32

Tubes also sometimes develop a microphonic response — they will respond to ambient noise and vibration. This can be an insidious problem since measurements in a quiet room will indi- cate perfect performance. Gently tapping the tube shields while listening to the output at a normal monitor level should reveal nothing more than a slight “clank.” On a peak-reading meter connected to the VT-15 output, with 50 dB gain, any microphonic response above -55 dBm is excessive. Replacement is indicated unless the VT-15 always operates in a quiet and vibration-free environment.

Remember that vacuum tubes may be quite hot during operation. Protect your fingers dur- ing tube replacement. The VT-15 should be turned off before removing tubes. Allow at least one minute for the filter capacitors to discharge before tube removal or insertion.

Tubes are made of glass and will break if dropped or even bumped in a critical area. Handle with care. There are only a limited number of NOS (new old-stock) 6072A tubes in the world. Treat each one carefully.

Electrolytic Capacitors

The VT-15 is designed and built to last for a long, long time, and it is possible that some com- ponents (e.g. electrolytic capacitors) may reach the end of their life long before the equip- ment becomes obsolete. The electrolytic capacitors used in the VT-15 typically will last at least twenty years. If there is a measurable and/or audible increase in 120 cps noise, the fil- ter capacitors should be suspected. They should be replaced with new capacitors of equiva- lent capacitance and voltage rating, and the replacements should be specified for a minimum ten-year service life.

Electrolytic capacitors are also used as plate decouplers. In choosing a replacement, the same considerations as with the filter capacitors should be followed.

VU Meter Calibration

The meter amplifier circuit is stable and re-adjustment of the meter calibration is not normal- ly required. If the output reference level needs to be changed, here is the procedure:

1.Remove the VT-15 top cover (see above).

2.Terminate the output in the same impedance as the VT-15 will normally be feeding. This will almost always be a high-impedance bridging termination of 20k ohms or more.

3.Apply AC power. Allow at least an hour warm-up. Feed a 1 kc tone at about -45 dBm into the VT-15 input.

4.Measure the output and set the Preamp Gain control for +4 dBm (or whatever refer- ence is desired).

5.Set the Meter Calibration trimmer potentiometer for 0 VU on the VT-15 meter. Be sure the meter is in the VU position. This control is a 25-turn pot so the adjustment is easy. A small screwdriver or alignment tool is necessary. The control is located on the relay printed circuit board on the top of the chassis, on the far left side and it is the only trim pot on this board.

6.Replace the top cover. Be sure the ventilation slots are over the tubes (towards the back).

D.W. FEARN

VT-15 Recording Channel

Page 29 Page 31
Page 30
Image 30
Crown Audio VT-15 manual VU Meter Calibration
Page 29 Page 31

VT-15 specifications

The Crown Audio VT-15 is a robust loudspeaker system that has gained recognition for its advanced technologies and impressive audio performance, making it ideal for various applications, including live events, installations, and broadcast scenarios. Built with durability and sound quality in mind, the VT-15 serves both professionals and enthusiasts in the audio industry.

One of the standout features of the VT-15 is its high-efficiency design, which is achieved through advanced driver technology. The loudspeaker is equipped with a premium 15-inch woofer that delivers powerful low-end reproduction, ensuring a solid bass presence that enhances the overall listening experience. This, combined with a high-frequency compression driver that operates at 1 inch, ensures clear and articulate treble frequencies, resulting in a balanced sound profile suitable for different musical genres.

The VT-15 incorporates Crown’s patented Waveguide technology, designed to provide consistent and controlled sound dispersion. This feature allows the loudspeaker to effectively cover large areas without experiencing significant drop-off in audio fidelity, making it a great choice for medium to large venues. The controlled directivity also means that listeners in various positions within the space can enjoy a similar audio experience, enhancing the performance of any event.

Another defining characteristic of the VT-15 is its rugged construction. Built with a reinforced enclosure, it is designed to withstand the rigors of touring and heavy use in fixed installations. The cabinet is constructed from high-quality materials that minimize vibration and resonance, further contributing to the clarity of sound. Its lightweight design allows for easier transport and setup, making it a practical choice for audio professionals on the go.

In terms of connectivity and compatibility, the VT-15 features versatile input options, allowing it to integrate seamlessly with various audio systems. Whether used in conjunction with other Crown products or compatible equipment from different brands, the VT-15 ensures flexibility in configuration.

Overall, the Crown Audio VT-15 stands out as a high-performance loudspeaker that combines advanced technologies, durability, and sound fidelity. Its powerful drivers and innovative design make it a reliable choice for any professional audio application, providing users with the quality and performance they demand. Whether for live music, presentations, or installations, the VT-15 delivers exceptional audio experiences that elevate any event or setting.
Top Page Image Contents