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15th January 2005, 12:06 PM  #1 
diyAudio Member
Join Date: Aug 2001
Location: Ingolstadt Germany

NEWER AlephX calculationsheet AXE1.2
Hi all,
after so much interest in the first sheet I thought that the time has come to introduce an update Basically itīs the same with a few more things to calculate, a few more digits in some places and a korrektion faktor for voltage limited power calculation at lower impedances. Hugo, I will sent you the file by mail so you can have it converted to open office (would do it myself if I would know what open office is) and put it in the WIKI. Have fun William
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15th January 2005, 01:38 PM  #2 
diyAudio Member

wuffwaff
Oh, I'm sure that you're being modest in describing the changes you made. I've just downloaded the update and it appears that the changes implemented were not trivial. The power/speaker impedence relationship calculator has been altered to suggest that less bias will be necessary to drive a given speaker load which in turn seems to suggest that you will dissipate less heat driving that speaker load. I wonder what prompted you to do all this additional work? Everyone seemed perfectly happy accepting the numbers of the previous version. I still can't quite reconcile the numbers on the AC current gain of your calculator but perhaps it is my failing. Please refer to the questions posted here in this thread about AC current gain. BobEllis had attempted to explain it to me but I suspect he would admit that his understanding of it is not complete. You seem to have the matter well in hand so perhaps you can address those questions? 
15th January 2005, 03:43 PM  #3 
diyAudio Member
Join Date: Aug 2001
Location: Ingolstadt Germany

Hi,
no changes whatsoever are made concerning bias and power. These are laws of physic and canīt be changed.......... The only changes made are that the voltage drop over fets and source resistors has been increased for the three lower impedances (4, 2.3 and 2 Ohms). It was pointed out in the first version that here a small error was made. The values assumed here a a bit more accurate but can also vary with used fets and source resistors. The good thing is though that almost all AlephX (also the factory ones) are current limited into lower impedances what means that these changes have ABSOLUTELY NO effect on the outcome of the calculations. To understand accurrentgain it would be good to read the patent on the active current source and one of the Zen Variantion articles. Iīll try to tell it in a simple way: The current source is modulated by the output signal so for positive voltages it will up the bias and for negative voltages it will lower the bias. The mean current stays at the dc bias. The accurentgain is the describes how big the changes in bias current are in relation to the output signal. At 0% the bias stays the same whatever output signal there is. At 50% the current source will deliver 50% of the total accurrent to the load. This is also the point at wich it will shut off at twice the dcbias current on the negative swing. The active current source makes it possible to deliver twice the dc bias into a load and therefore ups the maximum efficiency of a single ended class A amp from 25% to 50%. You can go even higher as 50% but than it will shutt off completely during negative peaks > 2x dcbiascurrent. I really hope this helps, William
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15th January 2005, 09:02 PM  #4 
diyAudio Member

wuffwaff
Since you had mentioned in this post that you were going to make an adjustment, I naturally assumed that would be one of the items you would fix. When I looked at the lowest ohm impedences and didn't see the power degrade as much as it did in the previous version of your spreadsheet, it looked like you had reconciled most of the disparity but I guess I should have looked at all the ohm values before complimenting you I just used the new spreadsheet to input in the following example: Voltage at the FETs: 15V Bias: 4A AC current gain: 50% Number of FETs: 4 Power at 8Ω: 42W Power at 4Ω: 32W By my numbers that power profile looks more like what you will get with 2.2A (ie:1.1A on each FET). The power numbers I get for the configuration above are 40Wrms into 8Ω and 80Wrms into 4Ω. In other words, something still doesn't add up to me. With that much bias the amp should be stiff to 4Ω. It's probably just me but I just have a feeling that something isn't right about that spreadsheet. Is there anyone out there that built out their AlephX similar to what is described in this post? If so could you please measure its reaction to the various loads as William did in the post referred to above and report to us your findings. 
15th January 2005, 10:15 PM  #5 
diyAudio Member

