Transcript
Common Terms for the Musician Often Heard and Not Understood by Dave Montalbano I teach at guitar schools and online and I must frequently explain terms to students that are not easy to explain to a non-technical audience. I have prepared this handout to try and clear-up confusion.... but to make it simple I have had to try and simplify a technical subject by not providing a complete technical explanation. The reader is encouraged to research these items. HEADROOM: also called “Dynamic Headroom”. The capacity of an Amplifier (Amp) to reproduce loud sounds without distortion. This means the range in amplitude of the signal from a nominal to peak value before the signal distorts or “clips”.
Distortion: Any change in a signal as it passes through a device. The type of distortion we are most concerned about in audio is “clipped signals” through compression or amplitude.
Distortion, Overdrive, Fuzz: Introducing clipping into the signal to produce a desired sound. At low signal levels, Transistors and Tubes behave linearly but when the signal from a transistor reaches a limit it will provide a “Hard Clip” with the waveform peaks cutoff. Tubes behave non-linearly when saturated and will provide a “Soft Clipping“ of the signal in which the wave peaks are compressed to the limit. Each sounds different to us. Tube “Clipping” (Overdrive) creates odd-order harmonics which may be desired depending upon the type of music.
Impedance: Impedance is AC Resistance measured in Ohms. Impedance changes with the Frequency of the signal. In an audio amplifier, the Speaker is a device that will provide AC resistance that varies with the frequency of the signal. If the Amplifier does not provide the proper signal some of the energy will be reflected back into the Amp power section (transformer). The job of the Output Transformer is to provide the current to drive the speaker and the impedance matching to efficiently transfer the energy with little reflection. Device impedance is usually specified by the manufacturer at a specified frequency (IKHz).
Compression: Also called audio level compression, in which the dynamic range, the difference between loud and quiet, of an audio waveform is reduced, used in guitar effects racks, recording studios, etc. Signal compression always introduces distortion, which is usually not objectionable, if the compression is limited to a few dB. Presence: A Presence control boosts the upper mid-range frequencies. Original Presence controls used a negative feedback loop so more of the higher frequencies were dumped to ground but by using negative feedback, it affected the dynamic range
rather than the amount and so it differs from a standard tone control. Later Presence Control systems used an active filter to “boost or cut” frequencies in the 3.9khz range.
Capacitance: Capacitance is the ability of a body to hold an electric charge. A Capacitor is an electronic component designed to hold a charge and perform a function in a circuit. The amount of charge a Capacitor can store is its “Capacitance” and the design and material it is constructed from determines characteristics like what frequencies it can pass and how fast it can change its charge state. Basically any conductor surface can store a charge and so capacitance is present in most areas of a circuit. Audio capacitors are capacitors with a value and dielectric material (A non-conducting material separating the two charge areas in a capacitor) that can pass audio frequencies in a pleasing manner. A Filter capacitor is a capacitor that is designed to store and discharge a large charge into a circuit usually during the process of rectifying AC to DC. When used in several stages with diodes, they can remove the ripple current present during the rectification process. They are usually made of an electrolytic material. Filter type capacitors are generally not suitable for audio signals.
Vacuum Tube: or thermionic valve (elsewhere, especially in Britain) is a device used to amplify, switch, otherwise modify, or create an electrical signal by controlling the movement of electrons in a low-pressure space. In its simplest form it can be used to conduct or stop the flow of electrons in a circuit (hence reference as a “valve”). An example of this is a rectifier tube. Tubes that can amplify a small signal (voltage) are preamp tubes like the 12ax7 (dual triode) which has a gain factor of about 100. Other examples are the 12AT7 with a gain factor of about 60. Power tubes are designed to amplify current to drive a high current device like a speaker. Tubes like the 6L6 can dissipate about 30 watts, KT88 42 watts, 6V6 20 watts, EL84 6 watts, etc.
