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Old 11th December 2009, 09:28 PM   #61
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Thanks for the link d to the g It's funny I was originally thinking I didn't want to do sallen key and would prefer state variable, because I thought it performed better than sallen key (but had decided it required too many opamps) as it turns out sallen key is probably better anyway. I think using a sallen key for the low pass and SY's Gyrator circuit for the high pass looks like a very good alternative, and it doesn't really add much complexity either. I like the fact that it is basically a shunt type arrangement as well. The only thing I'm uncertain of is it looks like SY's one is configured as a third order?? not second... (I'm assuming that it works just like it's passive equivalent, therefore C3 turns it from 2nd to 3rd order, and that C3 could be omitted for a second order electrical slope??).

Tony.
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Old 11th December 2009, 10:11 PM   #62
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I agree that the low pass is far less demanding, unless you're using it to bandpass your midrange. For a dedicated woofer a sallen key seems more than fine.

The gyrator circuit at the bottom of page 2 ends up as a resistor and inductor in parallel, which shunts after the capacitor in the 2nd order filter.

The last circuit diagram on page 3 is a full implementation, and I'm pretty sure C3 is the capacitor for a cascoded filter. Hopefully SY jumps in here eventually, as I'm just a hack.

I had been planning on using on of the SE borbely jfet circuits for discrete simplicity, but if someone did boards up with a B1 based gyrator circuit on it I'd be all over them.
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Old 11th December 2009, 10:37 PM   #63
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Quote:
Originally Posted by d to the g View Post
SL uses the Sallen-Key topography...
Topology... topography is part of a map...

I agree with SY about avoiding SK filters, i've not been happy with any i've tried, led me to what i'm using now, PLLXO (buffered or unbuffered)

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Old 11th December 2009, 10:44 PM   #64
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Tony, thanks for the heads-up. C3 is just a coupling cap, needed because of the servo. The time constant with the 1M resistor (R9) is probably a decade below the high-pass frequency.
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Old 11th December 2009, 11:09 PM   #65
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Thanks SY I'm a bit challenged electrically I think I should invest in some of the books you mention on your site... though the calculus reference scares me a bit The fact that the two caps were both 100nf made me think c3 was part of the filter

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Old 12th December 2009, 01:39 PM   #66
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Quote:
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Topology... topography is part of a map...
the map is not the terrain, the map is not the terrain. 5 points for whoever knows which [huge hint]De Niro[/huge hint] movie this is from....

Quote:
Originally Posted by planet10 View Post
I agree with SY about avoiding SK filters, i've not been happy with any i've tried, led me to what i'm using now, PLLXO (buffered or unbuffered)

dave
That's great for a first order electrical... but when you need a second order you still run into the giant inductor issue. Which is why SY's gyrator circuit is just that good. It also fits with the high bandwidth simplicity that the B1 circuit does.
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Old 13th December 2009, 12:00 AM   #67
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SY's circuit is something i want to try... only works for hi-pass thou

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Old 13th December 2009, 12:04 AM   #68
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Hi Everyone,

I've been thinking a little about a version based on the BF862, mostly SMD although maybe not entirely so. I really haven't got a clear idea yet of what I'd like to build, but I've prepared a schematic so that I can ask a couple of questions. I'm thinking about doing a 24db/octave version, so following the basic ideas mentioned above (and from Rod Elliot;s ESP site) I have repeated the filter "blocks", and have the same output buffer as before, so (unless I have missed something) the circuit is still unity gain. I have drawn it with the same filter point as Jacques used, although I will probably change this if/when I build it.

Here are some questions.

1. The gate stopper resistors are shown as 220R, as Jacques used. Other circuits using the BF862 use 470R (including juma's BF862 preamp), whereas somewhere Scott Wurcer talks about using just a ferrite bead. How does one decide the best value to use?

2. Would the lower BF862 ( the current source) in each buffer also need a gate stopper? Again, juma's preamp has one, and if I have understood other threads about the BF862, it is a little prone to oscillations, so maybe the answer is yes? (These aren't drawn in the schematic)

3. I presume that by repeating the filter to get 24db/octave there is no longer any phase inversion? (Or rather, it has been inverted twice?)

4. If I want gain, say of 2, can I just replace the final output buffer (after the trimpots) by juma's preamp circuit? If I do, this means four copies of it, which is OK, but increases the complexity of things a bit. Is this sonically preferable to using just a stereo pair on the input of the ciruit? (Jacques mentioned a few dsays ago that he thought putting one on the input would increase distortion - would putting them on the output be preferable?)

5. Did I miss anything? (Probably... )

Thanks in advance for any answers, and for any suggestions you might have.

Cheers

Nigel
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Old 13th December 2009, 09:57 AM   #69
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I've made a few of B1-like buffers with BF862 (I like them more than k170).
This is the way it works best for me:
- R3 increases Zout a bit (to about 35 Ohms) but it gives you the possibility to adjust the DC offset in wider range, and eliminates the need to match the JFETs - although they come in pretty tight range of Idss right out of the factory (I bought 500 pcs. and they all have Idss ranging from 12.5mA to 14.5mA with 80% of them being in 13ma to 13.5mA range)
- P1 sets the Id of Q1 (CCS) and consequentially the DC offset on output
- The gate stoppers (R6 and R1) are there to add some resistance to JFET's input capacitance in order to prevent possible oscillations. With bf862 their value is critical only in low-noise applications (MC & MM phono preamps) or in radio circutis. Anything from 100R to 1K will do the job without affecting the bandwith we need (because the input capacitance of BF862 is even lower than one found in k170). I just happen to have a lot of 470R 1/16W resistors so I used them on SMD adapter boards.
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Old 13th December 2009, 10:30 AM   #70
njepitt is offline njepitt  United Kingdom
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Hi juma,

Your circuit is still unity gain, right? (If I understand your BF862 circuit it's the first jfet that gives the gain of 2, which isn't present here... Sorry if it's a stupid question...)

Also, in this application it won't be very practical to have a trimpot on each of the six buffers, since I can't see how you would do the adjustments. Any reason not to use 10R in place of P1, like R3 and as you do in the BF862 preamp? Maybe keep the trimpots on the last buffers to handle the DC offset, I guess...

Cheers

Nigel
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