GB2562709A - Operation of an electrodynamic gramophone pickup as a displacement-sensitive transducer by detecting short-circuit current - Google Patents

Operation of an electrodynamic gramophone pickup as a displacement-sensitive transducer by detecting short-circuit current Download PDF

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Publication number
GB2562709A
GB2562709A GB1705370.3A GB201705370A GB2562709A GB 2562709 A GB2562709 A GB 2562709A GB 201705370 A GB201705370 A GB 201705370A GB 2562709 A GB2562709 A GB 2562709A
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United Kingdom
Prior art keywords
pickup
electrodynamic
design
constant
operated according
Prior art date
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Withdrawn
Application number
GB1705370.3A
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GB201705370D0 (en
Inventor
Arthur Brice Richard
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Individual
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Individual
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Priority to GB1705370.3A priority Critical patent/GB2562709A/en
Publication of GB201705370D0 publication Critical patent/GB201705370D0/en
Publication of GB2562709A publication Critical patent/GB2562709A/en
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R9/00—Transducers of moving-coil, moving-strip, or moving-wire type
    • H04R9/12—Gramophone pick-ups using a stylus; Recorders using a stylus
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
    • H03F3/181—Low-frequency amplifiers, e.g. audio preamplifiers
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
    • H03F3/181—Low-frequency amplifiers, e.g. audio preamplifiers
    • H03F3/183—Low-frequency amplifiers, e.g. audio preamplifiers with semiconductor devices only
    • H03F3/187—Low-frequency amplifiers, e.g. audio preamplifiers with semiconductor devices only in integrated circuits
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
    • H03F3/45—Differential amplifiers
    • H03F3/45071—Differential amplifiers with semiconductor devices only
    • H03F3/45076—Differential amplifiers with semiconductor devices only characterised by the way of implementation of the active amplifying circuit in the differential amplifier
    • H03F3/45475—Differential amplifiers with semiconductor devices only characterised by the way of implementation of the active amplifying circuit in the differential amplifier using IC blocks as the active amplifying circuit
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R11/00—Transducers of moving-armature or moving-core type
    • H04R11/08—Gramophone pick-ups using a stylus; Recorders using a stylus
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R3/00—Circuits for transducers, loudspeakers or microphones
    • H04R3/04—Circuits for transducers, loudspeakers or microphones for correcting frequency response
    • H04R3/08—Circuits for transducers, loudspeakers or microphones for correcting frequency response of electromagnetic transducers
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F2200/00—Indexing scheme relating to amplifiers
    • H03F2200/03—Indexing scheme relating to amplifiers the amplifier being designed for audio applications
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F2203/00—Indexing scheme relating to amplifiers with only discharge tubes or only semiconductor devices as amplifying elements covered by H03F3/00
    • H03F2203/45—Indexing scheme relating to differential amplifiers
    • H03F2203/45526—Indexing scheme relating to differential amplifiers the FBC comprising a resistor-capacitor combination and being coupled between the LC and the IC
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F2203/00—Indexing scheme relating to amplifiers with only discharge tubes or only semiconductor devices as amplifying elements covered by H03F3/00
    • H03F2203/45—Indexing scheme relating to differential amplifiers
    • H03F2203/45544—Indexing scheme relating to differential amplifiers the IC comprising one or more capacitors, e.g. coupling capacitors

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Power Engineering (AREA)
  • Acoustics & Sound (AREA)
  • Signal Processing (AREA)
  • Multimedia (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Electromagnetism (AREA)
  • Electrophonic Musical Instruments (AREA)

Abstract

An electrodynamic gramophone pickup comprises a displacement-sensitive transducer which senses the short-circuit output-current generated by the pickup when tracing a gramophone record. A voltage generator (1, fig 1) feeds an internal impedance comprising a resistive component (2, fig 1) and an inductive component (3, fig 1). Two such circuits are present in a stereo pickup. In normal operation the open circuit voltage of the pickup 4 is developed by across a resistor 5 and amplified by a low noise amplifier 6. By sensing the output current rather than the voltage, the electrodynamic pickup may be made displacement sensitive.