Rectification/Rectifier: The process of converting an AC power to DC. Most circuits require a DC voltage to operate. By using large value Capacitors and 2 or more diodes, AC current can be smoothed out to a DC form. In guitar/bass amps, the process is the same except that in lower wattage tube amps the rectification can use a tube instead of diodes. Large filter capacitors are still required. Tube rectifiers cannot handle the high current required for amps usually above 40 watts and in these amps you will find diode (solid state) rectifiers and not tubes. Which sounds better in an amp is a personal preference. Because a tube rectifier can “sag” when it begins to draw a large current and because many people like this sound, tube rectifiers are preferred by many musicians.
Ground: This is a very complicated area but there are really just a few important things for the musician (not the technician) to know: a. Audio equipment must be properly grounded to minimize noise. b. Grounded electrical systems are referenced to the “Earth” ground. c. Multiple audio sources tied to the same ground can form a “Ground Loop”. Audio equipment must be properly grounded: In the 70ʼs, the US Electric code changed to a three prong grounded system. The earlier two prong system contained the Hot wire and a Neutral wire. Both wires carried current and because the plug on a device could be “plugged in” either way, the current was present in all parts of the device. Early radios and tube equipment had the chassis completely covered with a wood or plastic cabinet to protect the user from a potentially lethal shock!!! The modern system uses a polarized plug for safety. Grounded electrical systems are referenced to the earth: At the electrical meter where the power enters the building, the power company drives two 8 foot copper rods into the ground which becomes the “earth ground”. This is the bare copper wire that is attached inside the electrical panel and throughout the building. It is connected to the round lug opening on the plug. When a device is attached it is directly attached to the copper rods “earth ground”. Where the confusion comes in is the “white or neutral wires” are connected to ground at the panel but they carry current in the system. In modern audio (and other) equipment the earth ground is connected to the chassis and the neutral is connected to the other end of the power transformer. Ground loops: A ground loop occurs whenever there are multiple attachment points and ways a small current can flow into the circuit. This can be a small induced current from the magnetic fields and can be the source of hum and noise. Ground loops must be addressed in amp design and construction but can also occur when multiple audio sources are connected to the same ground connection (i.e., plug). Grounds in the Guitar: It is a common practice to solder all ground connections and connect all the backs of Potentiometers (POTs) to the ground connection on the output jack. This works great for unshielded cavities and humbucker type pickups. If the cavity is shielded, and the POTs are in contact with the shielding, there is a potential for a ground loop if the back of the POTs are also joined. In many cases this makes the guitar noisy. You notice this when your hand comes closer to the strings and you change the magnetic field. Ground in an AMP: Vintage amps with two prong plugs are dangerous and if the amp is to be played safely, it is important that it be changed over to a properly grounded 3 prong system. I have seen basically two ways that vintage amps are rewired to a three prong system. Both work but one is right and one is wrong. In the “wrong way” you will see the neutral wire wired to one end of the fuse. Many times this is because they have left in a functioning “ground” polarity switch... dangerous. The correct way is to remove all wires to the ground switch (disconnect the “death cap”) and wire the “Hot” wire to the
end of the fuse receptacle and the neutral wire should be connected directly to the correct power transformer wire. The Hot goes through the fuse on to the power switch. The Hot wire is always switched and fused in a modern circuit. If a quiet amp develops noise and is plugged into the same outlet (i.e., at your home) then the problem is most likely a tube especially if the hum occurs after the amp comes to temperature. If the amp is solid state and develops noise or the noise is always there it may be a capacitor. These situations are electronics related and generally not “ground” related. If the noise develops in a new location (i.e., the band is playing at....) then try another outlet and see if its quieter. They may have a noise source like a “blender” or electric motor device plugged into the same branch circuit which affects your amp. In this case you need effective power filtering devices.
