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Old 2nd July 2013, 04:20 PM   #421
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Join Date: Sep 2005
The main reason I asked you to mount the 2 power BJT and heat sensor on one heatsink that will give you offset stability!
Those must be thermal coupled (connected)..
The size of your heatsink is another thing.
Did you try different value feedback resisters or just the 6.8K?
If yes what is your experience, how much influence has on the sound?
Probably U use only a few mA bias that is why your amp remain "cold".
Can you let us know your total bias/channel when you start up the amp and after an hour listening at normal level.
Thank you
Greetings G
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Old 3rd July 2013, 01:15 PM   #422
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I'm going back to the Darlington setup to study the bias issues and also to see if the sound is any different. I'm not worried about small changes which could be influenced by many factors. Mainly looking for difference in bass performance. I think the LatFET HF has a bit more attack in the right way.
Will also try out the bipolar assisted scheme that I've been using right now.
Current pcb has already been chopped up in a few places !

The last check will be the 2SB/2SD drivers. After that I'm going to make a one board stereo amp. I have a large enough heat sink which I will have to anodize black.

Haven't seen anyone reporting on some exploratory work yet !
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Old 3rd July 2013, 10:37 PM   #423
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I just wrote to borys, he designed the VAAS with BJT and hexfet.
I want to know if he has any bisa issue when he uses bipolars.
in these page you can find the schematic

FET-hex explendit amplifier

To me to do any exploratory work I would need a scope, sim software and some other tools.
I did what I could without those tools over the years..
Actually I'm more than satisfied with the sound of the amp I get with the provided transistor set and other parts I use right now.
If the amp would be more stable that would be an extra but again I do live the sound so much even with the bias issue I will use it.
Of course I will test it with the LatFet maybe with Toshiba 2SK1530/2SJ201 to.
In case I can achieve the same sound quality I will switch to the power FET. Otherwise I will build the amp as solid as possible so even with these bias drifting the heatsink and power transistors can handle the extra heat..
After two and half decade it would be time to settle done with one or two system and enjoy listening to music.
Ashok you have scope, can you please check if the amp has any oscillation with the darlingtons.
Of course after you return to the darlingtons.
Can you keep one channel with the best performance you get and use another PC board for A-B test.
That way you can avoid any mind tricks etc.
Also please take a look at borys schematic if we can use some of his idea here.
To me to do that I would need to invest into scope etc or go back time to time to the TV shop I used to help. The owner has all the tools I would need.
I do not have time for hat
Thanks for your work on these project.
Greetings G
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Old 4th July 2013, 12:30 PM   #424
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Note that Borys suggested to use a Darlington for bias spreader. Might be worth following up. I have some BDX53/54 . Wonder if that would do or they have to be TIP142/147. Maybe I'll try a TIP147 ( PNP) !
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Old 4th July 2013, 01:55 PM   #425
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If you have BDX I think that is good enough. Or BDW93C & 94C those are also in TO220 case.
To be honest I really do not understand why we should use darlington for bias spreader..
But we have nothing to lose if we test it.
If you read borys answer to my question about how stable the bias he mentioned also it has some drifting.
Now if we bias the darlington only up to 50mA or less we get also les bias drifting.
For years I didn't realised the amp has these issue because of the low bias set up!
I will follow that thread to see what happened if someone will use darlingtons or some other guy build the amp with any type of BJT.
It would be great to get more feedback on that.
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Old 4th July 2013, 02:06 PM   #426
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Hi G,
At least you know the LatFET works fine and has stable bias AND sounds very good. I feel reluctant to disconnect it and put back the Darlingtons.
This time the bias circuit is going to be outboard so that I can try out variations. Yes, I'm going to make one more board ! I have an extra heatsink with the same size. The IRF240/9240's are also beckoning me ! Better I use them or blow them . Otherwise they might stay in the box for another 20 years !
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Old 7th July 2013, 01:39 AM   #427
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QUOTE=djk;3545045]The close-spaced fins will do little of nothing without a fan.[/QUOTE]

If the heatsink will be not efficient enough I will ad 2PC 4" fan connected in series under the heatsink.. That would handle a Class A 30W amplifier with the fans.
Before I used the black heatsink mounted on each side now (I cut those two 2 piece) easily handled 150mA at 40V rail voltage.
The heatsink after few hours was only 40C degree so I increased the bias to 250mA .
After listening two CD I felt seep and I blew up the amp several hours latter.
Look at the picture..
I woke up at the middle of the night the amp was on fire. The CRC 10W resistor was hot red.
On the PC board burnt a hole.
Thank God my Visaton handle the 40VDC.
Next time I'll be more careful.
In case you think the heatsink still not enough efficient I can use these amp case I just work on it.
It is about 50LB or more.
Just kidding. That for a F5T with Toshiba mosfets or high power Krell KSA50..
I worked on these case all week but now I have to start to test these amp and the Hitachi LatFet amplifier to.
Time to do some test to improve the amp stability..

boyet0173 how is going? Any improvement..
Greetings Gabor
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Old 7th July 2013, 02:53 AM   #428
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I think the thermal stability should be fixed. Having a very large heat sink might help but isn't a good idea. I really haven't been looking at the problem closely. We've all been attacking the problem casually !

My out of town trip starts on Tuesday and from then onward I'm going to be really busy for several weeks. Maybe I can think things through during the trip.
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Old 7th July 2013, 11:18 AM   #429
AndrewT is offline AndrewT  Scotland
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Originally Posted by gaborbela View Post
.......... the amp was on fire. The CRC 10W resistor was hot red.
On the PC board burnt a hole.
This is where close rated fusing shows it's hand.
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 8th July 2013, 02:50 AM   #430
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I only use fuse on the AC..
May be the fuse was a bit over rated.
All together is my fault, we can not set up the bias to 250mA leave the amp on and fall asleep.

you right about the heatsink, but in my case clearly it was undersized for the 250mA bias
My friend used the amp more than 9 years until one channel started to distort for some reason.
He biased the amp over 300mA but he used huge heatsink, lower rail voltage and regulated PS.
I do not think the regulated PS has anything to do with the stability.
Much more the size of his heatsink and lower rail voltage.
Again the Mosnter Hiraga acted the same way or worst to me, when I started up it had 0.8A bias after warm up it had 1.5A and I run the amp 12V rail voltage.
These amplifier has much less bias drifting, sound much much better..
Thousands built the Hiraga Monster even do it has that issue.

Of course I'm interested to somehow make the amp more stable, in case we can't succeed.
If not I still need these amp because I do love the sound of the amp a lot.
I believe the best I ever built with the mentioned parts.
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