To SM400 users, amp tech, those in the know

[will33]

I gig with an SM400S and like it but noticed something wierd recently. The last few gigs I've been bridging it into a single larger 8ohm cab instead of running my two small cabs one off of each side.

Whenever I've run a cab off each side (amp set to stereo) it always gives a good thump when you turn the power switch on. My gain and master are always down whenever I power anything up as well as having the controls on the bass down. It's always done this and other SWR's I've had always did the same thing too. Never any damaged speakers or other amp trouble though.

Now when it's set to bridge mode, no thump. Absolutely quiet when you hit the switch. No other discernable difference in sound, performance or anything else, other than more power/headroom obviously.

I guess my question is why no thump in bridge mode?

[Jim C]

I don't think mine does that but haven't used stereo out in years
What is the impedance of the 2 cabs and do you need the stereo for balance control?

If the 2 cabs are well matched and 4 ohms (or greater) total impdance, run them from bridged (you of course knew that, but just in case)

[will33]

The 2 small cabs are an 8ohm 112 and an 8ohm 210. I do use the balance control to kind of even them out in some cases, in others it sounds fine in the middle. The single large cab I bridge into is an 8ohm 212. All that stuff works out fine, I'm just puzzled as to why it thumps on powerup in stereo and it's dead quiet in bridge. It's a lowish frequency thump, not a clack or a snap, if that makes any sense. It's always done this and still works fine, I don't think it's a sign of any amp problem, just something weird in the circuitry I guess.

I did get the thing used for cheap and don't know it's history. The DI doesn't work, shorts out and blows tweeters, probably why it was so cheap but the rest of the amp works great, always has.