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Old 27th November 2010, 04:18 PM   #711
qusp is offline qusp  Australia
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thanks owen, yeah hard to say how many i'll get, but indeed many of the guys with me have just sprung for 0.01% zfoils so best get them as close as I can, but thanks thats encouraging.

i've done up the excel file for gm match from VDS so just need to work out how i'm going to label them, maybe little sticker spots or something. thats pretty much what I said to the guys too, that pairs was more important than quads, i'm sure there'll be a few quads though.

bought some larger pin sockets too for connecting up the DMMs to, so all set for when everything gets here
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Old 27th November 2010, 05:46 PM   #712
opc is offline opc  Canada
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That spreadsheet looks great! Just add a column for the FET number, and one for the Vgs DC measurement.

You'll want to measure the DC voltage on the centre leg of the pot to ground once you have exactly 24VDC across the 200R resistor. You might get quite a bit of variance in this value, but it's nice to try and match it along with transconductance. Just add a column and measure it before measuring the other two values. You have a large enough number that you can probably get at least a few groups of four that match exactly in both TC and Vgs. A group of four like that would be worth it's weight in gold, since you can only get matching that close with a very large supply of fets. If it were me, I'd be keeping those for myself

It's probably also worth mentioning that there are a few threads on here regarding some PC connected matching boards. They actually measure each device and give you a curve for each at your selected operating conditions. This would be the ultimate since it would save a lot of time and allow near perfect matching. I don't know how far along they are though:

DIY MOSFET tester

For identification of fets, I used little sticky paper circles about 3/8" in diameter. They come in different colours, so you could just do something like blue 1-99 and red 1-71 and stick one to the front of each fet.

Cheers,
Owen

Last edited by opc; 27th November 2010 at 05:57 PM.
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Old 28th November 2010, 01:15 AM   #713
qusp is offline qusp  Australia
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thanks, yeah OK will do that.

I came across that project some time ago and they were still stuttering along, seems they are getting there, but now Patrick has got a hold of it, its likely to be a very excellent, but also very expensive device made with somewhat difficult to procure sense resistors and sand, in a fully CNC enclosure not really a direction I want to get side tracked with at the moment.

I dont plan to make a business out of making amps, or matching devices, so just knocking up a test circuit with adjustable power supply, socketed devices and source resistors so I can test and match jfets as well, will do for my purposes for the moment. if it turns out to be cheaper than I think, or there is a PCB on the offing i'll rethink that. at the moment there is only a couple of options that will take the current we want for testing power devices for signal use.

yep sticker dots, a large benchtop and a marker
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Old 28th November 2010, 01:35 PM   #714
qusp is offline qusp  Australia
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oh and yeah this

Quote:
Originally Posted by opc
snip.../A group of four like that would be worth it's weight in gold, since you can only get matching that close with a very large supply of fets. If it were me, I'd be keeping those for myself
**** I hadnt thought of that...... best match highest gm quads are for myself and Flocchini, you guys really oughta thank this man too for making this all possible. it was just a fleeting idea I had and was hesitating about, until he offered to buy some and send them over. so I will either be thanking him for the angels coming down to my listening room from above and visiting upon me as I weep tears of joy, or cursing his name as I wake sweating in my bed, with nightmares of 5 decimal place LCDs 10 years from now

Last edited by qusp; 28th November 2010 at 01:45 PM.
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Old 28th November 2010, 03:34 PM   #715
NicMac is offline NicMac  Italy
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Default psu electrolytics

opc,
which make/rating of smoothing caps are you using/recommending for the PSU section of the bipolar rails? I cannot see them in the BOM.
I guess a 65V rating is on the limit and I'm therefore inclined to use Pana FC's (100V, 1000uF). I'm not really on a budget so every suggestion is welcome.
Thanks,
Nic
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Old 28th November 2010, 04:23 PM   #716
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crazy question... what's the feasibilty of under-sinking the to-220 regulators (with a fr-4 shim), like the other parts?

edit: other than one possibly hanging out a little bit at one edge..

Last edited by res07njc; 28th November 2010 at 04:34 PM.
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Old 28th November 2010, 05:00 PM   #717
NicMac is offline NicMac  Italy
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Quote:
Originally Posted by res07njc View Post
crazy question... what's the feasibilty of under-sinking the to-220 regulators (with a fr-4 shim), like the other parts?

edit: other than one possibly hanging out a little bit at one edge..
Could you elaborate a bit on this? What are you referring to with respect to the "other parts"?
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Old 28th November 2010, 05:08 PM   #718
qusp is offline qusp  Australia
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I think he means using the same sink under the board as is used for the IV section. I dont think its feasible, not enough room

nicmac, there isnt any rail caps
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Old 28th November 2010, 05:44 PM   #719
qusp is offline qusp  Australia
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well not really, I should clarify that, there is 1800uf before and after the regulator, but remember, these are floating regulators, they are not specified to cope with the voltages we are feeding them, so shorting them to ground is a BIG nono. I havent looked into the onboard supply much, so forgive me if i'm missing something here. aha, you are right, owen hasnt specified a cap for this position in the BOM. C2, C24, C1, C23, C4, C26, C5, C27 are all 1800uf as you have noticed, I really think you can probably get away with 1000-2000 depending on quality, but remember, none of these caps connect to ground, so they do not have to cope with the voltage to ground and pana FC is going to be fine IMO
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Old 28th November 2010, 05:48 PM   #720
qusp is offline qusp  Australia
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wow thats something that really should be GB'd, sorry i've made my order at DK, i'm making a PCX order right now though or at least in the next 24hrs, I suppose everyone is going to have a favorite here, so maybe not the best thing to GB, what you guys need tpo know though is the pattern for the placement, who has the board and can tell us what the lead spacing is?

sorry Owen, I really should have picked that up
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