Description

(71) Applicant(s):
Richard Arthur Brice
Mazere, Ecueille 36240, Indre,
France (including Overseas Departments and Territori es) (72) Inventor(s):
Richard Arthur Brice (74) Agent and/or Address for Service:
Richard Arthur Brice
Mazere, Ecueille 36240, Indre,
France (including Overseas Departments and Territori es) (51) INT CL:
H04R 3/08 (2006.01) H03F 3/181 (2006.01)
H04R 11/08 (2006.01) (56) Documents Cited:
GB 1535938 A GB 0160223 A
JP 2017041683 A (58) Field of Search:
INT CL H03F, H04R
Other: EPODOC, WPI (54) Title of the Invention: Operation of an electrodynamic gramophone pickup as a displacement-sensitive transducer by detecting short-circuit current Abstract Title: Electrodynamic gramophone pickup (57) An electrodynamic gramophone pickup comprises a displacement-sensitive transducer which senses the shortcircuit output-current generated by the pickup when tracing a gramophone record. A voltage generator (1, fig 1) feeds an internal impedance comprising a resistive component (2, fig 1) and an inductive component (3, fig 1). Two such circuits are present in a stereo pickup. In normal operation the open circuit voltage of the pickup 4 is developed by across a resistor 5 and amplified by a low noise amplifier 6. By sensing the output current rather than the voltage, the electrodynamic pickup may be made displacement sensitive.
1/3
2/3
3/3
Operation of an electrodynamic gramophone pickup as a displacement-sensitive transducer by detecting short-circuit current
This invention relates to improvements in the reproduction of sound by operating an electrodynamic gramophone pickup as a displacement-sensitive current-generator rather than as velocity-sensitive voltage-generator as in normal practice. The invention is applicable to any gramophone pickup in which a changing magnetic field is used to generate an electrical signal, including (but not limited to): moving-magnet type; moving-coil type; variable-reluctance type; and moving-iron type.
Electrical equalisation of the signal from an electrodynamic gramophone pickup has been employed for nearly 100 years. Since the 1960s, this replay equalisation is often simply termed RIAA equalisation because of that organisation's recommendation of a particular recording characteristic as the industry standard. This well documented technology applies substantial equalisation to the audio signal; boosting the bass signals 100 times more than the treble signals to restore the original musical balance.
The impedance of the electrodynamic gramophone pickup is principally inductive and this creates a number of problems in normal (voltage-mode) practice - especially if the pickup is a moving-magnet type. The inductance interacts with the capacitance of the connecting cables to produce a resonant network with a frequency-response peak in the audio band, and, the higher impedances of the pickup operated in voltage-mode compromise stereo performance due to capacitively-coupled crosstalk between the right and left signals in the unscreened wires of the gramophone tonearm.
Operating the gramophone pickup as a displacement-sensitive current-generator secures two significant advantages over the standard mode of operation. Firstly, displacementsensitivity greatly reduces the requirement for replay equalisation. For, although it is widely assumed that this equalisation is required because the information recorded on a gramophone disc has the inverse of the replay characteristic - with extensive bass cut and treble boost - this is not the case. There is a substantially constant-amplitude characteristic engraved in the disc's grooves and the electrical RIAA replay equaliser principally corrects for the rising frequency response due to the velocity sensitivity of the pickup. Ceramic, strain-gauge and optical gramophone pickups, which are sensitive to the amplitude of the groove deviation, do not require extensive electrical equalisation and are highly prized for the concomitant simplification of the replay electronics.
Secondly, current-mode operation eliminates the deleterious effects the cable capacitances. These effects are abolished because the pickup, when operated in current-mode, feeds circuit input-nodes of near zero impedance (a virtual earth) where no signal voltages can develop.
The invention will now be described solely by way of example and with reference to the accompanying drawings in which:
Figure 1 illustrates the simplified equivalent circuit of the electrodynamic pickup.
Figure 2 illustrates normal operation of the electrodynamic pickup and associated amplifier.
Figure 3 illustrates the method by which the output current of the pickup may be sensed using a trans-resistance or trans-impedance amplifier.
Figure 4 illustrates the overall amplitude vs. frequency of the signal engraved on the gramophone disc.
Figure 5 illustrates analogue circuits for achieving electrical equalisation of the highfrequency proportion characteristic illustrated in Figure 4.
Figure 6 illustrates a digital filter architecture for achieving electrical equalisation of the high-frequency proportion characteristic illustrated in Figure 4.
Figure 7 illustrates analogue circuits for achieving electrical equalisation of the lowfrequency proportion characteristic illustrated in Figure 4.
Figure 8 illustrates a digital filter architecture for achieving electrical equalisation of the lowfrequency proportion characteristic illustrated in Figure 4.
Figure 1 illustrates the simplified equivalent circuit of the electrodynamic pickup in which a voltage generator (1) feeds a complex internal impedance comprising a resistive component (2) and an inductive component (3). In reality, these components are not discrete and, in a stereo pickup, two such circuits exist.
Normal operation of the electrodynamic pickup is illustrated in Figure 2 which illustrates that the open-circuit voltage of the pickup (4) is developed across a resistor (5) which is substantially greater in value than the resistive component of the pickup (2) and amplified by a low-noise amplifier (6). Here, as in all the figures and the circuit descriptions below, a single channel is illustrated or described for clarity but this invention applies equally to single-channel (monophonic) and two-channel (stereo) gramophone pickups.
An electrodynamic pickup may be made displacement sensitive if the output current is sensed, rather than the output voltage. The short circuit current in any electrodynamic generator obeys Ohm's law such that the current is determined by the induced voltage divided by the impedance of the generator.
We know from Faraday's law that the induced EMF in a coil of thin wire is,
d(FB} dt
where E is the electromotive force, FB is the magnetic flux, and k is a negative constant which also includes the number of turns in the coil.
