high gain 2nd order high pass active filter topology

[infinia]

Post #4&6 he is looking at Linkwitz Transform circuit in an active eq. So bass boost is correct.

[theChris]

why not just use a lowpass filter with a gain? i'm not sure i see what benefit you get with a highpass function in a feedback network.

in general, there is a balence between component values and gain. for a sallen key filter, i have the analysis done in generic terms, R, C, k, Fc, Q for a few cases. for equal value components, you can't get a gain of 4, but for unequal components you can choose almost anything. but note that in a sallen-key the component variations will cause the filter to be further from the design on paper!

3 opamp designs can often have better tolerances, but are a bit excessive.


for learning, look for nuhertz filter free (or pro or lite). these programs are excessively expensive, made more for real engineering firms over the casual user. the pro version is an excellent trial and error learning tool. you can see a passive version of a design, move it to an active implentation, or make a digital implementation. view component tolerences, ect... filter free lets you make up to 3rd order filters, making it a great tool.

[sreten]

Originally posted by theChris
why not just use a lowpass filter with a gain? i'm not sure i see
what benefit you get with a highpass function in a feedback network.

Hi,

Low pass ? at bass frequencies ?

What I was wondering was if you wanted to build a speaker with
overdamped bass and then add some bass boost and very low
frequency rejection, i.e. a 2nd order highpass with Q=2 would
give 6dB boost and low frequency rejection, could this be built
into the power amplifiers feedback network avoiding an op-amp.

Seems that you can do this, and it is a very useful technique.

/sreten.

[theChris]

ok, now i understand. I'd get a copy of filter free, and look into the various topologies. i think you want a chebychev filter from what it sounds. they have postive and negative single amplifer biquads generators.

[consort_ee_um]

Sreten's idea for the speaker is a good one. Don Keele wrote about in in 1975 see:
http://www.dbkeele.com/papers.htm
Scroll down to paper no 8.
The filter can simply be an underdamped Sallen and Key type see the data sheet for the TDA2030A for implenting these types of filter in active speakers
http://us.st.com/stonline/books/ascii/docs/1459.htm

[jcx]

active filters often require fixed drive impedance (usually assumed to be ~=0 )

not a great idea for a general purpose amp which may see anything from near 0 to over 1 K source Z from even desktop cd players - depending on manufacturer

throw in boutique tube preamps, transformer or resistor stepped attenuator/passive preamps and the input may go beyond 5 K and may vary with volume setting

ps: avoid Sallen-Key positive feedback filters at high gains - for typical power amplifier gains Q sensitivity is impractically high, trimming required even with 1 % components (and when did you last try to buy a 1% cap?)

[AndrewT]

I think that's why MFB is suggested for higher Q filters.
Less sensitive to errors.

[sreten]

Hi,

from what I understand you take a unity gain sallen and key filter,
but instead of taking the feedback from the output (which is the
same as the - input) you add the gain setting resistors of the
power amplifier and take the filter feedback from the - input.

As it a moderate Q 2nd order filter (Q = 2 to 3 typically) it could
be regarded as a high ripple Chebyshev, but not really the point.

You would need to ensure you don't compromise the input
impedance, i.e. make the filter sensitive to source impedance.

/sreten.

[infinia]

Not sure what you are trying to accomplish exactly?
Here is a link for some speaker EQ stuff at line level with out op-amps?


http://www.linkwitzlab.com/proto.htm
Hope it helps.

[theChris]

he is looking to combine the following:
1.) bass boost, boost audible bass.
2.) subsonic (aka rumble) filter, attenuate inaudible bass in hopes to reduce excursion.
3.) amplification.

all into a single design. a high Q highpass filter with gain will give a boost to some bass frequencies while rejecting lower bass frequencies and the gain allows for the amplification.


If you don't need a _good_ 2nd order function, you should be able to accomplish the other two goals easily. you could get away with a highpass on the input, followed by a highpass on the inv to gnd network, and a resistor in parallel with a (resistor - capacitor series). should be able to get a Q = 0.5 highpass, and a variable bass boost.

other variations to concider might be placing a LOW QUALITY twin-t network in the feedback, or on the input. these should be fairly component tolerant.

there are many possibilites that acheive the 3 goals stated thus far. the MFB thing might work as well.