The term "pre-amplifier" applies to any signal amplifier installed in the signal path before the main power amplifier and may include:
Pre-amplifiers are a very important and vital component of any hi-fi system.
This page presents a small selection
of tube based Pre-Amplifier designs for phono equalisation and tone control.
1. Historical Background to Pre-Amplifiers, Equalisers and Tone-Controls
Once upon a time, music lover - audiophiles had the simple choice of listening to music played either live on an instrument - usually requiring a trip to a concert hall, opera house or such place (eg for the wealthy in the "salon") - or, for the masses, listening in the privacy of one's own home to program information recorded mechanically on a cylinder or disc then played back through a mechanical phonograph or gramophone, amplified acoustically through a wooden horn, called a "loudspeaker".
At that stage there was no contest between the quality of live and recorded music.
The recording process was constantly improved (tweaked) by a variety of ingenious devices, but always the limitation was the mechanical recording process itself and the inherent natural resonances of the component parts.
Truly ingenious techniques were devised to balance the volume of a single human voice with the relatively enormous sound pressure generated by a full orchestra, when recording direct to disc via an air horn.
Playback was at a nominal 78-80 rpm by means of a steel stylus that mechanically actuated a mica diaphragm that in turn generated sound pressure through an extremely efficient acoustic horn.
Up to this point in history the entire recording and playback process was mechanical.
But in about 1925 electric recording was invented and everything changed.
Electric recording, made possible by the vacuum tube, enabled the program to be electrically recorded on a gramophone disc and played back electronically through an amplifier and electrical loudspeaker.
The mechanical turntable was replaced by an electric motor driven unit that did not require constant rewinding of the drive spring and, more importantly, provided constant and correct speed for recording and playback. The mechanical diaphragm playback head was replaced by an electric cartridge. Thus the "Record Player" was born.
The introduction of the 45 rpm vinyl record in 1949, then the 33 1/3 rpm long-playing microgroove record in the early 50's, produced the 3 or 4 speed Record Player, which was soon integrated with a Radio Tuner unit. Thus the "Radiogram" became available to the masses. The Radiogram offered reasonably pleasant listening quality at affordable cost and became very popular during the 1950's.
The more exotic Radiograms featured AM/FM radio tuner, 4 speed Record Player and two loudspeakers or sets of loudspeakers. Typical user controls would be on-off switch, source selector switch, volume control, bass control and treble control.
Lower quality Radiograms usually limited the tone controls to either a single treble-cut control or a treble-cut/bass-boost combo. After all, listeners had been conditioned to this form of tone adjustment for more than forty years!!
Stereophonic two-channel sound, introduced in 1958, required an extra "balance" control to compensate for uneven channel amplification arising from component tolerance variables, tube and component wear and tear, and unevenly recorded programs (which was very common at the time). The "balance" control also enabled manufacturers to produce amplifiers having "sloppy" tolerances resulting in uneven gain between channels. It also enabled users to compensate for vacuum tube wear and tear in one channel compared with the other - by a simple turn of the knob.
Stereophonic sound ensured two sets of loudspeakers as a mandatory feature. One benefit was enhanced bass reproduction, which is a natural consequence of two-channel sound.
Stereophonic sound recordings typically incorporated treble boost to enhance the stereo effect, which is more evident to the human ear at higher frequencies, so tweeters became a standard feature in this class of Radiogram.
Radiograms just got bigger and bigger - and sounded better and better!!
This was the standard product format until the introduction of the (almost) high-fidelity stereophonic Cassette Tape and record/playback deck in about 1964.
In Australia, the complication of a tape deck and introduction of FM radio (1976-1980) caused the demise of the Radiogram and created demand for modular systems - see http://www.radio.adelaide.edu.au/intro/history_OZ-radio.pdf for details
Thanks to the Japanese economic revolution that resulted in lower unit prices coupled with obviously better quality and quality control, modular system components became available, and then the norm - allowing the user a wide range of mix and match options.
