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Old 18th August 2013, 03:09 PM   #11
fpitas is offline fpitas  United States
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Quote:
Originally Posted by Magura View Post
In my experience, active cooling for stuff that needs to be in the listening room, is something invented by the devil in anger.
All of a sudden, things gets expensive and complicated real quick, if it has to be safe, silent, and reliable.
Especially if you run your fets on the edge.
I've seen a few do it, and none that worked in the long haul.

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Points taken, although pro amps do it all the time, using common muffin fans.
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Old 18th August 2013, 03:18 PM   #12
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As things get hotter it's not only the semi's that turn to toast. The lifespan of the power supply electrolytics will also be reduced.
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Old 18th August 2013, 03:18 PM   #13
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1.5 amps is a lot of bias current.
I found I was getting away with 10mA/transistor and not getting any cross over distortion. I believe Peavey have the same attitude towards bias current, the less the better.

Having a smaller bias current means there is more current to provide real signal power.
It also means the amp runs cool at moderate output levels.

However if you think it sounds better with 1.5amps then go for it.
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Old 18th August 2013, 03:21 PM   #14
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10mA is hardly running the F5 in Class A though.
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Old 18th August 2013, 03:22 PM   #15
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Quote:
Originally Posted by KatieandDad View Post
10mA is hardly running the F5 in Class A though.
My apologies I thought it was class AB.
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Old 18th August 2013, 03:27 PM   #16
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OK I run an Aleph 4 which is pure 100W Class A. The F5, I believe, lives in the "First Watt" category where the first watt is Class A and the amp then degrades into Class B.
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Old 18th August 2013, 03:48 PM   #17
Magura is offline Magura  Denmark
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Quote:
Originally Posted by fpitas View Post
Points taken, although pro amps do it all the time, using common muffin fans.
They also they have all sorts of safety circuitry, that we normally don't see in DIY amps.
If you ask me, they're hardly comparable to a DIY hifi amp.

For a DIY class A amp to work well and be reliable, there are hardly any alternatives to passive cooling, and plenty of it.
Keeping things at a reasonable temperature, makes the entire amp a lot more reliable, and increases the life expectancy dramatically.
If you take a capacitor for instance, you'll see that mean time to failure is divided by 2, for every 10C of increased working temperature.
At least running things hot, asks for some pretty hefty capacitors, if they're meant to survive more than a test run.


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Old 18th August 2013, 03:57 PM   #18
Magura is offline Magura  Denmark
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Quote:
Originally Posted by KatieandDad View Post
OK I run an Aleph 4 which is pure 100W Class A.
The Aleph amps are based on an active current source, thus not "pure" class A" (or constant bias as we usually call it around here), but yet another active bias Class A.
It's fairly simple to define, as with class A without active bias, the highest efficiency I know of, is just shy of 20%.
Once you're past that figure, you've either done something that requires immediate patent application, or you've left the "no active current source" department.
To get close to the 20% factor, the only way I know of is inductor loading.
As I recall, that gets big, heavy and tremendously expensive real quick.


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Old 18th August 2013, 06:46 PM   #19
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Quote:
Originally Posted by KatieandDad View Post
The F5, I believe, lives in the "First Watt" category where the first watt is Class A and the amp then degrades into Class B.
where did you get this?
First Watt is a brand not a category.
none of the First Watt amps only stay in class A for the "first watt" and most of the first watt amps ar SE amps. not push-pull.
the F5 gives 25W class A at 8ohm. and around 40W class A/B at 4ohm (13W class A at 4ohm)

Last edited by AudioSan; 18th August 2013 at 06:50 PM.
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Old 18th August 2013, 09:12 PM   #20
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Yes, the F5 is 25w/Class A.

Regarding bias here's what I've found:

At 1A, this amp is lacks punch and life. It was about on par w/a T2050 tripath amp I replaced it with, with better highs. This was the burn-in period.

At 1.3A now we are cooking. The amp came alive. It was different.

I am at 1.6ish amps right now. It's better. Not subjectively better. It is better. Mosfets like current I believe Nelson has said. Heatsinks are 45C.

When I first received my 5U diyaudio case I was shocked. It's huge. Wife was mad, friends made fun of me, mocking the extra space inside. Now I am glad I got it. People spend hundreds (thousands?) on capacitors, resistors, wire, interconnects, etc. that aren't going to make as much difference as jacking up the bias. I'd rather have more heatsink so I could bias higher than a $700 power cable.

Regarding lifespan, yes, it's probably shorter. I can live with that. The DIY stuff I build never lasts more than a few years before I replace parts with something better or move on to something else. My caps are 105 degree computer grade and they are far from the sinks. I don't worry at all about it. What I do worry about is blowing a Mosfet and sending DC to the speakers. But that is the risk you take in DIY. I figure with the $$$ I have saved doing DIY I can take that risk.

I am going to get an IR temp guage and measure the Mosfet. If it's not to hot I am going to go up to 800mv (1.7A) bias. I am sure the heatsink is enough but I want to make sure the mosfet is thermally well-coupled to the sink first. I am using K-10 silpads which some say are not as good as mica.

As-is this is the finest amp I have heard. Ever. I haven't heard them all, or even a lot of them but I enjoy it a lot. And I built it. So if it breaks I can fix it.
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