Just a story.
So you heard about building your own amplifier. Maybe you must give it a try. Then you search the NET for a good circuit, but not difficult, this will be your 1st time so you want to have it easy. You get 100's of circuits, then you get to Pass Labs. You read about "Class A" amps, "Pure Class A", "Single Ended" and it just sounds better and better. And then.... you find their circuits. Now you really hooked, start searching more about how good they are (those Pass amps), read the reviews, read the forum articles. You MUST have 1. O.K. decide which 1 you like the better, or can afford the better
, you start asking THE questions, (allthough others asked it already, you want to make sure and know yourself), What size transformer will do the job, what quality caps must I use, what size heatsinks do you need, are there PCB's available somewhere, where must I get all these things????
So you get all the parts, order a custom made transformer, for which you must wait 3 weeks in RSA, find some sort of heatsinks and start to assemble. Assembly done, checked and rechecked component lay-out and orientation, now to switch on.
I could not wait for the transformer, so just hooked up 1 I already had, a bit under rated, but will do for now. (Gives 20V on the rails)
1St switch on with no input and no load, . Hookup a little 10ohm 10 Watt resistor, (all I had), still no input and switch on again. Within 3 sec. this resistor start smoking. No, what now?? Switch off, check all again, can find nothing wrong, switch on - get 16V d.c. on load resistor, still without an input signal. What, what, what??? After going over the circuit again and again, there it was. Shamed to say, but suppose it can happen and DID happen, all Zeners was turned around
Corrected mistakes, praid all else was still well, switch on again. Resistor cool as an Eskimo. Voltages perfect. Connect an old speaker, which friend borrowed to me with the understanding that he might get back a magnet with a paper cone in front, connect the input, switch on again, what did I see, hear, feel...., beautifull, sweet, gorgeous, pure NOTHING!!! 
Check speaker, dead. Measure all again, all good. Connect nother speaker. Perfect! (or near perfect anyway). Turned out that speaker was dead before I connected, not funny. What a relieve and experience. All voltages measured just fine. What is true, these Class A amps are PURE, within 30 minutes I measured 70°C direct on MOSFET's. Must have big heatsink. So not to BURN them in , just put some 12v fans on them till bigger, better, smarter heatsinks arrive.
So far this project is very enjoyable. Matching the FETs were no problem, out of 24 IRFP250's, the biggest differance was 27mV.
Next is the other channel, which is just a repeat of the 1st 1, with zener's the correct way I tested with my CD player (Yamaha CDX480) connected direct to amp, without any pre-amp. Works beautifull. HOW??? CD player got own output level control
1 Question:
I see a tiny little signal on output when input are disconnected, only 10mV pp @ 100Hz, where can it be from and is it normal?
Thanx to all for all help so far, and Panos, your PCB's work perfect.
So you heard about building your own amplifier. Maybe you must give it a try. Then you search the NET for a good circuit, but not difficult, this will be your 1st time so you want to have it easy. You get 100's of circuits, then you get to Pass Labs. You read about "Class A" amps, "Pure Class A", "Single Ended" and it just sounds better and better. And then.... you find their circuits. Now you really hooked, start searching more about how good they are (those Pass amps), read the reviews, read the forum articles. You MUST have 1. O.K. decide which 1 you like the better, or can afford the better
, you start asking THE questions, (allthough others asked it already, you want to make sure and know yourself), What size transformer will do the job, what quality caps must I use, what size heatsinks do you need, are there PCB's available somewhere, where must I get all these things???? So you get all the parts, order a custom made transformer, for which you must wait 3 weeks in RSA, find some sort of heatsinks and start to assemble. Assembly done, checked and rechecked component lay-out and orientation, now to switch on.
I could not wait for the transformer, so just hooked up 1 I already had, a bit under rated, but will do for now. (Gives 20V on the rails)
1St switch on with no input and no load, . Hookup a little 10ohm 10 Watt resistor, (all I had), still no input and switch on again. Within 3 sec. this resistor start smoking. No, what now?? Switch off, check all again, can find nothing wrong, switch on - get 16V d.c. on load resistor, still without an input signal. What, what, what??? After going over the circuit again and again, there it was. Shamed to say, but suppose it can happen and DID happen, all Zeners was turned around
Corrected mistakes, praid all else was still well, switch on again. Resistor cool as an Eskimo. Voltages perfect. Connect an old speaker, which friend borrowed to me with the understanding that he might get back a magnet with a paper cone in front, connect the input, switch on again, what did I see, hear, feel...., beautifull, sweet, gorgeous, pure NOTHING!!! Check speaker, dead. Measure all again, all good. Connect nother speaker. Perfect! (or near perfect anyway). Turned out that speaker was dead before I connected, not funny. What a relieve and experience. All voltages measured just fine. What is true, these Class A amps are PURE, within 30 minutes I measured 70°C direct on MOSFET's. Must have big heatsink. So not to BURN them in , just put some 12v fans on them till bigger, better, smarter heatsinks arrive. So far this project is very enjoyable. Matching the FETs were no problem, out of 24 IRFP250's, the biggest differance was 27mV.
