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Old 6th January 2002, 11:17 PM   #1
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Default About replicating a Pass X series amplifier


From the backengineering thread I got interested in building an X series amplifier myself. Someone seems like doing it already, and Mr. Pass said couple of times he has no objections.

I have some practical questions concerning the details of the construction process, the X circuit, it's performance and inner workings. This is why I founded this thread. While planning this and the amplifiers in sauna some time ago I found lots more to say, but below is what I still remember.

1. I wonder how far do the X amplifiers work in class A, and how is it related to quiescent dissipation. In A40 and ordinary push-pull amplifiers I know that for every 40W of class A output one must dissipate 100W heat. As both sides of the end stage conduct current, the amplifier is in class A, and if one side ceases conducting, then the amplifier is no more in class A. So one must use bias that is half of the maximum output current.

I get this intentionally wrong here, but don't know exactly where, so please tell me. So... the X circuit is internally bridged. X150 dissipates 100W per channel. 50V output swing is 25V to one direction, that is like operating from 70V or +/-35V supplies (just a guess). That gives me about 1.4A of current per channel, divided between 2 bridge halves, 0.7A in each. That would mean class A operation up to 1.4A output current, which is about 15W peak into 8 Ohms. Not even close to stated 50W in class A.

2. How close must the end-stage FETs be matched? If I buy a batch of 50 IRF9240 and use the 12 of them that are closest to each other, do you think that'll do it? What kind of measuring equipment will I need? Is a homemade circuit with a cheap digital voltmeter good enough?

3. If I happen to win a million in lottery, then how much do you think would it cost to get one X350 to Finland? Of course all X series models deserve my advocacy, in case someone else here gets wealthy.

4. I hope it will not be a problem if I use much smaller transformers than NP uses in the originals. If I did a copy of X150, I'd use one 540VA 2x24V transformer for end stages and one with higher voltage output (maybe 2x38V) for the drive stages. The drive stage would also get good regulation and/or a capacitance multiplier.

5. How about replacing all the current sources with resistors? Multiple (maybe nine) 1/2W 1% metal film resistors made to look like one, to get less thermal noise. I have some reservations about the effect a constant current source has to the sound.

6. Power supply capacitors. One 100V 10 000uF from positive rail to negative rail equals two a) 5000uF, b) 10 000uF or c) 20 000uF caps from the rails to ground?

7. The virtual grounds next to the inputs are connected to each other and to nowhere else? Maybe to the input cable ground?

More later. If you want to tell me something, but at the same time keep it out of public domain, I welcome emails to musher@mbnet.fi

-Kimmo Sundqvist
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Old 7th January 2002, 12:20 AM   #2
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OK here goes,

X-series is push-pull and is inherently bridged so ...

X-150 idle dissipation is 200W which is 100W per channel which is 50W per terminal which is 25W per mosfet bank.

Ok now the output swing is 50V which is about +/- 28V

25/28~0.9Amps bias per mosfet bank.

now lets calculate the class A point into 8ohms.

1.8x1.8x4=12.96W peak / terminal which is 6.48Wrms
12.96x2=25.92W/channel which is 12.96Wrms / channel.
25.92x2=51.84W total.

Therefore the 50W Class A must be 25W peak Class A per channel. Either that or Nelson's done something I dont know about.... (wouldnt surprise me if he has )

You'll need big transformers as Nelson Pass uses because it has a high bias point.
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Old 7th January 2002, 12:41 AM   #3
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5. Yes constant current sources can be replaced by resistors but the idle dissipation of the circuit will increase and the performance can actually be worse depending on the situation.

