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20th January 2002, 11:28 PM  #1 
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Join Date: Jan 2002
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What is the Aleph4's true power capability?
Hello all,
I recently did some SPICE simulations of the Aleph 2 and 4 circuits and found some disturbing results........... Here is what I did: I ran a transient analysis of the amp into different resistive loads. I increased the input voltage until clipping occurred. At clipping I recorded the peak output voltage delivered by the amp into the various loads. Here are the results........... Aleph4 (bias came out to be 2.25 amps total) (R29=619 ohm) 81 Wrms @8 ohm (162 peak) 60 Wrms @6 ohm (121 peak) 40 Wrms @4 ohm (81 peak) 20 Wrms @2 ohm (40 peak) (R29=523) 100 Wrms @8 ohm (200 peak) 92 Wrms @6 ohm (184 peak) 62 Wrms @4 ohm (123 peak) 30 Wrms @2 ohm (60 peak) Aleph2 (bias came out to be 2.5 amps total) 100 Wrms @8 ohm (200 peak) 133 Wrms @6 ohm (266 peak) 140 Wrms @4 ohm (280 peak) 71 Wrms @2 ohm (142 peak) So what does this mean??? It appears that according to simulation the Aleph4 is not exactly capable of it's ratings. Of course, simulations can be wrong (those damn computers!) I have found that only slight changes to the bias current can affect the power output a great deal. Maybe the simulation is just off slightly, I'm not sure (I would think DC bias currents would be pretty accurate). Has anyone tested their Aleph constructions or commercial offerings for maximum power output?? I would be interested to know if they really do perform as outlined in the user manuals. If no one knows I guess I will have to wait until I finish mine. I will run some tests (when I get the thing built of course) and post when I get some experimental measurements to back my simulations. Perhaps Mr. Pass can help us with this one. I am very interested in this. Happy Listening Jake
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21st January 2002, 04:21 AM  #2 
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Location: Columbia, SC

The Aleph 2 actually biases at about 3A.
There is some question as to what the bias current is on the Aleph 4. Being a stereo amp with a shared power supply and less heatsinking available per device, I believe that the bias was somewhat less than the 2. Nonetheless, based on scratch calculations I just did, I think you'll find that the 4 will deliver 100W into an 8 ohm load as per the specifications. Looks like it'll deliver peak power around 6 ohms, then begin to progressively current limit below that. The 2, with more current available, continues to increase power down to a lower impedance before it runs out of steam. If nothing else, keep in mind that you can increase the bias somewhat as long as you've got the heatsinking to handle the increased power dissipation in the output devices. Grey 
21st January 2002, 07:02 AM  #3 
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Location: Lethbridge, Alberta

The service manual for the Aleph 4 says the output is biased at slightly greater than 3 amps. This doesn't seem right however, and likely occured as the result of Passlabs adapting the service manual form the Aleph 2.
With 0.6 volts across the 1.5 ohm source resistors wouldn't the biasis be 2.4A? 
21st January 2002, 05:46 PM  #4 
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Location: Northern California

Aleph 4 Power
The Aleph 4 is actually capable of about 115 Watts per channel.
This is what they measured at as shipped. The supply rails are higher than indicated in the manual. The current source on the Aleph series needs to be looked at closely and may not simulate properly at higher levels. Wayne 
21st January 2002, 06:25 PM  #5 
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I checked, and the Aleph 4 figure is about .6 volts
across the source, resistors, which by measurement and calculation gives a 2.5 amp bias and a 5 amp peak output, for 200 watt peak, 100 rms into 8 ohms. As discussed on a previous thread, if you base your results on the schematic, you suffer from the cut and paste error of the Aleph 4 schematic pulled from the preexisting Aleph 2. We corrected the X6 vs X3 figure, but did not adjust the NOMINAL figures on the Source resistors. The Aleph 2 comes in more like 3 amps bias, which gives about 144 watts rms into 8. 
21st January 2002, 08:36 PM  #6 
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Thanks guys,
I guess the simulations may have some trouble with calculating the bias currents. I will see how my version does when it is finished. Jake Wayne.............. You mentioned the simulations being off at higher current levels. Is this typical of SPICE? Can you elaborate?? Do you have some experience with this type of thing? You got me interested..........
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21st January 2002, 09:32 PM  #7 
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I've gotten on this soap box before on simulations,
but part of the problem is simply that there is quite a bit of variation from part to part on MOSFETs, and of the models available, the MOSFET models seem to be the most problematic. In simulating an Aleph 4, I would screw around with the bias values until you get the .6 volts on the soource resistors, and adjust the value of R29 until you get the best results, which is approximately where the current source supplies 1/2 the AC output current. 
21st January 2002, 10:35 PM  #8 
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Join Date: Oct 2001
Location: Lethbridge, Alberta

Questions.
To increase the bias on an Aleph 4 would one simply decrease the value of the source resistors and then adjust R29? The value of R29 being where the current source supplies 1/2 the AC output current. How is this measured/comfirmed? Brett 
22nd January 2002, 01:39 AM  #9  
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Brett,
Perhaps this might help... Quote:


22nd January 2002, 02:21 AM  #10 
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Location: Philadelphia

Thanks for the responses guys.
So Nelson....................... I can tweak the values in the simulation to get the circuit to perform as intended, but when constructing the circuit is it necessary to tweak bias and the value of R29? Or have you already done that in the design? Will the MOSFET device tolerances require the DIY'er to tweak R29? Perhaps devices are different from batch to batch? Did the original 'tweaked' design use the same output devices? It seems logical that if the simulation models are not exactly right, then couldn't the real devices be different from when the circuit was designed? What would you recommend? JAke
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