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Audio |
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Balanced |
A balanced signal means that neither connection is grounded. The theory is that any outside interference will be induced equally into both conductors, and will arrive at the receiving end and get cancelled out. Some early systems used centre tapped transformers where this centre tap was grounded. These days, many inputs and outputs are ‘electronically’ balanced – so avoiding the use of transformers. Pity! However, some of these don’t like to have one side grounded. The ‘A’ range of desks all had balanced inputs and outputs – both transformer isolated. |
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Capacitor |
As transistor amplifiers work at low impedance, most coupling, and de-coupling, capacitors were electrolytic. These work by the DC potential difference across the capacitor to produce the insulating layer. If an electrolytic capacitor has not been used for over a year or so, the insulating layer will disappear! The capacitor then (preferably) needs to be replaced, or ‘re-formed’. Do not power up any device if you suspect that power has not been applied for some time, without replacing, or re-forming, the capacitors. Under these conditions, capacitors are quite likely to explode! ) See the page on the site map on re-forming capacitors.) |
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Db |
Decibel. The standard method of measuring an audio level. It’s a log scale where an increase of 3db (or 6db, depending on whether you’re looking at volts or power (I’m getting tired now!)) doubles the volume of what you’re listening to. (This is not English, though since the BBC has relaxed its standards, I can too!) There, I’ve done it again! |
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Diode |
Diodes, or diode logic, were used to control relays – IN2004. They were also used in the transformer less output stages in System 12. |
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Distortion |
transf |
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Dbm |
This is the standard method of measuring audio levels. In most professional equipment the standard reference level is 0 VU, which is the equivalent to +4 dbm. At the time (late 60’s) the BBC used PPMs as these measure peaks - to stop the transmitters from blowing up - whereas the record business used VU meters, which give you a better indication of how "loud" the sound is. (The USA used VUs even for broadcast, but, even now, I believe that the BBC set standards that the whole world followed.) |
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EQ |
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Fader |
The initial desks used either Elcom, EMT or Painton, quadrant, 600 ohm constant impedance, stud faders. Later we used Penny and Giles carbon track (flat) faders. The faders used to make up about 50% of the component cost in any desk. Faders are actually potentiometers, but are associated with the master gain control of an input or output channel. They are also linear in action (though log in characteristic) and bigger than an knob! |
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Feedback |
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Fuse |
The trouble with fuses is that they blow! All power supplies should be fitted with ‘slo-blo’ fuses that can tolerate an initial inrush several times there actual rating. |
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Ground |
It’s vital that that the desk, and all other units, are properly and effectively grounded. Essentially this means that you have two grounds, one for mains earth and a separate one joined at one point only to the mains earth. It is vital that you only ground each lead once. (See grounding.) |
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Hum |
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IC |
Integrated circuit – a collection of transistors packaged in one small case. |
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Inductance |
EQ |
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Impedance |
600 ohms |
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Insert |
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Jacks |
One of the great things about life is ‘standards’. For example, a jack plug is a standard – isn’t it? Yes, it is – or rather it isn’t! Jacks fall into A, B, micro , and mini – which can be 2.5mm or 3.5mm. And, balanced or un-balanced! Jack plugs, like everything else, get dirty and need to be cleaned frequently. (See WD40) Also, the sockets, and you can get jack field ‘tooth brushes’ for the purpose. |
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Knob |
We went to great lengths to get ‘pretty’ knobs, and knobs that no other mixer manufacturer was using. The ‘A’ range used largely Bulgin knobs. |
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Logic |
LS’7400 |
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Loudness |
Equalisation built into some HI-FI amplifiers that accentuates the bass as the level is decreased. This is (was) an attempt to compensate for the fact that our ears tend to recognise less bass frequencies at lower levels. |
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Meter |
Most desks had VU meters (which see), but we were sometimes asked to fit PPM meters (which see). |
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N |
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OP amp |
‘Operational Amplifier’! Once thought to be the solid state answer to all audio amplification problems. They have a ‘balanced’ input which is ideal for professional studio applications. However, unless they are discrete amps, they suffer from a number of problems. First, they very noisy compared to a conventional low noise transistor. Second, they don’t like having one of their input legs grounded. Third, the output is not balanced and will go into a most unpleasant distortion at the drop of a hat. (Also, diodes tend to be used in the output stage. With a discrete OP amp, the diodes, and their biasing can be chosen with due care.) |