6. 10000uF between +ve and -ve will look like 20000uF between +ve and ground and 20000uF between -ve and ground

7. the virtual grounds are definately not connected to the input cable ground because the other end of the cable will be @ true ground which will be different.
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Old 7th January 2002, 01:08 AM   #4
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1) There's nothing inherent in the X/Supersymmetry circuit that means that you have to hit the same bias point that Nelson uses. All the "special" stuff that happens is in the front end. You can use any back end or operating class you like. Feel free to bias it for class A, AB, or B if it makes you happy. I wouldn't recommend going so far as class C.
2) The Aleph devices were specified as being matched to within .1V. Although closer is better, that would be a good goal for output devices for an X.
3) There are Pass dealers listed in several northern European countries according to the Pass Labs website.
4) A 24-0-24VAC transformer would give you roughly +-30V rails. That should get the job done. The VA rating will depend on how hard you intend to bias the output (see #1). There are several options for the front end rails, including voltage doublers, separate transformers, tagging smaller transformers onto each end of the main transformer, etc.
5) Although I have expressed reservations about current sources from time to time, they do have their uses. Depending on how you assemble the front end, and whether you intend to use unbalanced or balanced inputs, you may need to use them.
6) J=.5*C*V**2, where
J= Joules of storage
C= capacitance (in Farads)
V= volts
The V squared part is where the kick comes in. Note that if you use one cap across both rails, you'll need to create a ground one way or another. Never met a circuit yet that didn't ground <i>somewhere</i>.
7) Ah...the mystery of the grounds. Actually, you can do all sorts of things with the grounds. It's not important to the X front end.

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Old 7th January 2002, 08:56 PM   #5
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1. This is not important, you can configure anything you want and even have a low/high switch if you like.

2. This depends on a number of factors, one of which is the value of your source resistor. Naturally, matched units work better together -- if you want perfect matching, try using single devices ... It is most important to match each 4 sections (top left, TR, BR, BL) internally. Bear in mind that the whole point of the X circuitry is to make it take advantage of cancellations in very similar components. You can find information on what matching results I got out of a population of 60 in the reverse engineering thread (X100 ...)

4. You can use any transformer you like, it is not a very important question. However large transformers seem to have a very positive effect on power amps in general. NP has info on power supplies on his sites (commentary for consumers?).

5. Sorry, this will not work well unless you increase the voltage and value of resistors radically. I have no idea why you would want to do this as there are very few components to make a decent current source. A resistor is only a constant current source if the voltage drop is very large or the change of voltage is low (which it is actually is here, but hey!). The folded cascode still needs to be there, but it is not a cc anyway. If you build a preamp, you might want to experiment with constant current diodes, but the circuit still needs to be trimmed to work well. An alternative to trimming is matching current mirrors. (top to bottom)

6. If the caps are identical, 4 times the capacitance at 1/4 (the impedance when eyeballed from a circuit diagram (this ties well with the stored energy calculation as well). Tip: use 2 caps and see what happens when you put them in parallel instead of series.

However, you would typically use larger caps with lower voltage ratings and so it is impossible to say for a practical case without choosing components first.

7. One place to go looking is at the original patent found at www.passlabs.com. As long as you reference something to some ground, you are OK. I use my own grounding scheme where I reference center tap of transformers to source component ground. Your mileage may vary.

When you make boards, don't make them too small like I did with my first set! Focus on the important stuff (getting the circuit right) and make it as a preamp first. That way you are experimenting at low cost and you can learn from your results with a simpler circuit. It is quicker to build a prototype than to make everything perfect before you start (look who is talking!)

Good luck!
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Old 7th January 2002, 09:59 PM   #6
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Nice of you guys to save me all the labor
of answering this question!
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Old 8th January 2002, 01:41 AM   #7
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Yes i agree it is not important how much bias the X-150 output stage has as you can change this to suit your needs but i believe the original question was trying to understand how Mr. Pass got the 50W Class A figure.
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Old 9th January 2002, 05:59 PM   #8
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The calculation resulting in the 15 watt figure looks
to be the correct one.

Is there a 50 watt figure being quoted by Pass Labs
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Old 9th January 2002, 07:34 PM   #9
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Nelson, there is such a statement on the Pass Labs Products page, next to the link for the X150 product lit. I'd also assumed that it referred to the total peak class A output, rather than the RMS from one channel.
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Old 9th January 2002, 10:00 PM   #10
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I'll get that fixed next week.
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