Vibrato and Tremolo: These terms are sometimes used interchangeably but are actually different by definition. Vibrator refers to the periodic variation of the “Frequency” of the note while Tremolo refers to the periodic variation in the “Amplitude” of the note. The “Tremolo” arm on a guitar is actually a “Vibrato” arm because it varies the pitch (frequency). Phaser and Flanger: In the Phaser effect, the signal is split (usually into two parts) and one part is processed and placed out of phase and mixed with a low frequency oscillator. It is then mixed back into the original signal creating the effect. When the signals are delayed it created the “Flanger” effect.
Reverb: This is a “delay” effect that simulates the reflections of a sound arriving at your ear at slightly different times. In the guitar amp it can be generated digitally by mixing delayed signals or using an analog system like a spring reverb and mixing the delayed signal back with the original.
Chorus: In the real world with a human chorus, many people sing and it is not perfect. Some people are very slightly off key and others sing at different pitch. Electronically, the signal is mixed with combinations of itself that are slightly delayed and off frequency. Noise: Anything that is mixed with the signal that is not desired. Noise can be generated in an electronic circuit in many ways but in audio equipment it can be “thermal” (generated by the heating of components themselves) or “RFI” (radio frequency interference) generated in the environment by the Sun, cell phones, gas discharge lights like fluorescents, etc.
Shielded Cable/Shielding: The signal produced by a guitar pickup or a microphone is very small (generally less than .6 VPP). Because the noise in the environment can be just slightly less, it is necessary to “shield” the signal as it travels to the preamplifier. If noise is allowed into the signal before it is amplified, it will also be amplified. Shielded cable is used to protect these low level signals by surrounding the
signal carrying wire (called the center conductor) with an aluminum or copper braided wire that is connected to earth ground. This can protect the signal from RFI noise since it is collected and sent to ground by this shield. Thermal noise is not a concern at this stage. Guitar cavity shielding performs the same function by protecting the unshielded wires and POTs from RFI. Shielding is connected to earth ground closest to the ground connection at the output jack. If the POTs are grounded then the shielding should not be in contact with the POT bodies. Shielded cable is designed to carry small current and voltages. This is why a shielded instrument cable cannot be used to connect the amp to a speaker. It could seriously damage an amp (Output Transformer).
Preamplifier: A Preamplifier is the first stage of the amplification process that takes a very small signal (like from a Mic or guitar) and increases its “amplitude” (voltage) to a level that can then be sent through the other effects processing like tone controls, etc. In a tube amp this is frequently a 12ax7 tube with an amplification factor of 100. As the signal is processed through tone controls it loses amplitude and must be re-amplified (usually the other half of the 12ax7) to continue to the power amp section. Noise that comes into the preamplifier is also amplified and so care must be taken to make sure the signal coming in has very little noise. The most common noise in the Preamp is caused by thermal currents. This can be from a defective component or a failing tube.
Power Amp: Once the signal has be sent through the Preamplifier it is ready to go to the Power Amp so a large amount of current (amps) can be added to drive a Speaker. A Speaker is a large electromagnetic “pump” designed to move a lot of air and reproduce the frequencies of the sound that was initially collected by the mic or guitar pickup. At this stage the signal is very large compared to any noise and so Speaker Wire does not need to be shielded cable. Speaker Wire must also be of a gauge (diameter) sufficient to carry a large current. The Speaker and Output Transformer form a “system” that must efficiently send a current with minimum reflection of the energy back into the amp. Thus the impedance between the Amp and the Speaker must be matched as closely as possible. Power ratings for a Power Amp are subject to interpretation in how a manufacturer measures the power. Look for the “RMS” rating. Speaker Power Ratings: This is a tough one because there is no independent rating authority for Speaker power ratings (WATTS- unit of power). Speaker manufacturers are known to exaggerate their ratings. There are two types of ratings you will frequently see. The truest rating and the one you should consider when pairing with an Amp is the “RMS” ratting. RMS is a mathematical way of averaging the power dissipation at the speaker coil- RMS stands for “root, mean, square”. The other type of
rating is “PEAK WATTS”. This is generally 2 times the RMS rating so a 300 watt peak rating means about 150 watts RMS.
