JPS6124317A - Cut-off frequency variable type low cut filter circuit - Google Patents

Abstract

PURPOSE:To reduce automatically cross modulation distortion by connecting low cut filter circuits in series for a proper number of stages comprising capacitors of series connection and voltage controlled variable resistor circuits connected in parallel and controlling each cut-off frequency with a DC voltage rectifying a low frequency component of an input signal. CONSTITUTION:A junction transistor (TR) where it is inoperative with a base- emitter voltage VBE is a prescribed value or below and a collector-emitter resistance RCE is large, is used for a voltage controlled variable resistance circuit Rv, then the cut-off frequency Fc of a low cut filter circuit LCF is increased gradually when the base-emitter voltage VBE is a prescribed value or over and gradually decreased at fall-down. Then the characteristic is cut partially depending on the base-emitter voltage VBE at a low sound frequency within an audible band. The circuit is a low cut filter circuit whose cut-off frequency Fc is adjusted automatically depending on the level of a DC voltage ed rectifying the low frequency component of the input signal and generation of cross modulation is prevented to the utmost.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field to which the invention pertains) The present invention relates to a low-cut filter circuit used in audio amplifiers and the like.

(Prior Art) A low-cut filter circuit used in an audio amplifier or the like is provided for the purpose of cutting unnecessary extremely low frequency signals to prevent distortion such as cross-modulation distortion from occurring in necessary audio signals. Conventionally, it has been inserted into the front stage of an audio amplification circuit, and its cutoff frequency has been manually switched as necessary. In addition, in the recording amplifier of a tape recorder, since the amount of bass compensation is large, the linearity of the amplifier can be maintained sufficiently. may cause cross-modulation distortion, and to prevent this phenomenon, the cutoff frequency of the low-cut filter circuit should not be set in the ultra-low frequency band, but rather be raised to a frequency that sacrifices the bass range of the audio signal to some extent. The cutoff frequency was determined by balancing the reduction of cross-modulation distortion in the middle and high frequency ranges, but there was still a drawback in terms of cross-modulation distortion.

(Object of the Invention) This invention was made in order to eliminate the drawbacks of the conventional filter circuit as explained in the above section.
An object of the present invention is to provide a variable cutoff frequency type low-cut filter circuit that can automatically reduce cross-modulation distortion by variably adjusting the cutoff frequency & of the low-cut filter circuit according to the level of the low-frequency component of 1□. I have to.

(Structure, operation, and effect of the invention) As shown in the block diagram of FIG. 1 and the characteristic diagram of FIG. When an appropriate number of low-cut filter circuits LCF consisting of variable resistance circuits B and V are connected in series, and the respective cut-off frequencies F and DC voltage e, which is the rectified low frequency component of the input signal e, rises. This configuration is characterized by variable control such that the height is gradually raised and gradually lowered when moving downward.

In addition, in FIG. 1, the symbol l indicates a low-cut filter circuit L C
' A control voltage generation circuit for F is shown, including a low-pass filter circuit LPF, a buffer amplifier circuit A2. and a rectifier circuit DET.

FIG. 3 is a circuit diagram showing the main parts when the variable cutoff frequency type low-cut filter circuit of the present invention is implemented in a two-channel stereo amplifier. In this circuit diagram, symbol TR is a junction type transistor, type transistor T
This is a part of the low-cut filter circuit LCF connected in series with the filter.

For all junction type transistors, the base-emitter voltage BB is a certain value, for example 0.6V or 0.4
When the voltage is higher than V, the operating state is reached, and in this case, as the emitter voltage VBE increases, the collector-emitter resistance hole CE gradually decreases, and the base-emitter voltage VBF increases.
As , decreases, the collector-emitter resistance R6B gradually increases. Also, the base-emitter voltage VB
When E is less than the above-mentioned certain value, the device becomes inactive and operates so that the collector-emitter resistance R68 becomes a large value.

When the junction transistor TIL that operates as described above is used for the voltage-controlled variable resistance circuit 1 (,v), the low-cut filter circuit LCFO power/iteoff frequency FC is expressed as follows.This cutoff frequency F is the base・When the emitter voltage V is higher than the above-mentioned certain value, it gradually increases when it rises and gradually decreases when it falls.For this reason, in the low frequency range within the audible band, the base emitter The characteristics are partially cut depending on the value of the voltage VB.

Further, when the base-emitter voltage (■8°) is less than the certain certain value, the cutoff frequency F. becomes very low, and the frequency characteristics in the audible band become flat.

In this manner, the present invention allows the low-frequency components of the input signal to be rectified to increase the level above the cutoff frequency depending on the level of the DC voltage ed. This is a low-cut filter circuit that can be automatically and variably adjusted to prevent cross-modulation distortion as much as possible.

[Brief explanation of the drawing]

FIG. 1 is a block diagram explaining the invention, FIG. 2 is a characteristic diagram explaining the operation, and FIG. 3 is a circuit block diagram showing an embodiment.

Claims (1)

[Claims] 1. An appropriate number of low-cut filter circuits each consisting of a series-connected capacitor and a parallel-connected voltage-controlled variable resistance circuit are connected in series, and each cut-off frequency is set to a DC voltage obtained by rectifying the low-frequency components of an input signal. 1. A variable cut-off frequency type low-cut filter circuit, characterized in that the cut-off frequency is variably controlled so that when the DC voltage rises, it gradually becomes higher and when it falls, it gradually lowers due to the DC voltage exceeding a certain level value. 2. The variable cutoff frequency type low-cut filter circuit according to claim 1, wherein the voltage-controlled variable resistance circuit is constituted by a junction transistor.