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Old 19th February 2001, 10:57 PM   #1
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Thumbs up X100 backengineered

Click the image to open in full size.

I am very interested in feedback on this design (which is based on patents held by http://www.passlabs.com and thus not available for anything but research). To scale power, simply change rail voltages (and increase value of feedback resistor to get enough front-end voltage gain to drive outputs to saturation). To change level of Class A drive, increase bias resistors. To improve "supersymmetry" and reduce gain, increase value of resistor between bottom of input FET's.

The 1M resistors are there for simulation purposes and would ordinarily be smaller. And yes, I simulated this design using a Spice CAD package.

We are probably looking at something very close to a commercial X-series amp (without embellishment). If you like feedback, place the feedback loop around the output transistors.


[Edited by Petter on 02-21-2001 at 07:52 AM]
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Old 20th February 2001, 12:22 AM   #2
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Can't see it.
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Old 20th February 2001, 02:50 PM   #3
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yeah the image doesnt come out.
too bad.
can you send it be email maybe.
ilkam@online.de
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Old 20th February 2001, 03:24 PM   #4
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Hi Petter
Sorry, but can't see it - Send it per email - thanks.
jens
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Old 20th February 2001, 04:01 PM   #5
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Default x100

HI
Please send me the GIF on my email
janey2k@email.si

Thanks
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Old 20th February 2001, 04:16 PM   #6
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Unhappy No Show

Tried MSN too, but still the gif won't come up. I have a MSN account to log on. The page comes up blank.

I'd like to see this. Email it to me.

vincenet3@juno.com


Vince
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Old 20th February 2001, 11:14 PM   #7
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OK, I saw it this time. It's interesting. So, the PSPICE software told you this would work?? Are you going to attempt it Petter, or are there things that need to be worked out yet?
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Old 21st February 2001, 07:46 AM   #8
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Hello all,

I too had problems seeing the picture. I signed in using my ordinary hotmail-account but still got up a blank page. When I joined the community (note: "Stax OTL DC coupled amp design (fun with fibrillation)", created a new member) it all started working, et voilą, there it was.

Petter: I'll look at it later, but I guess there are people on this forum that can give you more intelligent input than I can since I'm not an EE.

/Matti



[Edited by Matti on 02-21-2001 at 02:48 AM]
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Old 21st February 2001, 01:15 PM   #9
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Default Got the image up, and some answers

Finally managed to get the image to show (i.e got the file hosted on this server).

I simulated using Circuitmaker which is a great piece of software -- so easy to use (and they do have a demo version available at http://www.microcode.com). The frequency response, I eyeballed to 30KHz or so -- it would be higher with fewer output transistors (number of output devices is somewhat arbitrarily chosen -- that is I did some quick calculations but no deep analysis, and to allow for future expansion if I need more power), or feedback placed at speaker terminals. A particularly cool feature about this design is that it scales so well -- hardly any changes other than PSU voltages are required for different power output level. Also note that output transistors need only be mathed in quad's when you cut to the bone.

The key to this design is accuracy of current sources. I strongly believe they should be FET type sources since any gate current will mess up the balance between halves. I am probably being a bit paranoid about this "base current" and the accuracy of current sources, but I really don't want an amp that is going to need adjustment other than for quiescent curren in output stage. I have two basic designs ready for this, one relying on significant voltage drop (degradation), the other on active device physical parameters. Op-amp based versions would work, but I don't like them very much.

What is missing is ajustment for quiescent current (now shown as fixed resistors), gate protection for transistors (zeners) and schematics for current sources. Note that the design requires balanced input signal like the original. It is very easy to make an unbalanced to balanced input stage and improve performance (frequency response etc.). What is also missing is compensation to control phase margin at very high frequencies.

I am indeed planning to build this design, but will tinker with voltage levels, number of transistors and resistor values, particularly the one between input transistors, as well as feedback resistors (gain selection). As I will be needing a single-ended to balanced front-end anyway, I will probably change these values to reflect the gain I can get out of this input stage as well.

As a concequence, I am very interested in feedback whch you can post here or send to me by mail.

