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In Reply to: RE: Looking for a high pass filter kit posted by Cleet Torres on July 04, 2014 at 18:53:01
I don't want to remove my speaker crossovers, I have no intention of removing my speaker crossover and I am not going to remover my speakers' crossovers.
My need is very simple. Use a SET for the tops but spare it from having to consume what little energy it has to amplify frequencies the speaker crossover will block, but leave enough so that the speaker crossover will work as designed.
Follow Ups:
You have not mentioned the speakers' passive crossover frequency or slope. That's important, your high-pass filter muse not disturb the phase of the tweeter signal in the range where it will affect the acoustic crossover performance. It is not difficult to imagine a situation where a simple first-order highpass two octaves below the speakers' crossover will still have an audible effect on the tonality. If it's a commercial speaker, you might have to get the attention of the actual designer to get a useful answer to these questions.
Hi Paul
I believe the crossover frequencies is around 2,500Hz and first order slope. I am thinking that for the signal I feed my SET if I keep the cutoff at below 400Hz I should keep all the phase issues I will introduce well away from being audible or bothering the tweeter.
The bass driver and signal I am not touching.
thanks
Cleet
That helps a bit. A first order crossover has only 90 degrees of phase difference between the highpass and lowpass signals, and (ignoring temporarily the response of the drive units) the tweeter signal is still substantial two octaves below the crossover (625Hz in this case).
Without writing a book on crossover design, I think you'll get the fastest and most reliable answers by experimenting. Put in the capacitor that Tre' described, but lay in a supply of several values. Use clipleads or a switch so you can compare the sound with the cap in and out, or between caps. Don't listen for sonic quality, just the frequency response/tonality.
It is not uncommon to reverse the tweeter phase, especially in first and third order crossovers, looking for the best sound. Add that to the experiment if you have thee time and patience.
You may well end up with something different from the designer's intent, but that works better in your room with your music etc... But if you do the experiment, you will know what compromises and choices you have made, and will have no need to worry whether it might have been done better.
The post from Tre' to which I referred seems to have changed or disappeared, so my reference has no meaning anymore. I was talking of an RC filter and so was the post that has gone away. Sorry for the confusion.
For what it's worth, higher order filters have more phase shift.
I took a class in filter design in grad school; it barely scratched the surface of what matters in speaker crossovers. That's why I suggest experimenting, and I suggest keeping the experiments simple with few variables at first. Or, of course, asking the designer of the speakers you are considering.
It didn't go anywhere and I didn't edit it.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
That's teh one. It didn't show up yesterday for some reason. Thanks.
The problem isn't the signal your amp has to handle.....it is the signal your speaker (individual drivers) have to handle. Let that sink in real good and hard.
The simple solution you are after leaves you with a two outcomes. One is you have such a shallow slope that your amp and tweeter sees a lot of energy below this 500 hertz spot you pick (and that is a rather bad spot BTW). The other is, if your speaker crossover is such, that you leave your SET driving a complex impedance that changes with hertz (because they don't make a tweeter that can run 500-20,000 hertz without serious problems/crossover help). Neither is what you want.
Bottom line, you will walk away thinking bi-amping is flawed. That would be a shame. It is hard to get right but the rewards are there. You just have to think it through a lot more and get the right parts.
I'll suggest you look at what speakers (drivers) can do on the flat and level. We are talking about what range of hertz they can handle while keeping a ruler flat impedance. I doubt you will find much (in a single driver) that can handle more than about three octaves.
What you should be looking for, unless you want to tri-amp, is a mid-range driver that can run down to around 125-250 and up to around 1250-2000 hertz at 100db/watt (minimum) and mate that to a compression driver/horn that can run from there to 20,000 hertz (more or less). You would then have a passive crossover between mid-range and horn. Your SET would drive those. You'd be surprised what a couple/few class A watts can do.
You would use an electronic crossover after your preamp to send everything below 125-250 hertz to a solid state amp (say a 15" speaker on it) and everything above to the SET. If your musical tastes dictate it, a dedicated subwoofer will be needed.
You are better off using pro-audio drivers. Pick a crossover that allows you to adjust slope and cutover point. There are many things to take into account from room conditions to mechanical slopes of the drivers so you need adjustablity to begin with.
Don't think this will be easy. Chances are high you will decide amps were never your problem and that speakers were. There are some 12" and 10" vintage (alnico) speakers that will do and a few newer 8". DSS and 18sound made some good (cheap) horn lens.
Did you actually read my post at all?
That's very simple to do.
Just put a cap in series with the input of the tube amp.
A cap chosen against the input impedance of the tube amp (the grid resistor value of the first stage of the SET amp).
Here's the math. The cap value in uf = 159000 / (the frequency times the resistance)
If the input impedance of the amp is 100k ohms and you want to limit the amp's low end frequency response to 500Hz then a .0038uf cap placed between the input jack and the top of the grid resistor of the first stage will give you a 6db per octave high pass filter with a -3db point of 500Hz.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hi TreYep, I was thinking of making a little switch-able setup with a couple of cap values allowing me to adjust the cutoff frequency.
I'm trying to see if there is an easy way to get a steeper slope. I am not worried about phase issues as I intend to keep the cutoff frequency way below the speaker crossover point which I believe to be 2500Hz. So if I have some phase shift anomalies at 250Hz - 300Hz I'm not particularly worried.
Thanks
Cleet.
Edits: 07/06/14
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