GB2274367A - Audio preamplifier using high voltage MOSFET - Google Patents

Abstract

An audio preamplifier circuit utilises a MOSFET device biased from a 250 volt HT rail to emulate conventional vacuum tube circuits used, for example, in guitar amplifiers. <IMAGE>

Description

AUDIO AMPLIFIERS This invention relates to audio amplifiers and in particular to audio amplifiers for use with electronic musical instruments.

The traditional and preferred method of amplifying electronic musical instruments, in particular electric guitars, is by using an amplifier that incorporates thermionic valves (otherwise-known as "tubes") in its circuitry. Such an amplifier would normally incorporate two sections in its signal path between the input and output. That is to say, a preamplifier dealing with voltage amplification and sound shaping and processing of the incoming signal, and a power amplifier, enabling the preamplifier signal to drive a loudspeaker or any other suitable load. Alternatively, the preamplifier and power amplifier can be separate items and used either jointly or separately depending upon the requirements of a particular user's system requirement.

This invention is concerned with preamplifier circuitry, whether in a separate unit or combined with power amplifier circuitry.

As stated earlier, the traditional and preferred amplifying element is a triode type vacuum tube, most commonly an ECC 83 type (known as 12A x 7 in some countries, notably USA), although other types are sometimes used. The traditional and largely continuing pre-eminence of valve circuits compared with typical solid state amplifying circuits is that valves exhibit a far greater degree of "musicality".

This "musicality" can be defined and quantified in the case of a typical ECC 83 triode circuit, found in many guitar amplifiers for instance, and can be listed in terms of input and output impedance, headroom, perceived gain, distortion (percentage and type), power supply compression, and voice (which is related to the frequency response characteristics).

There have been many attempts at synthesising some aspects of valve behaviour by solid state means primarily in terms related to distortion, but these attempts have usually neglected, or have been at the expense of, the other parameters of the valve which contribute to "musicality".

They have thus not achieved a true equivalence of a vacuum tube circuit.

It is an object of the present invention to replicate more closely than has hitherto been possible the parameters of a vacuum tube that contribute to "musicality", whilst avoiding the disadvantages of a vacuum tube in terms of expense, wasteful heat energy, microphony, and unreliability.

Accordingly, the present invention consists in an audio preamplifier circuit for amplifying an input signal from an electronic musical instrument and comprising at least one semiconductor amplification element, characterised in that the semiconductor element is of a type adapted for operation at high voltage and in that the circuit provides to the semiconductor element a bias voltage comparable with a conventional vacuum tube bias voltage.

Preferably, the semiconductor element is a high voltage MOSFET device.

Advantageously, the amplifier circuit is arranged to provide a bias voltage of around 250 volts to the MOSFET device.

The principle behind the invention lies in substituting a solid state device for a vacuum tube triode, whilst retaining the higher voltage regime of a vacuum tube. This limits the selection of semiconductor devices, but devices are available.

The principal work has been done on a family of MOSFET devices intended for high voltage switching, thus enabling them to run on the typical 250-300 volt HT rails found in valve circuits.

By evaluating these devices and developing the traditional valve circuits to incorporate adequate biasing and control means, suitable equivalent circuits have been built and tested, both objectively in quantitative terms and subjectively in terms of audio performance to a practised listener.

Quantitatively, the test circuits have not only matched the performance of an ECC 83 circuit but have the scope to be more flexible in parameter control.

Subjectively, these circuits have been judged to have the "musicality" of their valve counterparts.

From a technical viewpoint there is only one difference in the biasing circuit used in the present invention as compared to conventional valve circuits. Whereas a valve requires a negative bias voltage on the grid with respect to the cathode, which is relatively easy to provide, the MOSFET devices so far found suitable are described as being of the ENHANCEMENT MODE type which require a positive gate voltage with respect to its source terminal. This requires an alternative bias supply circuit, but one which retains the aspects of a valve bias circuit - and particularly the high voltage - which are judged critical to audio performance.

The use of ENHANCEMENT MODE MOSFETS is a practical requirement arising from the need in manufacture to use only devices which are commercially available at reasonable costs.

Whereas a DEPLETION MODE device would be of greater benefit in the circuit, suitable devices are not generally commercially available.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a prior art triode circuit as used for example in an electronic guitar preamplifier, and Figure 2 shows a circuit forming part of a preamplifier according to the present invention.

Comparing the circuits of Figure 1 and Figure 2, it will be seen that they share many common features, for example: the supply +HT, the load resistance R1, the input ground reference R2 and the frequency dependant biasing and feedback components R3 and C1. The principal difference between the circuits, apart from the replacement of the vacuum tube with an ENHANCEMENT MODE MOSFET device, is the addition of a negative rail -LT and suitable resistance R4 providing the necessary positive gate voltage with respect to the source. Capacitor C4 is added to AC couple the signal to OV whilst resistance R5 serves to preset the required gain. The value of resistance R5 for a specified gain, would depend upon the parameters of the selected MOSFET.

In operation, a preamplifier according to the preset invention will receive from an electronic guitar or other electronic musical instrument, an audio input signal typically ranging in amplitude from 0 to 1 volt. The preamplifier would be capable without difficulty in providing an output voltage of from 0 to 60 volts, that is to say a similar voltage to that generated by conventional vacuum tube preamplifiers.

It should be understood that this invention has been described by way of example only, and a variety of modifications are possible in circuit configuration and component selection without departing from the scope of the invention.

Claims (5)

1. An audio preamplifier circuit for amplifying an input signal from an electronic musical instrument and comprising at least one semiconductor amplification element, characterised in that the semiconduction element is of a type adapted for operation at a high voltage and in that the circuit provides to the semiconductor element a bias voltage comparable with a conventional vacuum tube bias voltage.

2. A circuit according to Claim 1, wherein the semiconductor element is a high voltage MOSFET device.

3. A circuit according to Claim 2, arranged to provide a bias voltage of around 250 volts to the MOSFET device.

4. A circuit according to any one of the preceding claims adapted and arranged to provide a voltage gain of 60.

5. An audio preamplifier circuit substantially as herein before described with reference to and just shown in the accompanying drawing.