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Old 19th November 2008, 10:41 PM   #1111
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Thanks Paul. Yes I do take what I read on ESP with a grain of salt but what he says about heatsinks seems sound (it was on DIYAUDIO I read about the fets performing better when cooler).

I have been to that link you provided before but I will read through it again!

Just one point tho, you said "Given that I=V/R raising rail voltages will increase the bias". However I have raised the rail voltage a couple of times (reducing value od CRC resistor in PS) and each time the voltage accross the source resistor got a little smaller (same source resistor value .33) thus the bias current got lower. Effectively changing the rail voltage did not raise the dissipation which stayed on 25watt. I assumed I would need to adjust R13 upwards to increase the bias? Must say it did confuse me!

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Old 19th November 2008, 11:07 PM   #1112
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after a bit of googling it seems the relationship between case and junction temperature is:

case temp [degC] + Rth(j-c) * total power dissipation [W] = junction temperature

For the IRFP140 junction to case thermal resistance is 1.1degC/W (max).
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Old 19th November 2008, 11:16 PM   #1113
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Cool, (OK, not so cool!)

So the overall thermal resistance is 1.1degC/W junction to case and about the same case to heatsink (using sil-pad). Thus mosfet running at 25watts the junction is around 55C greater than the heatsink temp. With a max junction temp of 150C the heatsink can get pretty darned hot!

Thats good to know.
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Old 19th November 2008, 11:23 PM   #1114
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If I do go the route of trying to add some fans - and right now its a big if, am I better blowing into the case or out of the case? Seems they both should be the same, but what do I know??!

I have 2 small 2" fans that I could mount easily enough, but I want to hook them up to the bench supply first and try them out for noise. I think if I drop the feed voltage just a little they should get quieter. I also have a 4" and a couple of 5" fans but they would be much harder to accomodate in the case.

Still sounds really great tonight, this is quite the revelation for me.. never thought this amp would sound so good. Like, it sounded good, but not top notch, ie I would be keeping the valves going, fine for a second system etc. But not since this upping the bias. Bowls me over completely. Really a big jump.

What needs to happen now is for some of you with mini-alephs to up the bias and see what you hear. This should be replicated and tested. If you want to get the same value as me, with 12V rails, you need to parallel a 0.33 with the 0.47 that was stock.

(Its interesting that a value of 25-30W is desireable for the mosfets - and thats what I am just at now. Makes me wonder what it would sound like if the supply was 14V instead of 12 (although when I measured it tonight it was 13V, which would give me 26W per device)

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Old 20th November 2008, 12:03 AM   #1115
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yes you are right - my bad. The rail voltage doesn't effect the bias, just the dissipation.

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Old 20th November 2008, 03:24 AM   #1116
troystg is offline troystg  United States
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"If I do go the route of trying to add some fans - and right now its a big if, am I better blowing into the case or out of the case? Seems they both should be the same, but what do I know??!"

Fans exhausting hot air fail sooner due to the higher operating temperatures. But we are talking slight differences here.
Thinking positive doesn't make things better, it makes you a better person.
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Old 20th November 2008, 09:36 PM   #1117
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Just on the question of which is better, higher bias current or rail voltage I will post this quote from Grollins:

"...The thing about reducing the rail voltage is that distortion will rise rapidly; I was pushing it a bit by building the circuit with 15V rails. The problem being that the MOSFETs' Gate capacitances increase below roughly 25V or so. That, in turn, makes the MOSFETs harder to drive, particularly at high frequencies. As a direct consequence, distortion increases.
... It's just a question of being aware of the tradeoffs before you start making changes.
On the other hand, reducing the output bias will also increase distortion a bit. There's no such thing as a free lunch.
Just for the record, the lowest 'optimum' rail voltage for MOSFETs often turns out to be 25V or so. At that point, the Gate capacitance tends to make a dogleg and begins flattening out. Further increases to the rail voltage tend to produce only incremental reductions in Gate capacitance. ..."

Which is why I decided on a higher rail voltage (23V). But Nelson says in the "leaving class A" article higher bias also reduces distortion. Best of both worlds results in more heat to get rid of. I guess at some point there is an optimal balance between bias and rail voltage for a given heatsink capability.

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Old 21st November 2008, 12:54 AM   #1118
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Default an cooling

With* just the 1" fins and* 0.75*C/W (?) it would be better to use the larger diameter fans (5") and run them slower - best results are obtained by pushing the air directly onto the centre of the sinks opposite the fets (increased thermal transfer) although this is more inconvenient and also requires a larger space* between fan and fin.**

The more common method is to mount the two sinks (L & R) face-to-face and blow the air quietly down the fins, and using a simple temp control switch, can easily maintain 55*C..*

* Usually, most of the noise from the super quiet computer fans comes from too tight/ridgid mounting methods (use silicon blocks)or not keeping the blades away from the fins (minimum is 1/4 fan dia) and also need to "round off" the heatsink* fin edges.*
* A lot of pro gear does this and the fans and sinks are cleaned periodically when the amps are serviced - it works very well and they often run 24/7, too.

A most surprising thing is that nearly all computers have appalling heat transfer systems resulting in noisy air cooling despite the quiet fans - you have to go "after market" to shut them down - go figure that one!**

* Many people "shy-away" from fan cooling but it's been used for years with very little problems and the new super quiet fans make noise a thing of the past with just a little effort.
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Old 21st November 2008, 04:03 AM   #1119
amp_guy is offline amp_guy  United States
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as far as using fans. they push air better then they pull. the most effective method is to arange the sinks as was previously described then place the fan on the bottom letting the fan and natural convection lift the heated air out the top. larger diameter fans generaly make less audible noise. i find that enermax enlobal computer fans are very quiet, plus the are magnetically suspended so the bearing lasts seemingly forever. I boght the last ones at Mcm electronics. ifind nothing wrong with using fans either other than the periodic need to clean out rhe accumilated dust. My deciding factor was when the wife saw how big the new amp would be she freaked ,so fans were the only option to get the new amp out of the shop.
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Old 22nd November 2008, 01:49 AM   #1120
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Default Fans

I am going to run two of these fans
I am thinking of using Rod Elliott's Thermal Fan Circuit.
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