Instrument Cable: A low capacitance shielded audio cable designed to conduct a low level signal. Audio cable has a characteristic impedance of 50 ohms. It cannot be used to connect a Speaker to an Amp and such use can seriously damage an Amp. Speaker Cable: Designed to carry larger signals, this is usually unshielded wire 18 gauge or larger depending upon the output power of the amp. The use of speaker cable as Instrument Cable will not cause damage but because it is unshielded, it can be very noisy.
Potentiometer(POT): A Potentiometer is an electronic component that introduces “resistance” into the circuit. A Potentiometer is a “variable” resistor with a shaft connected to a moving conductor that slides along a “resistive strip”. POTs come in various values (ohms the unit of resistance) and in circuits they are designed to attenuate (reduce) the signal level. When used as a Volume Control in an Amp, the value is chosen to cover the range from signal quiet to the maximum value of the output signal. Quiet is maximum resistance and full volume is zero resistance. When used as a Volume Control in a guitar, the value is chosen to completely quiet the signal level produced by the pickups. For single coil pickups the normal value is 250 kohms and for Humbuckers the value is 500 kohms. When used in a RC tone network a portion of the signal is routed through the Tone POT and through the audio capacitor to ground. Since high frequencies pass a capacitor easier than low frequencies, the higher frequencies are passed to ground. Another way to say this is that the tone circuit removes the higher frequencies and the sound has more “bass”.
Linear Taper and Audio Taper POTs: When you turn the knob on a POT it rotates a certain amount from maximum resistance to minimum resistance. A linear POT reaches 50% of itʼs rated value half way through the knob rotation. You find linear POTs used mainly on amps and the exact controls can vary depending upon the designer. Blend controls and Mix controls are usually Linear. Some Amps use Linear POTs as Volume Controls and some use Audio Taper POTs. Audio Taper POTs mimic how the human ear “hears”. This is not linear but is “logarithmic”. You may find Audio Taper POTs as Volume Controls on Amps (ever wonder why you canʼt get the Amp up past 4?). Treble and Bass POTs are usually Audio Taper; Middle usually Linear; Presence and Boost usually Linear.
Tone Stack, Tone Control: Using the properties of the audio frequency capacitor and a resistance to create the proper impedance in the circuit, the tone control colors the sound by passing higher frequencies to ground. By using a variable resistor (POT) the amount of frequencies sent to ground can be controlled. This is called a Tone Stack or Tone Control and it is made up in most circuits by a simple RC network (resistor/ capacitor). There are some Amp circuits that use induction and some circuits that use more unique tone control like the “Baxandall” circuits in the 1972 Orange Amp. Guitar tone controls use simple RC circuits where the higher frequencies are sent to ground through a POT. Some designs like some Gretsch and Gibsons used a more complicated network of multiple capacitors and resistors on a rotary switch.
Guitar Pickups: A transducer is a device that changes one form of energy into another. The guitar pickup is a transducer designed to change vibrational energy (from soft-magnetic strings) into an electric analog of the frequency. Composed of permanent magnets wrapped with thousands of turns of fine enameled coated wire, the pickup is like one half of a transformer. As the string vibrates in the magnetic field, the charge component is transferred to the coil and sent to a preamp stage to amplify to a usable level. The number of turns and the design create the “sound” characteristics of that pickup. A Single Coil pickup has a single coil which is separated spatially on the guitar from other pickups. The output is usually from 100 mv to 500 mv. A Humbucker pickup is basically two single coil pickups placed next to each other and wired in Series. The wire on the individual coils is reverse wound to each other and thus noise is out of phase and cancels. A Humbucker can output anywhere from 250 mv to 1 volt. The above describes a “Passive” pickup system. “Active” pickups are magnetic coils similar to the above but low impedance (less windings) and to make up for the smaller signal it must be “pre-amplified” in the guitar before it can be sent to the Preamp in the Guitar AMP. This involves a transistor network or “OP AMP” and generally some equalization or tone control (this requires power to run the circuit hence the need for a battery on-board). Which sounds better is usually a matter of taste but most musicians do not like the cold sound of an Active system but those playing heavy distortion types of music like that sound.
