Capacitor as Crossover for High Pass?

Al,

Thank you Al. It makes sense. Not sure why I've read comments that running the satellites through an electronic crossover has a detrimental effect, while it is OK on the woofers/subs? I guess it would be not having to use an additional cable and the cross-over? For the electronic crossover I was considering the Behringer to get things set and then replace with a Marchand? (or is the Behinger good)?

Also hope I was using the right terminology. I was looking for the satellites to take care of somewhere between 125-150HZ and up. If I was to use a capacitor to roll off the frequencies below say 125HZ not sure what frequency I would be shooting for with the capacitor. Would the capacitor have a 6db slope?

Just a bit more info if I may.

The sats are tested at 91db, 5-10 ohms between 100HZ and 25KHZ and the bass panels would be, still looking for them, approx 87db 4 ohms. The amps for the sats are 24 watt PP SETs, while the amps for the panels would be SS about 400-500 watts. The sats go down to 55HZ. Not sure what it is down by at that frequency. -6db? Also, the room is not large 15x19, but pretty much dedicated to 2 channel listening.

I understand that the electronic crossover would let me set the crossover freq, volume and phase. My thought was to crossover at 125-150HZ, with a high slope 36db? (Not sure until you try I guess) The satellites would be in front of the bass panels, so I would need to be able to adjust the phase.

Thanks again,

Lee

Not sure why I've read comments that running the satellites through an electronic crossover has a detrimental effect, while it is OK on the woofers/subs? I guess it would be not having to use an additional cable and the cross-over?

I would imagine so, particularly with respect to the electronic crossover. Any sonic degradation or artifacts it might introduce would figure to be more significant if they affected the mid-range (and adjacent) frequencies, as opposed to just the low bass.

Would the capacitor have a 6db slope?

Yes. If placed between preamp and SET amp, the high pass filter would be comprised of the capacitor and the input impedance of the SET amp, which presumably is essentially resistive at audio frequencies. An RC filter has a slope of 6db/octave once you get beyond the point at which it is 3db down (that point being referred to as the 3db bandwidth). So if you set the 3db bandwidth (which in this context is the same as the "crossover frequency") to 150Hz, then at 75Hz it would be down 9db relative to the upper bass, the mid-range, and the treble, and at 37.5Hz it would be down 15db.

You would calculate the required capacitance by using the formula for capacitive reactance (impedance), Xc = 1/(2 x pi x F x C). The 3db bandwidth point would occur when Xc equals the input impedance of the amp. So you would substitute the value of that impedance (which might be perhaps 50,000 or 100,000 ohms) for Xc, set F equal to the desired crossover frequency, and re-arrange the equation to calculate C. If you use units of ohms and Hertz, the calculated C will be in Farads, so multiply the answer by a million to get microFarads (uF).

The specs you provided for the amps and speakers seem generally reasonable, but I still have a concern about the disparity in power between the two amps. A 400W amp driving 87db panels will be able to play about 8db louder than a 24W amp driving 91db speakers, which is a substantial difference. Even with an electronic crossover and a sharp rolloff, I think you might find yourself unable to use perhaps 1/2 or more of those 400 watts without clipping the SET amp. (I'm assuming the 87 db figure is for 1W input and at 1m, not 2.83V input and at 1m. Given the 4 ohm impedance, if the 87db is based on 2.83V, that corresponds to 2 watts, or 84db/1W/1m, which would reduce the concern).

Together with the fact that the satellites are reasonably sensitive, and the room is not large, that would seem to suggest (assuming 87db/1W/1m) that perhaps a 200 or 250W SS amp (into 4 ohms) would be a more appropriate choice.

I have no particular knowledge of the quality of the Behringer or Marchand (or other) electronic crossovers. But having the flexibility to adjust the crossover slopes over a wide range, as you mentioned, would certainly seem to be desirable.

Best regards,
-- Al

All of your points sound reasonable.

What do you think of running the monitors "full range" and then just using the electronic crossover for the lower frequencies? I could always experiment with a capacitor as I go along here. I'll also take your advice on the bass amp.

Also want to say that being new to this audio, (I wouldn't call it a hobby as it's become a passion) experience, how really supportive everyone one is and how willing they are to share their knowledge. It is a great help.

Thanks again.

Lee V.

Lee, the one issue I see with that (aside from the fact that the monitors might sound better if the deep bass were kept out of them, which as you say can be evaluated later) is along the lines of my earlier comments. The power capability of the tube amp will constrain how much of the SS amp's power can be utilized.

If the bass panels are 4db less efficient than the monitors, and the monitors are run full-range from the 24W tube amp, and you gain-match properly, then the maximum amount of SS power that can be utilized without clipping or overdriving the tube amp (which would result in a highly distorted signal being supplied to the monitors) is:

24W + 4db + whatever amount of headroom the tube amp can provide above its 24W continuous power rating.

4db above 24W is 60 watts. So on a sustained basis you would not be able to supply much more than 60W to the bass panels, or perhaps (just guessing) 75 to 100W on brief transients, regardless of how powerful the SS amp may be.

And those numbers are based on the optimistic assumption that when high volume bass notes occur, there is no significant musical energy present at higher frequencies. If there were, that would add further to the burden on the tube amp, and thereby further reduce the amount of power from the low-pass filtered SS amp that could be utilized without clipping the tube amp.

Best regards,
-- Al

Got it. I'll plan on rolling the lower frequencies off the monitors. If its OK, once I find the panels and start working through the process I'll send an update.

Lee