Simple start-up delay circuit

Rohit Balkishan Dubla

This page describes an ultra-simple start-up delay circuit. This is a very useful addition to a power amplifier where the speakers are disconnected from the power amplifier for some time after powering up. Some amplifiers produce a thump (from the speakers) at power on, and a delay of a couple of seconds before connecting the speakers to the amplifier eliminates it completely. Some pre-amplifiers or active crossovers also produce a power-on transient and when this is amplified by the power amp, the resulting thump can get very loud and annoying. This is especially true when the preamplifier(s) and power amplifier(s) are switched on together.

Figure 1 shows the schematic. It uses a simple RC timer to activate a couple of relays via a transistor switch. The start-up delay is set to about 2.5 seconds. Transistors Q1 & Q2 are connected as a Darlington pair, and with the values for R1 & C1 as shown, the time taken for the voltage across C1 to reach about 1.4V (the turn-on voltage of the Darlington pair) is about 2.5 seconds. This can be increased by increasing R1 or C1 or both and vice-versa - however do not exceed 560k for R1 as this risks limiting the base drive to the Darlington pair which may result in the circuit not functioning properly. C1 is electrolytic, its tolerance means that the timing will no be accurate, but is sufficient for this particular application.

The relays are 12V/200Ω DPST (normally "OFF") or DPDT types rated for at least 5A AC. The schematic assumes two DPST relays. If using DPDT relays, the normally "OFF" terminals must be connected in series with the amplifier outputs. Note that applying power immediately after removing it will activate the relays almost immediately simply because C1 has not discharged sufficiently via the base-emitter of Q1/Q2 - wait about 1-2 minutes before reapplying power. This circuit allows simultaneous delayed switching of up to four speakers. If only two speakers are to be switched, then the switching contacts of the relays can be connected in parallel for improved current handling.

As can be seen, the relay coils are connected in series and the supply required is about 30V which can be taken easily from the rails of a standard ±15V dual supply. The circuit will consume about 60-65mA of current, however. For a supply greater than 30V, R2 must be increased to limit the current to about 60mA. For a supply less than 25V, the relay coils must be connected in parallel - assuming two relays, this will double the current consumed by the circuit to about 120-130mA. R2 will also have to be changed to maintain about 60mA per coil (or 120mA total).

This circuit can be further enhanced to provide muting and speaker protection by connecting a muting transistor (or relay, if isolation is desired or the circuit is operating without a ground connection, as from a ±15V dual-supply) across C1. When the transistor (or relay) is turned on as a result of fault detection (such as DC on the speakers) or muting signal, the capacitor will discharge through the transistor (or relay) thereby turning off the Q1-Q2 pair, which in turn will deactivate the main relays.