Power Amplifiers for modular systems generally incorporated a pre-amplifier and selector control, to enable the whole system to string together into something that worked.
This concept has stayed with us in the mass market to this day, particularly in respect to audio-visual modularised systems.
Later in 1982, the CD Compact Disc was introduced, then games, then in 1997 the DVD Digital Video Disc, then 5.1 channel "surround sound", and now computer interfaces and who knows what else?
Modularised systems offer flexibility in selection of individual component elements and ease of repair or replacement if a component fails.
Modularised systems also provide the user with a relatively low-cost option for upgrading elements as better products become available as a result of technological advances - usually at lower cost through international competitive forces.
Consequently there is need now, more than ever before, to provide a facility to select program sources and compensate for program tone variables - see http://www.wavecor.co.uk/oldtopics.htm.
All of this is can ideally be accommodated
in the AUDIO PRE-AMPLIFIER CONTROL UNIT.
1. THE GRAMOPHONE RECORD
Following the Edison Phonograph of 1877, where sound could be stored on a rotating wax coated metal cylinder, invention of the "Gramophone", where sound is mechanically stored in and played back from a flat rotating disc, is attributed to Emile Berliner (1851-1929) in 1887 - US Patent # 372,786 - see http://www.opinionjournal.com/la/?id=110006171
Emile Berliner went on to establish the Berliner Gram-o-phone Company in Montreal, Canada, in 1897. In 1908, he used Francis Barraud's painting of 'His Master's Voice' as his company's official trademark, but later sold the rights to his "Gramophone" (patent #372,786) to the Victor Talking Machine Company (later RCA), thereby providing them their first major product.
For a more complete history of the gramophone and the early recording industry, please see:
Although electric recording was introduced about 1925, the use of continuous tape as a storage and playback medium was introduced much later on.
Since the commercial introduction of the electric tape recorder, "phonograph" or "gramophone" records (nowadays referred to as "vinyl records"), have been originally recorded using analogue tape-recording and mixing processes having a relatively "flat" frequency response output signal - see http://home.flash.net/~mrltapes/equaliz.html.
This signal was then transcribed directly to a master disc that was used to produce the moulds needed for mass production of copies.
A variety of tape recording methods and processes have been very successfully used over the years since but although offering outstanding fidelity and realistic playback, in general, the signal format recorded with a high-speed high-fidelity analogue "reel to reel" tape recorder is not convenient for most users.
Although tape equalisation was incorporated in the recorder itself and there was no need for external equalisation devices, reel to reel tape playback is expensive and cumbersome.
The user friendly "Cassette" tape recorder-player system was introduced in the 1970's, but the sound quality - although adequate for many users - has never qualified as true "high-fidelity (except in the very best and usually very expensive Cassette record/playback decks).
A generic problem with tape recorders is that when playing back, they are not able to accurately match the recording characteristic or bias used in the original recording and sound quality suffers. As a general rule best results are obtained when the record and playback are performed on the same machine - OK for the recording studio but not practicable for mass produced tapes.
Later digital technologies enabled auto bias detection, as used in VCR's, but that came too late to save the Cassette Tape and Deck system.
So for user convenience and mass production of copies, the taped signal is transcribed to a cutting lathe, which produces a "Master" phonograph recording for production of a mould suitable for volume pressing in vinyl plastic.
In a gramophone record, the signal information is mechanically recorded in a groove that wiggles from side to side, with the amplitude of the groove offset being directly related to signal voltage amplitude.
Nowadays, most recordings are in digital format, which means they must either be transformed to an analogue format to drive the analogue recording head (groove cutter or recording lathe) or used with a digitally controlled recording head.
Monophonic (single-channel) records were originally 78 rpm in 7", 10" and 12" diameter format.
