Thump when switching amplifier on (guitar amp)

[maurycy]

I have Washburn BD40R solid state guitar amp which this great community helped me bring back to life (see this thread). The amp works just fine but every time I turn it on with the guitar plugged in, I hear a very loud thump. It doesn't really matter if the amp volume is set to min or not. Thump is always present. Now, when I unplug the guitar, I hear no thump. Turning it off produces no thump.

Here is the schematic.

Any ideas if that's a normal operation or if something is still wrong with the amp?

Thanks.

 

[Enzo]

I have no idea why the input jack would matter if the amp thumps even with ALL controls at zero.

However, turn-in thumps are VERY common in solid state amps. Unless it sounds darn unusual, I ignore it.

 

[thaumaturge]

Thump is from a sudden DC swing on output. As stated quite common. Also don't see how having an input plugged in would cause it to do so, or more to the point why not having one would cause amp not to thump. But if guitar had a condenser mike pickup you possibly could get a DC offset coming out of it. Check guitar output connector with a DVM. I see nothing in the schematic that would cause a serious output swing on power up.
Doc

 

[maurycy]

I have checked the amp again and you guys are right, there is a thump even without the guitar on but the thump volume is a significantly lower than when the guitar is plugged in. That's probably why I haven't noticed it before.

Since we are on the DIY forum, is there anything I could do to limit the thump?

 

[thaumaturge]

Not easily. But yes. You'd have to install some sort of soft or delayed start circuit between power supply and amp. Most new stereos have a delay circuit that lets supplies all get up to spec before connecting to amp. To really do it right you'd want to slowly ramp up supplies into the amp. Take a look around here for anybody that might already have designed a ramp circuit.
Doc

 

[maurycy]

Thanks for the info. I think I am going to try to add another on/off switch for the speaker. I have seen such option on some of the guitar amplifiers.