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Old 25th February 2012, 06:14 PM   #11
Eric is offline Eric  United States
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That output measurement sounds a bit better... At this point, I don't see where you would need more power into a 7ohm load.

I know that the 044's can handle more heat and more current than the 240's, but you want to consider doubling up your output mosfets for greater longevity. Have a look at the datasheets and the Safe Operating Area for the voltage and current level you are using, and derate their current capability according to your temperature measurements. I don't remember the specifics, but it seems to that you should be running your output mosfets at less than 50% of their maximum SOA...
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Old 27th February 2012, 02:46 PM   #12
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Funny thing, the word ďneedĒ

Yeah, I know I'm pushing my FETs hard, but Iím making a hot rod here lol...

I couldnít find where I read this, but somewhere I read that the temp you run the FET at + the max power you run the FET at should be less than the FETís max rated power.
So, for my 120W 044ís if I run them at 50W I have to keep them below 70 degrees C.

Anyway, you are right. Doubling the FETs would be the "responsible" thing to do. Iíll try it and see how it goes. Should get me more power into 4 ohms, which is something I "need"
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Old 27th February 2012, 05:39 PM   #13
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Hmmm.. if you don't increase your bias current, you just might be OK for the long run.

According to the IRFP044 datasheet, the device is rated for 180w max power dissipation. This figure has a linear derating factor of 1.2w/c above a case temperature of 25c. Give the heatsink temperature of 52c that you show in your first image, lets assume that the case temperature of your mosfets are closer to 65c (they are probably a little cooler than this, but let's be conservative). This is 40c above 25c, so we derate by 40*1.2 or 48w. Thus, at Tc of 65c, maximum power dissipation = 180w - 48w or 130w (again, rounding to be conservative in approach).

So, at a level that is higher than your measured temperature (assuming reasonably good technique for mounting your fets to the sink), you should be able to dissipate 130w from each mosfet. Given your rail voltage and bias current, it looks like you are pulling about 50w from each mosfet. Thus, you are running at about 38% of the maximum output at this temperature.

You just might be fine leaving things the way they are...

Having said that, you are likely to experience longer device life at a lower temperature and doubling your output devices just seems like a "kind" thing to do...

If you do increase your bias level, though, more mosfets is just about mandatory...
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Old 27th February 2012, 07:28 PM   #14
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Thanks Eric, I appreciate your advice. Now I have some decisions to make.
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Old 28th February 2012, 10:08 PM   #15
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I have been losing sleep over power calculations...

Here is the correct power formula for continous power of a sinusoidal waveform into a purely resistive load.
P = [(Vrms)^2]/R

So a 40V peak to peak sine wave into an 8 ohm resistor results in
P = [(14.142V)^2]/8ohm = 25W

This formula gives the same result
P = [(Vpeak)^2]/2R

Which would be [(20V)^2]/(2*8ohm) = 25W

So my 16Vrms signal into my 7 ohm resistor is
[(16)^2]/7 = 36.6W
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Old 29th February 2012, 12:15 AM   #16
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Quote:
Originally Posted by Boofers View Post
I have been losing sleep over power calculations...
Ha - welcome to DIY Class-A biasing for your first amp

Quote:
Originally Posted by Boofers View Post
Here is the correct power formula for continous power of a sinusoidal waveform into a purely resistive load.
P = [(Vrms)^2]/R
OK, this formula matches/confirms my own output power readings for my amps as well as William's AXE spreadsheet (linked on my Aleph-X page).

Quote:
Originally Posted by Boofers View Post
So a 40V peak to peak sine wave into an 8 ohm resistor results in
P = [(14.142V)^2]/8ohm = 25W
Ok, so how did you get from 40vp-p to using 14.142V in your formula? Looks like you are using peak one-way voltage, not peak-to-peak....

Quote:
Originally Posted by Boofers View Post
This formula gives the same result
P = [(Vpeak)^2]/2R
Hmmm, I used peak-to-peak voltage here - using p-p in this formula checks out with my data as well... Why have you used one way?

Quote:
Originally Posted by Boofers View Post
Which would be [(20V)^2]/(2*8ohm) = 25W

So my 16Vrms signal into my 7 ohm resistor is
[(16)^2]/7 = 36.6W
Might I suggest a different course of action... Rather than worrying about output power in terms of measurement, use your ears instead of you meter/scope. Is your amp providing enough power to drive your speakers to a satisfying listening level in your room without audible distortion or other obvious problems? If yes, then you're done. If no, then add an extra set of mosfets and increase the bias with your trim pots.
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Last edited by Eric; 29th February 2012 at 12:18 AM.
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Old 29th February 2012, 03:29 PM   #17
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Quote:
Originally Posted by Eric
Ha - welcome to DIY Class-A biasing for your first amp
It's an enjoyable pain lol.

