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Velodyne SubContractor Series SC-12 Subwoofer and SC-1250 Power Amplifier

Part III

August, 2006

John E. Johnson, Jr.

 

On the Bench

The SC-1250 is a switching amplifier, which means it turns the output voltage on and off very quickly to produce the musical signal. I utilized an Audio Control AUX-0025 filter that is designed for use in testing switching components. It sits between the amplifier output and the Audio Precision test instrument input. It begins reducing the signal above 20 kHz and rolls off rapidly. Below is a graph showing the frequency response of a loop through (Audio Precision output connected back to the input) with and without the AUX-0025 in the circuit. The idea is that switching amplifiers produce high frequency switching noise above the audible band that can get into the test circuit and contaminate the results. It has to be noted that the filter reduces this noise but does not completely eliminate it.

With the SC-1250 configured for the SC-12 driver, output voltage is restricted to 50 volts RMS. Using an 8 ohm load (equal to having one SC-12 driver connected), THD+N at 50 volts output was 1.38%.

Using a 4 ohm load (equal to having two SC-12 drivers connected), THD+N was 2.82% at 50 volts RMS output. This is rather high for a power amplifier within its rated output, but I suspect the noise component of this number - remember, the measurement is THD plus Noise - is a large percentage of the total value. In any case, I could not hear whatever distortion was there at low frequencies. That's what makes switching amplifiers so useful with subwoofers. They deliver high power and run cool, which is what a subwoofer needs. Also, note that I have tested several switching amplifiers, and the tendency is towards higher THD+N than typical A/B amplifiers, so the THD+N measured here is not unusual.

I was able to get 900 watts RMS with a 50 Hz sine wave into a 4 ohm load before the amplifier shut down.

The following six graphs were taken by placing our calibrated microphone 1 foot from the front-firing driver of one enclosure (the second enclosure was not connected for these tests).

At 20 Hz, I could get a maximum of 95 dB output. THD+N was 10%. This is an acceptable level of distortion from a 12" driver at 20 Hz, and lower than I thought it might be. Theoretically, a 6 dB increase in maximum output will be obtained with a second enclosure placed near the first one, or 3 dB if the second enclosure is placed somewhere else in the room.

At 25 Hz and 100 dB output, THD+N was slightly lower, at 9.1%.

31.5 Hz gave a much lower THD+N.

And, at 40 Hz, even lower.

By 50 Hz, THD+N was very low, at 2%.

80 Hz is a likely crossover point for many home theater systems, and at this frequency, THD+N was less than 1%.

The impedance of the SC-12 driver is well above 50 ohms for its general operating range of 20 Hz to 65 Hz. The peak at 70 Hz is due to the enclosure resonant frequency. This high impedance means the amplifier will never have any problems driving it. Note that this is reactive impedance, not the DC resistive impedance, which measured 8 ohms. The reactive impedance is high, in part, because of the huge magnet. When the voltage is AC, such as 20 Hz, the changing electromagnetic field in the voice coil, produced by the alternating 20 Hz current, is resisted by the driver's permanent magnet. Velodyne mentioned to me that they designed the drivers with high impedance so that several of them could be connected to the single SC-1250 amplifier in parallel.

The room response was collected using two enclosures several feet out from the walls, and 6 feet apart. The microphone was placed midway between them, 1 meter from each driver, pointing between them, forming a shallow triangle. The crossover in the amplifier was turned off for the measurement. The response is flat from 40 Hz to 100 Hz and rolls off below 40 Hz, but appears to stay close to within the spec of 22 Hz to 120 Hz 3 dB.

Conclusions

Velodyne has established a fine product line in the SubContractor SC series of drivers, all operated with the SC-1250 amplifier.

Coupling sealed enclosures with high impedance drivers insures the bulletproof setup that custom installers will find very appealing, but most importantly, will deliver deep, tight bass to consumers that will continue to play, and play, and play.
 

- John E. Johnson, Jr. -

Copyright 2006 Secrets of Home Theater & High Fidelity

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