Technological development and manufacturing cost pressures and the need a more robust format for produced the vinyl microgroove "unbreakable" phonograph recording, which has been produced commercially since about 1954 for 16 2/3, 33 1/3 and 45 playback speed, in 7", 10" and 12" diameter format.
Vinyl records are still produced in some countries.
It is of historic interest to note that the diameter of a computer floppy disc, Audio Compact Disc and DVD are all more or less identical to the original German Patent granted in the late 1800's for the first 78 RPM Gramophone Record format - so much for innovation!!.
Stereophonic (two-channel) records have been produced commercially since about 1960 in 33 1/3 and 45 RPM playback speed format, although it is now evident that many tape recordings were made in stereophonic format many years before they became available to the listening public.
For a while, four track stereo looked to be the next generation of technological advancement in audio listening experience, but it met its demise because of the need for users to replace the entire playback system - often a very expensive exercise.
Fortunately, re-issues in stereo
CD format have seen the manufacturers purge their archives for stereo material
and a fantastic range of great recordings are now available on CD - usually
at less cost and better quality than when first issued.
2. PHONOGRAPH MAGNETIC CARTRIDGE EQUALISERS
When the original "flat" response signal as recorded on tape is transcribed to a phonograph record format, the groove in the record has too high an amplitude - ie vertical and lateral wiggle - particularly in the low frequency region. This results in fewer grooves being stored across the playing radius of the recording, resulting in less playing time for a given record diameter.
Of course there is only one continuous groove per side on a phonograph record but for explanatory purposes, it is convenient to refer to the plural - "grooves".
Hence, since the introduction of (analogue) tape recording for gramophone records in 1947, the master tapes and gramophone records have been produced using a "Recording Characteristic"
For further info see also http://www.aes.org/aeshc/docs/3mtape/soundtalk/soundtalkv1n2.pdf
The primary purpose of this is to minimise noise (hiss) in both tape and record pressing and to minimise groove excursion amplitude in the record pressing.
A secondary, but very important benefit, is that more grooves can be squeezed onto a given record diameter because their amplitude is smaller. This results in a closer pitch between adjoining grooves and extends playing time per side.
In essence, the recording characteristic cuts low frequencies by 20 db and boosts high-frequencies by 20 db, about a mid-frequency.
The playback signal has to be "equalised" in order to obtain a flat response output voltage - see http://www.sfu.ca/sonic-studio/handbook/Equalization.html
There were several "Recording Characteristics" used commercially - largely based on nationalistic ideologies - including:
About 1956 the record industry decided it was time to offer a standardised product across the globe and the RIAA recording curve was universally adopted and progressively introduced.
Consequently amplifier manufacturers were able to offer a simplified product.
This was a great leap forward for listeners, because most recordings did not state on them what recording characteristic had been used in their transcription, so the listener had to experiment to discover the most suitable playback characteristic.
However in many situations it may not be practicable to exactly match the original recording characteristic - particularly when it is not know which one was used. For further detail on this see the informative article "Disc Recording Equalization Demystified".
A similar situation has happened with VCR and DVD players where, after many years' user suffering and in the "decline" stage of the product life cycle, the recording industry has finally allowed users to own a playback device that will play more than one format (eg NTSC and PAL systems and global zone carvup) - see "Grimwood's Laws of Technology".
To obtain a flat frequency response output from a magnetic cartridge, the RIAA recording characteristic must be equalised by an equal and opposite signal. This is achieved by a pre-amplifier having the following frequency response characteristic:
RIAA Playback Equaliser Frequency Response
The following Pre-Amplifer designs support this format and are suitable for use with magnetic phonograph cartridges of either the moving magnet or moving coil type.
This equaliser characteristic is also required when a step-up transformer is used between the cartridge and the amplifier.
In the case of modern audio equipment, the output of the Pre-Amplifier should be connected to the LINE INPUT socket.
