Negative feedback is a commonly misunderstood subject. Global negative feedback refers to the "feeding back" of a small amount of signal from a later part of the circuit to an earlier part, usually from a tap on the output transformer back to the phase inverter.
What does negative feedback accomplish?
The use of global negative feedback does several things: it flattens and extends the frequency response, it reduces distortion generated in the stages encompassed by the feedback loop, and it reduces the effective output impedance of the amplifier, which increases the damping factor. All of these things affect the tone in some manner.
The flattened, extended frequency response obviously changes the tonal character by removing "humps" in the output stage response and producing more high and low end frequencies.
The distortion reduction makes the amp sound cleaner and more "hi-fi", up to the point of clipping, with less output-stage generated noise.
Perhaps the main difference for the "feel" in a negative feedback amplifier, as opposed to a non-negative feedback amplifier, is the increased damping factor produced by the negative feedback loop. The decreased effective output impedance causes the amp to react less to the speakers. A speaker impedance curve is far from flat; it rises very high at the resonant frequency, then falls to the nominal impedance around 1kHz, and again rises as the frequency increases. This changing "reactive" load causes the amp output level to change with frequency and changes in speaker impedance (a dynamic thing that changes as the speakers are driven harder). Global negative feedback generally reduces this greatly. This can be good or bad, depending upon what you are looking for. Negative feedback makes the amp sound "tighter", particularly in the low end, where the speaker resonant hump has the most effect on amplifier output. This is better suited for pristine clean playing or a tight distorted tone, while a non-negative feedback amp has a "looser" feel, better suited to a bluesy, dynamic style of playing.
The other disadvantage of a negative feedback amplifier is that the transition from clean to distorted is much more abrupt, because the negative feedback tends to keep the amp distortion to a minimum until the output stage clips, at which point there is no "excess gain" available to keep the feedback loop operating properly. At this point, the feedback loop is broken, and the amp transitions to the full non-feedback forward gain, which means that the clipping occurs very abruptly. The non-negative feedback amp transitions much more smoothly into distortion, making it better for players who like to use their volume control to change from a clean to a distorted tone.
How much feedback to use?
The amount of voltage fed back determines the amount of gain reduction and the amount of distortion reduction, as well as the effective output impedance. The more voltage fed back, the less distortion, the lower the effective output impedance, the higher the damping factor, and the lower the gain of the stages enclosed by the feedback loop.
Typically, in a guitar amp, somewhere around 6-10dB of feedback is used. If you have 6dB of feedback, for instance, and it takes 2V at the phase inverter input to achieve output clipping, if you removed the feedback, it would only take 1V at the phase inverter input to achieve output clipping. In other words, there is a voltage gain reduction of 6dB, or a factor of two, in the stages enclosed by the feedback loop. This is achieved by feeding back a certain percentage of the output voltage to an earlier point in the circuit, the phase inverter. The more voltage fed back, the more the voltage gain reduction, as mentioned previously.
The series feedback resistor, in conjunction with the resistor to ground, determines the amount of voltage being fed back. If you want to feed back more voltage, you make the series resistor smaller, or the shunt resistor larger, or you use a higher impedance tap on the output transformer.
The actual resistor values used in the feedback attenuator aren't that important, as their ratio determines the amount of feedback. The shunt resistor value is usually fixed by the phase inverter design requirements, and the series resistor is then sized according to the desired amount of feedback, given the voltage available at the output. Note that Marshall typically uses 100K/5K attenuator, while Fender uses a 820ohms/100ohms. You can get the same attenuation from a 10K/500ohm pair as you would from a 100K/5K pair. In addition, if you were using a 100K/5K attenuator running from the 16 ohm tap, you would get roughly the same amount of feedback if you used a 47K/5K attenuator running from the 4 ohm tap. Note that the tap voltages are not linear with respect to the impedance, it varies linearly with the square root of the impedance, that is, the voltage on the 8 ohm tap is not half the voltage on the 16 ohm tap, rather, the voltage on the 4 ohm tap is half the voltage on the 16 ohm tap. It helps if you think of the equation for power: P = V^2/R. If you have 100W into 16 ohms, the voltage is V = sqrt(100*16) = 40V RMS. If you have 100W into 8 ohms, the voltage is V = sqrt(100*8) = 28.28V RMS. If you have 100W into 4 ohms, the voltage is V = sqrt(100*4) = 20V RMS.
Where to apply the feedback?
