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Large motional feedback system


Excursion limiter

The operating principle of the excusion limiter for the MFB woofer system is depicted below. It starts with a 2nd order Butterworth high-pass filter at 60 Hz for the frequency range that is not limited, nor affected in distortion by the dynamic limiter. This 2nd order slope corresponds with a line of constant excursion of the cone. At 60 Hz the cone can stand the excursion that corresponds with the maximum output voltage of the power amplifier. At lower frequencies the maximum excursion amplitude is limited to the same value. This meins that at lower frequencies the sound power level is reduced with a second order relation to the frequency because of the second order behaviour of the roll off below the first resonance frequency. Aat 16 Hz the sound power is then reduced to 10*10log(16/60)2=-23 dB which corresponds with the value as calculated in the introduction.

This HF signal part is subtracted from the input signal to obtain the low frequency part. This subtraction results in a first order response with a small peak which is due to the phase relation and well known to people who want to make a phase-aligned second-order cross-over filter.

The low frequency part is then attenuated by means of a gain cell and added to the high frequency part. Depending on the gain of the gain cell a total reponse as shown with the red lines is obtained.

Excursion limiter overview

The gain of the gain cell is controlled through a second branch where the low-pass signal from a normal filter is measured and limited with a fast attack-slow release hard limiter circuit around a second gain cell. This implies that the limiting action will only start when the cone excursion has reached its maximum allowable value and will be kept at that level even if the input signal rises. This might seem drastic but it is the best way to protect the expensive woofer and in practice this will hardly ever happen. The only example that I have experienced a problem with is a magnificent two organ transcription of Richard Wagner's "Ring der Nibelungen" by Hansjörg Albrecht on Oehms classics SACD(OC612). At the very beginning only a very low (subsonic) note is produced and one is tempted to turn the volume button to the right to better hear it but then the whole room starts to tremble and the limiter is heard to attack. But that is not a realistic level as "subsonic" means that you can only hear its higher harmonics.

The circuit diagram below shows the entire circuit of the limiter. OP1 is the second-order high-pass filter, OP2 is the first adder, OP3 the upper gain cell, OP4 the second adder. OP5 and OP6 make a full-wave rectifier while OP7 defines an offset value above which the limiter has to work. The transitor with diode and resistors creates the fast attach-slow release characteristic where the input resistance of 10 k of the NE571 determines the release time of 0,22s. The NE 571 is a compandor circuit from Signetics.

Finally OP8 determines the second-order low-pass filter together with R29 and C12 where the signal will pass the internal gain cell of the NE571 and its output is sent to the full-wave rectifier.

Excursion limiter

It should be noted again that this is not the most component efficient solution and the result of experimenting that was ended as soon as it worked sufficiently OK.

Power limiter for the tweeter

Since I burned one expensive tweeter by an oscillating amplifier I use an active power limiter for the tweeter. It works with an approximated power (squared voltage) detector based on a parallel diode/resistor network and a well known timer IC NE555.

The LT-SPICE circuit diagram below shows the principle and the component values are determined such that the circuit triggers when the average power on the tweeter exceeds P = I R = 4W, measured on the Sense terminal, being the tweeter connection. The averaging time constant is around 1.5 seconds (C1,R6) which is conform the measured thermal time constant of the tweeter that I obtained by measuring the resistance change of the voice coil over time with a constant input current level.
The circuit takes into account the complex impedance with 0.03mH and 3 Ohm coil resistance by means of low-pass filtering with C2,R7,R8. The effective minimum impedance of the tweeter is 3.6 Ohm. At 4.2 V/eff, (5.8V amplitude) a current of 1.14 A wil run over 3.6 Ohm. I R =(1.14) *3= 3.9 W

Tweeter power limiter

The LT-SPICE model can be uploaded here. Be aware then when modelling this circuit the value of the LSP voltage is the amplitude, not the RMS value. I made that mistake and in that case the limiter interrupted also in case of normal high-power music signals.

Other protections

Other protections include DC detection on the LF and MF power amplifiers (HF are capacitor coupled) and a more simple execution of the power limiter for the MF amplifier without squared detection of the voltage. Finally a dynamic limiter is placed over the total signal when severe clipping occurs in the HF or MF section in case of an erronic volume control setting. This is done with a small series resistor of 220 Ohms and a special J-FET (BSV78) to ground.


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