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Old 22nd May 2005, 11:40 PM   #11
ilimzn is offline ilimzn  Croatia
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Quote:
Originally posted by anatech
Hi ilimzn,
Correct on the 240 / 9240 's for the LV-105. I have easy access to the IRF parts in Canada, my good supplier for Japanese parts closed up shop due to economic reasons. These are from my Adcom stock .

I think my main point was to illustrate the driver circuit in the Luxman units was capable of driving the gates of the IRF parts ( the worst to drive as far as I'm concerned).

-Chris
Well, they are not THAT bad, unless you need to drive more than one pair. But, the IRFPs that the data says are complements, really are not, and that is usually the culprit for their reputation for being worse sounding (to put it mildly).
For ove 95V rail to rail, the best I have managed is IRFP9240 for the P part and IRF 340 for the N part. On average they differ about 7% on gm, and are pretty well matched capacitance-wise. In contrast, IRFP240 and 9240 differ by as much as 50% on gm, hardly what one would call complementary.
My reasons for using them were originally that I could get 5 IRFP parts for one 2SJ/2SK. Since then, having invested a lot of time into the complementarity issue, I have concluded that the myth of their inferior sound is just that - a myth. Yet, people still take IRFPxxx and IRFP9xxx to be complements if the xxx is the same number. The only thing they are complements on is the maximum Vds, and nothing much more than that. Once you get past it, they reveal themselves to be unbeatable price/performance wise, and not just because their price is low.
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Old 23rd May 2005, 12:15 AM   #12
anatech is offline anatech  Canada
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Hi ilimzn,
I'll believe that 100%. Even the International Rectifier shows them as complimentary pairs. Adcom used them that way (and many others). This is why I have them, for warranty replacement. I'll have to try your suggestion - thanks for the info.

My reason for calling them tough to drive properly is their gate drive characteristic. The gate abruptly changes charge rates as you increase the gate source voltage. The standard TO-92 type transistor, or worse, a vacuum tube will not properly drive these. You need a lower impedance drive circuit and more feedback to compensate. The lateral fets have a much more linear gate characteristic. That means you can make a design mistake (low drive current) and it will not sound horrible.

It would be interesting to swap the fets in a Counterpoint SA-100 and see how it sounds. It just has to sound way better.

-Chris
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Old 23rd May 2005, 03:15 PM   #13
ilimzn is offline ilimzn  Croatia
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Chris,

The situation is not that bad because it largely happens only under speciffic circumstances. In particular:
- Vgs close to Vgs@Idmax for the part - typically 5-6V on VMOS.
- Low Vds - typically below 10V for VMOS, this is when capacitance nonlinearity becomes very evident
- Common source connection

What this sums up as is, Vd ~~ Vg, in a typical amplifier this only happens when the output voltage and output current are close to maximum simultaneously. That being said, LMOS has a different gate geometry, but it still suffers from the same basic problem, just less - and not even a whole lot less. In fact, for rail voltages below +-45V, one can often use one pair of IRFP VMOS instead of two pairs of LMOS, at which point LMOS loses it's advantage on better gate charge characteristics. On higher rail voltages, in domestic use it is less likely you will encounter operating conditions as above, when gate charge problems become evident on VMOS (and LMOS, just less intensive).

The above is one good argument or DC coupling the tube stage to the MOS output (though it does generate a different set of problems to deal with downstream). Unless very large coupling caps are used (and this can be a problem becvause these often need to be high voltage), the gate charge step tends to reflect on the voltage of the coupling caps, in particular, for small coupling caps, you get a 'rectification' effectsimilar to what happens when the driving voltage on the SS side is clamped. For a while (which the amp needs to spend amplifying signals well below clipping), the bias current is suboptimal, until the coupling caps recharge. The gate charge step generates a less pronounced version of the effect well before clipping, and also produces dual 'notch' type distoprtion, which means high harmonic content (4th and over) - compounding the possible low bias problem.

As you say, the only real way around it is having a lot more drive current than raw gate capacitance figures suggest, by lowering gate capacitances and gate charge storage effects, and/or using larger tubes or an extra SS driver stage for the MOSFETs.
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Old 23rd May 2005, 03:40 PM   #14
anatech is offline anatech  Canada
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Hi ilimzn,

I will agree and add that most mosfet output stages are common source. On top of this, most designs deliver inadequate drive current to the gates. I've often said that you need a bipolar power output stage to properly drive gate charge in the vertical type fets. This is to avoid the double notch type distortion. Obviously you can use a fet driver as well but I am just making a point. The "power" stage I refer to is enough to dissipate the standing power from the quiescent current required.

An intermediate stage between the coupling cap in a tube stage and the output fet stage would solve those problems and bring a higher degree of safety to the design. The aim of the driver would be to present a more constant impedance to the VAS stage and thus linearize the output stage. I did this with a bipolar output with very positive results (driven by a tube stage).

I like using Counterpoint as a model because it falls short in many areas and illustrates well the problems with this topology. It's also an easy design to understand for those who are learning.

-Chris
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Old 28th October 2005, 08:48 PM   #15
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Send a message via MSN to 16vRocket
Sorry for hi-jacking this thread, but I cant believe I've found some people that know about the LV-105. I've been trying for the past 8 or 9 years to get my fathers LV-105 repaired as I damaged it while playing my music a little loud, but have never been able to find a place that will fix it. The amplifier comes on and the tubes light up and what not but it's supposed to "click" when it's ready, but it never does. MOSFET's possibly?? I brought it to a place to have it looked at when it first went, and they said it was some overload chip that died and they could not get parts. Does anyone know of a place that will service LUX or know how to fix it?? Thanks, and look forward to hearing from you.
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Old 28th October 2005, 09:08 PM   #16
anatech is offline anatech  Canada
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Hi 16vRocket,
PM me. You are reasonably close to me. I have the manual and used to service them under warranty. bhome at sympatico dot ca

I'm in Georgetown

-Chris
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Old 21st October 2006, 05:50 PM   #17
Euge555 is offline Euge555  Ukraine
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Default Manuals for LV-105

Dear Chris!

Send me, please, any information (manuals) for lv-105.
Eugene< Ukraine ea@ipnet.kiev.ua
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Old 21st October 2006, 09:02 PM   #18
anatech is offline anatech  Canada
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Hi Euge555,
They are on paper, not scanned. I'll have a look around ....

-Chris
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Old 22nd October 2006, 12:14 AM   #19
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Hi all.
On this link you will find a service manual for the LV-105U

http://fileshare.eshop.bg/downloadsm...n_LV-105U.html

You have to download 0,1 and 2 because its a multipart winrar file and otherwise its asking for the other parts.

JP
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Old 22nd October 2006, 04:46 PM   #20
Euge555 is offline Euge555  Ukraine
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Default Manuals

Dear friends.

I need a manuals for amplifier LV-105 (not 105U).
Also I need a servise manuals for CD-player LUXMAN (tube) D-105U.
If somebody has this documents, please, send me.
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