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Old 2nd September 2008, 03:50 AM   #1691
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595mV across 0r47 is just 0.75W.
What size (dissipation) of emitter resistor are you using?

Andrew, I'm using 3w resistor which is same as schematic.

My F5 is very stable on both channels. I turned it on before I gone to work this morning and I have almost 2 hours listening section after I came home - totally 11 hours. I took the measurement again after the listening section and everything remains the same. The temperature is still 38C.
Many guys talked about how good the sound is. I thought I'm not qualified to say anything about the sound, but I'm really happy with this amp. Only after the first 2 hours listening, I like this amp even more than my tube amps. It sounds really mellow and tubie and the sound stage is wide. Bass is deep and tight.
I'll let it run for another week and will try tune the bias to 1.5A. ( 0.705v across 0R47 )

Albert
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Old 2nd September 2008, 04:45 AM   #1692
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Congratulations Albert! Wow, you really are fast at putting this one together. Glad to see you have it running and definitely keep us posted on how it sounds (especially how it sounds compared to your tube amps)
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Old 2nd September 2008, 09:14 AM   #1693
juma is offline juma  Germany
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Albert, some bigger pictures would be nice too !
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Old 2nd September 2008, 10:23 AM   #1694
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Quote:
Originally posted by albertli
temperature on heatsink is 38C
You should call it the Fridge-Five.

Biased at 1.8A, the dies of IRFP output devices would still be under 100C, assuming 25C ambient and say 0.5C/W added thermal resistance for the insulator pad.

Fairchild FQA devices would be a more convenient choice for such a high dissipation number in that setup, nearly 10C more slack.
100C die temperature may not be the ticket for serial manufacturing, but should work flawless in most cases and DIY is pretty service friendly.

I've got Sanken power devices operating at 100C die temperature in an ExtremeA power amp, there's no way for me to drop it to a lower value, but none have pushed up daisies yet.
Statistically, there's not a longevity issue for a die temperature at 100C, just the rotten apple will pop up his head.
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Old 2nd September 2008, 03:55 PM   #1695
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Juma,

I did try to up a bigger picture, but the site didn't allow me to do that. Is there any other way I could try, or I email you.

Jacco,

I've 56C on both IRFs now. Do you mean I won't kill the IRFs within 100C????? I'd love to run it hot.

Can you tell me the different how much better if I run it at 1.5A and 1.8A

Thanks

Albert
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Old 2nd September 2008, 04:32 PM   #1696
juma is offline juma  Germany
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Quote:
Originally posted by albertli
Juma,
I did try to up a bigger picture, but the site didn't allow me to do that. Is there any other way I could try, or I email you.

Jacco,
I've 56C on both IRFs now. Do you mean I won't kill the IRFs within 100C????? I'd love to run it hot.
Can you tell me the different how much better if I run it at 1.5A and 1.8A
Thanks
Albert
I sent you an e-mail about pictures.

With 56C on IRFs you are in the safe zone. Generally, class A amps tend to sound better with higher bias.
How much better is 1.5A/1.8A than 1.3A is up to you to decide after you try it
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Old 2nd September 2008, 04:55 PM   #1697
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Vertical MOSFETs are used for a load of other stuff besides audio, operational lifetime cycle of these devices is pretty steep, think 50 years.
At less elevated die temperatures you'd be dead and burried before they (all) pop, 100C is acceptable if it's not the last DIY power amp you'll ever build.

A number of wellknown serial production amps were designed for really steep die and heatsink temperatures.
Best example is the 30 Watter Hiraga with BJT output devices, more than a few needed repair but a lot of them managed +20 years.
The 60W power devices of the Hiraga each ran at a 30W dissipation level, with small heatsinks good for 125C die temperature.
High die temperatures of BJT power devices usually destroy them because of the combination of reduced safe operating area and second breakdown effect.
BJT power amps such as the Hiraga and ExtremeA manage to survive due to the very moderate rail voltage levels.

Vertical MOSFETs do not suffer from second breakdown, just thermal runaway due to a positive tempco.
Do a search, some of the paesanos here have run vertical MOSFETs at very high die temperatures for long periods without any hassle.
(d., i'm a walking talking phone book)

Ask Papalala about bias level of V's and SQ.
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Old 2nd September 2008, 05:22 PM   #1698
Magura is offline Magura  Denmark
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Quote:
Originally posted by albertli

Jacco,

I've 56C on both IRFs now. Do you mean I won't kill the IRFs within 100C????? I'd love to run it hot.

Can you tell me the different how much better if I run it at 1.5A and 1.8A

Thanks

Albert

Hmm, I'm positively sure I'm not Jacco (at least I like to think that the guy in the mirror is somewhat younger)

Anyhow, you can run an IRFP240 at 3A 40V for a looooong time, without problems. Just keep in mind that you need some respectable thermal mass to keep them from going over the edge, and naturally some pretty massive heatsinks.

Been there, done that.. (actually am still there), and love the sound of it.

I don't think you can tell 1.5A from 1.8A, but 2.5A or 3A will sure make a (to me) positive difference.


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Old 2nd September 2008, 05:24 PM   #1699
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Old 3rd September 2008, 07:58 AM   #1700
juma is offline juma  Germany
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Pictures of Albert's amp, with no reduction in resolution:

set 1
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