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Active filter board GB

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#681
Hi Bob,
I had not picked up that subtlety in the schematic. Thanks for the clarification.
However, the ECV S&K has independance between settings for Frequency and Q factor. As for reducing the gain, I wonder if tapping off the input where effectively one is sampling the error signal in the opamp filter actually gives a clean output. I don't know, just a thought.

Can I presume that the unity gain is required because the shifted tapping point is not of low impedance, whereas the normal output point is low impedance and thus normally does not need an intervening unity gain to feed the subsequent filter?

Can you explain why the sim is showing overall gain of the two stages at +16db?
 
#682
AndrewT said:
Can I presume that the unity gain is required because the shifted tapping point is not of low impedance, whereas the normal output point is low impedance and thus normally does not need an intervening unity gain to feed the subsequent filter?
Click to expand...

Hi Andrew,

Yes you are right, the unity gain buffer is needed to isolate the high Zout of the circuit.

I know it is an extra opamp in the signal path, but I think the added flexibility of the circuit is worth it - IMHO.

You can use the circuit as a unity gain filter with odd values of R and C, or you can use it as equal value R and C with the Q set by the gain.

I hope this helps.

\Jens
 
#683
Converting spice to speakerworkshop

Hi i am currently modeling my active filter data.Does anybody know where to get the conversion sheet for converting the export data from spice to a FRD format so i can import it into speakerworkshop?

kro5998
 
#684
Re: Converting spice to speakerworkshop

kro5998 said:
Hi i am currently modeling my active filter data.Does anybody know where to get the conversion sheet for converting the export data from spice to a FRD format so i can import it into speakerworkshop?

kro5998
Click to expand...

There's a conversion sheet in Jay Buttermans SW manual 2.0 - the chapter regarding active XO.
 
#693
Active circuit board
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To do a small run of this board is quite expensive in Australia, maybe someone in Hong Kong(I heard it is very fast out there) or elsewhere wants to do small run of this board is welcomed.

I am interested in buying at least 4 boards.
 
#696
Currently building up my part list. Can I drive 2 board with 15VAC 15VA transformer? What about 4 board? Well, I hope to use 2 board for something like NaO but if that doesn't work out, may end up with 3 way active which uses 4 board.
 
#697
That depends on how much of the boards you use and the power supply used. Figure at least 10 mA per opamp, so if you use all the sections around 90 mA per board. With low filter and/or load impedances you may go higher.

Remember that the transformer will see around twice the draw of the load, due to capacitor charging currents. So if you have two fully stuffed boards drawing 180 mA your transformer will see around 360 mA. That's a bit over 10 VA. I'd bump it up to at least 30 VA.

The group buy PSU 2.2 seemed a little sensitive to smaller transformers, so the group buy offered 50 VA transformers for up to 4 boards.
 
#698
I've just gotten around to calculating component values for my boards. Maybe I'm getting this all backward, but when I was calculating my values for my input buffer I came up with this:

An externally hosted image should be here but it was not working when we last tested it.


And when I simulated that circuit I got the following graph:

An externally hosted image should be here but it was not working when we last tested it.



So... It looks like it's starting to roll offer even before 20K, and I was under the impression I had chosen values for 150KHz. Whats going on here? Also, does anyone have a completed schematic with component values that I could reference as a sanity check?


Thanks
 
#700
Hi,
you have two single pole roll-offs in your circuit.
100r & 10nF = 1uS (=-3db @ 160kHz)
10k & 100pF = 1uS (=-3db @ 160kHz)

The total cut @ 160kHz is thus -6db and falling beyond that @ -12db per octave (if the caps keep behaving at very high frequency).

Look again at your graph, I think it confirms that.

BTW,
a filter operating at 1us WILL be audible. It starts to have an effect from about F/10, in this case 16kHz, and you have two of them doubling the effect.

I would increase the first to 0.5uS (~300kHz) and stagger them so that the feedback operates at least an octave above the passive filter (~600kHz). That opamp must be unity gain stable for that 100pF to operate.

If you find that these higher frequency cuts allow too much interference through then consider lowering them, but initially open the window fairly wide and test the wind ( some builders suggest going beyond 0.3uS and even as high as 0.1uS).
 
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