THE AMPLIFIER

 

Vox combo amp, model AC30

 

In the series about rock music hardware, we take a look at the amplifier.
Technical and musical greetings from
Kurt Starlit
- aka CykelKurt

 

 

This series includes:
1. The electric Guitar 2. The Stomp Box 3. The Microphone 4. The Amplifier 5. The Speaker 6. Organ & Synth 7. Record & Music Production 8. Drums & Rhythm Boxes

 

 

Latest update: 2016.June.21

 

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Amplifier
Amplifier or amp is an ambiguous term, which in a rock context means:

EITHER an amplifier without (built in) speaker(s). Often, the amp is further divided into two parts with the pre-amp in one cabinet, and the power amp in another (separate) cabinet.

OR (in most cases) a combo amp, meaning a single cabinet with built-in amplifier and speaker unit(s). A combo amp is sometimes also called a speaker amp.

 

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VOX was the dominant amplifier brand in European rock music in the 1960s. It was used, among others, by The Shadows, The Beatles, Manfred Mann and many more. In this way, the VOX brand got itself an effective (and cheap) promotion of its products.

A key reason for its position was, that the Jennings (VOX) company offered sponsoring amplifier equipment to a number of well-known groups. Jennings paid installation and service, and in return, the group had to use VOX equipment exclusively.

The Selmer company used nearly the same procedure with, for example, Gerry & The Pacemakers (see page 11 of Danish Alfred Christensen 1966 catalogue).

Previously, though, the group had made advertising for the VOX brand, e.g. take a look at this picture with the subtitle: 1964 VOX promotional photo of Gerry & The Pacemakers with two AC-50 "big box" amps and a Foundation Bass enclosure.

Selmer amp

Selmer Zodiac

Looking back, it's easy to see that Selmer made a mistake by offering the chosen few (groups) to sponsor their equipment partially. The groups had to pay part of the cost. So, as a result, most English groups selected VOX as their chosen brand. As a result, Selmer never reached the same sales figures as VOX, but had to stay way down the sales list together with most other manufacturers, such as Laney, Sound City and WEM.

From an objective point of view, VOX amps were no better (or worse) than any other. First of all, it was a matter of IMAGE, that so many groups chose VOX. For the major famous groups, it was also a question of sponsorship, which brand to choose. It had a high price for Selmer and others to neglect this fact.

By the way, talking 'bout VOX, they also made guitars. Take a look for example at this survey of equipment - VOX Gear.

About myself, in 1965 I fell in love with the VOX Symphonic Bass, looking very much like the Fender Precision bass, but at a much lower cost. Together with a VOX Foundation bass amp (equipped with an 18" Goodman Audiom 91 speaker) this equipment lay a nice bottom at small venues.

 

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Tube sound
People are talking 'bout the clear crisp sound of a valve amplifier. And there is a reson for this sound. It is due to the fact, that a tube is - apart from amplifying the original signal (sound) - also produces an amount of second harmonics.

If you, as an example, send a clean ("clean" meaning without any harmonics) 100 Hz tone into the amplifier, you will of course get 100 Hz on the output as well. But also an audible portion of second harmonics 200 Hz. Listening tests have demonstrated, that this is something the human ear likes!

An amp that doesn't distort in any way, is sounding "dull".

 

Idle Current and Classification
How much distortian an amplifier produces, is depending on its idle current. Idle current means current drawn by the output stage, when no signal is sent to the amplifier input.

You probably have heard about "Class A" amplifiers. This classification means, that there's a hell of a lot current drawn by the amplifier - even when no signal is sent into the amp.

The explanation is, that the valves in the output stage are adjusted in such a way, as if the volume was turned fully up.

The advantage of a high idle current is low distortion.

The disadvantage of a high idle current, unfortunately, is that only a small amount of the current (from the power supply) is left for the output stage - and thus the sound from the loudspeaker.

Class A output amplifiers generally have low output power (because most of the power is used by the idle current) and low distortion.

To the best of my knowledge, class A amplifiers are best used by hi-fi enthusiasts or maybe as a control amplifier in a studio. Used as PA and guitar amps, they are hopeless.

Vox amplifiers are probably class B (= low idle current), meaning that most of the current an output tube is consuming, is used to produce output power.

As already mentioned, class B tube also produce a portion of second harmonics.

Finally we have Class C, meaning that the amp don't consume any power or current at all (zero idle current). When there is no signal (sound) on the input, there will be no current drawn in the output stage.

This means that allmost all current (or power) in the output stage is converted into output power (sound). This is called high efficiency - the amp has a high efficiency because most of the current is converted into output power (sound from the loudspeaker).

But alas, everything comes at a price: distortion (and we are dealing with a multitude of harmonics and disharmonics).

At least this is theory, because my experience tells me, that it is possible to produce combo amps and power amps close to class C, without anyone claiming 'bout any kind of distortion.

