70W Class A amp.

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#21
if you decide to build a DC coupled amplifier, then I suggest you fit a DC servo to maintain the low output offset. That you fit a DC detection circuit with "alarm". That you fit a DC triggered isolation circuit to protect your speakers.

Otherwise I recommend an AC coupled amplifier.

Have you clicked on the "April" hint posted earlier?
 
#22
AndrewT said:
if you decide to build a DC coupled amplifier, then I suggest you fit a DC servo to maintain the low output offset. That you fit a DC detection circuit with "alarm". That you fit a DC triggered isolation circuit to protect your speakers.

Otherwise I recommend an AC coupled amplifier.

Have you clicked on the "April" hint posted earlier?
Click to expand...

I am sorry, but I do not know what is a DC or AC coupled Amplifier? Also, what is the April hint?

What I am referring to, is to install a capacitor (C-8 & C-9) before the output, like on this version:

http://i496.photobucket.com/albums/rr324/ericrc30/IMG_6060.jpg
An externally hosted image should be here but it was not working when we last tested it.



What are the Pros and Cons of these capacitors? (Apart from DC protection for the speaker)
 
#23
http://i496.photobucket.com/albums/rr324/ericrc30/70Watts.jpg

70Watts.jpg


Hi everyone,
Here is the complete +- 70 Watts completed and running! At this time the the bias is set at 1,85 Amps on DC rails of +- 40 volts. I made a few modifications to the magazine, the first one been the addition of two 24 mH chokes with bigger capacitors on the power supply. I also used an OPA 445 instead of the LM144H as I can not find any! Also the plan called for 4 LM 317 HV, I used 6 LM 117 HV instead, because the design used them to the point of the self protection limit. I tough it was not wise on the long run.

Now the sound,
I am running this puppy using a B-1 (Thank you again Mr. Pass🙂 ) and the speaker I am using are my old SD Audio SD- 1, I find the sound very smooth very close to tubes. All the frequency's are there, even the loud base. Overall I find it is a great amp! ...of course I am biased😉, so I will bring a friend of mine who writes at time for a local Audio magazine to comment on it 😀
I know as a fact he is not shy to express his comments:nod:
For now its:cheers: time for me.
Also, I have a couple of board left If someone is interested to build it and comment. I will give them away, just pay the shipping (the real shipping cost...😉)
 
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#26
Hi Eric, congrats on getting the 70 W Class A up and running! Look forward to hearing how it sounds. So far you have a good proof of concept, and the amp is running well..so I guess it is safe to say that type LM317 as a constant current sink can be used successfully, providing they are not run past the safe operating limits. By the way I will submit the claim that the LM317 used in this type circuit they normally should not have any big affect on the sound quality (there could be a bit of roll-off at 10 to 20 Khz)
 
#29
Hi cyclotron, than you for your post, I read the bit of thread and it is interesting. For your info I am using LM117HVK and right now the voltage rail is +- 40volts DC. I am planing on increasing it to +- 50 volts and a bit of current increase as well. But first I have to increase the size of the "gold" heat sink as it is to hot for my taste! When I set up the lay out I did not grasp the fact that the 2 transistors were going to heat as much as all the LM's...😱
 
#30
Congrats!

Congratulations! Nice job, most people would not have even finished this difficult project.

Until you make a class A amp you don't really understand how hot the transistors get. So it is understandable that you need to adjust the heat sinks a little.

Can't wait to see the finished product.
 
#31
Thank you Igreen! your comment is appreciated🙂
I will post all the evolution as I go, I should get started on the new case in the beginning of next year. I will also need another pair of transformers as the one I am using, was already old and does BUZZ!
Eric
 
#32
I have a question for you guy's, yesterday I tried to increase the voltage to very close to the normal supply rail +- 41 V DC(at this time I have it on a lower voltage for testing +- 34 V DC) and I burned two resistors and it's respective LM-117 HVK. I used metal film resistors are they suitable for this kind of use? or I should revert to wirewound???
Thank you,
Eric
 
#33
Hi Eric, Thanks for the listening test on the amp..I was impressed with the performance. The amp sounds very ''smooth'' across the spectrum, I did not perceive of any ''edginess'' or ''graininess'' to it. Overall to me the amp sounded ''easy on the ears'' without any harshness from bottom end to high end frequencies. Another impression I could make would be that the amp did not sound ''stressed'' at any point.. I guess that would be more to do with the way it handles dynamics.. look forward to hearing it again soon!
 
#35
Arick said:
I have a question for you guy's, yesterday I tried to increase the voltage to very close to the normal supply rail +- 41 V DC(at this time I have it on a lower voltage for testing +- 34 V DC) and I burned two resistors and it's respective LM-117 HVK. I used metal film resistors are they suitable for this kind of use? or I should revert to wirewound???
Click to expand...

Maybe at the higher rails you are exceeding the ratings of
some of these chips. Does it do this at the +/-34V rail value?

😎
 
#37
Nelson Pass said:
Maybe at the higher rails you are exceeding the ratings of
some of these chips. Does it do this at the +/-34V rail value?

😎
Click to expand...

Hi Mr. Pass,
It is fine at +- 34 volts.
The voltage rating of the lm117 HVK is 60 volts, they are getting half of the supply voltage so they still below what they are rated for. However what I am now wondering, and I will try to measure it during start-up, is what IS the voltage at star-up? (my educated guess here) Because it may take a while for the OP-amp circuit to be "operational" maybe it is letting an abnormal amount of voltage slip through when in transient? (please bear in mind I built the amplifier of post 1 schematic). Maybe this is not a good idea:devilr: but, because I have some transformer limitations, I have to start this monster in two steps:
1) power up the capacitors
2)Then applying the load of both channels with the capacitor voltage stabilised
Otherwise the combination of anemic transfo + load, trips my 15 amps breaker.
 
#39
Nico Ras said:
Hi Arick, could this happen at high peaks, if you swing close to rail voltage then you may exceed the maximum voltage across the regulator.
Click to expand...

Sorrrry I think I was not clear enough.
It happened at start-up, I never got to the higher voltage running stage. I blew 5 out of 12 LM's, OUCH...! I replaced them with "standard" LM-117(40V limit) and I am running it back to the lower voltage rail.
 
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