User manual SUMMIT TPA-200B

[. . . ] This completely variable potentiometer changes the input gain for the TPA-200B. Use in conjunction with the output gain to get the desired amount of tube saturation in the signal. Turning the output gain down and the input gain up increases the amount of tube distortion. This completely variable potentiometer changes the output gain for the TPA-200B. [. . . ] The 990 op amp used for the low impedance output stage is made of discrete components and then potted for stability. The line and mic inputs are transformer coupled and are wired for balanced operation. A ¼" unbalanced plug inserted into the Hi-Z input disconnects the line and mic inputs and feeds the grid of the tube directly. The second stage amplifier has a cathode follower for lower output impedance. The output gain control is between the cathode follower and the 990 output stage. This design was chosen because it works best in the "overdrive" mode, and sounds most like tube microphones in the clean mode. The power supply is all solid state and regulated and the tubes heaters are D. C. The overload indicator circuit is made of a Hi-Z amplifier that is used for signal pick off from the first tube stage. The amplifier is designed so it does not affect the operation of the tube circuit. The sampled signal is then fed to a comparator circuit that turns the LED on when the signal is ­5dBM with respect to clipping. The overdrive indicator works the same way, but instead of picking up the signal on the plate of the first tube, it picks up the signal on the cathode of the last tube. A Word About Tubes The tubes that are used in the TPA-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. 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. If a tube turns micro-phonic, the distortion will be obvious and the tube must be replaced. [. . . ] This filtering was developed so that sound pressure level (SPL) measurements can more nearly match human's non-linear hearing characteristics. 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. Specifications: · Output: +4dBM corresponds to 0 VU. [. . . ]