Class A amplifier build (need a lesson in audio engineering)

[Marko25]

Hello everyone,

This is my first post, I have noticed such a wealth of knowledge on here and thought someone may be able to give me some advice.

After sometime I've decided to pull my finger out and have a go at some DiY audio. So I have been looking at a class A push pull which is a simple design and I am quite keen to hear.

I currently have Wilson Benesch Arc speakers rated at 6 ohm nominal and 200W Peak unclipped. Trying to match these requirements my calculations assume 100W RMS so say 96 Watts equating to 4 amps 24 volts @ 6 ohms.

This would give me 4 x 1.414 = 5.656 amps and 24 x 1.414= 33.936 volts at peak load.

If using a push pull we would get 5.656/2=2.82 amps per transistor producing 2.82x33.936=95.97W per transistor.

Given this is a lot of heat to dissipate and I will get a nice 200w heater per channel along with the electricity bill but that doesn't concern me so much.

As I have only started learning this in the past 2 days and it seems remarkably straightforward my questions on this are thus;

1. Are my calculations correct/on the right track?
2. I would intend to use a power supply with capacitance smoothing am I correct in assuming this would follow the full sinewave peak cycle i.e 34-0-34 (plus losses) rather thans 24-0-24?

Many thanks for your help in anticipation,

Mark

[pacificblue]

1. Your calculations are correct, but you are still not on the right track. The power rating of a speaker has nothing to do with the amount of power your amplifier should have. Start with a small 5 or 10 W class A amplifier, e. g. the JLH. You will be surprised, how little power is needed for sonic fulfillment. Check http://www.passlabs.com/ for a start into class A as well.

2. Also correct.

3. Ask a mod to move this thread to the Solid State Forum.

[AndrewT]

Hi,
a 100W ClassA amplifier is not the place to start learning.

Try a small ClassAB amplifier. 50W? with one output pair fed from a simple PSU @ +-35Vdc. Have a look at the ESP site for a cheap starter amplifier.

Then, try a small ClassA amplifier, Pass is a very good site for ideas, knowledge and circuits that perform. Or the little JLH, or the ESP ClassA. There's lots of choice.

ClassA costs a lot more than ClassAB. Start cheap, if it goes wrong there's less money down the drain.

First job: build a mains light bulb tester. See Nuuk's site for details.
Learn how to use the mains . SAFELY !!!!!!!!!!

ps,
I have asked for this thread to be moved.

[Marko25]

Start with a small 5 or 10 W class A amplifier, e. g. the JLH. You will be surprised, how little power is needed for sonic fulfillment. Check http://www.passlabs.com/ for a start into class A as well.

Thanks for the advice.

2. Also correct.

Good to know I got something right

3. Ask a mod to move this thread to the Solid State Forum. [/B]

I would but I can't e-mail them to tell them, sorry real noob mistake.

[AndrewT]

simply "report" your own post with a request.

[adason]

Hi Marco25,

to me the most important parameter of the speaker is its efficiency. You did not mention that. Peak power is quite a useless parameter. You never listen at full power. As a matter of fact, most listening is done with less than watt. Thats right. Furthermore with highly efficient speakers all you need is few milliwats. Why we need more power? Just a reserve to spare to keep music dynamic.

Statement that speakers can handle 200watts peak means nothing. It might mean that once during testing they survived 200 watts impulse. But the music they produce at 50-100 watts may not be listenable at all. They might have such horrible distortion, that you will not be able to wistand it.

Good start is 5-7 watts classA amp, preferebly well known designs, JLH for instance. Or 20-40 watts classAB, there is too many outhere, majority very good ones. Even if you will built 100watts classA eventually, you will never need it. It will be waste of transistors, since you will burn quite a few of them on the way. It helps to learn from small stuff, where voltages and currents are not serious. At 100 watts classA, your power supply needed will be able to weld. Trust me, its not fun to mess it up.

ed

[Williams Audio]

hi marko25
a class a is a task that as told by others is a difficult start
but you can get help from 'signal pcb' the Self Douglas AMPS will be an excellent start for class a
there is '' le monstre'' by hiraja French staff as well
I did build the Douglas amp, is very good, I extended the pwr to ~45watts/8 ohms
Best regards
Williams

[Marko25]

Well I've finally managed to make it to the correct forum section

thanks Ed for your considerate response.

to me the most important parameter of the speaker is its efficiency. You did not mention that.