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Old 22nd February 2001, 10:08 AM   #10
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As a non-EE who briefly thought of doing something very similar, but soon realised it was beyond me, I'm very impressed!

Have you tried the values for the 'virtual ground' resistors cited in the patent, and was there a particular reason for using 1M/5k for the purposes of the simulation?

Don't know whether NP will be giving his usual support to this one!
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Old 22nd February 2001, 11:00 AM   #11
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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
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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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Old 27th February 2001, 09:25 AM   #21
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And of course any old regulator + resistor can do the same thing with great precision and probably not all that good sound. I have seen some pretty good info on this on http://www.tubecad.com.
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Old 3rd March 2001, 06:48 PM   #22
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Default Buy it from Allied Electronics

These are the approximate prices I obtained from Allied. They are not exact, but I am posting early to enable others to get good prices as well.

IRFP240 -- $2.5
IRFP9240 (these are hard to get from IRF right now) $3.5
IRF610 -- $.5
IRF9610 -- $.65

Method of getting these prices:
1. Go to http://www.irf.com
2. Search for any part number such as IRFP240
3. Check out all the supplier links
4. If one co. has high prices on one part, you can usually haggle the offending part down to the normal level.

In my case, it could have been even cheaper, but I needed the parts now and had to go to those who had all parts in stock.

Now I have 50 of each type of output device and 30 of each type of input device. This should enable totally tight matching.
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Old 4th March 2001, 01:53 AM   #23
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I watch this with enormous interest ;-)

Petter, would you be able to e-mail me the Circuitmaker file for your X100. Neat toy and quite instructive for the non-EE types like me. I could draw it all again, but figured you probably wouldn't mind.

Thanks in advance. Mark
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Old 4th March 2001, 07:03 PM   #24
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Default Cirkuit maker file in my MSN site.

Go to the file area.
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Old 4th March 2001, 07:23 PM   #25
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Petter all this is incredible.
Very cool.
I got by the way a demo cd from circuit maker 2000.
It was a 30 day promo and I did something on the computer and now it says date expired.
This has nothing to do with this thread here, but can any body help with this.
There is some file saved somewhere and i can“t make the demo cd work anymore.
Anybody know a crack or something?
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Old 4th March 2001, 07:50 PM   #26
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Default Change date backwards does not work?????

Install your machine from scratch :-(

Borrow another disk and do it there

Install a dual boot should work

There is a Student demo which should work for this small circuit available for download as I recall without expiry.

Probably many other good fixes.

Good luck
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Old 5th March 2001, 12:37 AM   #27
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A very simple hack that often works (and doesn't involve decompiling the code, with all the attendant hassles & legal questions) is to simply set the date on your computer back a year or whatever.
Just remember to set it back to the present date when you're done.
Jason: Why can I sometimes see other posts...and sometimes not? Right now I can't, for instance.

Grey
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Old 5th March 2001, 12:45 AM   #28
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Actually changing the date doesn't work. CM specifically tells you this when you download the demo.

I'm still in the 30d trial period, so don't know if I will get around this, but I suspect they write to the registry on install, then each day simply decrement some counter (IF today <> yesterday THEN decrement turns_left). This is likely "invisible", ie hidden deep in the registry somewhere where it is not obvious = unlikely to be under a "CircuitMaker" key.

The student version is limited to 30 devices which eliminated all but the really simple circuits.

mark
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Old 5th March 2001, 03:05 AM   #29
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Bummer about the date trick. I no longer have the time to fiddle with reverse-engineering the code. My position these days is just not to mess with crippleware unless I absolutely have to. I don't suppose you could export it to the student version of Spice, could you? (Or some other version of emulator...)

Grey
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Old 5th March 2001, 11:09 AM   #30
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Well thanks to everybody. I just downloaded the students version of circuit maker and see what I can do with that. Petter good luck with your attempt to make this amp work. I think everybody is waiting for more on this.
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Old 7th March 2001, 07:49 PM   #31
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Default Notes on Thermal Washers

I have researched the field of Thermal Washers for some time now, and made some conclusions which I would like to share with you. I have also ordered the units.

