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Old 8th July 2013, 06:32 AM   #431
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Do you remember me mentioning that the same amp was stable when it had a slow stream of air blowing across the heat sink. So the alternative to a large heat sink is a smaller heat sink with fan. But then one has to study it carefully. I had it working like that about 3 to 4 hours. The solution would be a combination of the thermal resistance of the system with or without fan and an optimum thermal solution for the bias spreader transistor. Need time to do that ! Maybe we need more than 24 hours a day ! ( feasible on another planet I guess !)
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Old 8th July 2013, 09:49 AM   #432
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Quote:
Originally Posted by gaborbela View Post
.......All together is my fault, we can not set up the bias to 250mA leave the amp on and fall asleep.........
That is not a "fault".

Equipment designed to safely operate for year after year should never catch fire because it has been left ON all night.
It should operate just as well after 7days of continuous running as it does after 7hours of continuous running.

If you have an amplifier that consumes 50W from your 110/120Vac supply then using a T5A fuse in the mains supply is virtually guaranteeing continued operation for a LONG time after a fault has developed.
Whereas using a T1.6A for a 200VA transformer running on 110/120Vac will likely blow quite quickly if a serious fault occurs.
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Last edited by AndrewT; 8th July 2013 at 10:05 AM.
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Old 8th July 2013, 02:05 PM   #433
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In my case I had a 1 watt 10 ohm resistor in the fuse holder on each rail. However the power transistors were still dead !
The resistors burst into flames and it took me several moments to get up and reach the power switch. The ( loud) music had stopped some time before that and there was nothing playing but the bias current must have been building up slowly. Never had a breakdown after that as I kept the bias lower than 80 mA.
I'm itching to try out the pnp bias spreader and the Darlington one but I have no time. I'm off in a few hours and will be gone for a few days.
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Old 8th July 2013, 03:25 PM   #434
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Quote:
Originally Posted by AndrewT View Post
That is not a "fault".

Equipment designed to safely operate for year after year should never catch fire because it has been left ON all night.
It should operate just as well after 7days of continuous running as it does after 7hours of continuous running.
Andrew
I agree with you 100%
Please understand me, may be I was not clear enough.
I made several test on the amplifier, I tested different components.
I set up the bias 150mA and it work fine for several hours, I check the heatsink and was cold around 40C degree.
So I increased the bias up to 250mA and kept listening music until I felt sleep.
Now when I increased the bias by an extra 100mA I supposed to wait until the amplifier totally warm up and settle done!
That is why I wrote "it was my fault"
When I felt sleepy I supposed to shut of the amplifier! We can not set up a amplifier and leave it unattended.
Several years back with 3 pair power transistor, 18VAC transformer and huge heatsink I biased the amplifier to 3A.
Day after day I left the amp on with out any problem.
Again for that 3A bias I used the right rail voltage, and more than enough heatsink!
I think 90% of the amplifiers would have thermal runaway if the heatsink does not handle the heat or the power transistors is pushed over the limit.
If we have protection so the amp shut off in case if over heat that is another think.
I hope you agree with these.
In my case the amplifier was under test, it was not a finished amplifier!
Back in Europe I used the amplifier for a while with out any problem.
Of course when I immigrated to Canada I didn't brought with me.
Now after 15+ years later those transistors I used no longer available.
My friend also built these amplifier and more than 9 years had no issue with.
There is a Hungarian corp. who built commercial amplifiers to the market based on a very SIMILAR circuit.
Our problem when we build a amplifier even if it sound great we want to get better result.
So we start to switch components, looking for the sweet spot.
Many of us who used to it to Class A amplifier circuit we right away play with the bias to.
Yes the sound get better by increasing the bias but we must play safe!!!
I think I made a mistake I posted that picture because sometimes people does not read the comment and now they think better not to do anything with these circuit or these amplifier
I can assure you with 35V rail voltage 50mA bias with the right size heatsink the amp will operate years and years.

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Old 8th July 2013, 04:06 PM   #435
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A close rated fuse would probably have blown when the heatsink became too hot from the massively increased bias current.

