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ANALOG GUY 13th February 2006 07:18 AM

Pass X250 concept ?
2 Attachment(s)
I seen UGS4 module on PASS X250, does it as same as UGS2,3 module in PASS X2 ?

I think it's cascode jfet+transistor and drive with little mosfet (zvn\zvp) but I don't understand how to use zvn/zvp stage to drive power mosfet by use irfdxxxx for vbe bias in balanced aplication.

I want to apply this module for input stage of AYRE V3 PROTO is it posible ?

Please me Mr.PASS (only simplify)

Best regard
analog guy

Nelson Pass 13th February 2006 09:14 PM

I'll see if I can draw up a diagram that will answer some of
your questions.


Nelson Pass 14th February 2006 06:25 PM

Next day: I am informed that someone will shortly post their
version (here, I think).

GRollins 14th February 2006 09:32 PM

I was tempted to release this JFET power amp I've been working on as a "hot follower" circuit, meaning just the output stage and leave the voltage gain to the user. During development, I was using a front end not unlike an Aleph--IRF610 differential, but biased with a (single) current source. It worked well, but was somewhat limited in voltage swing for anything more than a modest power amp. In addition, the bandwidth (-3dB circa 90kHz) was not as wide as I like. And so on and so forth. With that in mind, I set out to put together another front end.
I wasn't explicitly trying to emulate either Nelson or John Curl, but found myself with a complementary JFET front end, which then needed cascoding (I did not fold) for voltage reasons. As things stand now, it will work with a wide range of rail voltages with minimal modification (two resistor values). I believe it satisfies "X" requirements.
The downside is that it uses a 2SK389 & 2SJ109, both of which are difficult to find and comparatively expensive. I have a source for new 2SK389s, but they make only N-ch devices, hence no 2SJ109s. Bummer.
My intention is to fiddle with this front end a bit more, then release it as part of the JFET amp. I will then do one of two things--either go on to the next power JFET topology on my list, or pull together an entirely different front end that uses more commonly available parts (the J310, perhaps--lower gain, but a nice part in its own right and can pass a fair amount of current).
If all goes well, this front end will stand on its own as a John Curl-style transconductance line stage, or work with another, non-power JFET amplifier output stage; a normal push-pull N & P-ch MOSFET output, for instance.
If someone does post another front end, it will be interesting to see whether we end up with a case of convergent evolution.


gl 14th February 2006 10:24 PM

So Grey, I take it you're not the mystery poster we're all waiting for here? Whatever happened to that line level pre-amp project you had burning in several months ago? You said you were going to post it if all went well. What's the story?

If it's any help I have UGS type circuit I've built but not yet powered up. It's based on acaudio's work from last summer and uses cascoded 'k389's and 'J109's with four current mirrors at each corner and 610/9610 followers lifted from the Aleph Ono. The whole thing is set down in a context lifted from the X2 pre-amp ver 2. It was designed to replace the gain block of the Aleph P with a stepped attenuator hung on the output. I was planning to post it in a couple of months (if it worked).

I would be happy to post the schematic and maybe even a photo if the other poster doesn't show up. It demonstrates what ANALOG GUY requested but it ain't tested.


Nelson Pass 15th February 2006 12:02 AM

The more the merrier.


gl 15th February 2006 12:10 AM

2 Attachment(s)
OK here is a UGS type line stage. It is meant to operate in an Aleph P type of setting with input selection up stream and an attenuator on the outputs. It's based on acaudio's UGS posted last summer. To that has been added an array of NP breadcrumbs. It's been built but not tested.

Here's the circuit in a nutshell.

1) The input JFET diff pairs are cascoded with BJTs. The cascodes put about 7 volts across the JFETS. V1 is adjusted to give 4ma of operating current through each JFET.

2) The current mirrors are level shifters and provide no gain. I was going to use 857's and 957's but couldn't get one of them at the time. These transistors are matched for gain and Vbe.

3) The output followers are lifted from the Aleph Ono and the bias is set by measuring across the 33 ohm resistors.

There are a number of unanswered questions:

1) I don't know if the offset adjustments are the right ones or if they have enough range. I am hoping that all will be revealed when the power goes on.

2) I don't know if load resistors are required off the outputs of the current mirrors. We'll just have to see.

Hey ANALOG GUY I hope this answers some of your questions. You're on your own trying to fit this into the V3! But keep the board posted on what happens. Sounds like a very creative project.

Mr. P - thank you again for the crumbs.


GRollins 15th February 2006 12:43 AM

The line stage was a made-to-order thing with 24dB gain. I used a 2SK389 as a straight differential driving an IRF610 follower. Cap-coupled the output rather than stick in a level shifter. The signal only went through two active devices. If I recall correctly, bandwidth was around 300kHz. Distortion .07%?...something like that. Only 7dB NFB--down in tube territory. Sounded very nice (scared the dooky out of a buddy of mine who has a big CAT preamp). The circuit was single-ended, so it wouldn't be of use for this application. Not that it would be all that difficult to make it balanced, but I opted to do complementary differentials this time for four times the fun.
Incidentally, that line stage caused me to give up on the Panasonic film caps. WIMA polypropylene is the ticket. They're harder to get, but sound notably better in the midrange and high end. Mouser carries a rather eclectic assortment of values at reasonable prices. I ran the IRF610 fairly hard for a line stage (50mA?) and used a 3.3uF WIMA PP bypassed with a small polystyrene. Nearly as good as the big MIT tin & polystyrene for a fraction of the cost. The Panasonic film caps are roughly equivalent to the WIMA in the bass, so perhaps I'll use them for something to do with my subs. Who knows?


gl 15th February 2006 02:11 AM

Hi Grey,

The line stage sounds really nice. Any chance of posting a schematic? And thanks for input on the caps. I've been using Panasonic PP's for a while but I do like to hear about what others are actually hearing.


GRollins 15th February 2006 02:21 AM

I don't know about the Panasonic polypropylene--I was using the 10uF polyester. It's no real surprise that polypropylene sounds better than polyester, but I needed larger values in order to get the frequency rolloff down.


EDIT: You want the schematic? No problem. Remind me when I'm at home and can get to it.

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