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Old 19th November 2010, 12:02 AM   #671
opc is offline opc  Canada
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Hi Guys,

Whew... what a week. Pretty much everything that can go out, has gone out. I've updated the list yet again with ship dates, and there are only a few more boards left to ship.

I want to extend a big thanks to everyone for their patience... I know this all took much longer than planned. I also want to thank those on the waiting list for stepping up and taking the boards for those who dropped out. From the looks of it so far, I don't think I'll be left with any unplanned extras, which is a big weight off my shoulders.

I have sent emails out to richluvsound and TheShaman to see if they'll take the last two extras, and if so, then every board will have been sold.

Check out the list, and let me know if there are any issues.

Cheers,
Owen
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Old 19th November 2010, 02:12 AM   #672
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Hi Guys,

I decided to post the transconductance measurement method on here so everyone could use it if need be.

I've attached a drawing that shows the circuit used, and the calculation.

It's not exactly quick, but a good power supply (one that doesn't fluctuate),a good signal generator and three DMM's will make things much easier.

You'll need the following to do this correctly:

1 Lab PSU set to 45VDC
1 Breadboard
1 DMM (two is better, three would be best)
1 Signal Generator set to 1kHz

Hook up the circuit as shown, and hook one DMM across the 200R, one across gate to source, and one from the wiper on the pot to ground. If you only have one DMM, you'll have to keep moving it around.

Set the DC operating point for the fet by disconnecting the signal generator and adjusting VR1 to get 24VDC across R1. At this point, you'll have 120mA Id and 21VDS which is very close to what the fet will see in the actual circuit.

Let it stabilize and adjust as needed to get as close as possible to 21VDC across R1. Measure the voltage at the wiper of the pot, and write it down. This will be your measurement for VDS matching. If you adjust each fet to the same 21VDC across R1, then you know that VDS will be the same, and the DC voltage on the gate (measured from the wiper on the pot and NOT the actual gate) will be the voltage value that gives you the set VDS for that fet. If two fets have the same voltage on the gate to get same VDS, then they are matched.

Once you have the VDS matching done, connect the signal generator through an AC coupling cap and adjust the output to get 3VRMS across R1. Once you have 3VRMS, measure the AC voltage from gate to source (VGS) and write it down.

If you're doing a lot of fets, you'll want a way to number each fet , and then make an Excel spreadsheet that will calculate Gm from the numbers you measure, namely VGS and VR1. You'll also want a quick way to mount each fet to a heatsink. I used a screw and a spring clip with a hunk of aluminum. The 200R resistor will also need a heatsink, and you'll probably want to mount everything on a breadboard to make it easy to swap fets in and out.

With a large enough number of fets, it will be easy to find pairs that have very close VDS values as well as Gm. It took me 2 hours to measure 14 devices and I found four that were very closely matched. I was using 3 Fluke DMM's and all the right lab equipment, which really helped to speed up the process.

As you can see, this whole process is VERY time consuming, and a serious PITA. All these values a temperature dependent, so each fet needs time to reach equilibrium. Please reward qusp accordingly for the time he puts into this. If I were doing it, I'd be charging a pretty penny for each matched pair.

Let me know if there are any questions or issues.

Cheers,
Owen
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Old 19th November 2010, 10:26 AM   #673
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I don't have the AmpOhm has been sold in the swap meet.
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Old 19th November 2010, 03:18 PM   #674
qusp is offline qusp  Australia
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hmm I dont have an actual tone generator, but I have lots of pro audio software and a RME pro audio card capable of 32/192khz balanced output. shouldnt be hard to produce a 1khz square wave (assuming it should be square) then can use a simple opamp circuit to allow amplification to give 3vac. sound like a plan OPC? or should I search for a used old tone generator?
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Old 19th November 2010, 03:25 PM   #675
qusp is offline qusp  Australia
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Quote:
Originally Posted by merlin el mago View Post
@qusp
Legato is a good contender, standard kit with no mods & no new parts sounds very good with B II....far better than IVY
yeah I thought about trying it out just for kicks, but I would end up blowing too much on nice caps for it, plus there are better options for IV of this type IMO, just build the borbely IV/line amp from the audio express artical and acko's upcoming jfet IV looks great, if I were pursuing the buff for home I would probably give it a try, but I think this here circuit we are discussing is superior anyway.

Quote:
Originally Posted by TheShaman View Post
Merlin let us know how it sounds with various "boutique" caps you might try. I was thinking AmpOhm.
yeah the ampohm copper and polypropylene is quite nice. best medium budget combo would be a large polypropylene bypassed with small teflon. thats what i'll be doing, havent fully decided on the large cap, but bypasses will be vcap CuTF copper teflon. already have 2, just need another couple. my amp has 1mo inputZ, so I can probably forgo the large cap altoghether, but I want to maintain compatibility with variety of amps
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Old 19th November 2010, 04:00 PM   #676
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I'm thinking about mounting caps and resistors (the one after the caps in the circuit) on the XLR so that I can change and compare the different IV boards.

May try the .1 Russian teflons as a bypass.

Thoughts

Thanks
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Old 19th November 2010, 04:55 PM   #677
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Quote:
Originally Posted by flocchini View Post
I'm thinking about mounting caps and resistors (the one after the caps in the circuit) on the XLR so that I can change and compare the different IV boards.

May try the .1 Russian teflons as a bypass.

Thoughts

Thanks
FT-3 is the better but are bigs.
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Old 19th November 2010, 07:26 PM   #678
qusp is offline qusp  Australia
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mistake

Last edited by qusp; 19th November 2010 at 07:42 PM.
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Old 19th November 2010, 07:50 PM   #679
qusp is offline qusp  Australia
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i'm also getting some of the red keratherm isolators from Zhoufang, I think they will work well for this build.
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Old 20th November 2010, 04:40 AM   #680
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I have a few FT3s at .1 and .22 and you are right merlin they are huge.
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