amplifier topolgies

[Andrew Pratt]

As some of you know I've been looking at picking up another amp. During my quest I've come across a few amps that claim to run at least partially in class A...I know this has something to do with the voltage rails but can someone please explain (1)how an amp works and (2) what the differences are b/t the various classes (and where does mosfet come in?)

[Lex]

My knowledge has slipped in this area unfortunately. I used to know something about it. Class A typically run hotter I think and are fairly low wattage, but typically is associated with audiophile grade gear. Class AB retains some of the benefits of Class A, but maybe overcomes the heat problems associated with Class A, and boosts output stage, if memory serves correctly in the class B area. Of course, it's amplifier topology 101. Unfortunately, I flunked out, lol. So, I could be off base here.

I know just the guy to go over this, but he may not be available until Monday. Jon Marsh can probably fill all the gaps if he gets around here before the down time.




Cable Guy DVD Collection

[ThomasW]

Well Jon is of course the person to address this question, but I'll throw in a couple remarks

Class "A" topology usually uses a significantly higher "bias" current to drive the output devices. This is the reason the amps run so hot even when idle. Virtually all class "A" topology amps switch to "AB" mode with low impedance loads or at higher wattage outputs. Understand that extremely high power class "A" amps run so hot you could fry an egg on the heatsinks. So if you don't want a free space heater included with your amp, best to look elsewhere.

Also class "A" amps have a specific sonic signature. I really really like them. To me they have the best qualities of solid state combined with some aspects of tube gear.

MOS-FET = Metal Oxide Semiconductor - Field Effect Transistor. These are one type of output device. Some people lov-em, others hate-em. They're just fine by me




theAudioWorx
Klone-Audio

[James W. Johnson]

My thoughts on amps are this, they need to amplify the signal without adding or taking away anything. Sounds simple but there are alot of things that can happen between the input and the output of an amplifier.

I accidentally started using a pro amp to power my mains, I say accidentally because I intended to use if for a subwoofer , well I never needed it so one day I tried it on my mains ...over a year has passed and the amp still powers the mains , it sounds better than any other amp I have ever heard. The guts in my amp are unbelievable, there must be 10 ot 15 output transistors and the power supply is HUGE, much beefier looking than an Outlaw 750, Anthem MCA 5 or most of the other home amps I have seen.
I plan on running 3 of these amps with an Outlaw 950 by years end.
Anyways its a QSC amp and here is a great article from QSC on amplifiers ....

http://www.qscaudio.com/support/library/papers/amptalk.pdf

[JohanK]

Class A amps are flowing current through the output transistors during the entire cycle of the signal. This differs from other classes (like class B) that stop/start current flow through the output devices.

Class B runs the output transistors for 180 degrees of the signal cycle; class AB is greater than 180 degrees but less than 360 degrees. Class C is less than 180.

Class D amps (also called PWM and switching amps) are a different breed. They transform the original signal into square waves, do amplifying and then reconstruct original analog signal. Output devices act as switches.

Class G amps use separate Class B stages; one for low voltage power supply rails and one for high voltage. Class H amps alter the actual output of the power supply depending on output demanded.

MOSFETs are a type of transistor. Other popular types are JFETs (usually only used on input stage of an amp) and Bipolar transistors. HEXFETs are sometimes used (Adcom 5802 amp).

[Eric S]

Andrew: You might want to have a look at Rod Elliot's web page. He has a description of Class-A amps as well as the remaining classes that are posted above.

Shortly, I'll be building a Pass Labs a40 amplifier (I've been building a parts list for a little while). Its 40wpc Class A output. In my research, I've come across a few "general" trends. In Class-A amps, MOSFET output stages seem to have a more "tube-like" sound (warmer) while amps that use Bipolar Junction Transistors (BJT's) seem to have a more "analytical" sound (brighter) to them.

The distinction of an amp being Class A does not necessarily refer to the voltage rails, but to the current flow through the output transistors. Where Class B amps only conduct maximum current through the output transistors under full load (reference level input signal, amplifier set to maximum output), Class A amps conduct maximum current through the output transistors 100% of the time. Class A amps are very heavy and get very hot, but they have a sound that is quite different from the traditional Class B amp that is available almost everywhere today.