If the change of flux is created by a sinusoidal groove modulation of amplitude (A) in an electrodynamic gramophone pickup, then the output voltage of pickup is given by, d(Asina)) = k.M .A cos Mt in which, the independent term ω indicates that the signal amplitude increases with frequency. It is exactly according to this relationship that electrodynamic gramophone pickups normally operate.
The short-circuit current is determined by dividing the above by the impedance of the generator which, to start with, we will consider to be entirely the reactance (X) of the inductance (3 in Figure 1). This is determined by the well-known equation, X= ω . L
Thus, the short-circuit current, (Is) will be,
k.M .A COS Mt Is =-------T----mL
In which L is the inductive component of the impedance of the pickup. It's clear in this equation that the ω terms cancel-out and the transfer-function of the pickup is rendered independent of frequency.
The circuit of Figure 3 illustrates an example of how output current may be detected; although other arrangements are possible and remain within the scope of this invention. The pickup feeds the virtual-earth of an inverting amplifier (7) working as a current to voltage converter, a circuit referred to as a trans-resistance ortrans-impedance amplifier. The result is a voltage generated at the output of the amplifier stage (8) which is an accurate analogue of the groove displacement.
In a practical application of this invention there are two complications. Firstly, the information recorded onto the gramophone record does not have an entirely uniform amplitude characteristic vs. frequency. To limit the velocity of the cutter-head when making the record, the high-frequency portion of the signal range is reduced in level in comparison with the lower-frequency range. The overall amplitude vs. frequency of the signal engraved on the gramophone disc is given in Figure 4 in which the time-constants relating to the various parts of the characteristic are annotated as follows: (9) = 3180pS ortl; (10) = 318pS ort2; (11) = 75pS ort3.
The equalisation of the signal amplitudes above the 318pS (t2) breakpoint may be accomplished by one of two possible methods, both of which fall within the scope of this invention. Equalisation may be secured either by means of a wide, damped mechanical resonance at higher frequencies, or by analogue or digital equalisation of the output voltage developed at (8) in Figure 3. An analogue circuit with the required equalisation is given in Figure 5; although many other implementations are possible and remain within the scope of this invention. In the circuit of Figure 5, the capacitor (12) causes the overall input impedance of the input leg of the inverting amplifier configuration to fall at frequencies above l/(2.n.t2). Because the gain of this type of amplifier is defined by the ratio of the feedback resistor (13) to the input-leg impedance, the gain of the stage rises in the required way and by the required amount by the selection of the ratio of resistor (14) to resistor (15).
A digital filter arrangement with the same transfer function as the circuit in Figure 5 is given in Figure 6; although many other implementations are possible (either in hardware or software) and remain within the scope of this invention. Here the signal feeds both a main path and a side-chain. In the side-chain, the signal is filtered by a high-pass filter (16) with a turnover frequency of l/(2.n.t2). The signal in the side-chain is then scaled by a constant (17 in Figure 6) and added to the main path at (18).
It is worth noting that the maximum range of equalisation required to return recorded frequencies above the 318pS time-constant to the correct level is defined by the ratio of the two time-constants t2/t3, thus
318pS/75pS = 4.24 or 12.5dB.
This is only four percent of the degree of equalisation required in the normal RIAA equaliser circuit.
The second complication arises due to the RIAA recording characteristic which reverts to constant-velocity in the portion of the frequency-range below the 3180pS time constant (tl). Figure 4 illustrates that the displacement signal rises with falling frequency below this breakpoint. The pickup must therefore be returned to velocity operation in the lowest octaves of the frequency-range to compensate for this change. This may accomplished by two possible methods both of which fall within the scope of this invention. The first method requires the ability to design or select specific electrical parameters of the pickup.
Returning to the equation for Is above, we see that it is incomplete because the short-circuit current is determined by both the real and the imaginary parts of the pickup impedance. The equation below incorporates the resistive part of this impedance (r).
k. ω ..4 cos ωί |/S| = V((«L)2+r2)
At the point in the low-frequency range when the reactance of the inductive coil (2 in Figure
1) falls to that of the resistance of the coil (3 in Figure 1) and, in the equation above, r dominates u>L, the pickup gradually returns to velocity operation with the magnitude of the response falling away to zero at DC as required. The pickup may thereby be designed with the ratio of resistance (r) to inductance (1) such that
L = r. tl and this alone will compensate for the recording characteristic.
If this ratio cannot be contrived by design of the pickup itself (and generally it will be found that the ration of L to r is insufficient), electrical equalisation may be employed to shift the frequency point at which the change to velocity operation is accomplished. An analogue circuit for so-doing is given in Figure 7; although many other implementations are possible and remain within the scope of this invention. In the circuit of Figure 7, the turnover frequency is set by the lowest time-constant of the RIAA specification, such that the value of capacitor (19) multiplied by the resistance of resistor (20) = 3180 x 10-6. The ratio of the values of resistor (20) and (21) depends upon the degree to which the ratio of the values of the pickup DC resistance (2 in Figure 1) and the inductance of the pickup coil (3 in Figure 1) differ from the ideal ratio. It may be noted that the circuit of Figure 5 and Figure 7 may be combined around a single amplifier.
A digital filter arrangement with the same transfer-function as the circuit in Figure 7 is given in Figure 8, although many other implementations are possible (either in hardware or software) and remain within the scope of this invention. Here the signal feeds both a main path and a side-chain. In the side-chain, the signal is filtered by a low-pass filter (22) with a turnover frequency equal to 1 /(2.n.tl) scaled by a constant (23) and added to the main path at (24). The value of the scaling constant of the side-chain filter depends on the degree to which the ratio of the values of the pickup DC resistance (r, 2 in Figure 1) and the inductance of the pickup coil (L, 3 in Figure 1) differ from the ideal ratio.

