SWR SM400 not working in bridged mono mode

[nsubulysses]

I rarely get SWRs and I have this one that does the same output power in bridged mono mode vs stereo mode.

This SWR seems to have one fuse taken out of the circuit and I would guess this has something to do with why bridged mode does not work.

Why would it be wired this way? Was there some sort of factory recall on bridged mode or something I don't know about.

here is a picture of mine vs a picture I found online. As you can see mine has the fuse wiring altered for the white wire. The online pic clearly has the fuse in series with the output where as mine has the fuse bypassed.

Is there something I should be taking into account before I put this white wire back on the other side of the fuse and see if it works bridged then? Why would someone disable bridged mode??

my amp


online amp


schematic : swr_sm400_bass_amp_sch.pdf

[Enzo]

Stop. Why would bypassing a fuse make something NOT work? A good fuse is the same thing as a wire to the circuit. If the circuit fails, instead of blowing a fuse, it now burns something up. I doubt the fuse wire is causing your issue.

How are we determining the amp produces the same power in both modes? In plain old stereo, each side produces some output voltage, you can tell us what the power rails are that limit that. In bridge mode that doesn't change. The difference is that in bridge mode, the signal is inverted to one channel, so when one swings positive the other swings negative. We then use the hot from each as the two speaker terminals. Note how it even says "Do not reference to ground". Neither side is ground. So from either red terminal to ground you have the output of one of the channels or the other. Only from one red to the other do you have the double voltage of two amps added. The quarter inch jacks remain one of the channels or other alone.

[The Dude]

The schematic won't open for me, but it does look like someone bypassed the fuse. Maybe it was an emergency repair and they didn't have a fuse? Do both channels work as they should in stereo mode? When you try to run it bridged, are you using the banana jacks? It appears all of the 1/4" jacks are chassis grounded, so the banana jacks would be the only ones capable of bridged output. Again, I can't see the schematic, so this is a bit of a SWAG.

[Enzo]

I opened my own schematic, this one is the one pixel thing.

[nsubulysses]

Fortunately I stopped. Thanks for explaining how bridged vs mono works because I really didn't undertand that. I was given this amp saying bridged mode doesn't work. To me it just seems like it works, but works the same as stereo, meaning no additional power output

When I scope the output I get an unstable signal on the screen. Is this because it is referenced to ground through the scope? When I scope with positive probe only, I get same output in bridged mode as I do in mono.

What would be the proper way to scope the amp then in bridged mode? Float the scope.....or?

https://elektrotanya.com/swr_sm400_b.../download.html

[The Dude]

Yep. As Enzo described, in bridge mode speaker negative is not referenced to chassis ground. Bridge mode is not the same as Parallel mode, wherein the inputs are tied together to make the unit a dual mono amp so that both amplifiers can be driven by the same signal. Bridge mode, as the moniker describes, actually "bridges" the two power amps into a single amp. It's an often misunderstood configuration and I can't tell you how many amps I've repaired because someone has their amp configured incorrectly. In bridge mode, one channel is put out of phase with the other. If you then run the amp with a speaker on each channel, they will be out of phase. The harder you drive the amp, the less sound you will get where the speakers couple in the room- until......"BOOM".....done. I generally tell customers there's no reason to bridge an amp unless you are using only one speaker and want to get the full amp power to that single speaker.

[Enzo]

If you connect your scope to the red posts by clipping the scope ground clip to one of the posts, you are grounding that output, assuming the power cord is also grounded. The amp will not like that. If you scope the posts one at a time all you see is the regular stereo output. What doubles the power is the stacking of the two channels, the channels themselves do not change their output. (Other than one is inverted in polarity)

[TimmyP1955]

Enzo is correct. I occasionally forget that I have to test a bridged amp one terminal at a time, instead of using a normal cable. Then I stand there saying WTF for minute, then I call myself an appropriate name and do it rightly.

[g1]

Do you have a 2 channel scope? If so, in bridge mode you should be able to see the 2 outputs are out of phase with each other.
Or use a floating load, scope one of the outputs, run the amp up to full power, and measure output voltage with a floating multi-meter.
In both methods, neither output terminal is connected to ground.

[Jazz P Bass]

Interesting.
The manual states that the 1/4" speaker out jacks should only be used in "stereo" mode.
The banana jacks should only be used in "bridged" mode.

sm400om.pdf

[Enzo]

And good reason, if in bridge mode, you use the 1/4" jacks to left and right speakers, they will be out of phase, and will pretty much cancel.


If you have a sturdy iso, you can plug the amp into it, then the amp floats and you can hook a scope ground to one post. But make sure the signal comes from something also isolated, or the signal ground will reference the amp circuits to ground.

[g1]

Yes, I believe the whole reason behind bridge mode was so you could use the entire capacity of the amp into a single cab.

[The Dude]

Agreed. It's really the only way it can work as configured (1/4" jacks and banana jacks). Some modern amps use NL4 connectors for outputs and you can use the same jacks for stereo or bridged. The caveat is that to use those jacks in bridged mode, you have to wire a special speaker cable that uses pins 1+ and 2+ instead of the normal 1+ and 1- that are used for stereo mode (on the amp end). This is often overlooked. My clue that a customer is not configuring their amp correctly is when they have the amp set to bridged mode and then have colored tape or stickers by the 2 stereo speaker jacks indicating that they are wired in stereo mode. When I see that, I will have a conversation with the customer about how to configure the amp so that it doesn't come back to me after the first use.

Edit: That was in response to Jazz and Enzo's posts. g1 popped in while I was typing.

[g1]

Another aside is that the minimum load for bridged is always double the minimum per channel when used stereo.
This is just one more reason so many problems arise from improper use of bridging. Unless the user is a pro, bridging is often a recipe for disaster, and often not even necessary for the user's purposes.

[The Dude]

Another aside is that the minimum load for bridged is always double the minimum per channel when used stereo.
This is just one more reason so many problems arise from improper use of bridging. Unless the user is a pro, bridging is often a recipe for disaster, and often not even necessary for the user's purposes.

Agreed,...... and God forbid they should read the manual.

"But,... but,... I read somewhere that it doubles the power"