Before discussing the individual subassemblies and their functions, we need to define the most important performance concepts: noise figure, gain, output power and distortion (distortion = lack of linearity).
4.1.1 Noise
This should be easy to understand but actually the subject is complex. There are various forms of noise (AM noise, FM or PM noise), there are various forms of noise distribution (white noise, pink noise, just about any color noise you might imagine) and there are different contributors that are mixed and summed to make
up a noise spectrum, thermal noise, flicker noise, random walk noise, and so forth.
We are going to keep things simple by considering only thermal noise. This can be justified by the fact that it’s the major contributor to audible noise – noise that we can hear – in a guitar amplifier. This form of noise is universal, it’s
everywhere. We know this and we even know how much noise is everywhere.
The expression that relates noise to some other factors of an amplifier is:
Noise = (4 x K x T x B x R) 1/2
Where noise is in volts, K is Boltzmann’s constant (a constant is an
unchangeable number, frequently a physical limit, this one is named for the German scientist that first established it from his study of thermodynamics), T is temperature in degrees Kelvin, B is bandwidth in Hertz and R is resistance in ohms. The 1/2 term means the same as taking the square root of the entire expression.
Let’s not get bogged down in the mathematics, we’ll simplify this term to represent AVERAGE noise at room temperature and over the loudspeaker frequency range, say 100 to 5,000 Hz (allowing for harmonic content) and
assuming a nominal input (pickup) resistance of about 10,000 ohms. That leaves us with a simpler, more manageable expression:
Thermal noise at the input of an audio amplifier is approximately 1 microvolt.
This doesn’t include noise contributed by the amplifier. This is important to understand: ALL AMPLIFIERS HAVE NOISE. A theoretically perfect noiseless amplifier would still have an audible “hiss” at the speaker output because the
“hiss” results from universal thermal noise (remember that it’s everywhere).
4.1.2 Noise figure
This term is used to describe the amount of excess noise in the amplifier. Since different amplifiers have different amounts of gain, the amount of noise at the OUTPUT of any amplifier can’t be directly compared to another amplifier. In order to make comparison easier, noise figure is always referenced to the INPUT of the amplifier.
OK, let's make sure that we "get" this: the amount of noise emanating from a 100 watt amplifier will be much louder than the amount of noise emanating from a 5 watt amplifier even though the noise at the "inputs" of both amplifiers is equal. Intuitively, we should be able to understand that a 100 watt amplifier has a lot more gain than a 5 watt amplifier (at least 13 dB more, we'll get to that in a moment). Repeating: to make a "fair" comparison, we must always reference noise at the INPUT of an amplifier.
Noise figure does NOT include the thermal noise voltage that we discussed in the previous topic. Amplifier noise figure is determined roughly by three things:
The circuit resistor values, as modified by the amount of circuit gain at the resistor location.
The noise contribution of the gain devices in the circuit (whether tube or transistor) and where the gain devices are located in the circuit.
Location and turns ratio of transformers (more on this later) in the circuit.
Some general rules for designing a low noise figure amplifier follow. Use low-noise tubes (or transistors), biasing them for optimum low-noise; low-noise is highly dependant on bias conditions. Keep resistor values low and distribute gain in the circuit so that gain always precedes resistance (or loss) if possible. We will discuss this in more detail as we get into preliminary design discussions.
Before we get too absorbed in trying to keep the noise figure low, there are practical reasons for not worrying excessively. If the object is to design a guitar amplifier, it is a low-fidelity, bandwidth limited and not very linear amplifier to start with. We don’t require a high standard of performance from a guitar amplifier, so let's not obsess about noise for now.
4.1.3 Gain
Another easily understood concept, it’s just the ratio of the output power divided by the input power. (Power is usually used because it is independent of input and output load/impedances.) The industry standard for power gain is the decibel. The name comes from inventor of the telephone, Alexander Graham Bell, an immigrant to the U.S. from Scotland. The definition of one Bel is an awkward unit of measure (it’s not sensitive enough for most purposes) so we use 1/10 of a Bel therefore obtaining the ”deci-bel” term. Determining the gain in decibels if the input power level and the output power level are known is as follows:
Power gain, in dB = 10 x log (Pout/Pin) Voltage gain, in dB = 20 x log (Vout/ Vin) Current gain, in dB = 20 x log (Iout/ Iin)
where the output and input power levels are in watts, the output and input voltages are in volts and current is in consistent units of amperes.