Next is the other channel, which is just a repeat of the 1st 1, with zener's the correct way
I tested with my CD player (Yamaha CDX480) connected direct to amp, without any pre-amp. Works beautifull. HOW??? CD player got own output level control 1 Question:
I see a tiny little signal on output when input are disconnected, only 10mV pp @ 100Hz, where can it be from and is it normal?
Thanx to all for all help so far, and Panos, your PCB's work perfect.
Pride!
Hey man, I remember when my amp looked the same... I can see some similarities. ;=)
I also used a fan and I was soooooo proud I had it up and running. Now I've played it in my livingroom for some hundred hours and I'm still happy as a puppy.
Man, isn't it a good feeling, the first time you fire it up and IT WORKS!
Good luck! Good job!
And, Panos29!!! Thanks for your PCB artwork!
//magnus
Hey man, I remember when my amp looked the same... I can see some similarities. ;=)
An externally hosted image should be here but it was not working when we last tested it.
I also used a fan and I was soooooo proud I had it up and running. Now I've played it in my livingroom for some hundred hours and I'm still happy as a puppy.
Man, isn't it a good feeling, the first time you fire it up and IT WORKS!
Good luck! Good job!
And, Panos29!!! Thanks for your PCB artwork!
//magnus
My 1st Time - Part 2:
I eventually got my REAL transformer and assembled A30. Tested and all is beautifull. This amp is plenty more then I ever expected it would be. I don't want to switch it off, it is like if I just got 200 new CD's, unfortunately some of which now sounds terrible, quality wise - poor recordings revealing. The heatsinks are slightly under rated, but I think I can mannage with them, after 4 hr's no smoke comming out anywhere. Here follow a couple of pic's I took. Also a data sheet on readings I got, and calculations, hopefully correct. By looking at them, any conclusions, recommendations you guys can give me??
Thanx again to Mr.Pass and all you other very helpfull guys for helping to build this beauty (sonic wise at least)
I eventually got my REAL transformer and assembled A30. Tested and all is beautifull.
This amp is plenty more then I ever expected it would be. I don't want to switch it off, it is like if I just got 200 new CD's, unfortunately some of which now sounds terrible, quality wise - poor recordings revealing. The heatsinks are slightly under rated, but I think I can mannage with them, after 4 hr's no smoke comming out anywhere. Here follow a couple of pic's I took. Also a data sheet on readings I got, and calculations, hopefully correct. By looking at them, any conclusions, recommendations you guys can give me??Thanx again to Mr.Pass and all you other very helpfull guys for helping to build this beauty (sonic wise at least)
Final last update.
Just to complete all info I can supplie:
I got hold of really huge wire resistor, 8ohm 300watt, and did max output test. I do not have signal generator but went about as follow:
Recorded 200Hz triangle signal , created with sound software, onto CD and 200Hz sine signal. (Is this just as good as with signal generator?) Connected 8ohm resistor to output of amp, hooked up oscilloscope and true rms meter. Play CD with test signals, 1st 200 tri. Turn up volume till 200 tri start to clip. Play 200 sine track. Took reading on multimeter.
Result:
R/channel = 38Watt
L/channel = 35Watt
Now I am done.
Just to complete all info I can supplie:
I got hold of really huge wire resistor, 8ohm 300watt, and did max output test. I do not have signal generator but went about as follow:
Recorded 200Hz triangle signal , created with sound software, onto CD and 200Hz sine signal. (Is this just as good as with signal generator?) Connected 8ohm resistor to output of amp, hooked up oscilloscope and true rms meter. Play CD with test signals, 1st 200 tri. Turn up volume till 200 tri start to clip. Play 200 sine track. Took reading on multimeter.
Result:
R/channel = 38Watt
L/channel = 35Watt
Now I am done.
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