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 7th October 2003, 03:38 AM #1 lumanauw   diyAudio Moderator Emeritus     Join Date: Oct 2002 Location: Bandung Aleph formulas I'm very attracted to aleph design. They are unique and good. I'm about to built aleph power amp, but my primary limitation here is heat dissipation. So I want to built aleph based power amp just as much as I will need. I have 2 questions here, hope you elders can help me. 1. If I built aleph with +/-25 supply, how big will I get clean sinusoidal? Will I get full +/-23 clean sinusoidal in my scope? What if I built with +/-15 supply, how much clean sinusoidal trace will I get? How can I calculate the relation between supply voltage and clean output trace (below clip)? 2. What is the relation between steady current vs speaker load? I will use 3 way 4 ohm speakers, how much steady current (in second stage) will I need if I built +/-25V aleph? What is this number if I built +/-15V aleph? What is the formula for determining constant current for aleph vs load impedance for certain voltage? Is it as simple as V/R or not?
 7th October 2003, 05:23 AM #2 Nelson Pass   The one and only     Join Date: Mar 2001 Re: Aleph formulas 1. If I built aleph with +/-25 supply, how big will I get clean sinusoidal? Will I get full +/-23 clean sinusoidal in my scope? What if I built with +/-15 supply, how much clean sinusoidal trace will I get? How can I calculate the relation between supply voltage and clean output trace (below clip)? You will lose about 3 volts, 2 to the Mosfet and 1 to the resistor. 2. What is the relation between steady current vs speaker load? Inversely proportional for best results. I will use 3 way 4 ohm speakers, how much steady current (in second stage) will I need if I built +/-25V aleph? 5 amp peak = 2.5 amp idle What is this number if I built +/-15V aleph? What is the formula for determining constant current for aleph vs load impedance for certain voltage? Is it as simple as V/R or not? It's that simple.
 7th October 2003, 04:26 PM #3 jh6you Account Disabled   Join Date: Aug 2002 Location: Near to the Pacific Ocean The answers are much simpler.
 9th October 2003, 12:21 PM #4 lumanauw   diyAudio Moderator Emeritus     Join Date: Oct 2002 Location: Bandung Thank you Mr. Pass, I am honoured to have yourself reply my questions. Actually I have made trial pass amp on simple heatsink (but they are burnt now). They are very hot amp. I tried to make the current smaller, it results in distoreted sound, so my conclusion the steady current must be sufficient to flow into speakers. I just dont know how much is enough. I've been studying the aleph schematic for months now. Your second stage constant current is very unique design. The output is taken from the Source from upper mosfet, but still it can gives full swing. I really do not understand this, because usually if we want to get full swing, the output is taken from the Drain, not from emitor (uses P channel in upper mosfet). Also usually ordinary amp uses output from collector-top+collector-bottom from current mirror (instead of 390ohm drop) to be fed into second stage. This way, the output point of the first stage is "hovering", and determined by the gate voltage of VAS transistor, because the upper VAS has already determined quiscent current. But in aleph design they are unique. I've tried to adjust the 390ohm resistor to reduce DC offset, but nothing happens until some value (>450ohm) the output is high DC. I read the forums, and found out that the dc offset is not determined by the 390ohm resistor, but from matching the differential mosfets, and they work. My conclusion is that the value of 390ohm is changeable, as long as it gives smaller voltage than the second stage VAS+resistor drop of4-5volt. Is this true, Mr pass? I observed in the inverting section of ONO preamp, you use 470ohm, not 390ohm, because the drop in second VAS is higher, due to bigger source resistor than aleph. I asked this question, because I have an idea to eliminate the lower VAS resistor (Source directly to -VCC, not via resistor). I tried this in ordinary amp (not aleph) and I think I hear better detail. What happens What if the 390ohm resistor is much smaller, like 200ohm (gives voltage drop way smaller than second stage gate voltage of 4-5volt), will aleph still works fine? I'm not trying to challange the well time-proven design of yours, or tobe much clever than you, I'm not, I just want to try new things. One more question. In second stage, there are various placement of 1nF stabilizer capacitor. In aleph design it places in lower G-D of mosfet, but in volksamp, it is on the top (on the constant current G-S of mosfet). Which is the right one, Mr pass?
nobody special
diyAudio Member

