SWR SM-400 amp issue. Need expert input. FIXED

[Deleted member 165050]

A wild stab in the dark here, could the special speaker cable be faulty?

 

[40Hz]

@agedhorse nailed it. It was the bridge mode switch.

Fixed now. Thanks everyone for your input.

 

[agedhorse]

Excellent. I'm pretty familiar with these amps, as I spent some "quality time" studying them during our time under Fender's rule.

 

[40Hz]

Excellent. I'm pretty familiar with these amps, as I spent some "quality time" studying them during our time under Fender's rule.

Yeah. I’m happy. Much as I want another amp, I’d rather not buy one just yet until I have a better idea of exactly what I realistically need. We’re thinking of getting a little more serious about a home recording studio of sorts, so it’s balancing requirements against available cash. Good exercise in personal discipline if nothing else.

 

[Omega Monkey]

Damn, an amp that cost 900 bucks in the early 90s had it's first (fixable) problem after 25 years? What a piece of junk!

Seems like a labratory amp and not suited for music IMO

This is just silly. All the SWR gear I've ever used has sounded fantastic. I will ALWAYS remember being about 14 and playing an Urge bass into a Baby Blue and just being in bass tone heaven. I don't think high and low shelving controls and 4 bands of semi parametric mids is all that complicated. And you can just zero it all out if that's what you want. But the control and flexibility is there if you want it. And yeah, you can used it in stereo, biamped, bridged, etc... or not.

Anyway guys, keep trashing SWR. My wallet and gear collection thanks you. I'm actually planning to pick up an SM series one of these days, maybe I'll find a high quality killer full featured hybrid amp for 100 bucks or so by the time I get around to pulling the trigger.

 

[Jim C]

Laboratory Amp

This is just silly. All the SWR gear I've ever used has sounded fantastic. I will ALWAYS remember being about 14 and playing an Urge bass into a Baby Blue and just being in bass tone heaven. I don't think high and low shelving controls and 4 bands of semi parametric mids is all that complicated. And you can just zero it all out if that's what you want. But the control and flexibility is there if you want it. And yeah, you can used it in stereo, biamped, bridged, etc... or not.

Anyway guys, keep trashing SWR. My wallet and gear collection thanks you. I'm actually planning to pick up an SM series one of these days, maybe I'll find a high quality killer full featured hybrid amp for 100 bucks or so by the time I get around to pulling the trigger.[/QUOTE]

They are a great deal on the used market and my SM400 is approx 30 years old.
It had a problem in the first month and hasn't missed a beat.

The SM400 has NO HPF
It will go down to DC and kill speakers without extreme care; just like the old Crown DC300's; very little HP filtering
You can set the parametric to it's lowest position (I think a 50 Hz center freq), pull the slider down all the way, and still have LF problems

For those of you with a SM400, set the EQ and volume the way you like it, and then without playing, push the strings toward the pup, the cones will move.
Same thing with another amp and very little if any cone movement

 

[agedhorse]

Note that the SM400 won't go down to DC, it does have shallow high pass filtering that is 3dB down somewhere around 18-20hz. Too low and shallow sloped to be effective compared with today's more sophisticated designs.

 

[Jim C]

Note that the SM400 won't go down to DC, it does have shallow high pass filtering that is 3dB down somewhere around 18-20hz. Too low and shallow sloped to be effective compared with today's more sophisticated designs.

You engineers; no sense of humor
OK so you can't run a flashlight with it but it's a trouble maker
What type of HPF do you design today for a MI amp?

 

[Al Kraft]

Note that the SM400 won't go down to DC, it does have shallow high pass filtering that is 3dB down somewhere around 18-20hz. Too low and shallow sloped to be effective compared with today's more sophisticated designs.

An SM400/400S with an external HPF works quite nicely IMO. Had I been the designer of that amp, first off it probably wouldn't have worked at all, but more importantly, in and among all those tone shaping controls I would included a variable HPF.

Those heads had a control for just about every aspect of the tonal spectrum and I suspect that many of us thought that we were accomplishing something meaningful by using the lowest semi-parametric EQ to reduce the inherent low frequency problem. In reality I guess it may have helped some, but a HPF is an essential tool to get the most out of those heads IME.

 

[agedhorse]

You engineers; no sense of humor
OK so you can't run a flashlight with it but it's a trouble maker
What type of HPF do you design today for a MI amp?

Depending on the intent, anything from a 2 pole to 4 pole filter, typically with a Butterworth of Chebychev type alignment. At least 28-30Hz as that's still well below the F6 of just about any bass cabinet.

 

[Jim C]

Thanks for this; I understand and can look up much of this tech info but don't know what F6 means.
I appreciate the free engineering eductaion we get from most of your posts.

 

[Passinwind]

Thanks for this; I understand and can look up much of this tech info but don't know what F6 means.
I appreciate the free engineering education we get from most of your posts.

