Switch on thump

I have a guitar pre-amp and separate power amplifier . When the power amp is already on for a while , should I then power up the pre-amp , there is a loud ' thump ' . Can I modify the switch on the pre-amp to get round this problem . I'm in 230VAC UK .

Thanks Chris

The thump probably comes because the preamp outputs are AC-coupled through capacitors to the power amp. (There are probably also caps in the power amp input.) Thumps are especially likely if the preamp runs off a single supply so its internal output is at half the supply, and relies on the cap to block this. When you first turn it on, that internal output point rises quickly to half of the supply, and that fast rise is essentially AC that passes right through the cap to appear as a thump. To prevent the thump, you'd need a special slow-turn-on circuit design.

If the preamp has balanced + and - supplies, this is still a possible problem if the circuit doesn't come on in a controlled fashion.

So no, there is no simple switching arrangement to cure this. A complicated arrangement would be to have a time-delay such that the preamp is allowed to power up and charge its output cap into a resistor to ground simulating the power amp, and then after the voltage across the cap is reduced to a very low value the switch connects the power amp.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Turn on the pre-amp first and let it settle for 10 seconds or so before applying power to the power-amp. Do the reverse when turning off, turn the power-amp off first then the pre-amp.

The problem is caused by large swings in voltage in the circuitry as power comes up and amplification is established. Many amplifiers and pre-amplifiers suffer from this defect. Circuit designers usually go to great pains to insure their products don't do this by incorporating some form of muting circuitry in the design. Apparently your pre-amp does not have this.

One way to solve it is to use a time delay relay to connect the speakers to the power-amp only after the power has been applied for some number of seconds. Then any power up thumps will simply not get to the speakers no matter where they originate. Some amplifiers may require a dummy load when not connected to the speakers which can be accomplished with a double throw relay. Bob

Bob might have it but, if the preamp is of any quality, it really shouldn't do this. If you have a Digital volt meter you might want to check and see if the is any DC at the output of your preamp, there should be none. When you power up the preamp the output might be jumping from 0VDC to 1 or

possibly. more likely you'll need to add a de-thump unit inside the preamp.

something that will disconnect the output until the pre-amp has stabilised (probably about 1/4 second after turn-on)...