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Tube power amplifier negative feedback

We're strong advocates of vacuum tube amplification. We think tubes do a lot of things right--superior rendition of depth and ambiance, realistic timbres, lively dynamics, warmth--the usual. But conventional vacuum tube amplifiers, as well as sounding warm and endearing, also do a whole lot of things wrong. Output tube drift, soft top octave, mushy bass, high noise levels, slow overload recovery, plenty of euphonic distortion for that rich fat sound, and erratic frequency response into difficult speaker loads. Much can be forgiven for the sheer musicality of tube designs, but no one can pretend they're close to being perfect. What if we told you that our very different tube amplifier had highly accurate autobiasing, a nearly infinite damping factor, acted as a pure voltage source into almost any real world speaker load, had a full power bandwidth of nearly 60kHz 3db down point , had distortion below 0.

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Tube power amplifier negative feedback

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WATCH RELATED VIDEO: Negative Feedback Explained / Tube Amp Circuit Fender 5F1 Champ Clone

Feedback and Fidelity – Norman Koren

A higher feedback resistor value is among the design aspects that make an old-school Marshall amp have more grit or grind than an old-school Fender. Hi Jeff, First, I want to tell you how much I enjoy your articles. That said, let me try to answer your questions, although it will be technical at times. Although there may need to be some changes, deletions, or additions of smaller capacitor values in one type of construction verses another in some amps, these suggested values 2x 0.

Sorry, but as with most changes modifications, improvements, etc. In a nutshell, negative feedback is incorporated in an amp design as a way to minimize distortion in the output stage of an amplifier. This is done by sending an out-of-phase signal into the phase inverter circuit, reducing its gain and, thereby, distortion. This is especially important in hi-fi amps to reduce THD total harmonic distortion and possibly other types of distortion as well.

We like distortion. So, why not just remove all the negative feedback from your Deluxe Reverb? If we take a look at the feedback resistor in a typical Fender-style amp, we would generally find a value of ohms. So what do we do? You can see the schematic for this in Fig. Now you have a variable feedback signal that can go from a stock setting CCW to far less feedback, yielding a more aggressive-sounding output stage.

With the current setup, the signal from the output stage is fed back equally over the full frequency range of the amp, thereby reducing the gain over the entire frequency range. But what if we could be selective about the frequency range? Well, we can, if we add a capacitor in series with the resistor.

This will begin to reduce the amount of low frequency signal that is fed back. See Fig. The smaller the capacitor value, the less low frequency is fed back. This way we can reduce the gain of the higher frequencies while keeping the low end more full. Anything with a 25V or greater rating should work fine. Start with maybe a 0. If you want to get more creative, you could even use a multi-position rotary switch to be able to select between values Fig. If you decide to install both and drill a hole in your amp , you would then have full control over the frequency and level of your feedback loop.

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Tone Tips. Bass Bench. Joe Gore's Recording Guitarist. Vintage Vault. Esoterica Electrica. Mod Garage. Bottom Feeder. PG Giveaways. Stay Connected. Enter email address Subscribe. Wanna shake up an old Fender? Try modding the negative feedback loop.

Is this effective or do you think the Deluxe Reverb needs a certain amount of negative feedback? Maybe add a switch for this function? Best regards, Mark Hunter Lebanon, Ohio. The most dangerous voltages are stored in electrolytic capacitors, even after the amp has been unplugged from the wall.

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Audio distortion and feedback

You build it, and you get the exact response you were asking for. But only if they start with a circuit and find out what it does afterwards. This method takes out the guesswork. Just work backwards from the result you wanted. Ask for a stable amp, you get a stable amp. They only care about the high pass filter response, which in their lingo is called the Noise Transfer Function.

Fundamentally, all electronic devices that provide power gain (e.g., vacuum tubes, bipolar transistors, MOS transistors) are nonlinear. Negative feedback.

How to Voice a Tube Amplifier

A further object is to provide a high fidelity amplifier with a high degree of reliability and capacity to deliver high overloads at frequencies down to substantiallyzero frequency or direct current as well as in the video region of the frequency spectrum. A further object of the invention is to obtain prompt and faithful response to transient conditions and high fidelity amplification of speech and other signals of constantly varying frequency and amplitude which make the transient characteristics of the amplifier of as great importance as the characteristics under steady state conditions. Another object of the invention is to make'the magnitude and phase of the output voltage substantially independent of load impedance variations that would normally be encountered and to provide a generator having the power to be the principal factor in the determination of magnitude and phase of the output voltage. Still another object is to apply corrective feedback for the reduction of distortion Without creating undesirable transients. A further object is to eliminate the necessity for directcurrent flow in the load circuit, transformer coils, choke coils or devices which may be subject to magnetizing or to other unilateral effects. An additional object is to permit grid excitation toenter the positive grid region without creating distortions 'or transients in the coupling circuits. A further object is to accomplish maximum coupling elficiency and minmium reactive eifects in a simple circult.

Audiophile Myths About Amplifier Negative Feedback Conclusion

tube power amplifier negative feedback

Skip to content. All Homes Search Contact. This amplifier is shown because it's the one I started this series with. What you are trying to do is obtain a certain amount of negative feedback, which is determined by the ratio of the two feedback resistors the series feedback resistor from the output and the shunt feedback resistor that goes to ground, which form an attenuator. I designed and built a watt per channel amplifier in the first half of the 90s.

Updated: Apr 18, Part of a blog series Tube Amplifier Circuits Explained.

local and global feedback in amplifiers

What does negative feedback accomplish? 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.

Oh no, there's been an error

This being the entertainment industry, I hope everyone is having a good time. Feedback is very large subject, and I am going to limit myself to some simple tutorial comments and a discussion of phenomena associated with complexity in distortion created by nonlinear gain stages, negative feedback, and the audio signal. Taken singly, these phenomena seem simple enough, but when they interact, they create distortions out of proportion to what you expect from the specifications found in product brochures. There are linear and non-linear forms of distortion. Tone controls are a good example of circuits with linear distortion.

In order to use large values of negative feedback to reduce distortion in a circuit, the circuit has to produce a lot of voltage gain.


Tube sound or valve sound is the characteristic sound associated with a vacuum tube -based audio amplifier. Many electric guitar, electric bass, and keyboard players in a range of popular, rock, funk, blues, reggae and jazz genres also prefer the sound of tube instrument amplifiers or preamplifiers. Before the commercial introduction of transistors in the s, electronic amplifiers used vacuum tubes known in Great Britain as "valves". By the s, solid state transistorized amplification had become more common because of its smaller size, lighter weight, lower heat production, and improved reliability.

US2777020A - Direct coupled high fidelity amplifier - Google Patents

Global negative feedback takes a small portion of the output signal and re-injects it back into the input stage which greatly improves linearity and distortion performance of an amplifier. A tasteful amount of negative feedback is the key: too much and you're into poor transient response and phase shift problems. We generally use somewhere between 4 to 14 dB's of feedback in our line preamplifiers and within that window is a definite point of maximized stability with optimum sonic performance. So we give you the choice of where you want to be in several of our products.

There are many ways to tweak the tone or "voice" of a guitar tube amplifier.

About Negative Feedback

LOG IN. I am not talking about the kind you get as a flaky seller, but as used in amplifier design. It just seems to me that a lot of amp designs advertise "zero negative feedback" as a selling point. As I understand, NFB is a loop taken from the amplifier output and fed back into the input to keep the amp stable. This sounds like it should be a good thing.

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  1. Dayle

    What charming message

  2. Jorrel

    Well done, you have been visited by the remarkable idea