F6 = 6dB down point on the cab's low frequency rolloff curve. At least back in the old days that was more or less considered the lowest point where corrective boosting EQ could still be effective in getting to flat response.

 

[Ant Illington]

Depending on the intent, anything from a 2 pole to 4 pole filter, typically with a Butterworth of Chebychev type alignment. At least 28-30Hz as that's still well below the F6 of just about any bass cabinet.

With all due respect, please explain what y'all are saying in layman's terms. Am I at risk of destroying something with this amp I've had for 25 yrs? I did have to replace the power switch once so there's that. Thanks.

 

[agedhorse]

F6 is the -6dB point in the low frequency roll-off of the speaker. At this point, the speaker is becoming un-damped and beginning to "flop around in the box". At the F6 point, the speaker's mechanical power handling is also falling (by at least 3dB [power] per octave, usually more).

So, ideally (well sort of, there's no ideal) we would like the power applied to the woofer to fall faster then the power handling falls. This is exactly what the HPF does, it decreases power with falling frequency, allowing the driver to stay reasonably powered over most of it's bandwidth.

If you are careful, and have plenty of power handling in your speakers, it's not a big problem BUT with amps getting bigger and bigger it has become a huge problem for some players (and types of music).

 

[beans-on-toast]

I thought that amps are getting smaller amd smaller and power output is getting bigger and bigger.

 

[agedhorse]

I thought that amps are getting smaller amd smaller and power output is getting bigger and bigger.

Good point, they are getting smaller and smaller while at the same time getting bigger and bigger!

 

[Omega Monkey]

Everything is DC, until it isn't. You make it sound like you need an advanced degree to make the thing work and sound good, and yet, here we are 30 years later and there are tons of these things still out there and in use. Sensible volume and EQ settings should have these things making music for a long time and sounding great doing it.

Anyway, I'm not sure I've blown a speaker (when I wasn't trying to do it on purpose) in 30 years of playing electric instruments of all kinds through amps and speakers of all kinds, with effects of all kinds. Maybe I benefited from being brought up in a house that was a little more aware of proper practices with that kind of stuff, but none of that knowledge is so out of reach as to be inaccessible, and it's largely common sense anyway.

 

[agedhorse]

Everything is DC, until it isn't. You make it sound like you need an advanced degree to make the thing work and sound good, and yet, here we are 30 years later and there are tons of these things still out there and in use. Sensible volume and EQ settings should have these things making music for a long time and sounding great doing it.

Anyway, I'm not sure I've blown a speaker (when I wasn't trying to do it on purpose) in 30 years of playing electric instruments of all kinds through amps and speakers of all kinds, with effects of all kinds. Maybe I benefited from being brought up in a house that was a little more aware of proper practices with that kind of stuff, but none of that knowledge is so out of reach as to be inaccessible, and it's largely common sense anyway.

Sensible volume is something that you might take for granted, but you are not representative of much of the market. Unrealistic expectations, combined with increasing amplifier power and lack of good common sense (plus simple accidents) are pretty rough on speakers.

Now add to this speakers that are unrealistically over-rated in power handling and it's a recipe for increased failures. This is what I have seen over the years.

 

[40Hz]

Damn, an amp that cost 900 bucks in the early 90s had it's first (fixable) problem after 25 years? What a piece of junk!

I don't recall anybody here calling it that. Are you reading the same thread I am?

The SM-400 has well understood design flaws. It's something that's been freely discussed and addressed in SWR specific places on the web. Not just here at TB. THAT was where I learned what the problem was and why an inexpensive HPF can make it a much better amp. Something I'm happy to say proved true. I've also owned and used the thing since the early 80s. So any comments or critiques I have about this amp are based purely on my own direct personal experiences with it and aren't me just regurgitating something I got off the web.

Minor point too... I paid around $900 for that amp in 1985. Adjusted for inflation that comes out to about $2200 in buying power with today's dollars. That puts SWR more in the boutique price range when new in my book. So if expectations are a little higher, I think they're perfectly justified based on that pricing.

And FWIW I can get more useful tonal adjustments using a 2-band Bandaxall design preamp stage than I can with the dashboard on my SM-400. More doodads and controls don't automatically translate out to more useful settings.

I've owned and played a lot of amps over the last 40 years. So I've got a pretty broad range of experience to fall back on when I'm talking about amps from a player's perspective. And one thing I've learned is go by your ears and don't buy into any marketing spiels.

 

[BadExample]

F6 = 6dB down point on the cab's low frequency rolloff curve. At least back in the old days that was more or less considered the lowest point where corrective boosting EQ could still be effective in getting to flat response.

How well does -6 db roll off respond to + 6 db of eq?

Nowadays a lot of us have a lot more eq at hand, which of course results in a lot more overextended speakers