Amp Compensation

Description: Adding negative feedback to a vacuum tube amplifier is more complex than it appears. The process involves connecting a resistor from the output transformer's secondary back to the cathode of the pre-phase inverter tube. However, it is crucial to ensure the correct phasing of the transformer to achieve negative feedback instead of positive feedback. Failure to properly compensate the amplifier can lead to oscillations, which may occur beyond the audible range for humans and even dogs. Such oscillations can damage expensive speaker systems, including super tweeters and crossover networks, resulting in significant financial loss. This article outlines a method for testing and modifying an amplifier using an oscilloscope, a function generator, and a calculator. Although no calculus is needed, the procedure requires careful measurement and is not suited for those lacking experience. Formulas will be provided without derivation, but further study in "Electronics for Non-Engineers" is recommended. Feedback in control systems is typically negative, as it serves to regulate a system. While this discussion centers on vacuum tube audio amplifiers, the methods may also apply to transistor amplifiers, though the reader must adapt the concepts accordingly. The terms "open loop gain" and "closed loop gain" are relevant but will not be the focus; instead, loop gain will be emphasized. A non-inverting amplifier configuration is used, where the audio signal from the volume potentiometer enters the grid of the 12AX7 tube, serving as the non-inverting input. The point marked "Inject signal here" connects through a resistor to the cathode of the 12AX7, which acts as the inverting input. The loop gain, relevant to control systems engineers, is measured by injecting a signal at the appropriate point and assessing the output at the break in the loop, usually at the output transformer secondary connection to the feedback resistor.

In the context of vacuum tube amplifiers, the implementation of negative feedback is a critical process to enhance performance and stability. The feedback loop's configuration must be meticulously designed to avoid introducing unwanted oscillations, particularly those that can occur at ultrasonic frequencies. These high-frequency oscillations can lead to catastrophic failures in high-end audio components, such as damaging tweeters and crossover circuits, which are sensitive to excessive power.

To begin the feedback implementation, a resistor is connected from the secondary side of the output transformer back to the cathode of the pre-phase inverter tube, typically a 12AX7. This configuration allows for the feedback signal to be fed back into the amplifier's input, effectively reducing distortion and improving linearity. It is essential to ensure that the feedback signal is out of phase with the input signal, which is achieved by carefully considering the transformer phasing.

The loop gain, defined as the product of the amplifier's gain and the feedback factor, is a central concept in this process. The feedback factor, denoted as B, represents the fraction of the output voltage that is fed back to the input. The relationship between open loop gain (A) and closed loop gain (A') is fundamental in analyzing the performance of the amplifier. The closed loop gain can be expressed as A' = A / (1 + AB), where A is the open loop gain and B is the feedback factor. This equation illustrates how negative feedback reduces the overall gain of the amplifier while improving its stability and bandwidth.

Testing the amplifier's performance involves injecting a known signal at the designated point in the circuit and measuring the output to assess the loop gain. An oscilloscope is used to visualize the waveforms, allowing for the determination of the amplifier's response to feedback. Adjustments to the feedback resistor value may be necessary to optimize performance and eliminate any oscillatory behavior.

In summary, the addition of negative feedback to a vacuum tube amplifier is a nuanced process that requires careful attention to detail and a solid understanding of the underlying principles of feedback control. Proper implementation can lead to significant improvements in audio fidelity and amplifier reliability, making it a worthwhile endeavor for audio engineers and enthusiasts alike.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 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.

If you attempt to find supplementary information in an electronics text you are likely to find more confusion than help. If seeking supplemental studies find a book on analog control systems if you can in this age that is totally enamored of digital technology.

Control systems are usually found in factories, ships, and aircraft, and incorporate electro/mechanical transducers such as motors and position sensors. This is not a tutorial in such systems. It will be limited exclusively to vacuum tube audio amplifiers. The methods could also be applied to transistor audio amplifiers but you, the reader must make the transference yourself.

Where A` is the closed loop gain, A is the open loop gain, and B is the fraction of the output that is fed back to the input. These terms "open loop gain" and "closed loop gain" should be familiar to the reader. If not you should review the subject in Electronics for Non-Engineers. Use your back button to return here. Concentrate on the noninverting amplifier because that is exclusively what you will find in vacuum tube amplifiers.

Inverting amplifiers are sometimes used in transistorized amplifiers but as stated above they will not be covered here. A noninverting amplifier needs a noninverting input and an inverting input. In the circuit below the audio signal comes from the volume pot and goes into the grid of the 12AX7, this is the noninverting input.

The point labeled "Inject signal here" goes through a resistor to the cathode of the 12AX7. The cathode is the inverting input. We will not be using the concept of either open loop gain or closed loop gain here. We will deal with loop gain. The loop gain is how control systems engineers treat a feedback system. This is the gain that is measured by opening up the loop, injecting a signal at the proper point to allow signal to pass through the amplifier, and measuring the output at the point where the loop was broken. Theoretically the amplifier could be broken at any point but there are practical limits. The most practical point to do this is where the output transformer secondary connects to the feedback resistor as shown in