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Old 22nd February 2001, 11:00 AM   #11
Jason is offline Jason  Australia
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Please let me know if it is good for my Karma to have this image hosted on this server. If it isn't, it will have to be moved to somewhere else. Thanks.
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Old 22nd February 2001, 12:08 PM   #12
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Well, anyone who was going to exploit this commercially would have the skills and tools to do what Petter has done in any case. So I can't see any problem, I just meant that NP couldn't be expected to help Petter along in his usual generous manner.

I'm sure your Karma is fine, and you aren't risking coming back as an ant or something.

Jake
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Old 22nd February 2001, 02:48 PM   #13
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Default Karma

I am sure your Karma is fine. There really is nothing new in this image other than what is available on the patent (linked to from the Pass site), and schematics freely available on the Pass site.

Reason for 1M resistors is that you need a ground to run a simulation. I wanted to make sure that there was noe effect of the ground, and put 1M down for starters. Should probably be 100K instead.

The other resistor you refer to is the combination of the gate input resistor and the one connected as feedback resistor. The ratio of those determines gain (R40/R39 for each half).

The other resistor in question R43. This resistor if zero implies unimpeded gain. The higher this value, the lower the gain. I cannot remember what the virtual ground resistors you refer to (from patent) were, but I am totally convinced that ground resistors in combination R43 are useless and unnecessary and that the Patent is not totally correct with respect to real life use. You want ground where I have placed it, near input MOSFET gates.
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Old 23rd February 2001, 12:48 AM   #14
Jason is offline Jason  Australia
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Great
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Old 23rd February 2001, 10:04 AM   #15
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Hi again Petter,

Looked again at the patent last night, and I see where my confusion stemmed from, as pointed out it is the shift of the ground from between the source pins to between the gates. Would you mind expanding on the reason for so doing, in what way doesn't the patent reflect real-life (as I believe you said)?

I'm not disputing this by the way, just interested (and I might not understand the explanation for a few years, but I'm still intrigued!).

The values for those resistors in the patent are 300 Ohms (to ground), and 47 Ohm (coupling resistor), by the way.

The rest of the schematic looked (perhaps unsurprisingly) right on the mark - but then, of course my opinion is limited by ignorance. It is actually a remarkably simple circuit, no? One of my mistakes when I looked at this was to presume that the circuit used had a complementary symmetry input as outlined later in the patent, but I see from re-reading the literature that it ain't that involved at all.

Anyway, cheers Petter, there are lots of us rooting for you I suspect.
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Old 23rd February 2001, 11:23 AM   #16
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Thanks for the kind words. If you separate out the input section from the output, you will see that the amp is actually not that complicated at all. Think about it like this: Assume voltage at gate is zero. Assume gate draws no current. Place 1V at one gate. Current flow is 1V/R39 towards gate, right? This current must flow through R40 and makes the voltage at the other end of R40 equal to 0V - R40*this current.

Now to input impedance. i have said that R40/R39 is the limit to how much voltage gain you can have for each half. If the voltage at the gate is zero (reads ground). What then is the input impedance? R39 or 5K per half -- that is 10K in balanced mode. Rings any bells from X specifications???

The patent with 2*300 Ohms to ground + 47 across is equivalent to 47 in paralell with 600 Ohms. between sources and about 175 or so Ohms to ground. The advantage of putting something to reference sources instead of gates is that you don't draw current to ground from signal directly -- all current flows towards gate -- the same is essentially taken care of if you use 100K resistors, and you can accept a common mode error voltage without loading down the input too much. Very nice.

The problem I have with the patent grounding scheme is that all this grounding business kind of messes with the current sources operation -- which I would like to be pure and never managed to simulate successfully anyway. You don't need current to flow to ground (which you would get because the gates are probably 3+ volts higher in potential (and be the same at the output unless you mess with where you take the feedback from). What I believe you want is current flowing in the source resistor.

This section of the patent had me scratching my head ever since I saw it. When I looked at the preamp schematics available on the Passlabs site, it became very clear -- it is not done the patent way at all.

So what I have done is to set the mean voltage at the gate. Thus, the voltage at the sources and output will sort themselves out, and the voltage at the output will be zero which is what we want.

The circuit is simple. It is in fact very similar to certain op-amp circuits (look in patent) and there are many application notes available from op-amp manufacturers on such op-amp connections. The only thing you need to worry about is the balance of current sources -- IS1=IS2+IS3, then everything will fix itself -- if it is precise which is easy to do. In fact, if you were to place the input at +10V average, it would still work without a hitch (I like current sources!). R43 will handle some of the imbalance.