wuffwaff
I'm not trying to change physics, just the way you determine the required power delivered to a given speaker load. If a person building this amp needs only half the bias to obtain the same result (ie: have his amp double power to the value of his speakers lowest impedence dip) then his build requirements for heat dissipation are much less severe than what is suggested in your spreadsheet. The AC current gain must be so amazingly complex judging by the simplified explanation you claim to have provided above that it's a wonder anyone would even attempt to use it If I could please ask you to reread and answer the questions about AC current gain in this post that directly relate to your spreadsheet so I can better begin to understand the calculations and the purpose behind it, it will bring me and many of the other less gifted like me closer to actually building this amp instead of reading about it. 
15th January 2005, 11:54 PM  #6 
diyAudio Member

William,
Shouldn't the power calculation reference the peak current rather than the bias? It makes no difference at 50% AC gain, but at other gains the current/voltage limit changeover is at a different point. How about this for B16? =IF((2*B$144)^2/2/8... yldouright, Yes, power doubles at 4 ohms IF enough current is available. The catch with the AX is that the load is between the two halves. 4 amps peak in the load requires that each side of the amp produce 4 amps (it goes in one side and out the other). In your example. each half idles at 2 amps and with 50% AC Gain can deliver 4 amps peak to the load. for your example, I calculate 32 watts RMS output based on the current limit. If you plug the PEAK current of 4 amps into P=I^2*R you'll come up with 64 watts. What you may be missing is the conversion from peak to RMS. RMS current (or voltage) for a sine wave is peak/2^.5 Since you divide by the sqaure root of two, then square it again, your rms output is Ipeak^2*R/2=32 watts. the voltage limit is a little more complex. Since this amp is bridged, the voltage across the load is twice the voltage swing of either side. As the left side goes to 12 volts, the right goes to 12, for 24 V across the load. Again, you divide the peak voltage by sqrt(2) to get the RMS swing, or RMS Power =(1/2)*Vpeak^2/Load or 74 watts for your example IF the bias would support it  you'd need 6 amps bias at 50% AC gain. Rather than call the situation where the current source can provide twice its static current 50% AC gain, I think of it as a current gain of two. That's the way it came to me reading the patent, and it took me a while to reconcile that view with the nomenclature used in this forum. 
16th January 2005, 08:25 AM  #7 
diyAudio Member
Join Date: Aug 2001
Location: Ingolstadt Germany

Bob,
it does refer to the peak current. The first part of the calculation looks at the voltage B$12 and compares it with the available peak current B$16. Then decides if the power is current limited or voltage limited and prints the result. Youdou, if you could point out to me what precisely is wrong and donīt use phrases like "if a person......" "some members on the forum....." etc. maybe I could help you. Just because somebody changed the spreadsheat doesnīt mean that suddenly the amp needs only half the power. If the efficiency getīs over 100% you should take care otherwise your caps will freeze. William P.S. Youdou, I donīt like the way you phrase your questions, I donīt like the way you always assume something is wrong when you donīt understand it. Iīm on this forum to learn and sometimes to help people. Your posts make me very aggresive and that canīt be good. This is the last answer you are getting from me.
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16th January 2005, 11:19 AM  #8  
diyAudio Member
Join Date: Dec 2002
Location: Vienna, Austria

Quote:
Fantastic reply! Uli
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16th January 2005, 11:47 AM  #9 
diyAudio Member

William,
My mistake  I copied and pasted part of the sheet into a new workbook to verify the formulae and didn't place it where I got it  the absolute position references bit me. the circular reference error ought to have told me something. DOH! That said, thanks for putting together such a useful tool. 
16th January 2005, 11:58 AM  #10 
diyAudio Member
Join Date: Dec 2002
Location: Vienna, Austria

Hi William,
is it possible that your zipfile is empty? tried to download the new axe but nothing there Uli
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