Class A or AB?

[mace]

Hi,

How exactly are SS amps rated? I've seen some literature about A and AB ratings on some B&K equipment (2140 stereo power amp I believe) which I'm interested in.

Thanks,

Mace

[Deang]

The greatest majority of SS amps are run in class A/B.

The B&K has a Class A pre-driver stage (good) and Class A/B output stage.

Typically, the first few watts of a Class A/B amp will be class A, at which time it switches over to Class B operation.

You might want to look at some things which are probably more important when it comes to determining the performance and sonic signature of a SS amp.

Damping Factor -- the higher the better, and will tell you how much control an amp will have over the low frequency drivers.

Transistors -- are they Bipolar or MOSFET? Bipolar designs typically sound a little on the cool side, to bright -- but have great transients. MOSFETS will give you a darker, or warmer signature -- but you lose the transient attack of a good Bipolar amp. With Klipsch speakers, I think MOSFET designs are probably the best direction. B&K are MOSFET amps.

Amperage -- compare the amperage ratings. All watts are not created equal.

Parts quality -- you want to know if the manufacturer is willing to share with you the quality/kind of parts they use.

Along with the above, I have always felt SS amps should be bought by the pound. If one 200 watt amp weighs in at 35 lbs., and another 200 watt amp weighs in a at 75 lbs. -- I'm taking the back breaker. Most of that weight will be found in the transformer(s).

The B&K amps present a very good value. A good SS choice.

This message has been edited by deang on 09-11-2002 at 10:39 AM

[mace]

Deang - Thanks for the info.

Does anyone know exactly what qualifies an amp as "A" or "A/B"? What does it mean exactl?

It looks like I'll be picking up the B&K amp this weekend.

I'll see how it mates up with the Cornwalls.

Mace

[Jolly Roger]

Well, I'm not the person to answer this but I've seen this question a time or two here lately. Here are a couple of posts I found that may help explain.

http://www.tpub.com/neets/book7/25e.htm

http://db.audioasylum.com/cgi/m.pl?forum=general&n=28304&highlight=what+does+class+a+mean&r=&session=

[mace]

Thanks for the links!

I read them both and am somewhat concerned... I've got a B&K ST2140 on order and it is rated A at low powers and then AB for the rest. In one of the links above it stated:

Therefore, class AB amps with only a low amount of class A power, e.g. < 10W, are to be avoided. Especially when one wants to play at high levels or with low impedance speakers.

The specs on the ST2140 can be found at here.

Does anyone have any experience with this B&K amp and 1977 Cornwalls??

Thanks,

Mace

[jnorv]

There are a whole lot of us out here running 10 watts max tube amps into our horns with no problems. Cornwalls must be at least 100db with one watt. Unless you are using them in a very large room, you should have no problems.

Jim N

[Jolly Roger]

10 watts of tube power aint like 10 watts of ss. You can push a tube amp to near max output, where as if your pushing a ss amp much more than half way your probably going to start hearing some clipping. With corns I'm sure this would probably be a fine amp although I really have no idea. Ref series is a step up.

[Colin]

Mace, it is best to remember that the super and ultra-high efficiencies of horn loudspeakers changes almost every generalization made about amplifier and loudspeaker matching.

Unlike most volume knobs, which send out most of the available power by 2:00 on the dial, my pre-amplifier's volume control approximates the actual amount of power being sent to the speakers.

At the 9:00 setting, I get about 73 dB (+- 2 dB) at the listening spot with possibly about 1.6 watts. This is a 1 kHz tone, with both subs set at their normal setting, measured at back of my couch, 117" from the front of the speakers, on a "C" weighting, and fast response, using a low cost Radio Shack SPL meter. Noontime on the dial is a 91-dB average with about 3.2 watts. Three o'clock with this extrapolation is 101 dB with about 4.9 watts. At flat out, with the volume twisted far right to 5:00 on the dial, the Cornwalls will put out something like 6 watts, providing an unmusical pain at 110 dB.

Theoretical calculations estimate about four watts to get a average 95 dB in-room response. This appears to be exactly correct, for between one and two on the dial, I am getting something like 92 to 97 dB with about 3 3/4 to 4 1/4 watts.

An amplifier with 10-watts Class A power is going to be running Class A almost all of the time. The Pass Laboratories Supersymmetry Balanced Single-Ended Class-A X250 Stereo Amplifier is designed to give you "Class A" quality below 75 watts. (http://www.enjoythemusic.com/magazine/archives/) In fact, using the thunderous Pass X250 on a big old pair of Cornwalls Is, we never saw the needle in the single blue eye move into the Class A/B range, even with some painfully loud levels (>110--dB) and some pretty violent action scenes (The Matrix).

You don't need lots of power with Klipsch's big old horns - you need lots of good, clean power. Class A is good; tubes, IMO, is often better performance at a lower price, but with some learning required.

------------------

Colin's Music System Ak-2 Khorns & Klipsch subs; lights out, tubes glowing & smile beaming!

[NOSValves]

I'd have to agree with Colin. With my Lascalas and my 299 Scott amp. I have no way or havn't even tried to confirm this but I think I never push the amp out of Class A operation. I also have a 222C that I modified to operate in Class A triode and I have not been able to hear a difference between the 2 amps at all. In my small listening area 2 watts produces all the power I need 5 watts will make my ears bleed !!!

------------------

HH Scott 299 Amp

HH Scott LT-110B Tuner

HH Scott P-87 Turn Table

JVC JL-F50 Turn Table

Sony CDP-CA7ES CD Drive

1985 Walnut Heresey I W/Layne Audio Woofers

KSW-15 Subs>c>

[mace]

Colin: thanks for the post!

I've a question. It is my understanding that doubling power (watts) gives you an increase of 3 dB SPL. I have verified this with my radio shack meter, using a similar method as you described (C weighting etc). Are you sure that you go from 4.9 to 6 watts and see a 9 dB increase in SPL???

I really have no clue at what wattage the B&K ST2140 goes A/B?? Their literature states "class A pre-driver and class A/B output stage". Does anyone have a clue what this means? I wish the spec sheet would be more discriptive. The spec sheet also states "30 amperes current output peak to peak". Well, what does this mean exactly???? Literally, it could mean that the actual amperage has an amplitude of 15 amperes so RMS is something like 10 amperes RMS. If this is true the B&K can't keep up with my lowly NAD 3240 putting out 25 amperes for up to 200 ms. But I digress....

So... I'm thinking if the B&K switches to A/B at like 0.5 or 1 watt or something that could be a problem. I agree, with the Cornwalls of KLF-30's 10 W is plenty (unless you're having an outdoor street party for the entire city or something...)

Anyway, I get the amp Friday so I'll know for sure how it sounds when I get it hooked up!

Any more insight on this topic would be appreciated!

Jolly Roger: Yes... Reference series would be a step up but too much $$$ right now.

Thanks,

Mace