It is interesting that Nelson Pass has progressed from relatively complex circuits to rather simple ones. I own a Stasis 3 which I quite like, but it is a little complex albeit logical.

I will be buying components next week, but don't hold your breath for me to come up with pictures etc. as I am really busy at work + have some hectic travel plans. At some point I guess I should post pictures of how the project is progressing.
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Old 26th February 2001, 09:21 AM   #17
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There should be no major problem with the amp you designed. Since I've just finished building my own headphone amp with similar topology to yours (without the folded cascoding) and I used single-ended follower rather than push-pull), I have a few point to be careful about. Vgs difference between IRF610 can be quite significant ranging from 3.8 - 4.1 Volt at 50 mA. This causes DC offset ebwteen the drain pins of the two half (which will appear across the speaker finally). Not only that, the rising temperature will causes drift of Vgs which is unlikely to be the same rate between the two half. I chose to couple the two stages with capacitor because even 50mV DC offset is quite serious for headphones. In the worst case, the offset used to drift between +/- 200mV. The less-than-exact current source can also cause this offset variation and the current can also be drifted with temperature as well. Just a note.
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Old 26th February 2001, 11:16 AM   #18
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I see from the various owners manuals that the later (and lesser powered) X amps use dual JFET inputs to ease the matching problems. On other hand this project was always going to call for very close matching of the transistors, and presumably that shopuld be carried out at something close to their eventual expected operating temperature.

Re getting the current sources balanced, would it possible to use current mirrors to force equalisation, Petter?
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Old 27th February 2001, 08:49 AM   #19
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Default Thanks Namui

Namui:

I am not sure that mismatching of the input FET's will cause DC at output per se. The current sources and feedback resistors should still handle the DC aspects because they will "automagically" set voltages to what they need to be in order to have correct current flowing (or adjust to the required gate voltage if you will). The gate voltage is tied to input voltage since no current flows through gate.

However, in order to have good AC performance (and achieve some level of noise cancellation which is the point) I was always going to match very tightly. At about 50 cents a piece, and only requiring matching in pairs, this should be no problem.

However, unless current sources are designed properly, mismatch of Vgs can cause them to be inaccurate and this shold have a profound effect on output DC voltage.

If I am wrong, don't sue me.



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Old 27th February 2001, 09:19 AM   #20
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Default Thanks Jakeh

The later and lowered powered X models use JFET's but they also accept unbalanced inputs. The way I read this is that they have an extra gain stage or SE to balanced converter, probably with some gain at input (reads classical long-tailed pair). If you check out my "open source electrostatic speaker paper in my file cabinet page you will see what I am talking about. Basically classical long-tailed pairs.

http://communities.msn.com/StaxOTLDC...thfibrillation

In order to get high current going through the stage (to allow for high current drive at output), JFET's really don't cut it. If they do, you need to do a lot of cascoding etc. to get them to work without blowing up since their power-rating sucks. However, it is possible by cascoding -- but either way you are putting extra semiconductors in the signal path.

So base on all this, I would say that the JFET's are not running in Super Symmetric mode -- because they would then not be able to handle single ended inputs. Because there is some gain at this extra input stage, you are not quite so sensitive to matching, but still need to match up pairs.

As I recall, in the commercial units, the input devices are connected to the same heatsinks to maintain the same temperature for both.

Curren mirrors: Good idea. The critical matching is for top source to equal sum of bottom sources for each half. That they also match between halves is not critcal, but probably smart. Now the problem of having current sources matched at different voltage levels can be solved with mirror's I suppose but I would have to look at it and probably look up Horowitz & Hill's Art of Electronics. I think I have seen other methods of creating tracking sources as well in there.

Now, I had planned to use very high levels of degradation, possibly even in addition to physical parameter methods(reads BJT's on the control side) and then trim out any errors. Since there is not much power being lost in this segment, I have significantly increased the supply voltages of the input stage to allow for the simple method. Op-amps and precision resistors should be able to do the same, and the SuSy resistor helps a little too. One final method to do it is to use precision current sources from companies like Burr Brown (now TI) or Analog Devices to set up currents. You will find white papers on doing this kind of thing on their pages http://www.analog.com and http://www.burr-brown.com

I am beginning to think we are getting a little paranoid about this. However, I am thinking about putting a resistor between output terminals of say 50-500 Ohms to set up another current path, at least for testing purposes.

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