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Old 16th January 2005, 03:04 PM   #261
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Default Re: Few Questions if may

Quote:
Originally posted by keypunch
Hi Petter,

1) There was mention of a V2 and V3 in your postings. Is it correct to assume the circuirts and PCB you posed are for V1?
2) It seems you have made some refinements after posting the circuit and PCB re resistor values and added a Vbe. Any chance of posting or eMailing me those updates?
3) I like to see how you implemented the 10W Class A/100W AB and 50W Class A/100W Class AB so you can change via the external case switch you installed. Pic not needed just what points you tapped to, the values you used to switch between or add in parallel or series to enable the bias value change and method of value change as in did you just switch in different value or switched in a series or parallel value?
4) Is there a formula to determine different Class A/Class AB threshold points? If so what or where is this formula?
5) There seemed to be some reference to using output drivers in quadruplets? Is this a fixed requirement of this design that one has to use or can one choose to use 2 or one set of output devices per side of X? I realize this implies less power, that is ok, I am well aware that may be case.
6) Indication is this design scales well. What PSU or componet value calculations does one use to scale the design? For your implementation I cannot recall the Secondary Vac you used and resulting VRail of your supply design. I have a number of PSU based calculations, but seems the PSU calculations vary widely due to topogogy of the amp. Some topologies need certain key component values recalculated for different PSU rail voltages.
7) Any general thoughts on use of PSU chokes in terms or pros and cons that may be specific to this amplifier design?


1. Yes
2. I decided that the project was dead since there was limited interest and contribution from all parties dwindled as people became more interested in the rather vibrant newer Pass project (AlephX). I guess I will stick with that decision. Bear in mind that the original drawing (change the values of the components) with a Vbe multipler is likely close to the original. If you want to increase gain, reduce or even short out the "X" resistor.
3. The level of power you burn off at idle (which defines the class A drive level) is only decided by how much current passes in the output stage. This in turn is defined by the source resistors, the gate voltage of the devices (4-5V typically). So the method of changing this is to set up a relay which causes the Vbe (or other method used to set bias) to shift to the new desired voltage. Since constant current passes in the "Vbe leg", you could just as well use a fixed resistor as shown in drawing one. It is probably better to use a Vbe multiplier for reasons of stability, but the principle holds. If you use the drawing on page one, you can have a resistor of 140 Ohms with a relay switch between the top and bottom of both resistors and you get a resultant 70 Ohm total resistance. This yields half the voltage across the resulting compound resistor. You probably don't want to do something that extreme, but consider what happens if you just short with a relay and zero ohms - you have turned off the output stage completely. This is the principle for changing the level of "class A drive"
4. This is a general question and I really must refer you to the textbooks.
5. The way I understand, you are asking about number of output devices, and how you change the number of them. In the smallest configuration, you use 4 devices, one in each quadrant of the drawing. If you want to scale the current capability etc. you would add 4 more, then 4 etc.
7. Chokes would likely work well in ClassA designs. Bear in mind that the usage of chokes (or the "mode of the choke" if you will) depends on how you use it. Search for "Duncan PSU Designer" and use that to see the difference between a CLC design and an LC design. It will surprise you. The problem with units that draw highly variable amounts of current and an LC design as I see it is that (depending on capacitance at the end) the power supply might not be able to supply the desired current quickly. Still I am a fan of inductors, but I think it is important to think about what you want to achieve before applying them. Suitable goals can be: Lower ripple, more effective use of transfomer (longer charge cycles - i.e an LC design rather than a CLC design), less noise coupled into the system etc. Your mileage will vary. This is also a rather general question, and even though I believe this and many other designs is highly suitable for inductor usage in power supply I cannot answer your specific question with authority.

Also bear in mind that testing the X design is kind of hard. You will probably think that everything is broken even though it might be fully working when you have built it. Check out my tips written elsewhere in this thread about setting up the prototype.

One of my plans for the future is to build the input stage as a complete amplifier which I sense passdiy are either planning, or possibly have even done. This would be an interesting exercise, especially for sensitive speakers - the natural evolution of Zen.

Good luck.

Petter
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Old 17th January 2005, 04:26 PM   #262
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Adding on to question 3 and 4:

Class A can be though of as a standing current in the output stage capable of driving the load fully without shutting off the opposite leg. For a normal unbalanced amplifier with split supplies, this means that if your speakers desire 10 peak Amps at the level you ask of them, the standing current needs to be able to handle that. However since you are modulating both phases, you ned a standing current of minimum approximately 5 amps. For a balanced amplifier, you need half that per phase.


This current can be determined only by knowing what the load is. Typically one expects a speaker to be resistive at say 8 Ohms. Well that is typically far from the truth so it is impossible to say how much current is required without knowing something about the speaker.

