Switch on , speaker thud.

robgilmo

All the dials are at zero, I switch the amp on and get a thud from the speaker.
Is this a sign of a failing cap? DC offset? Normal?
This happens with the volume at zero.

ICBM

Normal.

Sassafras

I get that on my Tech 21 Trademark.
Doesn't seem to have hurt it and I've had it for years.

robgilmo

Is it DC in the audio path? I've only ever fettled hifi amps before.

ecc83

robgilmo said:

Is it DC in the audio path? I've only ever fettled hifi amps before.

Many hi fi amps have a protection system that consists of a relay in the speaker circuit. This delays connection for a second or two and is often associated with a DC offset sensing circuit. Later, more sophisticated designs can have slow rampup PSUs.

No such niceties in the guitar world! (make and model of amp please? IC says I gotta ask!) but then guitar/bass speakers are much more rugged than hi fi cones.

Might be worth slapping a DMM across the speaker terms? If the offset is more than 10mV I would investigate it. Maplin USED to do a delay/protection device, will have a varder. If it bothers you a delay relay is easy to implement or you could just have a 5A swtich in the speaker circuit. http://www.velleman.co.uk/contents/en-uk/p240.html  ; Maplin no longer do it seems.

Dave.

ICBM

As long as the transient doesn't make the speaker cone exceed its safe travel then it does no harm, it's just annoying - or useful, if you like confirmation that the amp has powered up properly!

Almost all solid-state amps do it to some extent unless they have a suppression circuit (often relay operated, or in a power chip) because it's usually caused by DC offset as the supply rails power up unevenly, which they nearly always do.

Some valve amps do it by magnetic coupling between the PT and the OT, so it will still happen even if the standby is on.

jpfamps

ecc83 said:

robgilmo said:

Is it DC in the audio path? I've only ever fettled hifi amps before.

Many hi fi amps have a protection system that consists of a relay in the speaker circuit. This delays connection for a second or two and is often associated with a DC offset sensing circuit. Later, more sophisticated designs can have slow rampup PSUs.

No such niceties in the guitar world! (make and model of amp please? IC says I gotta ask!) but then guitar/bass speakers are much more rugged than hi fi cones.

Might be worth slapping a DMM across the speaker terms? If the offset is more than 10mV I would investigate it. Maplin USED to do a delay/protection device, will have a varder. If it bothers you a delay relay is easy to implement or you could just have a 5A swtich in the speaker circuit. http://www.velleman.co.uk/contents/en-uk/p240.html  ; Maplin no longer do it seems.

Dave.

Actually there are plenty of guitar / bass amps with output muting relays, although I agree it's not common.

Hi-fi punters are more fussy about extraneous pops I guess.

Nerine

I don't mind a little thud. 

Makes me think "aaaannnnddd CONTACT!" 

Time to melt faces, dawg!