Switch-on delay and DC protection for Hi-Fi amplifiers - the ideal loudspeaker protection circuit

Switch-on delay and DC protection for Hi-Fi amplifiers - the ideal loudspeaker protection circuit

Updated by: Annabelle

On: 05 Agu, 2020

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Category: Circuit

Switch-on delay and DC protection for Hi-Fi amplifiers - the ideal loudspeaker protection circuit Rating:
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The circuit consists of a DC detector (T1-4), which was not developed by me, and a controllable flip-flop with automatic reset (T5-8) that delays switching on and delaying the loudspeaker relays immediately. An additional transformer is not requir...

The circuit consists of a DC detector (T1-4), which was not developed by me, and a controllable flip-flop with automatic reset (T5-8) that delays switching on and delaying the loudspeaker relays immediately. An additional transformer is not required. The circuit is designed for stereo.

Sense and purpose
Better quality hi-fi power amplifiers have a symmetrical operating voltage. This means that the ground connection (to which both the input and the loudspeaker refer) is not the most negative point in terms of DC voltage, but represents the midpoint between plus and minus. The power supply unit has three connections: plus, minus and ground, instead of just plus and ground. The midpoint voltage of the output stage is therefore at ground potential. In other words: in the quiescent state the output voltage with respect to ground is zero. Therefore, no decoupling capacitor is required to block the DC voltage to the loudspeaker. The audio frequency alternating voltage is created by alternately controlling the push-pull output stage in the direction of positive and negative operating voltage, which in itself makes no difference compared to an output stage with a simple operating voltage and decoupling electr. Large decoupling capacitors with their finite conductance in the direction of lower frequencies are therefore not required with symmetrical operating voltage. The attenuation of the loudspeaker by the internal resistance of the output stage and the quality of the deep bass are now optimal. Reliability increases because capacitors can age; their capacity can diminish or, under unfavorable circumstances, after long periods of storage and sudden full voltage, a short circuit can occur.

When switching on, however, there would be the dreaded switch-on pop if this is not suppressed by the circuitry, which turns out to be a fact on closer inspection. Much more serious is the risk of a short circuit in the output stage, be it due to heat or mechanical effects. In order to bring out the advantages of a DC-coupled output stage, you should install a protective circuit. When switching on, the loudspeaker is switched on with a delay via a relay. If a DC voltage is detected at the loudspeaker connection, the loudspeakers switch off immediately so that they cannot be damaged.


The function
A few seconds after the transformer secondary voltage is present and the DC voltage has run up on the charging electrolytic capacitor, the loudspeaker relay switches on. After switching off the mains voltage, it drops out again almost immediately, long before the DC voltage reaches a critical value. Various faults also lead to the relay dropping out immediately:

* with a DC voltage of more than + - 0.6V on the loudspeaker line
* possibly already with strong overload of the amplifier (can be influenced by the selection of the series resistor)
* if the operating voltage drops below the set relay operating voltage and continues to fall
* when the transformer secondary AC voltage drops.

The circuit is reset directly by the switch-on process, so that faults can never occur due to switching times that are too short. The relay voltage is also stabilized so that the relay is never overloaded. If the amplifier is to be operated with a DC voltage source, then you can simply connect the AC voltage input of the circuit to + UB.

The choice of components
All components can be easily and cheaply procured in the long term. The required relay voltage depends on the available electrolytic capacitor valley voltage at full load of the amplifier in which the LSS is to be installed. A 24V relay is best suited for a transformer with 2 x 24V ~. The circuit supplies any desired relay voltage in a stabilized form by means of a control loop. One Zener diode is switched off to the relay voltage and a second to exactly half. Any transistor of medium power is suitable for T8, e.g. BD140 or BD238. BD244 should also work. The transistor usually works without cooling and hardly gets warm, unless it is a particularly heavy relay or one with a relatively low voltage. For stereo you can of course switch relays in series or in parallel as you wish. The small signal transistors are B types; but this is not critical. The tested BD140 is a 16er type and also not critical. The relays are designed for 30V / 10A DC switching capacity. The current load of the circuit is tested up to 100mA. The power of the resistors R4 and R5 must be observed: From an operating voltage above 42V, 0.5 watt types should be installed.


Construction and installation
The switch-on delay time can be influenced with C3 (33µF). It is about 1s per 10µF. If the circuit already responds with loud music, you can increase R1 and R2. If there is a large difference between the operating voltage and the relay voltage, T8 can get hot; then you should buy a heat sink. The "U ~" connection of the module is connected directly to one of the connections of the transformer, i.e. before the rectifier. The "+ UB" connection comes to the + UB of the amplifier (charging capacitor), so only one pole of the symmetrical operating voltage is used. The connections "NF-L" and "NF-R" come directly to the loudspeaker terminals.

The circuit diagram of the loudspeaker protection circuit 8LSS27 (new version):

the circuit diagram of the loudspeaker protection circuit

the circuit diagram of the loudspeaker protection circuit

The drawing has been revised, nothing else has changed, C4 is not critical anyway. The "pierced" transistors T1 and T3 are simply connected to the lead through with their bases. In terms of drawing, this is allowed as a last resort. I can't find a better solution, otherwise it would only be confusing. - June 2019

Circuit description
If + UB and U ~ are applied, then U ~ is one-way rectified via D1 (1N4148) and charges the capacitor C3 (33µF) via R3 (100k). Since all the transistors of the trigger stage block at this moment, the cathode C3 is at least via R5 (2.2k) relative to R3 to ground. As soon as the UBE (0.6V) + UAK D1 (e.g. 12V) are reached at T7 (BC546B), T7 controls easily. As a result, T8 (BD140) also becomes conductive. The voltage rise is coupled to the base T7 via the voltage divider R4 (also 2.2k) and R5, as well as C3 and the circuit is about to tip over. When the desired relay voltage is reached, ZD2 (24V) becomes conductive and acts as a negative feedback on the base T6 (BC547). T6 then stabilizes the rectified U ~ to 13.2V (UZD + 2xUBE). This voltage is still smoothed by the charging capacitor C3. This now has a voltage of 12.3V ([UZD2 + UBET6] / 2) at its cathode; thus it is still charged to + 0.9 V, in other words: C3 has already discharged itself via T8, the voltage divider and T6, as promised above, during the switch-on process. Since the positive feedback was only capacitive via C3, the conductive state of the flip-flop now depends solely on U ~. If its charging voltage drops only slightly (namely because U ~ is off), T7 goes out immediately and the circuit switches back to its original state. However, if UB sinks on its own (although it shouldn't) fairly quickly, the falling relay voltage is coupled via C3 and everything is off. R8 is important to prevent unwanted feedback. In addition, it should drop 0.6V to idealize the relay voltage and it is a protective resistor in case there should be a short circuit. C5 and R8 form an attenuator that effectively suppresses interference and tendencies to oscillate. D4 derives the relay's no-load current. It provides protection against overvoltage at T6 if the relay used has more than 40mA. R9 ensures a defined collector current of T7 - ‚Äč‚Äčthis noticeably stabilizes the switching behavior.

Site plan:

site plan

site plan


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