The low idle current in a class C power amp helps stabilizing the amplifier temperature fluctuations. A class C amp will almost go into stand-by, when no signal is sent to the amplifier input. This is prolonging the life of any power amp and its components.

 

 

Line Voltage
Line Voltage is the cheapest way to install sound systems with multiple speakers.

The voltage switch can be set to different line voltages, while the mode of operation is the same for all voltages.

100 volts line voltage means, that the amplifier's maximum output voltage is 100 volts. To produce such a high voltage, the amplifier is equipped with a step-up transformer, which steps up (converts) the amplifier's maximum output voltage (15-30V) to 100V line voltage.

The main difference between a conventional system (low impedance 4-8-16 Ohms) and line voltage is the way in which the individual speaker is connected to the amplifier. In a conventional low-impedance system, the speaker is connected directly to the amplifier output. In a Line Voltage system, each loadspeaker (box) is protected by a step-down transformer, which steps down (converts) the Line Voltage to a voltage that each speaker can handle.

The advantage of this type of coupling is that you can use extra-long speaker wire (100 meters or more) without loss of power (Watts). This loss of power is the usual thing when talking about low-ohm systems.

Another significant advantage of the use of transformer coupling Line Voltage is, that you don't risk overdriving the voice coil of the speakers.

For further info, read more about Line Voltage.

 

 

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The digital Amp
In recent times, the digital amplifier have emerged. Known as Class D amplifier, the digital amplifier has overcome some of the problems with analog amplifiers.

 

 

Class D amplification is also called PWM - Pulse Width Modulation - or digital amplification. The digital amp has emerged because digital circuits are physical smaller, more efficient, and produce less heat than analog circuits. For this reason, the cooling profiles are very small, or in many cases completely omitted in Class D amplifiers.

 

PWM
- Pulse Width Modulation (PWM) is a technique
where you modulate (mix) an analog message into a digital carrier wave.

A transformation (or conversion) is taking place,
from the information being in the size of the input signal
to information being in the pulse width of the output signal.

 

In a digital amplifier, the gain is completely digital. This means, that the amplifying component (transistor, tube, MOSFET or whatever) is either completely open or completely closed. There are no intermediate stages. This is also the main reason, why the digital amplifier generates very little heat.

The input signal (e.g. from a guitar) is led to the input of a Comparator, where it is compared to a high-frequency signal (500,000 Hz or so) generated by the amplifier.

On the output of the Comparator, we find a digital signal with varying pulse width.

Comparator

 

Pulse Width
= the temporal relationship
between high (1) and low (0) states of an electric pulse.

If the pulse width is for example 50%, it means that
the pulse at 50% of the period time is high.

 

 
    An example of a PWM signal with varying puls width.
    D = Duration

As already mentioned, in a digital amplifier the output is completely open or completely closed - there are no intermediate stages. The time period the amplifier is open or closed, is changing with the size (volume) of the (guitar) input signal.

This is what they call PWM - Pulse Width Modulation: The pulse width of a digital (or square) output signal is changing with the size (volume) of an analog input signal.

Finally, the output signal is filtered through a low-pass filter (shown like a coil and a capacitor) in such a way, that you end up with a nice and clean (guitar) signal from the speaker.

The energy loss (heat generation) in a digital amplifier is very low, since each transistor works like a switch and not, as in an analog amplifier, as a variable resistance, which can be opened and closed more or less.

Because of its high efficiency (about 90% of the consumed current is converted into sound), it is quite common to see class D amplifiers with 500-1000 Watt output power. At the same time, a digital amp can be made very small mechanically.

 

The Comparator
Let's take a closer look at the centre of a digital amplifier, the comparator. It works like the drawing suggests.

Sine Wave Input = the analog (guitar) input

Triangular wave input = a high frequency (500,000 Hz) signal, generated by the digital amplifier.

The two signals are fed into separate inputs of the comparator. Once inside the comparator, the two signals are compared (by the comparator), and converted into a square signal with varying pulse width at the comparator output.

The comparator output is turning low (0), if the volume of the high-frequency signal is less than the analog (guitar) input signal.
greater than the analog (guitar) input signal.

 

Class D Amplifier

 

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LiNKS

Ampeg Danelectro Dynacord Echolette Fender Gibson

Hagström HH Electronics Hiwatt Höfner Laney Marshall

Orchester Electronik Orange Peavey Sound City Supersound Watkins Amplifiers Watkins Guitar World WEM

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Musikscenen i 1960'erne:
1. The electric Guitar 2. The Stomp Box 3. The Microphone 4. The Amplifier 5. The Speaker 6. Organ & Synth 7. Record & Music Production 8. Drums & Rhythm Boxes

 

 

This site is under constant consideration and expansion.
So, if you got an opinion on the topic in general or technical details in particular, don't hesitate to contact me:

Thank you very much!

Kurt Starlit
aka CykelKurt

 

 

 

 

 

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