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Old 7th March 2001, 08:49 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, 09:35 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, 02:17 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, 04:45 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, 11:45 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 11th March 2001, 12:00 AM   #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 11th March 2001, 12:09 AM   #37
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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, 02:44 AM   #38
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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, 05:52 PM   #39
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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, 05:13 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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