Join Date: Oct 2002
Location: USA
Quote:
 Dropping the gain from 20dB down into the mid-teens is going to require more voltage swing from your preamp.
That doesn't seem right...
It would reduce the open-loop gain, but the overall gain of the amp should stay the same. So, in effect, it would reduce the amount of feedback in the amp, which is already low, as you said.
__________________
Steve

 9th October 2003, 05:12 PM #7 GRollins   diyAudio Member   Join Date: Feb 2001 Location: Columbia, SC Look at it this way. Suppose you've got a 10mA AC signal going into a 1k resistor. You'd see a 10V AC waveform across the resistor. Okay, same current (remember that the Aleph front end is driven by a current source--one way or another that current is going to flow), but now use a 511 ohm load resistor. Presto! 5.11V signal, or roughly a 6dB loss of gain. Going from 392 ohms to 200 ohms will result in a similar loss. About 5.8dB. If you're using a CD player and a preamp with plenty of gain, that 6dB loss may not be a problem. You could, I suppose, reduce the feedback to compensate, but distortion will rise, damping factor will fall, and bandwidth will narrow somewhat. However, the sun will still rise in the east and the amp would still play music. Depending on how you feel about distortion and other sundry factors you might even feel that the amp sounded better that way. Grey
nobody special
diyAudio Member

Join Date: Oct 2002
Location: USA
Quote:
 Look at it this way. Suppose you've got a 10mA AC signal going into a 1k resistor. You'd see a 10V AC waveform across the resistor. Okay, same current (remember that the Aleph front end is driven by a current source--one way or another that current is going to flow), but now use a 511 ohm load resistor. Presto! 5.11V signal, or roughly a 6dB loss of gain.
Yes, that would be true of an amp with no feedback- running open loop.
But (and I could very well be wrong here) I thought the gain of amp with a feedback loop (which the Alephs have) is determined by the feedback resistor divided by the input resistor (this may be a crude approximation). It should remain the same, as long as the open loop gain is sufficient. By reducing the open loop gain, you would reduce the amount of feeback being applied, but as long as there is sufficient open loop gain, the voltage gain should stay the same. So, then the question is whether the Alephs have enough open loop gain with less gain from the front end to support the closed loop gain of the circuit (which, if I remember right, is around 26db?).
__________________
Steve

 9th October 2003, 06:15 PM #9 MikeW   diyAudio Member     Join Date: Sep 2002 Location: North of Boston Thank for the explanation. I was using Fairchild HUF75652G3 output devices. They needed a different value for the 392 ohm to bias them. __________________ MikeW
 10th October 2003, 02:56 AM #10 lumanauw   diyAudio Moderator Emeritus     Join Date: Oct 2002 Location: Bandung Thank you GR Rollins, for detailed explenation. So aleph will still works fine if I use 200ohm instead 390ohm in differential? How about eliminating the source resistor in the bottom mosfet in the second stage (source directly to -vcc), is this also do-able? Or it will gives bad effect? I observed that the quality of sound is somehow connected to how hot the devices are. Since the voltage I will be using is quite low, I will be able to get >20ma of steady current into TO-220 mosfets to get about 1W in open air. But since my background on electronics begin with bipolar transistors, I always imagine transistors as "water Valve". If you put 1ma current into base, with hfe of 50, you will get 50ma in emitor. The bigger current you get in the emitor, you will need bigger current in base. I know that mosfets and tubes are transconductance devices, they do not need current fed into their base. But it still worries me, for example : If I make constant current for aleph differential up to 50mA (instead of 20mA that aleph standard use), will it still have the same sensitivity to small signals? Or if I raise steady current into mosfets it will becomes less sensitive to small signals? This will result in lack of detail in music, in my imagination. From my experiment bigger quiscent current gives "stiffer" sound. But I dont have any equipment to measure distortion or frequency response, so what will be the effect if I raise quiscent current? Will it still have the same sensitivity to small signal and plays same detail in music?

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