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Old 24th February 2012, 12:59 PM   #1
Boofers is offline Boofers  Canada
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Default Aleph-X Waveforms and Questions

I am trying to join the Aleph-X builders club. I have successfully constructed one block.
I have 4 output IRFP044 FETs, 24Vrails and 2A bias.
Click the image to open in full size.

Here is the differential output signal with no load (just the 34 ohms to ground per output.)
I have intentionally driven it to clipping. As you can see I have about 96V peak to peak.
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Here is the same scenario with scope probes referenced to ground.
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Now here is the differntial output signal into a 7ohm load, only about 46Vpk-pk.
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Into 7ohm load with scope probes referenced to ground.
Click the image to open in full size.

Do these waveforms look normal?

I would obviously like to have more output voltage into a load, is there something wrong with my circuit?
Would increasing my number of output FETs from 4 to 8 give me more output voltage?
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Old 24th February 2012, 01:31 PM   #2
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Your circuit appears fine. You just need more bias current. With 50% AC current gain assumed, you'll need at least 3.5 A bias current for full voltage swing into 7 ohms.

Jeremy
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Old 24th February 2012, 01:33 PM   #3
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The solution: double the bias, double the FETs and double the heatsink.

Jeremy
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Old 24th February 2012, 04:04 PM   #4
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Aaaw man, I was hoping it was something more complicated. Thanks, will do.
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Old 24th February 2012, 04:28 PM   #5
Eric is offline Eric  United States
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Waveforms look OK, but something doesn't seem right to me with your numbers. 24V rails with 2A bias (total) yields less than 20w into an 8ohm load. Unless you mean 2A bias PER SIDE, or 4A total, then you should see about 60w into an 8ohm load and close to 30w into a 4ohm load. At this level, each FET is burning near 50w which is a bit high - 20-30w per fet is the typical target range.

Provided neither is a "starvation situation" your mix of rail voltage & bias current typically follow the pattern of:
-More output power into an 8 ohm load = higher rail voltage
-More output power into a 4 ohm (or lower) load = higher bias current

Bias current and rail voltage interact to large extent in the Aleph-X amps.

What is your output wattage goal and are you looking to drive 4ohm or 8ohm speakers?
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Old 24th February 2012, 04:47 PM   #6
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Oops, my miscommunication. I do mean 2A per side for 4A total. I'm also at about 58% AC current gain.
I am only getting 35W into my 7ohm load... 7*(16Vrms)^2.

My power supply is adequate, so I'm not worried about that. Each rail has a 400VA xfmr and 0.16F capacitance.

Yeah I know i'm running my FETs a lot hotter than recommended. Its one of the things I am "experimenting" with in this project.

I'll likely end up using 8ohm speakers.
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Old 24th February 2012, 04:48 PM   #7
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Oh and BTW Eric thanks for your fantastic Aleph-X webpage.
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Old 24th February 2012, 05:19 PM   #8
Eric is offline Eric  United States
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Glad you found my web page helpful - that's half the reason I created it. The other half is it was my own running set of notes over the years it took for me to get my own amps built and running.

I think something is incorrect in your calculations/measurement. Your 3rd scope trace indicates you are swinging 46vp-p into a 7ohm load with 24v rails - this sounds too high of an output voltage given your rail voltage, unless you are well into distortion/clipping already.

Are you sure you have connected your probes properly? Have a look at the output measurement section of my web page.

P-P v swing = RMS voltage swing * sqrt(2)
Output wattage = (p2p^2 ) / Load * 2

You should be seeing more than 35w out of your current set up - my guess is something closer to 60-70...
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Old 24th February 2012, 06:45 PM   #9
Eric is offline Eric  United States
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Hmmm... I posted too quickly. Your math seems to check out. This makes me think something is not right with your measurement technique. Grey's original design had 15v rails, 4.5A bias, and produced about 38w into both 8ohms and 4ohms.

Given that your rails are much higher and bias is only slightly lower (not to mention higher AC current gain), you should be seeing way more than 35w... Measurement technique is the easiest part to check before you go tearing into the your build.
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Old 24th February 2012, 11:35 PM   #10
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Thanks for the replies. The signal is indeed clipping in all 4 of my pictures.

Yes, it does seem that my scope is measuring a bit high.

After some reading, I found the formula I used for power is wrong. The formula I used was
P = [(Vrms)^2]/R

The formula you mentioned Eric is the correct one.
P = [(Vpp)^2]/2R

With this proper formula, my amp is putting out more like
P = [(40Vpp)^2]/[(7ohm)(2)] = 110W
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