[Lex]

That's a nice "Senior" level post there Eric!

He's no newbie, just a new member.

Now, where is my audio dictionary to try and figure out what he said, lol.

Lex

[Andrew Pratt]

Thanks eric the amp I ended up buying was a musical fidelity amp that runs in class A for the first 30 watts or so (120 watts into 8 ohms for the remainder) So far I'm very much in love with its sound and you're right its vry heavy and very hot

[Eric S]

Hi Andrew, Sorry I missed your "window". I didn't realize you'd already made the plunge! Glad to hear that you are enjoying your new amp! 30wpc should be plenty, unless you've got massively inefficient speakers (now that I'm thinking about it, don't you run Maggies?) I've go so many large projects up in the air right now, I don't imagine I'll get started on my amp until the end of the year (let alone the speakers I want to build to go with them!).

Lex: Thanks for the complement :B. I've been lurking for quite a while, and spending (probably way too much) time researching DIY amps, speakers, etc :W One of my goals before I die is to build plenty of amps and speakers just to be able to say "Yeah, well, I've built that one. It sounds OK, but THIS one really smokes!" Ever since I discovered the 10:1 ratio of retail price to parts cost :E, I've sworn off buying what I can build by myself!

[Andrew Pratt]

yes I'm running maggie 1.4's as mains, MMG centre and SMGa's as rears. The Musical Fidelity amp powers the 1.4's with a HK 5800 5 channel amp powering the rears, centre and DVC Sonosub. To say that the MF amp is better sounding then the HK wouldn't be giving the MF enough credit

I haven't got the manual for the Dr. Thomas amp yet so some of the wattage ratings might be a little of but from what the sales guy said it should be around 30-40 watts in class a mode before it switches over to AB. Also given that those are 8 ohm ratings and the maggies are 4 ohm how will that affect what range the amp runs in class a mode?

[Eric S]

Andrew: Hmmm... I can't accurately answer the question of how will 30-40wpc into 8 ohms change as it goes into 4 ohms. This depends upon the design of the amp. For example, the Pass Labs a40 amp that I am building features a "constant current" source for biasing the output transistors. This means that no matter what the load is, the output current is fixed. Following ohms law, 40wpc into 8 ohms turns into 20wpc into 4 ohms.

A more traditional Class B amp is usally able to increase its output into a 4 ohm load. The very best of which will exactly double its power when the resistance changes from 8 ohms to 4 ohms. Most others will probably give about a 50% increase. My current Marantz MA500s pick up about 50% more current into a 4 ohm load.

I would suspect that your new amp is somewhat different because it is able to switch back and forth between Class A and Class B modes. I further suspect that 30-40 wpc into 8ohms may turn into 15-20 wpc into 4 ohms since the switching of class is determined by currents through the output transistors. However, the total output of the amp may increase, say from 130wpc to 180-190 wpc in class B mode. The main limiting factors are the size of the transformer for the amp, and the more specific topology of the amp.

So, to sum it all up: you will likely get more power into 4 ohms (maybe 180 wpc), but don't be surprised if your amp drops from Class A to Class B sooner than 30-40wpc into a 4 ohm load.

Either way, you've got yourself a very nice amp there! I can't wait to build mine. I've also recently found a guy who has a web page describing how to build your own Pass Labs Aleph 4 amp (retails for $7000)

[Andrew Pratt]

thanks eric, I guess we can talk tech all day but what really counts is that this new amp is far superior sonically to the HK amp I was using before. Its like a whole new world

I have learned a little more about this amp and its got a pretty interesting history. Apparently this design was a very limited production and there were none imported into the US. The dealer I dealt with told me that he managed to get about 34 into canada which was as many as he could get. Anyway the model number says that its a Dr. Thomas MKII and my serial number is a hand written 808 so i figure if they were hand writting the serial numbers its not likely that there were never put into mass production...which in a way is a shame since I love its sound