If the forward gain is part of a feedback loop, the overall loop gain will also be high. The forward voltage gain is usually high (at least 40 dB).
VARIABLE BIAS VERSUS FIXED BIAS GUITAR AMP PC
Power capabilities vary widely depending on the application, from milliwatts in headphones, to a few watts in TV or PC audio, to tens of watts for “mini” home stereos and automotive audio, to hundreds of watts and beyond for more powerful home and commercial sound systems-and to fill theaters or auditoriums with sound.Ī straightforward analog implementation of an audio amplifier uses transistors in linear mode to create an output voltage that is a scaled copy of the input voltage. Audio frequencies range from about 20 Hz to 20 kHz, so the amplifier must have good frequency response over this range (less when driving a band-limited speaker, such as a woofer or a tweeter). The goal of audio amplifiers is to reproduce input audio signals at sound-producing output elements, with desired volume and power levels-faithfully, efficiently, and at low distortion. What are Class D amplifiers? How do they compare with other kinds of amplifiers? Why is Class D of interest for audio? What is needed to make a “good” audio Class D amplifier? What are the features of ADI’s Class D amplifier products? Find the answers to all these questions in the following pages. I don't understand why manufacturers haven't made one of those tube bias devices that gives a plate dissipation value along with the bias current it wouldn't be hard to do.Class D Audio Amplifiers: What, Why, and HowĬlass D amplifiers, first proposed in 1958, have become increasingly popular in recent years. Unless you are certain you are using the exact same manufacturers tube that the amp was designed with, and you are using the manufacturers grid voltage or cathode current bias specification, you should check your plate dissipation when you set your bias if you want to be safe. It's also a good idea to keep your plate dissipation to say 75% of the max rating. However, if you leave the screen grid current in the reading, and just assume the cathode current as the plate current, it's a nice safety factor of a few watts. Why is this important? If you bias a power tube in a seventies 50W head (with B+ 500V), you'll be driving the tube a lot harder in the Super Lead, and may burn it up.Īctual plate dissipation cannot be read from a cathode current reading as this is actually the plate and screen grid current summed together. So the bias current should be set with the B+ in mind so that you do not burn up the tube from excessive plate dissipation. The plate dissipation at idle is set by the bias level, and is basically the B+ voltage on the power tubes * plate current. All power tubes have a maximum plate dissipation value, rated in watts. This is done by adjusting the bias voltage until this target current value is reached, so the bias voltage itself is irrelevent once the quiescent current is right.Ĭlick to expand.If I may expand on this a bit, there is another parameter to take into account. The bias voltage itself is irrelevent, what you are trying to do is set the amount of current flowing in the valve, under quiescent conditions, to an acceptable level. One important thing that people loose track of on the subject of biasing their valves: In this way the circuit bias' itself but it's certainly not "fixed" bias voltage. Notice that this puts the cathode at a more positive voltage than the grid, this is effectively the same as having the cathode at ground potential and putting a negative voltage on the grid, like in fixed bias.Įven with a by-pass capactitor in place the "bias" voltage will fluctuate with signal and different valves will produce different bias voltages as they naturally draw different currents. The cathode resistor produces a voltage by virtue of the current flowing through the valve from cathode to plate. Remember the term "fixed bias" comes from the fact that the bias voltage remains fixed no matter what valve is there or whether there's signal or not.Ĭathode bias produces it's bias voltage supply itself. You cannot adjust the bias voltage on a Mesa because they want you to buy their valves which are known to operate well with their fixed bias voltage level. Most Marshalls use this bias technique, with the exception of the 18 watters and a few others.įixed bias is usually adjustable, as in you can set the "fixed" voltage by adjusting a pot. Fixed bias means that bias voltage is applied from an extenral source, usually a negative bias supply, and remains constant throughout the flow of signal or with different valve samples.
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