What causes that turn on "thump" ?

[Gliderguy]

Quick question. What would cause a through the speaker 'thump" when turning on a preamp?

It is driving me crazy. The pre has a warm up mute feature and it still thumps.

Thanks

GG

[Speedball]

Good question....maybe it's the same reason my amp or receiver being used as a pre thumps through the speaker? ....

....maybe the amp just amplifies the circuit noise or something...?.....?

[djk]

It can't thump, as you always turn it on before the power amp, and turn the amp off first.

[colterphoto1]

It can't thump, as you always turn it on before the power amp, and turn the amp off first.

djk has it right as rain.

Old PA guy rules, power up through the signal chain (amps last)

power down using the reverse of signal chain

You'll never have that THUMP again.

Michael

[Gliderguy]

Thanks for the replys.

I was looking not for a way to prevent it but more for the root cause of the thump.

What happens within the pre amp's circuits that produces this. Caps charging, caps discharging, flux capacitor spin up, electron spin discontinuity variance? What?

[DrWho]

Ideally when you turn off any piece of gear, everything discharges. Then when you turn it back on, current starts to flow until everything is full again. []

(the caps get charged, which results in some flow of current - and then inductors present large voltage swings as the current changes too).

[WMcD]

I believe it is related to power supply voltage ramping up at turn on. The transformer and rectifiers turn on immediately. But it takes a while for the filter caps to get charged up to full voltage.

Of course at shut down, it takes a while for the caps to discharge. This is why you turn things off and there is a slow fade of sound.

We have to look at how transistors work. They are basically diodes, in part. So they don't start working until the forward voltage gets above 0.7 volts. In normal quiescent (sp) state they are biased on. But as the power supply comes up to full voltage, they suddenly come alive. That may be part of the thump.

In my experience, the thump problem develops in older amp. I've got one. It may be that a cap which couples stages has become leaky. I'm not sure how that fits in exactly.

I suspect the problem is not seen in tube amps. The reason is that the heaters take a while to heat up. So all the transients from start up have occurred before the cathode gets hot. There is a slow ramp up not seen in transistors.

Gil

[Gliderguy]

SS diodes. I am wondering if it's the mute relay pushing some DC into the circuit. Activating mute well after the warm up cycle is finished produces the thump also.

Thanks

[Cornwalled]

It's the flux capacitor. Gotta be! []

[Andy W]

The mechanism for turn on thump can vary, but goes something like this...

There are op amps in the preamp, and when the power supply caps charge up, there is a point where the op amp will turn on. At this point in time output is in an unknown state.

[insert a bunch of technical explanation here]

Now the feedback sets about correcting the output of the op amp (very very quickly, I might add) and there is a DC "level shift" on the output of the opamp, which is fed through the amplifier. Even if the amplifier is not DC coupled (which is almost all of them), all the harmonics of the step function (the DC level shift) are still there and you hear the pop.

Even if the preamp has muting relays, if there is a persistant DC offset, when the relays close you will still get the pop.

DC offsets happen for a number of reasons from design to drift.