Amp Compensation

Posted on Feb 5, 2014

Adding negative feedback to an amplifier, especially a vacuum tube amp is not as easy as it may seem. What is there to it Just connect a resistor from the secondary of the output transformer back to the cathode of the pre phase inverter tube. Right Not so fast. Of course you have to get the phasing of the transformer right so the feedback will b

Amp Compensation
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e negative instead of positive. But there is a little bit more than that. If you don`t compensate the amplifier correctly it will oscillate anyway. What is worse the oscillation will be above the range of human and probably even dog hearing. If you connect it to a pair of 2, 000 dollar speaker systems with this condition, there goes 4, 000 bucks. Ultrasonic, not supersonic, oscillation will burn up the super tweeters and crossover networks in short order and may even do damage to the mid range and woofer. That`s a chance you don`t want to take. This article will explain how to test and modify an amplifier using only an oscilloscope, a function generator, and a calculator. No calculus is required but the procedure takes time and care to get the measurements correct. In other words this procedure is not for the faint of heart. Formulas will be presented without derivation however I recommend further study in "Electronics for Non-Engineers" which is linked from this page. The way in which the feedback equation is derived in electronics textbooks is to assume a general case in which the feedback can be either negative or positive depending on the sign of the gain. If the gain has a negative sign the feedback will be negative. If the gain has a positive sign the feedback will be positive. Control systems engineers on the other hand assume that the purpose of the feedback is to control something and therefore it MUST be negative....

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