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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Buffer for Pass' LXmini OX/EQ
Buffer for Pass' LXmini OX/EQ
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Old 15th September 2017, 03:57 PM   #1
Klarskov is offline Klarskov  Denmark
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Default Buffer for Pass' LXmini OX/EQ

Note to moderator: I hope I have posted this in the relevant section - otherwise I apologize, and please move it.

I was interested in trying out Nelson Pass' analog XO/EQ for the LXmini (Analog XO/EQ) . However I don't have enough of the 2SK170/2SJ74 JFets, which is problematic for two reasons: they are quite hard to come by, and if you find them they are very expensive. So I tried some Fairchild J113/J176 - they are not really complementary, but they might work anyways.



In the test set-up I used matched FETs with about 12 mA Idss.

With +/-12 Volts supply it'll supply around 7 Vrms without visible clipping. Soft clipping occurs around 8 Vrms.
THD and noise is below 0.025% at about 3Vrms (I cannot supply a higher voltage distortion free).
Frequency response: Flat from DC to at least 1Mhz. 100kHz square wave shows a tiny amount of ringing, but this could probably be removed if the bandwidth is lowered.
Rise time estimated at 0.5 µS.
Output impedance is around 35 ohm. This rises to about 70 ohms if a trimmer potentiometer (100 ohms) in installed in the sources to balance out small differences between the Fets.

These specifications are, of course, far from as impressive as the Toshiba pair - but we're talking about line level signals and how audible is 0.025% THD after all?

Is there anything I have over-looked?
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File Type: jpg FET_Buffer.jpg (5.3 KB, 1212 views)
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Old 17th September 2017, 01:40 PM   #2
samchen is offline samchen  Taiwan
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Sir, Where can I to buy 2SK170 with 2SJ74 of TOSHIBA conponents?
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Old 17th September 2017, 05:24 PM   #3
Klarskov is offline Klarskov  Denmark
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Hi samchen.

Diyaudio store has some Linear Systems devices. See Linear Systems Matched LSJ74/LSK170 JFET pairs (Grade B) – diyAudio Store
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Old 19th September 2017, 07:37 PM   #4
Nelson Pass is offline Nelson Pass  United States
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Buffer for Pass' LXmini OX/EQ
Quote:
Originally Posted by Klarskov View Post
These specifications are, of course, far from as impressive as the Toshiba pair - but we're talking about line level signals and how audible is 0.025% THD after all? Is there anything I have over-looked?
No, that's about right. By comparison the Toshibas or LSK's will get you into
below .01%.

You can tweak these parts a bit, but I wouldn't get too excited about it - that
.025% will be 2nd harmonic, not all that problematic, and for the LX mini
you are really listening at the <1 volt levels with a 30 dB gain amplifier.
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Old 27th October 2017, 12:46 PM   #5
Klarskov is offline Klarskov  Denmark
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Hi Nelson Pass,

Thank you very much for your comment. I went ahead and made a quick hook-up of your OX/EQ for the LXmini - and it works wonderfully. I'm looking forward to your final version - it's definitely the approach I'll go for!

Hans Jørgen
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Old 28th October 2017, 07:06 PM   #6
Nelson Pass is offline Nelson Pass  United States
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Buffer for Pass' LXmini OX/EQ
This morning I am finishing up the LX-mini plus sub crossover.

PC board looks like this:
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File Type: jpg LX MNI SUB PCB.jpg (521.2 KB, 1046 views)
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Old 29th October 2017, 10:29 PM   #7
JOHNNYCR100 is offline JOHNNYCR100  Abu Dhabi
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Default Thanks Nelson

Hi SIrz,

Will this be the hinted at PCB for the DIY Audio kit as i noted on Mr. Lankwitz website?....christmas is coming..

regards

Johnny
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Old 31st October 2017, 05:43 PM   #8
Nelson Pass is offline Nelson Pass  United States
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Buffer for Pass' LXmini OX/EQ
SL is happy with the bi-amp version, and will be playing with the 3-way at
his leisure, but I expect no more than a tweak, since it is essentially the 2 way
with a bass channel added.

I will have copies at BAF, although likely the LX display will be using Frank
Brenner's power box with analog crossover, which I am told is an op amp
version of similar topology.

I expect to be able to make kits consisting of boards, or boards + Jfets
available by the beginning of the year through the DIYaudio store. A more
complete kit is not out of the question.

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Old 31st October 2017, 05:49 PM   #9
Nelson Pass is offline Nelson Pass  United States
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Buffer for Pass' LXmini OX/EQ
Here's pictures of both:
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File Type: jpg lx mini 2way.jpg (847.5 KB, 953 views)
File Type: jpg lx mini 3 way.jpg (873.3 KB, 939 views)
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Old 1st November 2017, 03:02 PM   #10
Klarskov is offline Klarskov  Denmark
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Those boards look appetizingly neat - to say the least! Any chance of a schematic?
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