We divide the output power (Pout) by the input power (Pin) then take the logarithm (base ten - not natural log) of the result and multiply it by ten. Most pocket calculators can do this computation quickly and easily.
Let’s work out an example … suppose that we want to design a guitar amplifier that will produce 30 watts of power. Our input signal comes from a guitar that can typically produce 0.1 volt. We need to find the OUTPUT voltage level from the 30 watt amplifier. This is obtained from the simple expression:
P = E2 / R and E = (P x R) 0.5
where P is power in watts, E is voltage in volts and R is load impedance in ohms.
E = (P x R)0.5 = (30 x 8) 0.5 = 15.5 volts RMS
"RMS" means "root mean square" and is similar - although not identical - to
"average". We need to make sure that our voltage units are the same for input level and output level before calculating the gain. The input level was 0.1 volts
"peak to peak" - not the same as "RMS". To convert peak to peak to RMS volts:
Vrms = .354 x Vp-p = .354 x 0.1 = 0.035 volts RMS (35 millivolts) And the overall voltage gain required is
Av = 15.5 / .035 = 437.63
Av expressed in dB is 20 x log (437.63) or about 53 dB (from the calculation of decibels described above).
4.1.4 Output Power
This parameter determines how “loud” an amplifier will be when connected to a loudspeaker (obviously the efficiency of the loudspeaker is also a major
contributor). For optimum performance, the speaker impedance should be the impedance that the amplifier was designed for. Generally speaking, output power levels of a vacuum tube amplifier will decrease when the amplifier is connected to either lower or higher impedances than the design value. The amount the amplifier power will be reduced is approximately proportional to the ratio of the design impedance to the actual impedance.
For example, connecting a Fender "Bassman" designed to operate into a 4 ohm load, to an 8 ohm load will produce about 25 watts instead of the specified 50 watts. There are reliability considerations regarding incorrect impedances as well. Always try to operate the amplifier with a “matched load. If you’re not sure what impedance your amplifier is designed to operate with, it’s marked on the rear panel output jack to the speaker.
Over the years, and still continuing, manufacturers tend to pick their own definitions of output power, the reasons for this are usually apparent. All other electronic industries that specify this parameter do so in an unequivocal manner.
One can be assured that if a musical instrument amplifier is not specified the same way as all other equipment, there is a reason that the manufacturer doesn't want to specify it conventionally. The standard power measurement includes all of the following information:
Power level, watts, RMS, continuous Percentage distortion
Frequency range
The power level must be in RMS watts, continuous, to be meaningful and the other two parameters must also be specified for the measurement to be valid.
4.1.5 Distortion
Usually self-explanatory, except for the means of measurement. An easy way to visualize distortion is to observe (or imagine) the shape of the input waveform, on an oscilloscope for example, where voltage change is displayed as a function of time. If this can be visualized, then also imagine the output signal displayed on the same instrument.
Distortion is the difference between the shapes of the two sets of displayed information but NOT the difference in their magnitudes - which would be voltage gain (or loss). Observation of distortion on an oscilloscope is the grossest (least sensitive) form of measurement. Specialized instrumentation (distortion
analyzers, spectrum analyzers) are required for determining distortion less than about 5 percent.
Recall that we described “linear” operation above? Here’s a review: “Linear”
means that, if the input and output characteristics are plotted on graph paper, the result would be a straight line, hence “linear”. Another definition is now available to us: a linear amplifier produces an output signal that is an identical copy of the input signal, the only difference being “amplitude” or volume level.
Non-linear operation is distortion, whether it is caused by compression, clipping, phase shift/group delay variation, harmonic generation, whatever. If the output signal is not identical to the input signal EXCEPT for amplitude, it is distorted.
Distortion and output power parameters must always be used together to convey or obtain meaningful information. Describing a power level is meaningless unless the level of distortion present when the power was measured is also stated. The converse is also true. (Industry standard for vacuum tube amplifiers seems to be the measured RMS continuous power level at 5% distortion and with a 1 kHz test frequency.)