less nonlinearity than a typical 10 inch bookshelf speaker They have about 3 times less distortion due to their greater size and power handling capability, and about 30 times less distortion due to the Servo feedback loop The improvement is even greater if one considers exceptionally loud and deep tones such as organ pedal notes
An interesting result of this reduced distortion is that much of the bass, when played over a Servo F-Series Subwoofer, seems to actually be reduced. The truth, of course, is that the "missing" bass was never really there on the source material, and was never intended to be played by the artist, Distortion products introduced by conventional woofers actually make the entire bass spectrum sound louder.
Our Solution
The Velodyne Servo F-series Subwoofer System produces sound in the same manner as any conventional loudspeaker: a cone-shaped piston is forced to move by means of a linear motor composed of a coil situated between the poles of a very strong permanent magnet. In the Servo F-Series, however, this linear motor is a servo-motor, meaning that the motor is linked to the power amplifier as part of a control system The basis for the control action is the signal obtained from an accelerometer which is rigidly fastened to the voice coil The strategy of continuously monitoring the cone motion, and using this information as the basis for the corrective action, is called motional feedback This feedback technique is illustrated in the system block diagram, Figure 4.
The signal into the system is a time varying voltage which is an exact analog of the desired acoustic output from the loudspeaker The accelerometer output contains sufficient information to deduce the actual acoustical output of the woofer The servo circuitry compares almost instantaneously how well the acoustic output matches the command signal coming in. If and when there is a deviation, the servo circuitry modifies the drive signal to the amplifier so that the acoustic output from the speaker is a true representation of the input
In conventional loudspeakers, the drive signal into the speaker is virtually free from distortion but the speaker's acoustical output may be highly distorted. In Servo F-Series Subwoofers, the speaker coil drive signal is often highly distorted, or actually pre-distorted, in exactly such a way that the acoustical output from the speaker is free from distortion. Motional feedback has been used previously in an attempt to make small woofers sound like larger, more powerful ones. At Velodyne, the use of motional feedback is applied to make what is already a large and powerful woofer achieve a level of performance that has never before been reached in a home entertainment product.
HGS System Components
(Refer to Figure 4)
High and Low Pass Crossovers receive the incoming audio signals from your system pre-amp or power amp Frequencies above 85 Hz are diverted to your power amplifier while frequencies from the right and left channels below 40 to 100 Hz (depending on low pass crossover setting) are summed to monaural and passed on to subsequent stages in the subwoofer
Loop/Control Stage Incoming bass signals pass through a variable gain amplification stage designed to protect the subwoofer from being overdriven (see Supporting Circuits on next page) With a bit of added filtering, the bass signal is now ready to be compared with the feedback signal of the accelerometer. A composite signal is created which compensates for erroneous movement of the cone.
Figure 4: Servo Series System Block Diagram
Power Amplifier The Subwoofer's amplifier must provide power for distortion correction as well as basic music reproduction.
Custom Driver. Although our HGS electronics play the most important role in reducing distortion, the Servo F-Series driver must be carefully engineered and constructed so as not to undo the work that the HGS electronics have done A large magnet and voice coil are used to improve sensitivity, the cone is specifically shaped to reduce resonances, and ventilation holes are used liberally to eliminate whistling noises often associated with long-travel operation (essential for producing low frequencies at listenable volumes)
Accelerometer Perhaps the most critical component of the HGS system, the accelerometer measures the motion of the cone and relays it back to the loop circuits by means of an onboard charge amplifier and special feedback circuitry.
Supporting Circuits. A GAIN COMPRESSION circuit evaluates the amplitude of the incoming bass signal. The gain compressor is directly linked to the control stage and can thereby limit input signal gain in order to prevent amplifier clipping. If an extremely loud transient is detected, the gain compressor circuit will signal the COMPARA¬TOR circuit to immediately shut down the subwoofer's power amplifier for five seconds The comparator circuit will also shut down the power amplifier if it detects problems with the power supply
