User manual SUMMIT EQP-200B

[. . . ] Low Frequency Shelf / In / Out : This three-position toggle switch selects between a low frequency 6 dB per octave shelving filter starting at 50 Hz, or standard engaged operation, or bypass mode (which disengages the filters yet still passes through the tubes). When LF Shelf is engaged the other filters are still accessible. Low Frequency Boost : This potentiometer controls the amount of boost engaged for the 20, 30, 60, 100, 180 Hz frequencies. Maximum boost (a setting of 10) is about 16 dB. [. . . ] The equalizer has approximately a 20dB gain loss which must be overcome. This gain loss is overcome by the vacuum tube amplifier. The equalizer circuit feeds the first stage of the tube amplifier and the second stage of the amplifier has a cathode follower for low output impedance. The cathode follower drives the discrete 990 output stage. The tubes are operated class A with negative feedback. The power supply is all solid state and regulated. A Word About Tubes The tubes that are used in the EQP-200B are selected to give the best possible performance in the position that they are in on the printed circuit board. Switching them to different positions will cause performance deterioration on the audio path. Replacing them with "gold" or high end tube types may not be desirable. In cases that we have measured, these tubes have shown higher distortion and noise as compared to the tubes we have selected. In some cases, the so called "gold" or high end tubes have made the unit unusable. The reason for this is some of the "gold type" tubes are selected for high distortion in guitar amplifiers. Using gold tubes is no guarantee of better performance. All of these so called "Brand X" types are selected with 6. 3 volts AC on the heaters, whereas Summit Audio uses 5 volts DC on the heaters for longer tube life and lower noise. The reduced gain can raise the noise floor, increase distortion, and reduce headroom. For proper performance from a tube, the replacements must be selected using 5 volts DC on the filaments. The tubes we use are selected in the circuits that they are used in to ensure proper operation, long life, and low distortion and noise. In at least 50% of the cases we have tested, gear that has been used for several years actually have lower noise and distortion levels than when new. This makes the question of when to replace tubes difficult to answer. If a tube turns micro-phonic, the distortion will be obvious and the tube must be replaced. However, tube life will most likely be greater than 10, 000 hours of operation. Tubes are generally very reliable; don't replace them just because they are old. [. . . ] However, when used in noise, frequency response, and distortion measurements, weighting will alter the results. Completely flat frequency measurements are key to giving accurate specifications. Summit Audio devices are the highest quality professional audio gear and the specifications below are made with the flattest possible unweighted measurements, giving the most accurate results. OUTPUT: +4 dBu is the designated operating level. The output is balanced or unbalanced, using discrete 990 operational amplifiers with an impedance of 75 ohms. [. . . ]