Note: In some cases it may be necessary to instal an isolating coupling
capacitor between the Pre-Amplifier and the Power Amplifier - check your
user manual. Because this will diminish sound quality, the subject should
be well researched to determine optimum values of components.
Phono Equaliser Pre-Amplifier: RCA Design - Courtesy of RCA RC-23 Receiving Tube Manual 1964.
Phono Equaliser Pre-Amplifier: Richard Brice Design - Courtesy "Electronics" World Magazine 1985
SMART Phono ReMastering EqualizerTM
3. TONE CONTROL PRE-AMPLIFIERS
A very important element in an audio sound system is the TONE CONTROL.
The Tone Control offers the user the capability to vary the "tone" to whatever is preferred, within the limits of the Tone Control range.
A typical Tone Control frequency response curve looks like this:
Many purists reject the need for such an element in the signal path however this writer argues it is essential because:
The following TONE-CONTROL PRE-AMPLIFIERS are suitable for all tube amplifier applications:
RCA Tone Control Amplifier Stage RC-19 Tube Handbook 1959
RCA Tone Control Amplifier Stage RC-23 Tube Handbook 1964
RCA Tone Control Amplifier Stage "Radiotronics" Magazine 1960
STC Tone Control Amplifier Stage - Brimar Tube Manual #8 - 1959
STC Tone Control Amplifier Stage - Brimar
Tube Manual #8 - 1959
The above RCA Tone Controls are based on the "James-Baxandall" design.
Experienced constructors will find it useful to optimise circuit values for their particular system and room acoustics requirements.
Of particular importance - in fact extremely critical - is the value, construction style and material of the treble boost capacitor. This capacitor largely determines the "sound" of the entire system, because most of the sound will be in either the "flat" response or bass boost region.
Some experimentation will be essential to determine the best fit for any given system - a small change in the value of the treble boost capacitor will make a huge difference in overall "sound".
My recommendation is to try values above and below the design value shown in the your preferred schematic. Increments of only 20 pf will be easily recognised because of the effect on overall frequency and harmonic responses.
It may be beneficial to try capacitors in parallel or series when making up the desired value, because every capacitor has its own characteristics when "colouring" the sound signal.
In my experience, ceramic capacitors produce a hard but precise sound, whereas mica capacitors are softer but more musically pleasant - but the final choice depends upon the listener's personal preferences.
In the case of the bass tone control, capacitor values may be increased or decreased proportionately to taste - eg twice the capacitor value = half the boost or cut frequency, or half the capacitor value = twice the boost or cut frequency.
Ideally, bass and treble cut and boost capacitor values should be maintained in proportion as per the original design values, to retain the "flat" position in the centre of the control knob rotation.
The really keen audiophile can instal switchable bass and treble capacitor values, to enable optimisation of tone for each program source mode selected.
Regrettably, if optimisation of values is implemented, it will be likely noticed that the system will sound ideal on one format - eg CD, but awful on another - eg vinyl LP. In such a case it may be necessary to optimise for each signal source format and instal a switching device to select the desired configuration. Such an arrangement is effectively optimising equalisation for each format.
It is worth noting that many of the popular solid-state "integrated
systems" and "receivers" sold in the consumer market are fitted with tone
controls that have their peak boost and cut frequencies at 50 Hz and 10
kHz respectively - not exactly hi-fi but acceptable to the masses - one
quarter of whom have some form of measurable permanent hearing loss or
4. GRAPHIC EQUALISERS
This device is very important and should be incorporated in every home hi-fi system.
The reason is simple - even if every individual component in the system has a perfectly flat frequency response over the audio frequency range, the listener will still hear resonances, peaks and troughs in the sound as a result of different absorption rates in the surrounding room construction, furnishings, fittings, curtains and floor coverings.
The wavelength of typical low frequency audio frequency sound is greater than the length of the typical listening room so the listener will hear the sound differently, depending upon the relative relationship between the location of the listener's ears and the source of the sound. If a door or window is left open or closed, or if the room is not fully contained, there will be variances in the sound.
