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Old 6th February 2014, 02:27 PM   #611
Zen Mod is offline Zen Mod  Serbia
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Lilliya

with that Krell-like heatsink - lazy 12cm fan (cruising at anywhere between 5 and 8V) will give you what you need

do not worry too much for temp spreading
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Old 6th February 2014, 04:59 PM   #612
AVWERK is offline AVWERK  United States
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You could blow the fan at slow speed right at the center of that sink so the air splits and goes out both ends.
That effectively splits the sink in half. Has a somewhat same effect as cutting it in half.
The goal is even temps across all outputs as best you can.
Without any fan cooling you will get proper heat distribution, but will your final temp be in the proper range?
If you could place all outputs on thick copper bars and then bolt that to the heat sink, you get more options on placement and size.

Just as too much horsepower is almost enough...
To much heat sink is almost enough, meaning you can never get enough of it.
As NP says, heat sinks are first and foremost things.

Regards
David
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Old 6th February 2014, 05:24 PM   #613
Rush is offline Rush  United States
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Quote:
Originally Posted by AndrewT View Post
The "problem" if it exists, is that the air is heated at the beginning of the tunnel..
As Andrew said, if the "problem" exists.
As Zen Mod said, "do not worry too much for temp spreading"

Build it, the aluminium will conduct the heat and equal things out quite a bit. It will be what it is and the devises will work as long as the temperature isn't over the limit.

Rush
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Old 30th March 2014, 09:01 PM   #614
Omid is offline Omid  Canada
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Default Bias Question

I 'm just finishing up building my amp, an F5 turbo v2, with rails at ~34V. I've hooked it up and started the biasing procedure.

I manage to set the offset close to 0 Ohm, while turning the trim pots to get the bias up to ~470mV in the P channel, ~430mV on the N channel (I'm a little scared of going much higher to see at what voltage the diodes will kick in, my heatsinks are at 58degC near the MOSFET, ~50degC at the fins, and the MOSFETS are at 75degC).

The right and Left channels both have the same issue with P being biased at a higher voltage/current than N.
Is this expected? If not any ideas where the issue may be?

My Jfets are closely matched (no cascoding, Idss 7.5mA, max temp 37degC). The MOSFETS are IRFP240/IRFP9240, unmatched. I've compared the value of the resistors on the P vs N channel (feedback resistors, 10 Ohm Resistors, 1 Ohm resistors in parallel), they are all identical.
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Old 30th March 2014, 09:17 PM   #615
Zen Mod is offline Zen Mod  Serbia
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offset 0 Ohm ?

it needs to be 0mV
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Old 30th March 2014, 09:26 PM   #616
Omid is offline Omid  Canada
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Sorry. I meant 0mV
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Old 30th March 2014, 10:07 PM   #617
Zen Mod is offline Zen Mod  Serbia
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if you use two DVMs - one for output offset and one for voltage across any Rs , you'll prevent your self of bothering with discrepancies like that

later just check values of all Urs , to see if they're all in 5-10% bracket and , if they are , chill
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Old 30th March 2014, 11:17 PM   #618
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My reasoning may be flawed, but the reason I wonder is this:
Imagine amplifying a sine wave at a low current, you end up with a class A amplification. At really high amperage, you're in Class AB. At a current between what I have for my N and P channels, the positive side of the sine wave is reproduced in class A, the negative deflection in class AB. That's some potentially weird distortion (bad sounding, odd order harmonics?)...
The other worry is that the MUR3020s will kick in earlier on the P side than the N side, which again might be another strange distortion.
I realize these anomalies would only occur in a narrow range of currents, but I figured I should correct the problem if I can.
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Old 30th March 2014, 11:55 PM   #619
Zen Mod is offline Zen Mod  Serbia
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difference is less then 10%

say that you certainly have differences in FE halves xconductance , same as in OS halves xconductance , same as differences between diodes

go figure ...... you still have some feedback , to take care of

though , believing in old good Kirchoff , if you have 0V at output ......... same current is in entire circuit
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Last edited by Zen Mod; 30th March 2014 at 11:57 PM.
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Old 31st March 2014, 12:15 AM   #620
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Thank you. I guess I should just hook it up, and enjoy some music !
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