From memory, the Aleph's set up one of the legs as a constant current source (it is a little more complex because the current source did "help" a little by boosting current as required using a positive feedback system from the output). This current then becomes the maximal drive that the amplifier can provide from that phase and thus from both phases if one desires symmetrical drive. In the case of the Aleph, the efficiency is up to double the special case of the constant current source - often termed single ended amplifier (in the tube language). The most important thing is to grasp the concept.

So the formulae you asked for are pretty simple to deduce, but not all that practical unless you are willing to commit some values - such as load impedance. Current in the speaker is a function of voltage applied and impedance. Let's assume 8 Ohms. So 8 Volts applied yields V^2/R "power". In this case 8 "Watts" peak (less RMS). If this is what you want to achieve "power wise", you design at this point. In this case that means 1 A peak though the speaker. I think you get the point. Sounds like you want a whole lot more power, in which case just ramp it up. Remember that in the balanced amp you can halve the current from each phase and still remain with all devices in conduction continuously.

For a fully "loaded" amplifier, you want to set it up so that it can deliver the desired current. If you want class A, then make it so. You know the voltage and hopefully you know the load. Thus you can optimizie + add som extra headroom. So a 100 V split supply Class A unit driving 8 Ohms needs a standing curren of (work it out!)

Good luck!

Petter
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Old 17th January 2005, 08:52 PM   #263
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Quote:
Originally posted by Petter
For a balanced amplifier, you need half that per phase.
Actually, you need the same per phase.
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Old 17th January 2005, 10:52 PM   #264
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"Regular" amplifier outputs need all the voltage and half the current.
Balanced amplifier outputs need all the current and half the voltage.
A balanced amplifier's sides only see half the load. There is, for want of a better term, a sort of virtual ground in the middle of the load. Since one side of the amplifier only sees half the load, it needs sufficient current to drive it.
Others may think better at night--speaking only for myself, I find that I think better when I've had sleep. Not that it happens very often.
You can "X" any number of topologies. The basic requirement is that it hinge on a differential. Hence my fiddlings with the Aleph-X.
(Petter--more vibrant thread? Harrumph! I hereby move that this thread be pinned to the top of the Pass Labs forum. I never understood why this thread wasn't as...or more...popular than the Aleph-X thread.)
Hint to see if I can jumpstart Petter's thread: Take John Curl's complementary differential (later called the "Diamond Differential" by a mainstream manufacturer) and use it as the basis for an X amp. For those who aren't familiar with the topology, go to www.marklev.com and look up the circuits John designed for the then-young Mark Levinson company. It seems that John has had bad fortune regarding other people taking his ideas and using them for profit without compensating him for his effort. This has led to him being reluctant to share his ideas. With that in mind, please show some respect when using his topologies. At some remote time in the future, he might be persuaded to share some more schematics if people behave in a civilized manner.

Grey
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Old 17th January 2005, 11:26 PM   #265
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Quote:
Originally posted by Nelson Pass


Actually, you need the same per phase.

I won't make a habit of arguing with NP
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Old 17th January 2005, 11:28 PM   #266
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Quote:
Originally posted by GRollins

(Petter--more vibrant thread? Harrumph! I hereby move that this thread be pinned to the top of the Pass Labs forum. I never understood why this thread wasn't as...or more...popular than the Aleph-X thread.)
Grey

Thanks for your kind comment. I will have to go and look at your links.

Petter
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Old 17th January 2005, 11:30 PM   #267
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Quote:
Originally posted by GRollins
I hereby move that this thread be pinned to the top of the Pass Labs forum.
If we get a couple of people to second this proposal then I will.
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Old 18th January 2005, 06:54 AM   #268
MBK is offline MBK  Singapore
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Hey, I second this...

I always thought the regular X-amp was sort of the more elegant concept as compared to single ended class A (balanced or regular). With a variable class A point it can be adapted to all sorts of needs. Not everyone can tolerate 400W+ room heaters As nice as they may sound, there is an impractical side to them...

MBK
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Old 18th January 2005, 08:58 AM   #269
dveckom is offline dveckom  Croatia
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Peter, thanx for schematic.
Why dont you use feedback from output instead from driver. In that case you could have less distorsion, but amplifier may be less stable.
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Old 18th January 2005, 02:40 PM   #270
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Quote:
Originally posted by dveckom

Why dont you use feedback from output instead from driver. In that case you could have less distorsion, but amplifier may be less stable.

As I recall, this is covered in the Pass patent.

Feedback around stages (aka Global feedback in this case) is often not desired in high-end audio. One of the reasons I believe such feedback has gotten a bad reputation is that it is often used to correct a poor design by inflating the gain and culling it with said feedback.

Another reason might be that your reactive load might send junk back into the system.

Still, you may take the feedback where you desire. I imagine control in the lower octaves will improve if you do so to the detriment of the highs.

Petter
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