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Old 17th January 2002, 01:24 AM   #21
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Musher ...

you'll most probably need to use a voltage doubler circuit on the front end supply.
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Old 17th January 2002, 01:56 AM   #22
Geoff is offline Geoff  United Kingdom
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Why?

Geoff
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Old 17th January 2002, 03:37 AM   #23
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Cause Musher wants something like +/-40V for the ouputs and +/-60V for the front end supply from the same transformer.... the transformer only gives +/-40V or so when rectified and a voltage doubler is a reasonably easy way for him to get the extra voltage for the front end .... sure its noisy but it will probably give about +/-75 which could then be regulated down to a nice quiet +/-60V
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Old 17th January 2002, 12:13 PM   #24
Geoff is offline Geoff  United Kingdom
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Yes, I know what the voltage doubler would be used for. I asked the question because Musher indicated in his original post that he intended to use a separate transformer for the driver stages which would be a much better solution. Grey followed up with some further suggestions, including the voltage doubler, and then Joe Berry gave a fairly comprehensive coverage of the use this arrangement. I was wondering if you had any additional information to contribute or whether you were just reiterating that which had been said four posts earlier.

Geoff
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Old 17th January 2002, 01:18 PM   #25
maik is offline maik  Germany
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Default Re: Then to the problems with front-end

Musher,

using LED for the current sources has some advantages and some disadvantages as well. LED's
produce very less noise compared to zener diodes. On the other hand if you like to use mosfets for
the current sources you will need at least 3 LED's in series to create enough voltage drop for the
current source (4V for the mosfet, 2 volt for the resistor). In that case you have to deal with the
temperature drift of the 3 LED's.

From the manual of the smaller X-series amps like the X250 you can guess which type of input
device Mr. Pass is using. The manual describes it with as a double JFET with 0.02ms transconductance
figure. I guess it is a 2SK389 which is a really nice transistor for a differential pair. It has a
maximum drain/gate voltage of 50V. If you like to use it with +/-60V power supply you need to put
an additional cascode around it. This technique is very well described in the article about
cascoded amplifier design by Mr. Pass.

I do not really see a need to use parallel JFET for the front end of the X-amp design. It depends
on how much current variation you will need to drive your output section. a 2SK389 can be used
up to 10mA. But you should keep the voltage low on them because of the power dissipation.

Regards,

Maik
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Old 17th January 2002, 01:33 PM   #26
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My apologies... i had forgoten that he had intended to use a second transformer ... and yes Geoff i agree that this would indeed be a better solution ....
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Old 17th January 2002, 09:19 PM   #27
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We use 2SK389's and 2SJ109's for the input
differential pairs. They work beautifully, and
are my favorite such parts.
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Old 19th January 2002, 10:35 AM   #28
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What would you think of Semelab MOSFETs in the power stage? BUZ900 and 905? Those are quite expensive, but I think I'd be ready to get some if they are really good, which they much seem to be.

IRF9610 are obtainable here. 2SK389 also.

2SJ109 need to be hunted for a bit. Same with IRF610.

-Kimmo S.
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Old 19th January 2002, 09:29 PM   #29
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Default What is the deal with Finland?

Musher,

I don't understand the deal about those components not being available in Finland. Have you tried ANY of the multinationals such as RS Components, Future Electronics etc?

Now, if these components truly are unavailable in Finland, I suggest you get them from overseas. All these components are available in Norway -- perhaps we are just a much more advanced society than Finland

What is the cost of shipping to Finland when ordering from the US or UK? Surely it cannot be as bad as Norway!

Petter
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Old 28th January 2002, 08:49 PM   #30
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Obtaining transistors and so on...

It costs extra to get transistors by mail from Norway, Sweden or any other place than nearby Helsinki. It also more often than not requires getting a large batch at once. That really matters since I am a student. I am determined to get enough resources to finish the project, but I have no extra cents or percents to lose.

Then to the matching business...

My distant dream is to match within 0.5% as Mr. Pass does. I think that means the Vgs' of the transistors are within 15mV, am I right or not? I think the other possibility is that the Vgs' are within 30mV. I'm very afraid I will have to get 100 for every single matched.

Looking at Petter's schematic, you see one channel has one output section at the left and one at the right. And both sections have an upper and a lower row. Now, should the four rows be matched internally, or upper left also with lower left, or upper left with lower right, lower left with lower right, or every single transistor in the channel with every other?

And how many IRF240/9240 do you think I will have to get if I want to match 6+6 (or 3+3+3+3) within 15mV? How much easier will it get if I make four channels with 6+6? Or one channel with 24+24 (or 12+12+12+12)? International rectifier hasn't answered this yet, and it's been a while since I asked.

Maybe I will end up doing serious business selling matched transistors after all this. Unless Mr. Pass would like to start selling matched sets again. I know how this could help his business and maybe did earlier, but I also guess there are good reasons why he doesn't do it anymore.

-Kimmo Sundqvist
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