Feedback can be applied to either side of the phase inverter in a global negative feedback amplifier that has the feedback wrapped around to a long-tail, or differential, type of phase inverter, because the phase inverter has two inputs, one inverting, and the other non-inverting, with respect to the output. The overall effect of the global negative feedback on frequency response, output impedance, gain reduction, and distortion reduction will be the same with either form of feedback. The only thing that will change will be the input impedance of the overall feedback network, and the method of setting the overall gain (more on this in the following paragraphs).
In one case, the result will be a non-inverting amplifier, where the feedback resistor is connected from the output of the amplifier to the "other" side of the phase inverter (not the side with signal applied to it). The gain set by the ratio of the feedback resistor and the resistor to ground from that input. The input impedance of this type of global negative feedback amplifier is very high, so it won't load down the circuit driving it. Most early Marshalls are good examples of this type of global negative feedback amplifier. This is by far the most common form of global negative feedback amplifier used in guitar amplifiers.
In the other case, the feedback is applied to the "signal" side input of the phase inverter. This forms an inverting amplifier configuration, with the gain being set by the ratio of the value of the feedback resistor to the "input" resistor. The input impedance of this type of global negative feedback amplifier is set by the value of the input resistor, so care must be taken in the design of the circuit to avoid loading down the previous stage, as the value of the input resistor usually must be very low in order to get any decent amount of gain with not too large a feedback resistor value. In some cases, there is no input resistor at all, so the effective value of the input resistor is the output impedance of the previous stage. In either case, the overall feedback gain can be set by varying the feedback resistor. The Fender AB165 Bassman circuit is a good example of this type of global negative feedback amplifier. The "input" resistor is formed by the output impedance of the 7025 channel-summing stage, which uses local negative feedback to give a low, controlled output impedance so it functions well in the global negative feedback gain-setting function, and it also functions as a channel summing amplifier with very good channel-to-channel isolation, due to the "virtual" ground formed at the summing junction of the local negative feedback amplifier. A very efficient design, indeed!
See this paper for more details on the local negative feedback amplifier: http://www.aikenamps.com/FeedbackAmp.htm . Also, this paper has more details on designing non-inverting global negative feedback amplifiers: http://www.aikenamps.com/GlobalNegativeFeedback.htm .
Frequency response shaping via feedback
Closely related to the subject of negative feedback is the use of frequency-dependent elements in the feedback loop to shape the overall response of the amplifier. Most guitar amps have a "presence" control, which boosts the high frequencies. It accomplishes this not by actually boosting the highs in the forward path of the output circuit, rather by cutting the amount of high frequencies being fed back. This effectively reduces the amount of negative feedback at those higher frequencies, which results in a boosting of the highs at the output. Some guitar amplifiers have a "resonance" control, which does a similar thing, by cutting the amount of low frequencies present in the feedback loop, thereby boosting the low frequencies in the output. The amount of boost is equal to the amount of negative feedback. If the amp has 6dB of feedback, there can be at most a 6dB presence or resonance boost. This means that if you reduce the amount of feedback for more gain, you will also reduce the effectiveness of the presence and resonance controls, likewise, if you increase the amount of feedback, you will increase the effectiveness of these controls.
There is a danger in using too much negative feedback, however, as the amplifier can become unstable and oscillate, particularly with reactive loads. In addition, the more stages the feedback is applied around, the more likely the chance for oscillations, as there are more phase shifts within the forward path, due to coupling capacitors and other circuit capacitances.
Other types of feedback
Some amplifiers, most notably the Marshall Valvestate transistor amps, use negative current feedback in lieu of negative voltage feedback, or a combination of the two. Global negative current feedback has a similar effect on distortion reduction, but instead of decreasing the effective output impedance and increasing the damping factor, it actually increases the effective output impedance and decreases the damping factor. This makes the amplifier's output voltage vary with variations in speaker impedance. Since a speaker's impedance varies radically with frequency, a current feedback amplifier will tend to feel more "tubey" than a voltage feedback amplifier, because of this speaker/amplifier interaction.
In addition to global negative feedback, amplifiers usually have some form of local negative feedback, but sometimes this is not as apparent. A cathode follower is an example of an amplifier stage with 100% negative feedback. This is what gives it the high input impedance and low output impedance, and the near-unity maximum gain. Some amplifiers will use a single-stage inverting amplifier circuit with local feedback from the plate to the grid via a large resistor and coupling cap. The gain of these inverting stages is set by the value of the feedback resistor in proportion to the value of the input resistor.
Copyright © 1999, Randall Aiken. May not be reproduced in any form without written approval from Aiken Amplification.