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Old 22nd June 2010, 02:47 AM   #1
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Default JFET Active Crossover

Attached is a stab at an active crossover based on unity-gain Sallen Key filters built around a simple current source loaded JFET source follower. The circuit is designed to run on a single supply to accomodate my preamp setup, which uses simple single ended jfet/mosfet-based circuits running off an external 40V unregulated supply. Each functional block inside the preamp (RIAA amp, line amp) has its own on-board regulator. This will be the third functional block so I can finally biamp my system. I deliberately used a simple follower this time around - one obvious frill would be to add a load resistor and cap to each follower as well as changing resistors R16, R22, R27, R34, R45, R48, R54, and R62 to change the simple followers to "FETWhites" for lower output impedance. Folks using +/- supplies might want to consider complimetary followers and/or servo circuits, as the latter would allow eliminating some output coupling caps.

This one is already built up, and will go into my preamp test setup when time allows.
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Old 22nd June 2010, 02:49 AM   #2
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This is the simple on-board shunt regulator that sits on board to power things. I have some more ambitious designs I might use some time down the pike.
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Old 22nd June 2010, 08:52 PM   #3
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Left Side high pass and low pass response curves taken with an HP4194A gain--phase analyzer.
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Old 22nd June 2010, 08:56 PM   #4
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Right side response curves - essentially identical.
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Old 22nd June 2010, 09:57 PM   #5
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Da board...
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Old 23rd June 2010, 05:57 AM   #6
Calvin is offline Calvin  Germany
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Hi,

I like that very much and am pretty sure that it sonically beats any bunch of OP-Amp filter.

jauu
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Old 23rd June 2010, 06:18 AM   #7
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When I finally get to listen, that will tell the tale. I strongly suspect that the difference between biamping and a passive crossover will wipe out subtle (or less than subtle) differences in execution until I get used to the new sound - then It's time to play around.
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Old 23rd June 2010, 04:57 PM   #8
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Hi Wrenchone, Looks really great ! What are you using for speakers ? Could you give a little more detail about your set-up ect... Also would your cross-over work with the Pass B1 buffer ? Thanks for sharing your design !
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Old 24th June 2010, 06:38 AM   #9
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At present RIAA amp is a circuit from my "JFET SRPP" thread. The line amp is a "gm boosted" mosfet/darlington follower (I don't think I've published an explicit thread on that circuit). The power amp is currently my "Shrine" cascoded JFET/ultralinear 1625 SE tube amp.And a JFET/Mosfet class AB amplifier.It's sitting on top of my "Gain Clown" composite chip amp. The speakers are a pair of hexagonal prisms I built out of particle board in 1979, loaded with 16 each of a Vifa/JBL 4" driver from pars express and 3 X vifa soft dome tweeters. The setup is pretty much omnidirectional, with 4" drivers on 5 of the six faces of the prisms, and the tweeters on the three most forward-facing sections.
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Old 24th June 2010, 08:35 AM   #10
Calvin is offline Calvin  Germany
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Hi,

basically you can use any Buffer You like. The main characteristics of a buffer stage is a gain of unity or close to one, high input- and low output impedance.
When You look at different filter topologies, You will mostly find the triangular OP-Amp symbol as the active or gain-device. If the gain is one (direkt wire connection from the output of the device to its negative input) You can basically choose any buffer You like, wether it be JFET-buffers with single or dual supply, or triode tube buffers, or bipolar transistor buffers or MOSFET buffers. Wether it be very simple source-, cathode or emitter-followers with resistor load or current source load (as in this case).
In line level circuits I´d use JFETs in first place and triodes in second. They are the best devices for this application. The only advantage of MOSFETs is that there are a lot of complementary devices around, while there are no P-types with tubes and only a few P-JFETs. In a simple Buffer circuit this is of no disadvantage, because there is nothing like a true complementary transistor anyway and the N-channel only circuits work extremely well.
On Erno Borbely´s website You find nice descriptions of basic JFET-circuits that suite this application nicely.
If You´re intersted in music I can only recommend to try those current source loaded Source follower circuits. You probabely will never use OP OP amps in such applications again.

jauu
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