Similarly a heatsink temperature switch would have powered OFF until the sink cooled.
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Old 8th July 2013, 04:39 PM   #436
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Maybe this amp does need a thermal switch even it is made reasonably stable.
My tests were not really well controlled. I need to test it carefully.
OK ,I will be back next week and then do some more testing.
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Old 14th July 2013, 03:44 PM   #437
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I'm back but will be here only for 5 days.
Hi B.
Is the circuit shown at FET-hex explendit amplifier
enough to satisfy you that the discrete Darlington also works well ? ( post 123)
Cheers.
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Old 14th July 2013, 05:19 PM   #438
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Hello
Yes it does work I knew that, I tested way back 15 years ago.
There are several question
First of all the sound, we have to wait until someone build it and will compare with an acceptable amplifier.
Don't forget I built the amp with discrete darlington and it was only the shadow of the darlington but I didn't used Sanken BJT.
It can be because I used wrong transistor set up, who knows.
There is a similar amplifier called Canopus with discrete darlingtons but the sound of that amp not the same league, not even come close to these.
Second borys at the test he set up the bias around 60mA and when the amp warmed up it was the bias went up around 90mA or so.
He wrote the amp was after stable.
My amp is stable with low bias like that with proper size heatsink.
Also he wrote that little bias drift probably it happened because the heatsink compound was already dry..
Who knows??
I wish the discrate amp would be rock stable and the sound compare to the darlington at least the same level or so.
I sent my darlington circuit to borys, he asked me so he can see it.
He is on vacation for 2 weeks but until that he wrote to try the amp out with darlington heat spreader BD682 or other type
Also he wrote the 2SA1360/2SC3423 is not the best only linear until 20mA..
Al do right now I use the MJE243/253 set for VAS, I mentioned to borys but he forget that.
At the schematic I had the 2SA1360/2SC3423 , which was advised by wahab.
Borys wrote to try out the Toshiba 2sc4793 2SA1837 which I have at home.
I will do these tests soon.
The good news someone will build borys amp soon and will compare it with the LC LatFet version.
So hardly wait to get some info about the discrete darlington.
Otherwise I do not worried at all because my friend used these amp more than 9 years over 300mA bias.
I will use large heatsink, also I will test some other power darlingtons to, and let see if some of the borys idea will make the amp more stable.
These amplifier sing good at 150mA bias but over 250mA much better according to my friend and my ear to.
The reason I force the higher bias not only 50mA.
Another think the amplifier very sensitive to the components used in the circuit.
So carefully selected components will help to get closer to Nirvana...
My last setup was very-very good. The very best sound in my system.
The biggest help it would be if someone could run some sim. tests.
Since I do not own scope without sim I have no clue if the amp oscillate or not with the new transistors.
Greetings G
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Old 18th July 2013, 05:37 AM   #439
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I see a lot of people have interest in this amp but only a few seem to be reporting the progress. Maybe the others have not built it yet. There are so many designs on this forum. Must be truly confusing when it comes to picking a design to try out !

Well I'm off again for a week by the beach ! And soon after I get back I will be off again to another beach !
Meantime hope someone will check out the bias stability with the bias transistor fitted close to the power transistors. I felt that maybe we can use a power transistor itself ! It's got a much larger metal tab and so might have better heat sensitivity in this application. Maybe it's the time lag that's the biggest culprit in this case.

I found TTC5200 and TTA1943 in the market. Got a few to try out. I thought it was made in China but the printing says Japan !
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Old 30th July 2013, 04:21 PM   #440
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I'm back for a few days.
I placed the bias spreader transistor right next to the Darlington power transistor. I raised the bias till the heat sink got quite warm and bias level was about 180 mA. This took about 20 minutes. Then I dropped the bias down to 100 mA and it has been fairly stable for the last 20 minutes. Ambient is 29 deg C and no air flow around the heat sink. The heat sink is vertical and so vertical air flow is free. Even the slightest external flow of air drops the bias levels.

So I guess with a bigger heatsink all will be well for the bipolar implementation. With a fan there will be no problem at all with smaller heat sinks but unsafe in case the fan breaks down. I noticed that my heatsink is 'at least' 1/2 the size that some others have been using on a similar amp.

So now on to the next step. Have to get the changes completed in less than 2 weeks . Anyone completed the amp on a larger heat sink ?

Would it be worthwhile to incorporate the relay speaker protection on board ? I have it now on a separate board which contains the psu.
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Last edited by ashok; 30th July 2013 at 04:25 PM.
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