Claims (7)

Claims
1) The operation of an electrodynamic gramophone pickup as a displacement-sensitive transducer by sensing the short-circuit output current generated by the pickup rather than as a velocity-sensitive transducer by detecting open-circuit voltage as in normal practice.
2) The design of the electrical parameters of the pickup when operated according to Claim 1, such that the pickup may be returned to velocity operation in the frequency-range below the 3180pS time-constant of the recording characteristic.
3) The design of an analogue electronic equaliser operating on the signal derived from the pickup when operated according to Claim 1, such that the pickup may be returned to velocity sensing in the frequency-range below the 3180uS time-constant of the recording characteristic.
4) The design of a digital filter operating on the signal derived from the pickup when operated according to Claim 1, such that the pickup may be returned to velocity sensing in the frequency-range below the 3180pS time-constant of the recording characteristic.
5) The design of the various mechanical parameters of the pickup when operated according to Claim 1, such that a broad, mechanical resonance compensates for the reduced amplitude of the recorded frequencies above the 318pS time-constant of the recording characteristic.
6) The design of an analogue electronic equaliser operating on the signal derived from the pickup when operated according to Claim 1, such that the recorded frequencies above the 318pS time-constant of the recording characteristic are returned to the correct level.
7) The design of a digital filter operating on the signal derived from the pickup when operated according to Claim 1, such that the recorded frequencies above the 318pS timeconstant of the recording characteristic are returned to the correct level.
Intellectual
Property Office
Application No: GB1705370.3 Examiner: Mrs Margaret Phillips
GB1705370.3A 2017-04-04 2017-04-04 Operation of an electrodynamic gramophone pickup as a displacement-sensitive transducer by detecting short-circuit current Withdrawn GB2562709A (en)

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Application Number Priority Date Filing Date Title
GB1705370.3A GB2562709A (en) 2017-04-04 2017-04-04 Operation of an electrodynamic gramophone pickup as a displacement-sensitive transducer by detecting short-circuit current

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Application Number Priority Date Filing Date Title
GB1705370.3A GB2562709A (en) 2017-04-04 2017-04-04 Operation of an electrodynamic gramophone pickup as a displacement-sensitive transducer by detecting short-circuit current

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GB201705370D0 GB201705370D0 (en) 2017-05-17
GB2562709A true GB2562709A (en) 2018-11-28

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB160223A (en) * 1919-11-18 1921-03-18 Adrian Francis Sykes Improvements in telephonic and like apparatus
GB1535938A (en) * 1975-01-31 1978-12-13 Matsushita Electric Ind Co Ltd Electromagnetic transducers
JP2017041683A (en) * 2015-08-18 2017-02-23 株式会社オーディオテクニカ Voice reproduction device using capacitor pickup cartridge

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB160223A (en) * 1919-11-18 1921-03-18 Adrian Francis Sykes Improvements in telephonic and like apparatus
GB1535938A (en) * 1975-01-31 1978-12-13 Matsushita Electric Ind Co Ltd Electromagnetic transducers
JP2017041683A (en) * 2015-08-18 2017-02-23 株式会社オーディオテクニカ Voice reproduction device using capacitor pickup cartridge

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Publication number Publication date
GB201705370D0 (en) 2017-05-17

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