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Not a tubed crossover, but used between tube preamp and amplifiers.
3 pole Sallen Key active filter with TL071 op amps biamped for mono 811B two way.
From what I have been reading, the filters need to be optimized for the op amp that is used.
Been playing around with some filter design software, but not settled on anything yet.
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I use a 12db per octave low pass between my AVC (autoformer volume control) and my SS amp driving the JBL 2231a 15" woofers @ 200Hz.My mids cross themselves at that frequency and are crossed using passive 6db at 2500Hz into the Heil AMT tweeters.
There are at least 4 variations of the Sallen Key circuit.
One where the resistors are both the same value and the caps are both the same value.
Then, resistors different-caps the same and then caps different-resistors the same.
And then, what I use now, resistors different and caps different.
Does anyone know why?
BTW, I wouldn't place an Op-amp between by preamp and SET power amp for the mids and highs. That just doesn't seem right for a tube guy. If I felt like I needed to lose some lows in the midrange I would just use a cap in series with the input impedance of the tube amp for a 6db per octave high pass filter.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/01/14 08/01/14 08/01/14
I don't:) However, after reading everything I could find on the sallen key
circuit as well as John Broskie's blog on active filters it appears to me
the equal R and equal C, version can be used as an infinitely variable
crossover, with the R's being variable pots. For example, I am planing to build a low pass version that will use 10K pots as the R's and .2uF caps as the C's.
By theory, this should cover anything from around 80Hz to over 70KHz.
At the same time a Q of .5 is constant.
Of course this is only theory but it is a good place to start. I am
certainly not a SS guy, I like tubes but I guess we will see where this goes.By the way the caps can be different for a variable XO but the Q will suffer. Again I don't know why but that is what the calculators predict:)
Cal
Edits: 08/03/14
I used the non-inverting Sallen Key 2nd order Butterworth for a max flat response.Unity gain.
C2 = C1/2 . C1 is the cap in the feedback loop.
The resistor values are equal.
The Q helps determine the peaking of the response before roll off.
There appears to be a lot of complex math for those trying to optimize the circuit to individual op amp characteristics.
The battery power supply is quiet.
Tiny bit of DC offset on the output.
Edit: I ordered a quad of Burr Brown OPA227 op amps to replace the TL071.
The OPA227 is low $$$ , had superior noise characteristics and is unity gain stable.
Maybe I will hear some difference on the 811B.
Edits: 08/03/14
I am following your project with interest. My system likes ss below 80hz and I use ss on the mid-high's in the summer and 300b's in the winter. At the 12db slope do you know if the normal inverting phase characteristics apply when compared to an unmanipulated midrange signa? Option b for me would be the Marchand board at 24db.
A further complication is my concern about loading. My tube pre uses transformer coupled 26's for line out. Twelve foot cables follow to the amps with a pot in between fine tuning signal to noise. I totally love the neutrality of the 26, but the transformer output invites noise. Perhaps the Burr Brown chip will spell magic.
Hello,
2nd order graphs - phase vs. Hz ARTA Steps.
green - magnitude
blue - phase
The phase shift at the 1.2KHz x-over point is noted on the graphs.
I have a Marchand XM9 24dB octave 2 way x-over, but have not tried it out with the 811B horn.
The OPA227's came in the mail a few days ago but I have not got a chance to listen to them.
The battery powered x-over does not seem to be adding any extra noise to my set up.
There are a number of guys on the Altec Heritage forum using active X-overs on their horns like 511 or 811, so it is not that unusual to try it.
The TL071 is not a highly regarded op amp for audio, but they do allow for experimentation if nothing else is in the parts inventory.
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