The best thermal performance is from Beryllium Oxide washers. These are toxic when ground up, but should be safe if handled carefully. They appear to only be available in TO3 form factor. I would have got mine from an Aavid distributor such as Future Electronics, but am running TO-247 transistors and so these are out. Just as well perhaps as I understand they are illegal in Europe due to the toxicity of Beryllium dust (when inhaled causes Berylliosis similar to Astbestosis -- funny really that materials so totally opposite in thermal performance have the same characteristics to man).

BeO units are brittle, but since they are thick and non-conductive, they offer improved capacitance effects and should therefore increase high frequency response. They need thermal grease to mount.


The second best alternative (and there really is no need to look furher) is Bergqvist Silpad 2000 which is intended for High performance aerospace/Military/Hi-Rel projects. This is a very high performance film which does not require thermal grease, and so it should be clean to work with (usually people use too muc grease resulting in suboptimal performance anyway). It is also rated to +200C which is useful. Thermal performance really is very very good. The downside is that the film is thin, which will increase the capacitance of the devices used and impede HF response. Having said that, I still went for the thinnest unit since the conductivity still is way off BeO.

I ordered my units from http://www.bergqvistcompany.com and I got part number suffices -122 for my TO-247 units and -58 (-62 was out of stock) for my TO-220 devices. Having a high conductivity film for the low power input devices as well should further assist in keeping junctions at same temperature when hooked up to the same heatsink -- or possibly just to one another wit one piece of film between them.

You may want to order from a distributor as Bergqvist has a $100 per line order minimum. I ended up at $102 for 120 units of -122 and was sampled 15 units of the other type free of charge.
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Old 7th March 2001, 08:35 PM   #32
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Default Line filter -- best value in town!

I have researched the subject of line filters for some time now, and come to the conclusion that it is very very hard to do better than what you can get from commercial vendors.

There are two types available. Each are oftimized for the type of impedance that each side of where you position the filter is placed. I suspect what we really want from audio is a filter that absorbs what comes from the inside (low impedance to HF) and blocks out from the outside, but for digital polluters we want to block what is generated inside.

The best units cost a lot of money but offer differential mode and common mode attenuation of 60-80dB from 10KHz up. These cost about $60 a piece and are made by http://www.corcom.com -- typically their Q series.

The next best thing is Corcom 3SP1 now out of production similar to 3EP1 (http://www.corcom.com/catalog/filters/EP/Default.htm) with 50dB diff mode and 36dB common mode attenuation from 150KHz ... which is available for $3.25 from http://www.meci.com part number 560-0117. This is a three amp unit which may be a problem in the US, but they are conservatively rated and I should be able to get .75KW out of mine with 240 volt line (divide by 4 for US). The only problem with these is that it is not a power entry module and that it is physically a little large. However if you put one inside your CD player, I guarantee you will hear a difference -- biggest upgrade I ever did, and I used an el-cheapo from an old computer.

So I put my money where my mouth is and bough 10 ...

Mount it, test it and turn it backwards -- I suspect amps will sound best with it backwards and that the system will sound best when digital units are connected normally -- keeping noise from digital polluters way from the line.

[Edited by Petter on 03-08-2001 at 05:02 AM]
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Old 10th March 2001, 01:17 AM   #33
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Petter & Others,

An Aussie amplifier ( http://www.aussieamplifiers.com ) builder ran across my site and we began corresponding re the X_Series. He sent me his version from the patent, which with some minor alteration now looks like this:

http://www.adelaide.net.au/~mefinnis..._series_v3.gif

This is a lot closer to Fig.3 from the patent form and follows the normal grounding arrangement. Runs as advertised through Circuitmaker, for those having this the file is at:

http://www.adelaide.net.au/~mefinnis...series_np3.ckt

Obviously for the real thing you would parallel output devices etc.

Comments !! mark

[Edited by mefinnis on 03-09-2001 at 08:20 PM]
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Old 10th March 2001, 03:45 PM   #34
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Default Anthony's site

Mefinnis,

Excellent stuff!

I am familiar with Anthony's site and used to correspond with him about on the X-series about a year or two ago. Recommended site, nice guy! I hope to start building soon and am getting some help from a local guy who has the practical expertise I am lacking.

