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Old 7th October 2003, 03:38 AM   #1
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Default 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?
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Old 7th October 2003, 05:23 AM   #2
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Default 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.

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Old 7th October 2003, 04:26 PM   #3
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The answers are much simpler.

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Old 9th October 2003, 12:21 PM   #4
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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?
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Old 9th October 2003, 02:54 PM   #5
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lumanauw,
I've only got a few minutes and you've asked a lot of questions, but I'll try to get to as many as I can.
--Sufficient current for a loudspeaker isn't that hard to figure out. Really, the only variables are the impedance of the speaker and how loud you intend to play. All right, the efficiency figures in there, too, but let's leave that up to the owner. Keep in mind that most speakers have dips/peaks in the impedance curve, so you'll need to plan ahead for those. The minimums are the ones that will give you trouble. How loud you listen is something only you know. If you listen to string quartets, you're not likely to need a lot of power, even if your speakers are inefficient. If you're trying to crack the plaster in the wall, it's going to take a lot of power. Nelson's commercial designs went up to 200W/ch and it's possible to go even higher, but the heat is going to get pretty difficult to deal with in the practical sense; plan on three times as much wattage in heat as the amplifier delivers to the load.
--The voltage swing available from the Aleph current source isn't particularly hampered by the fact that it comes from the Source. Look at the output stage of Nelson's A-75...Sources on both sides. The ability of a current source to track a given signal is called compliance. The objective of the Aleph amps isn't really so much the ability to swing to within .000001V of the rail as it is to increase the theoretical efficiency of a single-ended class A amplifier. A topology that would ordinarily approach 25% effiency can now (at least on paper) approach 50%. Actual percentages are more like 30-40%, but it's still nice to have more power with less heat.
--The 392 ohm load resistor in the front end does double duty. It serves to generate a signal from the varying current coming from the 9610, and it also sets the bias point for the output MOSFET (i.e. the bottom one) by generating a DC offset against the Gate of the output--enough to make it "turn on." Trying to adjust DC offset at the output by changing the value of the resistor is going to be more trouble than it's worth. If you still want to try it, bear in mind that you'll only want a few ohms adjustment either way. Something like the adjustable triplet that I put into the Aleph-X current source would be the way to go, although you'd need to recalculate the actual values to center around 392 ohms instead of 221. Another possibility would be to adjust the current going through the front end, in which case you could simply plug in the Aleph-X front end current source. If you change the front end bias, the DC drop across the 392 ohm resistor will change too. In a 'regular' Aleph it's best just to use matching MOSFETs in the front end and let the feedback loop handle the rest. The only real exception is if you're using something other than IRF MOSFETs in the back end that might have a radically different Vgs. At that point, you'll have some fiddling to do to make all the pieces of the puzzle fall into place. In the real world, anything under 100mV of offset at the output is good enough.
--Using a 200 ohm resistor as the load in the front end will give you a number of problems. Assuming that you're using IRF MOSFETs in the output, they won't bias. Being enhancement devices, they need the Gate pushed a certain number of volts positive relative to the Source before they will conduct. Another problem will be that the gain of the amp will drop. Actually, this may be a benefit from your point of view, but the Alephs are fairly low-gain amps to begin with. Dropping the gain from 20dB down into the mid-teens is going to require more voltage swing from your preamp. More subtle things will include changes in the bandwidth, etc. In principle, you could increase the front end bias current in order to use a 200 ohm resistor, but be prepared to work with it a while in order to get the bugs out. Watch the heat dissipation on the front end MOSFETs if you increase the bias.
--The cap you mention isn't even in some of the designs. The stability of the amps will vary depending on the number of output devices (cumulative Gate capacitance), board layout, lead length, and a number of other factors. Just bear in mind that you may need it (or may not) depending on how your final amp is configured. Look at the output on an oscilliscope. Are high frequency square waves showing any evidence of ringing? Is the amp oscillating? Does it react poorly to reactive loads? If so, add the cap. If not, then you may not need the cap at all.
--As a final note, trying to analyze the Aleph circuit as though it was a normal amp will give you headaches. It's best just to approach it as a stand-alone design and understand it on its own terms. The front end is a completely ordinary differential. The only thing to watch out for is that the load resistance has to generate the right DC offset to bias the output. The output device (the one on the bottom) is a plain vanilla grounded Source amplifier. Nothing fancy. The current source is the unusual part. Left on its own, it produces a constant current. If a signal is present (read through the current sensing resistors at the output), it will vary its output somewhat. Any time I phrase it that way, Nelson comes along and notes that the current averages to a constant value over time, so I'll beat him to the punch and say it myself.
There's no way around it--you're going to need a lot of heatsinking for the outputs. Unless you just happen to like the little puffs of smoke as the MOSFETs burn up.

Grey
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Old 9th October 2003, 03:08 PM   #6
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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.
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Old 9th October 2003, 05:12 PM   #7
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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.

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Old 9th October 2003, 05:39 PM   #8
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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?).
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Old 9th October 2003, 06:15 PM   #9
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Thank for the explanation. I was using Fairchild HUF75652G3 output devices. They needed a different value for the 392 ohm to bias them.
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Old 10th October 2003, 02:56 AM   #10
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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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