This is all because what we hear is the result of sound pressure waves travelling through the air from the source to our ears.
The second problem is that of the loudspeaker system, which are notorious for uneven frequency response and discolouration of sound.
Two and three way speaker systems may well have uneven SPL performance between the individual units.
Two and three way speaker systems may also suffer a dip in response around the nominal crossover network freqencies.
The third situation is that of high or low loudness listening level. Many of the commercial home stereo systems incorporate a "loudness" control. This is a simple device that introduces treble cut to the system at low listeneing levels, to offset the change in the performance of the human ear at low sound pressure levels compared with that at high sound pressure levels.
Basically, the ear is more responsive (sensitive) to high frequencies than low frequencies across the range, but as the SPL increases, sensitivity to low frequencies also increases.
If the listener always listens at a particular SPL then the tonal properties can be also set to suit that SPL.
Unfortunately the ratio of high to low frequencies is not that provided by a conventional tone-control, so some other device is needed.
One easy way to compensate for all of the above conditions is to instal a Graphic Equaliser.
This device in all-tube construction was uneconomic for most users until the arrival of solid-state componentry - first transistors and now integrated circuits.
Graphic Equalisers are available as discreet plug-in units and should be installed immediately before the power amplifier - ie after any tone controls or fixed equalisation.
One technical reason for this is that the output impedance (not the load impedance) of solid state units is typically only a couple of hundred Ohms. This pulls the Control Grid of the first stage of the power amplifier down to nearly ground - a similar effect to setting a 1 Megohm volume control to a very low setting. The result is a very low level of hum and noise compared to the same amplifier with a high impedance input device installed.
My personal recommendation for a high-quality Graphic Equaliser is any of the range produced over the years ex the Tandy/Realistic/ Radio Shack/Gennexa organisation. Most have 7, 10 or 12 bands (up to 31 bands is commonly available), are stereo, and are very reliable. Some have DBX noise-reduction or IMX volume expanders installed. They are readily available at low cost from second-hand shops, pawnbrokers and eBay etc.
This is the 12 Band Stereo Tandy 31-2009 unit with IMX, independent level controls, tape monitors, dubbing and EQ functions. There is also a useful spectral frequency display.
In recent years, with the introduction of 5 channel surround sound (which
would require a 5 channel Graphic Equaliser), the Graphic Equaliser was
found to be beyond the capabilities or needs of average users and, despite
their technical contribution, they have lost favour as an essential element
of a home hi-fi system.
5. AUDIO CONTROL UNITS
This type of pre-amplifier usually incorporates all the above plus switching capabilities to select program input sources.
In modern (particularly solid state) equipment, these functions are usually incorporated with the Power Amplifier - particularly in "Receiver" units incorporating an integrated Radio Tuner unit.
The following tube AUDIO CONTROL UNITS are suitable for all tube amplifier applications:
Mullard UK - Pre-Amplifier
Quad UK - "Quad 22" Pre-Amplifier
RCA Control Unit Amplifier Stage RC-23 Tube Handbook 1964
© NOTICE: INTELLECTUAL PROPERTY COPYRIGHT © D.R.GRIMWOOD 2002 - ALL RIGHTS RESERVED.
IMPORTANT COPYRIGHT NOTICE:
Copyright and intellectual property in these audio amplifier designs and circuits remains with their original owner.
Their inclusion in this page as reference materiel is not a license to reproduce or use them for any purpose contrary to the terms of any original copyright notice or license.
These circuits are presented for historical information and education purposes only and are not intended for construction.
No warranty is given of any kind as to fitness for purpose or performance or rating.
Their source is not acknowledged here because
I am unable to determine their origin.
REMEMBER - ALWAYS TAKE CARE WHEN WORKING WITH
HIGH-VOLTAGE - DEATH IS PERMANENT!!
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This page last modified 11 June 2013
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