Ok, I think we are talking about the same thing, just using different terminology. The example in my previous post
Vpp = 40V
Vpk = 20V
Vrms = 14.142V (20/sqrt2)

Here is 120VAC from an electrical outlet displayed on the scope.
Click the image to open in full size.
The RMS voltage is approx 120V
The peak voltage is 170V
The peak to peak voltage is 340V

So imagine you plugged a powerful 8 ohm directly into an electrical wall socket. You would hear a nice loud 60Hz hum.
So, how much power would the speaker be dissipating?

P = [(120V)^2]/8ohm = 1.8kW
also
P = [(170V)^2]/[2(8ohm)] = 1.8kW


If we used Vpp = 340V in the equation, we would get
[(340V)^2]/[2(8ohm)] = 7225W

Now check this by calculating the current
I = P/E = 7225W/120V = 60.2A

Seems too high... We know that
I = 120V/8ohm = 15A


As for the amp, like you said, how much power do I really need? For me, I think the answer is as much as I can get!
Partly because when people see the amp they will think "man that must be a super powerful amp!" and I don't want to say "No, its just 30W per channel..."
More FETs & resistors are on order.
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Old 29th February 2012, 05:53 PM   #18
Eric is offline Eric  United States
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Yes, I think we weren't connecting in terminology... no worries. Something still seems strange to me about your power output. Grey's original design put out 38w into both 4 and 8 ohm loads with 15v rails and 4.5A bias. I think you should be seeing more power output with 24v rails.

Quote:
Originally Posted by Boofers View Post
"man that must be a super powerful amp!"
Responses:

Well, let's just put it this way: just be careful not to burn yourself on it

-OR-

Yep, when's the last time you felt an amp that puts out this much heat?!?

One of my friends thought I was building a radiator or space heater when he first saw the chassis in progress. I told him it was an audio amplifier for just one speaker...
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Old 29th February 2012, 08:09 PM   #19
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A space heater yeah, theyíre definitely winter amps!

Yes Eric, I believe you are absolutely right that I should be seeing more power out of my amp.

Ideally my amplifier into 8 ohms should be putting out
Vpeak = 44V
Imax = 4.76A
Pcontinuous = 91W

So, that leads me back to the original reason I started this post lol. Why is my amp choking?
Perhaps Iíve made an error calculating my AC current gain...
Unfortunately I can't make any more measurements right now because I have things torn apart so I can add more FETs.
I am going to double the output FETs from 4 to 8, and bias them at 1.5A each, for a total of 6A.
Hopefully I find something along the way that Iíve overlooked.

Anyway, thanks Eric for bouncing ideas around with me.
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Old 29th February 2012, 10:53 PM   #20
Eric is offline Eric  United States
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Hey, no problem! It's fun kicking things around. Many here have helped me, so I'm trying to give back to the community - though it seems that we've had our own private party here...

I have two primary thoughts about your output power. One, perhaps something is amiss with your AC current gain. This is a part of the circuit that I don't understand well. When I played with this before, I got exactly opposite behavior from what everyone else was posting. I am not quite sure and this part of my web page needs to be updated but I'm not really motivated to play around on my circuit board at this point (I'm too busy enjoying music and movies in my theater).

The second thought is that perhaps your output mosfets are simply current starved. For each rail voltage, there is an "optimum" minimum bias level for things to operate properly. Similarly, for each bias level, there is an "optimum" rail voltage that allows the amp to really sing. I wonder if 4.0A is just too low of a bias point for your rail voltage...

Other semi random questions:
Are you using balanced or unbalanced inputs? The amp is really designed for a balanced input and produces more output for a given input with balanced vs unbalanced. It is essentially one Aleph amp for the Pos input signal and a second Aleph amp for the Neg input signal.

Are you letting things warm up for an hour before you begin to do your measurements? What are your relative and absolute DC offset readings after the amp has warmed up? Large amounts of either will limit voltage swing on the output leading to premature clipping. Ideally, both should be below 200mV steady state.

Remember, when paralleling output mosfets, they need to be carefully matched so one doesn't hog power from the other.
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