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Old 21st April 2007, 10:45 PM   #431
Variac is offline Variac  United States
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The fans are 12v and 2 in series start fine. I put 3 in series and no-go.
At correct voltage they are very noisy, at half, they are hard to hear.
Mounted correctly they will be even quieter. I noticed you said WHEN I have fan failure I will put one of those AC thermal switches on the sink as a simple protection.
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Old 22nd April 2007, 07:40 AM   #432
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Variac,
That sounds like a nice solution, but are the fan's moving enough air with 6VDC each.

If not, supply them with 12VDC each and a 6.8 or a little higher zener in series with each.
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Old 22nd April 2007, 08:14 AM   #433
Variac is offline Variac  United States
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OK , I had another idea. I confess that I'm intrigued with the balanced configuration. I plan to build the Russ White balanced DAC when it matures. I have the X'ed Balanced Pass Linesource. A balanced power amp is something I've always wanted.

Even without a fan it appears that my heatsink is more than big enough for a single channel with its 6 mosfets. So if a made four boards, each with only two Mosfets, and combined them in pairs in balanced configuration, it seems that I would get about 35 watts per channel. If I decide to run them unbalanced at some other time, I guess they'd put out about 8 watts times four.

My current speakers are about 96 dB efficient, so those numbers sound like plenty to me.

That means eight mosfets total, which the heatsink should handle fanless..
Ok, I guess the question is whether to go with 2 unbalanced channels with 4 MOSFETS each or 2 balanced channels with 4 MOSFETS each(2 per board)

BALANCED (4 circuits with 2 MOSFETS each)
So the bad for balanced is:
only 20 damping factor?
Less distortion per watt ?
What else?

The good is:
Common mode rejection
Conceptually matches my other components
Power for 2 ch: 33w/ch?
What else?

UNBALANCED (2 circuits with 4 MOSFETS each)
So the bad for unbalanced is:
Slightly noisier
Doesn't match my other balanced components
Half Power for 2 ch: 16 w/ch?
What else?

The good is:
40 damping factor
More distortion per watt output?
What else?

THE MOST IMPORTANT QUESTION:
So either way, to have only 2 or four output devices per board (circuit) , how do I proceed? I suppose I just leave off the excess mosfets, and each one's two associated resistors.

Then I guess I have to lower the bias? I use P1 to do that? Is a different value for the pot and maybe other resistors required?
Other things to do? I just slowly bias it until it reaches the correct temp? That can't be right: if I had a bigger heatsink, I would end up biasing it more.

moving on...
Ok, now on to the question everyone asks Nelson in addition to "How big should my heatsinks be? " That is " I have a tube of IRFP 140's-that will be OK , right? or am I stressing them too much? I think he'll say it's OK.
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Old 22nd April 2007, 01:53 PM   #434
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I'm thinking about integrating preamplifier into this project. Few years ago I made such amplifier (it's basing on Dariusz Kulesza design):

Click the image to open in full size.

Idea is very similar to F4 but NFB includes the output pair to. Schematic can be found here:
http://www.unisonus.com/pdf/dracula.pdf
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Old 22nd April 2007, 06:46 PM   #435
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Quote:
Originally posted by Variac

A balanced power amp is something I've always wanted.

So the bad is:
only 20 damping factor
not as "manly" as the stock amp
What else?

The good is:
Common mode rejection
Adequate power
Conceptually matches my other components
No fans
Less energy used
Adaquate power.
What Else?

Then I guess I have to lower the bias? I use P1 to do that?

Ok, now on to the question everyone asks Nelson in addition to "How big should my heatsinks be?

First off, I have thought that you are a quiet old man. From today, you are not.

The balanced amp invites 3rd harmonics. Are you familiar with 3rd harmonics? I have thought that Papa developed F4 mainly for whom like 2nd harmonics.

Damping factor of 20 . . . what? Mine (not tube) is lower than that. I'm fine.

The good is no fan? Why fan with such big fins?

I think if P1 is adjusted lower, the bias would be more. By thw way, your speakers have 96dB SPL (I suppose 8-ohms). Why more bias?

Papa always likes to have heatsink temp about 50-55. This must be good temp with respect to all. My fingers are still safe, and I need some temp for a short heat-up time and for the fast reach to the steady-state heat after the turn-on.


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Old 22nd April 2007, 07:09 PM   #436
Variac is offline Variac  United States
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Quote:
First off, I have thought that you are a quiet old man. From today, you are not.
We have a saying: "Better to say nothing and be possibly thought a fool than to open your mouth and remove all doubt" But as I posted here before, this amp is just too great to ignore!


Quote:
I think if P1 is adjusted lower, the bias would be more.
OK I have rewritten my previous post that you are commenting on. I hope it makes more sense! I meant to say "Do I adjust P1 to lower the bias?" I think I need to lower the bias due to fewer output devices.

Quote:
Why fan with such big fins?
Because the people that have built this say you need a LOT of fins to keep it cool. I calculated my heatsink compared to theirs, and it is possibly not quite big enough for 2 channels.. OF course I can try it, and if the temp is reasonable with the fewer MOSFETS, I can add more

Thak you for your Babo-post. Quite an honor..but it wasn't dumb
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Old 22nd April 2007, 07:22 PM   #437
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Quote:
Originally posted by Variac


..............

OK I have rewritten my previous post that you are commenting on. I hope it makes more sense! I meant to say "Do I adjust P1 to lower the bias?" I think I need to lower the bias due to fewer output devices........................

look:

http://www.diyaudio.com/forums/showt...21#post1151521

you don't need to touch anything;
comparing to Papa's full F4 -with 2 pairs of mosfets you'll have 2/3 of bias;
with 1 pair just 1/3 of bias..........off course-presuming that you somewhere in the past put 3 pairs in it and adjusted bias as Papa commandeddddddddddddddddd........
conclusion- if you have them-put all 3 pairs in amp-spread them nice on heatsink and put that silly pot where you can- to not burn your little fingers

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Old 22nd April 2007, 08:01 PM   #438
Variac is offline Variac  United States
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Thanks Zen-Choky . I hadn't seen your post.

The problem is I wanted two channels on this heatsink, If I built it "stock" I would have 12 mosfets on it. I will try to study the Pass Commandment of Biasing.

EDIT: I found the Wisdom of Pass: " .25 volts across the .47 ohm source resistors" So there it is I think. I just build it with as many output device pairs as I think my heat sink will stand, then bias per
The Wisdom of Pass using P1 ?

Also, does the balanced have less distortion for a given output than the unbalanced? That seems to be what I'm seein in the specs...

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Old 22nd April 2007, 09:00 PM   #439
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Quote:
Originally posted by Variac
...............

EDIT: I found the Wisdom of Pass: " .25 volts across the .47 ohm source resistors" So there it is I think. I just build it with as many output device pairs as I think my heat sink will stand, then bias per
The Wisdom of Pass using P1 ?

Also, does the balanced have less distortion for a given output than the unbalanced? That seems to be what I'm seein in the specs...


you again didn't read my post.......see 0,5A mark somewhere up?

that's Papa's .25 across .47

anyway-put 4 +4 (two pairs/channel) on sink and then crank it as much you can ; even with 1A per vertical pair that's just 22W per mosfet

and yes-balanced always mean less distortion.....at least on paper
cancellation........
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Old 23rd April 2007, 01:07 AM   #440
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Just a thought… Usually Nelson lowers the source resistors when using less devices, so maybe it will be nice to try 0.33 Ohms instead of 0.47.

P.S. That is when using only two pairs.
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