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Old 22nd January 2009, 02:42 AM   #691
Salas is offline Salas  Greece
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Very practical board! One reason to be flexible with JFETs is their availability. And is good to have different pin out. Stronger JFETS for gain are always less noise. Measure the resistance between D, S. The less, the better for noise. I just measured a 5mA Idss 2N5459, 240R.
A 7.45mA Idss 2SK170, 35R. Measure the BF245A.
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Old 22nd January 2009, 02:43 AM   #692
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Here's a zip file with the pcb layout and supporting files. I used expresspcb which is free to download, so you'll need that program to print the layout. The zip file needs to be extracted in the folder My Documents\ExpressPCB

There's an interesting feature in expresspcb, after you import a netlist file (generated by ltspice in my case) you can click on any pin in the pcb layout and it highlights all the other pins that should be connected to it. Quite useful for correctness checking.

Any suggestions appreciated.
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File Type: zip sreg10.zip (16.5 KB, 112 views)
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Old 22nd January 2009, 02:47 AM   #693
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Cool, I'll measure and let you know. I was busy posting the rest of the files and didn't see your post. There is a .noise analysis in ltspice, I don't know how good it is, but when run with the bf245a it shows pretty much from 1Hz to 50KHz less noise (around 1nV/Hz1/2), whereas the 2sk170 shows double to triple the noise. In practice... probably un-noticeable. I'll probably do the board tomorrow as it's getting late now, plus, I'll wait see if there's any feedback or changes wanted.
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Old 22nd January 2009, 02:49 AM   #694
Salas is offline Salas  Greece
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Are BF245A easily available? More than 2SK170?
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Old 22nd January 2009, 03:02 AM   #695
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I found bf245a in a local store, at about 35 cents canadian and bought them for a different project. I figured it'd be ok to put them in the regulator (cheap), but I'm waiting for 100 2sk170 and will try both.

According to your rule they kinda suck, I measured the resistance between D and S and it was about 220R

Lately though, I've been thinking of using a double-gate fet in the first stage. If only I could find some cheap, to try.
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Old 22nd January 2009, 03:07 AM   #696
Salas is offline Salas  Greece
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Don't worry, if it is to lock a 1.6k resistor for constant Vdrop, it will be swamped and the 1.6k will dominate. With 3mA Idss and 10R3 it will give 140mA CCS. So the Idss sets the current. Do you get 3mA Idss BFs in the batch easily? As for my rule of thumb, think about it... When you short G,S, for making an Idss CCS, what can be dominating the self noise?...Self resistance.
Also the JFET under the Zener must work with VGS(OFF) less than Vbe of the BC BJT. IDSS JFET sources work well when the margin is more than double pinch off. The less the VGS(OFF) the better and always must stay under 0.7V in our case.
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Old 22nd January 2009, 03:17 AM   #697
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Would this not, then, imply that a power mosfet with really low Rds(on) be a really good choice to use in the CCS?

As for the matching, I haven't tried it yet, as I was positive that I wouldn't use them in the preamp.
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Old 22nd January 2009, 03:17 AM   #698
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Quote:
Originally posted by ikoflexer
Lately though, I've been thinking of using a double-gate fet in the first stage. If only I could find some cheap, to try.
Make a standard one, and then by all means, experiment. We need evolution. If you manage to find a workable better version with easier JFETS, it will be better for all. Only listen also, after you will be very familiar. What is your audio system comprised off?
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Old 22nd January 2009, 03:21 AM   #699
Salas is offline Salas  Greece
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Quote:
Originally posted by ikoflexer
Would this not, then, imply that a power mosfet with really low Rds(on) be a really good choice to use in the CCS?

As for the matching, I haven't tried it yet, as I was positive that I wouldn't use them in the preamp.
To my reasoning yes. That is why I use chunkier Mosfets than will ever be needed power wise, with low enough RDS ON. The bigger ones are just a little slower (cabling and Cout will slow down any faster, enough anyway) but less prone to oscillations with DIYers too.
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Old 22nd January 2009, 03:44 AM   #700
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Quote:
Originally posted by salas


Make a standard one, and then by all means, experiment. We need evolution. If you manage to find a workable better version with easier JFETS, it will be better for all. Only listen also, after you will be very familiar. What is your audio system comprised off?
Yes, for sure I'll first build the cascoded, buffered version with the 2sk170 that you recommended earlier.

As for equipment, I got nothing special. A few TTs, but only use a Dual at the moment, with a Grado green cart. The other TTs are waiting to be modded. Phono stage, a couple of variations on the opamp elcheapo. I was going to do the Gary Pimm tube phono stage next, but came across you jfet wonder and couldn't pass, so the tube one will have to wait. As for power amp, when I need to really hear the truth, I have a 21LU8 based head phone amp with the sony mdr-v6. My speakers and power amp just can't come close to the sound of the head phones, when I need to compare preamps.

I got a truepath kit from 41hz but after I blew a couple of ta3020 chips I gave up. The board from 41hz is IMHO can be GREATLY improved... just look at how many people burn their chips with the 41hz kit. Too bad, I had great expectations from that amp.

Also got a tube buffered chipamp (3886) which doesn't sound too bad.
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