Quite right sorry, 88dB spl at 1metre on axis. 2.83V input.

I assume that the 2.83V is db/w/m?

Max SPL is 111db at 1m

Peak power is quite a useless parameter. You never listen at full power. As a matter of fact, most listening is done with less than watt. Thats right. Furthermore with highly efficient speakers all you need is few milliwats. Why we need more power? Just a reserve to spare to keep music dynamic.

Appreciated, my speakers aren't the most efficient, my thinking of basing it around my speaker design was matching and providing plenty of headroom for the dynamics as you say. I don't listen to music very loud but I do like to fill the room, especially when the neighbours are away.

Statement that speakers can handle 200watts peak means nothing. It might mean that once during testing they survived 200 watts impulse. But the music they produce at 50-100 watts may not be listenable at all. They might have such horrible distortion, that you will not be able to wistand it.

Again I understand that, I only mentioned that above as my speaker didn't state the RMS so it was the basis of my calculation to get the 100 watts RMS.

Good start is 5-7 watts classA amp, preferebly well known designs, JLH for instance.

I was worried about the low level of watts with my efficiency speakers and any distortion which may occur within these limits. I understand the whole first few watts thing. I currently am using a 45 watt integrated amp and never have the volume beyond halfway.

It will be waste of transistors, since you will burn quite a few of them on the way. It helps to learn from small stuff, where voltages and currents are not serious. At 100 watts classA, your power supply needed will be able to weld. Trust me, its not fun to mess it up.

EEK Welding! don't want to go there not yet anyway.

Many thanks for your contribution.

Mark.

[Marko25]

Try a small ClassAB amplifier. 50W? with one output pair fed from a simple PSU @ +-35Vdc. Have a look at the ESP site for a cheap starter amplifier.

I suppose I'm worried about a) missing out on something better b) not improving on my current commercially bought integrated.

Then, try a small ClassA amplifier, Pass is a very good site for ideas, knowledge and circuits that perform. Or the little JLH, or the ESP ClassA. There's lots of choice.

Yes, the JLH seems to have a good following and build history so it may be a great place to start.

I think what I am after is extracting the detail which is what I love (as well as the music of course). Not so long ago I bought some decent headphones, AKG701's and noticed that on an opening track of a Mobile Fidelity record there is ghosting present i.e. the track starts before the track actually starts. I guess its the storage of the master tape thats leached through. Anyway back to the subject, this is never present through the speakers no matter how loud you set them, only the headphones. Now I know its there I want to hear it and feel I'm missing other details. I'm hoping a good type A would help extract this detail....I think I have just developed a tweaking bug

First job: build a mains light bulb tester. See Nuuk's site for details.
Learn how to use the mains . SAFELY !!!!!!!!!!

Thanks for the direction on the site, I've worked with mains power a few times albeit not power supplies so I'm cautiously careful with mains having been electrocuted a couple of times before

ps,
I have asked for this thread to be moved. [/B]

Thanks, I appreciate that.

[bear]

As long as you can mechanically build and find appropriate heatsinks, I see no reason not to build a Class A amp of any size you wish.

I mean really fellows, what in the world is the difference other than size and cost? You still stuff a board and connect to the outputs after all.

Assuming that one starts with a known design that is not to skittish (prone to instability) there shouldn't be any problems.

I'd suggest that a 25watt real class A amp (stays in class A into 4 ohms?) is a very powerful sounding thing... a 50watt is rather hefty, and a 100watt is a room heater, heavy and large.

Let's see if we are also talking about a real class A amp or one of those that merely doesn't cut off the output devices...

There are some proven designs online and on this forum, plus those of Nelson Pass to consider!

Pick one.

No reason not to start with a small one since it costs less, and the parts are smaller, so you can "get ur feet wet" with that one just to make sure you can handle it. But with the speakers you own I think you can likely handle a "loss" if the thing lets out "the magic smoke."

_-_-bear