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Class AB Simple Amp

#24
Blues Marvel said:
Great thread lots of cool amps mentioned. After so many mentioned the SWR stuff I found a deal locally for a vintage SM400 that looks real clean. Guy says it has some scratchy spots. Hopefully I can square those away myself. Planning on running it Bridged into an 8 ohm GK CX-210.
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I own an SM-400S and to me the SWR heads in that era offered a different voicing in a similar kind of way that GK was SS different from the Ampeg and Fender tube heads. I keep my old SWR less as a daily driver and more of a nostalgia piece that can still hold its own tone-wise against more heads.

I would also support an argument that says that the Mesa Walkabout is a bit of a classic that added something to the bass world that many found appealing. IMO it too has stood the test of time as having more than just a fad voicing.
 
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#26
100 bucks is a steal for that. Congrats

This always merits mentioning:
From the SWR SM-400 manual in the section on chassis heat:
"The only condition you should be aware of is if one or more of the power amplifiers in your unit becomes "over biased." This condition can be recognized by turning your amplifier on and letting it sit without speakers plugged in and without playing it. If, under these conditions, your unit becomes quite warm, it may be over biased. This situation should be attended to and can be easily remedied in about 15 minutes by a service tech. A power amp can become over biased through continuous vibration or by any large jolt received in shipping, etc. "
 
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#27
ihixulu said:
100 bucks is a steal for that. Congrats

This always merits mentioning:
From the SWR SM-400 manual in the section on chassis heat:
"The only condition you should be aware of is if one or more of the power amplifiers in your unit becomes "over biased." This condition can be recognized by turning your amplifier on and letting it sit without speakers plugged in and without playing it. If, under these conditions, your unit becomes quite warm, it may be over biased. This situation should be attended to and can be easily remedied in about 15 minutes by a service tech. A power amp can become over biased through continuous vibration or by any large jolt received in shipping, etc. "
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I make no representation or promise that this documentation is 100% correct, but someone like agedhorse could tell you if it is authentic and accurate. He has direct experience with SWR from his Fender days (perhaps earleir) and serviced my SM-400S a few years back.
 

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#28
Common causes of over-temp faults in the SM series include fan failure, blocking of the vents, driving a sub-nominal impedance load, a short circuited load and a bias fault.
 
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#29
Blues Marvel said:
So I picked this up this morning. Its in excellent condition, needs a little cleaning.
I paid 100 Bucks with the rack. Rack appears to be a fairly nice custom build 4 space with what looks like a grey duratex type finish.

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great time to be alive.
real amps are cheap
turds are expensive lol

I had one, always worked never heating problems
EQ took time to learn, but it worked.

Before I racked it the knobs got bumped in transport, so we always made a joke at sound check what random preset do we have tonight lol
I ended up buying his earlier designs a Acoustic 370 best amp ever
 
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#30
So got it home and played it for a bit. This thing is in great shape, definitely needs some love so going to take it by my Tech I been using for 30 years.

So the tone was great and I dont hear in farting low end troubles from this amp at all, plenty loud. I set the 31 HZ control for cut at about halfway down, I little boost at (not sure) around 100 HZ, and the other two Semi EQ Flat, Bass & Treble 12:00 and it sounded perfect. Just had a little bit so I ran it 4 ohms into two 12's. Sounded much louder than I would have thought. Using one side at 240 watts at 4 ohms in 2x12's sounded as loud as any 800 watt Class D amp I've owned, interesting. It seems we have gotten more watts but the decibels have stayed the same. Really like the tone both overdriven and clean.

 
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#31
Awesome, be happy that you are easily and inexpensively satisfied. You have found your nirvana, that doesn't discredit others who have had different experiences.
 
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#32
20170213_155211.jpg
 
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#36
BassmanPaul said:
Why is Class AB a requirement? As for simple, that concerns the pre-amp section not the class of the power amp. AB amps require more cooling than Class D so typically a bigger, noisier cooling fan.
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Oh, just been reminiscing about some cool old designs lately. All these that I know of would be class AB. The Class D thing has been discussed to infinitum should we discuss it further? I'm even toying with the idea of building a AB amp. For me something like an old SWR SM400S with one dedicated power amp (or bridged all the time), four band EQ (nice and simple), no FX loop (I never ever use one), transformer balanced DI. I like simpler stuff. A good example was my Thunderfunk, great amp too many knobs. I ran it flat all the time. I've thought about hiring someone to build me an amp design of my choosing. I take care of all the costs and they create my design. Even wondered if such an amp would be a viable piece in the market though it sure is crowded. It seems there's more builders than players these days. Not sure in the coming days there will be as many looks like we fixin to have a depression.
 
#38
Blues Marvel said:
I've thought about hiring someone to build me an amp design of my choosing. I take care of all the costs and they create my design. Even wondered if such an amp would be a viable piece in the market though it sure is crowded. It seems there's more builders than players these days. Not sure in the coming days there will be as many looks like we fixin to have a depression.
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While it's fun to dream and perhaps even a healthy thing to do now and then, correctly assessing market viability, being abel to build quality at a target price point, getting a new design through testing and certifications, etc. is not as much fun as one might imagine.

Having been heavily involved in major military aviation related design activities since the 1970's and more recently in some musical industry product designs as a beta tester for Mesa I can assure you there is a lot to consider beyond what may be pleasing/preferable to you as the designer. There is a good reason we see mostly class D with SMPSs dominating the market right now. People vote with their dollars and there is no way even the best class AB SS and tube amp designers can deny what market demand is telling them.

I own all 3 Mesa Carbine models, 2 Mesa Walkabouts and an SWR SM-400S that I genuinely enjoy every time I fire them up. However, my Mesa D-800 and WD-800, Aguilar TH500 and GK MB200, 500 and 800 see more gig action these days. Great sound/tone, more than sufficient firepower, rock solid reliability and convenience that no class AB head can achieve make thehse newer rigs my obvious first choice...OBTW, I paid less for my class D/SMPS heads than I did years ago for those other heads I mentioend and that's not even factoring in how inflation would make that difference even larger.

If you're looking for a super simple head that hits hard and sounds pretty good...try a Quilter Bass Block or a Mesa D-800. Personally I like a head that sounds good for my uses with controls aorund their neutral position, so having a few EQ and voicing filters available to adjust to different venues and styles of music seems like a bonus to me. When I'm just playing and not looking at the head, I have a hard time telling what class of amplifier or tpye of power supply is getting me my results.

I will admit that some of today's heads sound different than heads of days gone by. As an engineer though I chalk that up to voicing choices made in the design process that accomodated player demand and expectations. I also recognize that bass player preferences and tone goals changed/matured/evolved over time...and SWR (especially that SWR SM-400 head you and I like) played a big part in one of those times of change.
 
#39
BogeyBass said:
not really a fan is a fan.
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Probably true in many cases, but the higher heat loads inherent in less efficient class AB designs might require moving more air as part of the thermal management aproach - a bigger fan(s) moving more air could be noisier. As always though there are a lot of variables. With enough heat sink area and perhaps clever venting to create enough convective airflow, a totally passive thermal management approach (like many older Peavey procucts for example) is certainly possible.
 
#40
BogeyBass said:
Basically what im doing building a amp nobody builds.

Meaning eliminating pretty much every " fancy" unneeded feature
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It's great to have the skills, time, and tools/equipment to build your own custom tailored head, but for many of us that's just not in the cards...we're stuck "buying off the rack"! :)
 

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