Transformer: A device that transfers electrical energy from one circuit to another through inductively coupled conductors. Induction is a magnetic flux process and through a series of “windings” of wire, a varying magnetic field is generated in the “primary” winding. The varying magnetic field induced a voltage in a secondary winding. If a load is connected to the secondary winding, a current will flow. The ratio of the number of windings of the primary coil to the secondary coil will determine the
voltage and current capabilities. It is thus possible to create a primary coil hooked to 120 volts of AC power and through the secondary with more windings, create a higher voltage (note- current varies inversely with voltage). The Power Grid uses this principle to get the correct voltage and current to your house. The power is sent at extremely high voltages (Force) through transmission lines to your neighborhood. On a pole or box near your home is a transformer that takes the extreme high voltage and steps it down through a transformer to your electrical panel. Because there is an inverse relationship of current and voltage, we send a higher voltage to high current requirement appliances so we use less current. A device that might require 30 amps at 120 volts would require 15 amps at 240 volts.
Guitar Amp Power Transformer: In a tube Amp, the primary is 120 volt AC power input and the secondary is wound with “taps” into certain numbers of windings to get desired voltages. So you will have a high voltage tap for the B+ tube supply and other voltages for things like the filament heaters and rectified voltage requirements of the amp. For a solid state Amp, the power requirements are different and the high voltage and current requirements do not apply so solid state amps have smaller physical size transformers.
Guitar Amp Output Transformer: The output transformer on a tube amp must carry a high current load from the output tubes to drive a speaker. This is done by reducing the high voltage and thus increasing the current in the secondary. Transformer windings have impedance and so the transformer is designed so that the impedance of the secondary matches the impedance of the load. This reduces the amount of reflected energy and makes the system efficient. When there is a great mismatch or the load is infinite (no speaker plugged in), all the energy is reflected and damage to the transformer can occur. It is possible to take “taps” off various windings in the secondary for other impedances. So when using all the windings of the secondary you might have 16 ohms but tapped in the middle you use half the windings and get 8 ohms. Most musicians like the sound of all the windings and so they prefer to connect to the highest impedance setting (16, 8, 4 or even 2 on some amps). It is important to use the proper speaker impedance for that transformer. A Fender output transformer for a Blackface Twin Reverb for example is designed for an impedance of 4 ohms. There are no other settings or taps. The Output Transformers on some solid state Amps (like old Sunns) behaves differently. Here the Amp must “see” a minimum impedance but can work with speakers of various impedance without problems. Using various impedance speakers will change the output power so for solid state amps the output power is specified for an impedance (i.e., 300 watts into 4 ohms; 150 watts into 8 ohms etc.).
Soldering: The process of electrically joining two or more conductors by melting a lead/zinc metal known as solder at the connection point. Solder is a metal that melts at about 400 degrees F. Temperatures of about 600 degrees are needed for guitar wiring
up to 700 degrees to solder the Ground connection on the back of POTs. When the solder joint has not reached a temperature that forms a good electric bond, it is said to be a “cold” solder joint. This is a reference to the dull “spherical” appearance of the joint. A cold joint may have electrical resistance and cause spurious connection. As a soldering iron tip ages, it oxidizes forming impurities on the tip. This means it cannot get as hot and becomes more prone to cold joints. Frequent cleaning of the tip by rubbing it on a damp sponge prior to each connection is advised to remove the oxidation that naturally forms. For Guitar and Amp work, a variable temperature solder station is required capable of taking small to medium chisel or flat tips that are interchangeable. The station does not need a digital display but should be capable of 75-90 watts and temperatures to 800-900 degrees.