Comments:

1. You should seriously consider moving the feedback back to the input stage -- especially if you are expecting current coming back from the amp (I know I am)
2. You should also consider putting some degradation resistors on output stage devices.
3. Interesting voltage source to bias output devices. I suspect though that a variant using physical parameters (Vbe) of BJT's or even fixed bias might be even more effective.
4. Regarding Super Symmetric resistor, I can personally see no reason why we need grounding near it. I would still prefer it at the gates, esp. when considering interfacing to non-floating input devices.
5. You currently have very low voltage gain -- are you expecting a separate pre-input stage or do you have enough headroom from your source?
6. 3A Quiescent current seems a little high -- that's 150W dissipation per device.
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Old 10th March 2001, 10:45 PM   #35
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Petter,

I'll do the easy stuff first.

6&2. 150W/device? No, as stated for the actual amp I would parallel devices. I've put up a pic of what I would consider closer to production, rather than concept.

http://www.adelaide.net.au/~mefinnis...series_v3p.gif

OK, that dealt with .....

1. Feedback position? Please remember I am not an EE, so my theory is not what it could be. This is where NP placed it in the patent, and where he places it for his SE designs, ie from the output? What do you consider the advantages of making FB local to the input rather than global?

4. Re the SS resistor. NP stated in the patent that loss to ground at this point was required to prevent infinite iteration, the "hall of mirrors effect". I'm not sure placing this at the gates has the same effect. Does it?

5. Low gain ..... fair comment.

3. Voltage source = Anthony's (hell man, I'm not that clever!!)

regards, mark
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Old 10th March 2001, 11:00 PM   #36
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I took a look at design posted by mefinnis, and have a couple of comments.

First off, the mosfet output bias is a fairly commmon circuit, same one as used by Pass on the A75.

I'm not sure I understand Petters comment about a BiPolar devices Vbe being a physical parameter. The Vgs of the MOSFET, which this circuit uses as a reference, is also a physical parameter. Certainly there is more device to device variation than Vbe, but I would suspect that in a real world one of the resistors would be a trim pot.

In both this case, and in Petters schematic, this bias circuit needs to be bypassed with a cap.

Of more concern to me, is that this design appears to suffer the same problem as a similar circuit I'm working on. That is the output has a large DC offset. Now the offset is equal on both sides, so there won't be any current flow through the speaker, but it is going to limit the maximum power. I haven't simulated Petters circuit yet, but does it suffer this problem as well?
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Old 10th March 2001, 11:09 PM   #37
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Location: Adelaide, Australia
Offset is about 2VDC in my simulations.

This is less than I see in my Son of Zen, which sits about 5V above ground.

Yep, this limits output .... any suggestions ;-)
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Old 11th March 2001, 01:44 AM   #38
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Join Date: Mar 2001
Location: Ottawa, Canada
My simulations indicate a higher offset from ground...

Input coupling caps would result in 100% DC feedback, which would lower the offset, but I'd prefer a circuit that naturally zero's itself. I was hoping someone would post a clever solution. Time to pull out a text book or two and don my thinking cap!

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Old 11th March 2001, 04:52 PM   #39
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Join Date: Jan 2001
Location: Scandinavia
Default DC offset and a couple of items

I have a couple of quick points -- will attempt to get back with better answers when I have had time to look at it!

1. DC offset -- You can fix this by twiddling around with potential dividers R36 and R37 on page 1. Of course this is not a problem in that design ... the DC offset you are seeing is essentially the gate drive voltage of the input device.

2. I don't see the need for capacitors across R36/R37 on page 1. These are driven by a constant current source and will thus appear as a voltage source. I guess if one were include it there might not be a significant change, but I don't personally see the need for it. I guess the only way to find out is to try it.



[Edited by Petter on 03-11-2001 at 06:56 PM]
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Old 12th March 2001, 04:13 AM   #40
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Default Offset Drift

Just a note again on offset issue. From my experience with SOZ topology amp, even though you can adjust the offset to 0 VDC, as the temperature change, the offset will drift anyway unless the MOSFETs have identical thermal characteristics. And the drift value can be so different between the time that the amp is just powered on and half a hour later. This is not much of a problem for the follower, but quite substantial in the first stage.
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