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Overload |
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Plug |
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PPM meter |
Peak Programme Meter. Measures peaks of the signal, and has a very fast response time, and a log scale. Before the age of limiters, these were designed by the BBC to warn when their powerful transmitter valves would blow up. It’s a mystery to me why the Americans always managed to survive on VU meters! But, now we’re in the digital domain, peak signal meters have come back into focus as even 1db of overload will cause the most distasteful (to the human ear) distortion. |
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Potentiometer |
Rotary ‘pots’ were used for functions like echo (reverb) sends, fold back etc. Sometimes also for the EQ – where we didn’t used switches. Most pots were 10K log, still available. Sometimes a noisy pot can be ‘lubricated’ by drilling a small hole in the top, and squirting a very small amount of WD40 into the hole. However, a noisy pot can also be caused by the (electrolytic) capacitor feeding it starting to leak DC. |
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Power |
All our desks ran off +- 15 volts and +12 volts for relays and lamps. A couple of very large ‘A’ range desks that used TTL (LS7400 series gates) also had a separate +5 volt supply. Sometimes we made our own power supplies, but mostly we bought them in. We always used linear supplies, as I wasn’t prepared to risk noise getting onto the supplies or onto the ground line. They have to be kept well away from the microphone transformers or you’ll get high levels of induced hum. |
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Q |
The ‘Q’ of a coil or inductance is a measure of its efficiency. When an inductor is used in an EQ, as used in all our EQs, the higher the Q the more pronounced is the effect – be it boost or cut – at its resonant frequency. We used coils in ferrite cases to get high values of Q. If the Q was to high, we ‘slugged’ the coil with a resistor in parallel. |
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Resistor |
Resistors generally do not need reforming like capacitors, though some high value ones will age, and drift out of tolerance. Transistor amplifiers tend not to use these, so you do not have to worry when restoring Sound Techniques desks. |
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Relay |
I favoured reed relays, as they are small, take little coil current and are virtually bounce free. We found some Plessey relays that also had self wiping audio contacts. Some relay contacts need a few milli-volts across them before they will make reliable contact. With audio levels, you don’t have the necessary volts. |
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Switch |
Rotary switches are difficult to find these days. The best solution is to clean the contacts with WD40, which will both clean and lubricate. In really bad cases, clean with a small brush in white spirit, but you must then lubricate with WD40. |
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Transformer |
I had toyed with the idea of a valve (tube) mixer, but I didn't like the thought of making lots of metal boxes! There was only one type of amplifier used, and each had transformer output. I am a great believer in the use of transformers: they neatly avoid ground loops, the signal can be sent over some distance - the low impedance and balanced output helping to keep out noise. However, the most significant advantage is that if you let the transformer go into distortion first in the chain, the waveform will be smoothed off, rather than a straight clip. The distortion produced in much more acceptable to the human ear. radiometal |
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Transistor |
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Tube |
Our friends across the water call them tubes. Whatever, they have to be used with transformers (unless the output is a capacitor coupled cathode follower) and hence the inevitable distortion is more acceptable to our ears. |
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Un-balanced |
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Unity gain |
Where you can insert a device, e.g. a limiter, and the input level is the same as the output level. |
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Valve |
See tube above!! |
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VCA |
Voltage Controlled Amplifier. HUH! |
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VU meters |
A VU meter tends to measure the average level, whereas PPMs (which see) measure peaks. The VU meter is probably more useful in seeing a visual representation of the ‘loudness’ of the signel – not to be confused with the term ‘loudness’ – which see. We started with Ernest Turner meters, but later moved onto Sifam, mainly because they were illuminated with small festoon lamps. In order to read the correct level, VU meters need to be fed with a 3K6 resistor in series with the signal. It is important to remember that the signal sees a 3K6 ohm resistor in series with a diode. This may not matter if the amplifier output impedance is less than 600 ohms. Although we never got around to doing it, the ideal solution is to use a buffer amplifier before the VU meter. (Although you have to check this for unity gain at frequent intervals.) Zero VU corresponds to +4dbm where the impedance is 600 ohms, and 4 on a PPM meter. |
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WD40 |
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Y |
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Z |
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