more bass from a "class A" amp? (1 Viewer)

[Geno]

so I should just upgrade my a/b amp to a b&k amp then...
well if I ever get nother great deal at audiogon

 

[Yogi]

a pair of 38D's beats a pair of 32A's.

I kinda like a pair of 32A's more than a pair of 38D's. I like em light and firm rather than heavy and droopy. But again its subjective taste (no pun intended).

 

[Richard_B]

Just curious, Do they make any Class D amps for home stereo use? I am only asking because that seems to be the trend for "subwoofer" amps in the car stereo world right now. I have now idea what the differences are in the amp classes by the way.

 

[Michael R Price]

Richard, they do, and some people think they sound really good. I haven't heard any myself yet.

BTW, there may be some other factor contributing to bass quality besides damping factor and power supply. After all, my Zen has a damping factor of around 20 with my speakers, and the power supply is huge (it has as much capacitance as my bigamps now but it has 1/20th the output power). The capacitance didn't help... the bass stinks at any volume.

 

[Chu Gai]

Well that's zen for you...it was meant to be

 

[Lewis Besze]

I kinda like a pair of 32A's more than a pair of 38D's

Looks like you're into mosquito bites.

 

[Yogi]

Looks like you're into mosquito bites.

 

[Brad_Harper]

A class D operates very similarly to the other classes of amplifiers. Class A's have their output transistors fully "on" all the time. Since they are always on there is no delay during crossover from push to pull and vice versa. However even with no input signal, current still flows through the transistors. This obviously is very inefficient.

Class a/b or just class b has their output transistors partially on. Now there is a slight delay introduced between the push pull crossover causing distortion because the transistor has to go from partially on to fully on. Less current is wasted when there is no signal so this design is more efficient then class A.

Class D operates on the priciple that the output transistors act like a switch. Either fully on with a signal or fully off without. It also uses a PWM (pulse width modulation) scheme to change the analog signal into a high frequency switching signal (Mhz range). The ampilified signal is then filtered to eliminate the high frequecy signal and leave the lower frequencies intact. This creates a very efficient design but introduces a lot of distortion due to crossover and PWM harmonics.

Any amplifier design is a trade off. Class D done right can sound very good. I have a class D amp for my car subwoofers. Audio purists will probably avoid Class D because it completely modifies the analog signal during the switching phase. Class D's can be upwards of 80% efficient where a Class A is often only around 30%. To put that in perspective a 200W class A actually needs to draw over 600W from your outlet. A class D of the same power rating would only draw 250W.

 

[JohnSer]

Brad, you seem very knowledgable about the amp classes. Can you comment on the B&K, mentioned above, where it appears to operate in class A upto 25 watts, and then somehow switches over to class A/B? I don't quite understand how that is pulled off. I do understand how some amps, like the outlaw monoblocks run in class A/B upto a certain point, and rail switch to a different voltage.

JohnS

 

[Brad_Harper]

My best guess (I stress guess) would be that B&K are using a variable resistor in the forward biasing section of the amplifier. This circuit controls the amount the output transistors are turned "on". Using a current sensing device at the speaker outputs they could easily modify the amount of biasing on the output transistors turning them from class a opertion to class a/b very easily.

Most manufactuers control the forward bias of amplifier output stages for heat concerns. B&K probably just took this control one step further.

 

[Robert_Dufresne]

Geno

Class A amps are nice because they heat up a room nicely in winter. I had a friend who used to put a pan on his to cook eggs while we listened to music. It eventually got fried too.

Robert

 

[Yogi]

Robert, you mean this friend
http://img.audioasylum.com/cgi/view....e=&w=800&h=609

 

[Khoa Tran]

wow....and i was even thinking about building some alephs....it's already hot here....

 

[Geno]

well gees, my amps dont get near that hot.
Ive done demos for friends at rediculous levels but not for relaxing or movies. I usually listen to music at -20.

ive gotten my SPL meter out while listening and I usually have it peak in the high 90's and that is way loud for me.

 

[Michael R Price]

My Zen (15 watts into my speakers) is on a 20 pound heatsink and still around 55 deg C, or 130 F. Hurts to touch it. I think the Alephs are a few degrees cooler but still silly hot. The electronics will live at up to 85C though!