Why Preamps Thump on Turn on

I follow the instructions provided to me in my Aesthetix Callisto Eclipse line stage preamp. Power it up first and let it warm up for at least ten minutes prior to powering up my CJ LP27M monoblocks. Shut down is the reverse order. With this, I've never heard an in rush thump

Amplifiers should always be turned on 1st and last off! This will eliminate speaker thump due to large current inrush filling capacitors in the amplifier.

The inrush current is the same whether first or last.  However, turning it on first will cause the amp to amplify any thumps caused by upstream components, if any.

The amp may cause the AC voltage to dip momentarily, but that's not a problem for any other components that are already on.

IMHO, the correct way to avoid thumps is as follows:

Power on - source, preamp, power amp.

Power down - power amp, preamp, source. 

The above works well and is completely silent.

@erik_squires I should point out that in that article, the myth that a tube preamp produces a large Voltage spike is promoted. That’s usually not the case with them. Solid state has been around 60 years or so; I think we’d have been hearing about this way back in the 1970s or some such if it was really real.

The reason is that most preamps have a cathode follower output, which does not put any Voltage on the coupling capacitor at the output of the preamp until the tube begins to conduct. The Voltage thus applied to the coupling capacitor rises quite slowly, much more so than that of the power supply! So you don’t get that spike as shown in the graphic on your site.

You get a shallow dome-shaped waveform instead:

Since its a cathode follower, the Voltage spike will never be at full B+. And because proper design practice is followed, the output coupling cap has some sort of resistance to ground at its output, discharging the cap. So it might reach 0.5Volts during this. If a very large coupling cap is used you might see something higher, but that cap is very large because the designer meant the preamp to drive a solid state amp with a low input impedance. When it is doing so, the capacitor is discharged much more quickly. As a result, if you were to have the amplifier on while the preamp is warming up, you’re far more likely to get thumps because of prior circuits to the output warming up at the same time (like the phono section, which can be avoided by having the volume down) than anything to do with the output of the preamp.

@atmasphere   Thanks for the follow up... I'll have to go take a look when I have time, which may be a while.