Capacitor as Crossover for High Pass?

Putting a capacitor between the SET amp and the satellite speakers is not a good idea, for at least two reasons.

First, for frequencies below the point where the capacitor's impedance (capacitive reactance) becomes negligible, variations in the satellite speaker's impedance vs. frequency curve would interact with the capacitive reactance to produce an irregular overall frequency response.

Second, putting the capacitor at that point would force the SET amp to have to swing voltages corresponding to the full-range signal, including the deep bass components. If the satellites have sensitivity that is not substantially greater than the sensitivity of the bass panels, that would mean that much of the power capability of the SS amp would not be usable, because if you turned the volume up so as to utilize that capability you would clip the SET amp.

You might still have that same problem, although to a lesser degree, if you put the cap between preamp and power amp. The first-order (6db/octave) rolloff that the cap would provide would still result in significant bass energy being put through the SET amp.

To be more specific than that we would have to know the sensitivities of the speakers, and the power ratings of the amps you are considering.

Additional issues that you would have to deal with are gain matching (taking into account differences in sensitivity between the bass panels and the satellites), and matching of the crossover slopes (you would probably want to set the electronic crossover slope to also be 6db/octave).

It seems to me that the most sensible approach would be an electronic crossover for both the high pass and low pass functions. Among other advantages, that would give you the flexibility to choose sharper crossover slopes, and thereby keep more of the bass content out of the SET amp.

If you still want to use a cap for the high pass function, I would suggest putting it (actually them, for two channels) into a small metal enclosure together with rca jacks for input and output connections.

Regards,
-- Al

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

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

Hi Lee,

IF the satellites are reasonably flat to a frequency that is "significantly" below the chosen crossover point, and IF the bass panels are reasonably flat to a frequency that is "significantly" above the chosen crossover point, then you want to have the SAME 3db bandwidth point for both the high pass and low pass filters. What is referred to as the crossover frequency would correspond to that 3db bandwidth point, and at that point each filter will have rolled off by 3db relative to the response further into its passband.

So if you choose both "corner frequencies," as they are also called, to be exactly the same, and if the slopes are the same, then the overall response will sum to flat.

When I used the word "significantly" in the first paragraph, how significant that is will depend on the sharpness of the filter slope.

That is all a different situation than in the design of INTERNAL crossovers within speakers, where the choices of corner frequencies and slopes may differ between the high pass and low pass parts of the crossover, in order to take into account differing driver characteristics near the crossover frequency.

Assuming the criteria of the first paragraph are applicable, if you use a passive high pass filter, it seems to me that you also need to use a 6db/octave (first order) slope on the low pass active filter, or a mismatch will occur.

If you implement both the high-pass and low-pass filters actively, then you would have the flexibility to choose both slopes to be anything you want, within the range of selections that are offered by the electronic crossover.

My understanding is that while very sharp slopes may be beneficial in keeping the input frequencies to each speaker within the speaker's comfort area, the downside may be increased artifacts in the crossover, such as passband ripple in the frequency response, and phase anomalies. Those are complex areas that I'm not particularly familiar with; perhaps someone else will provide additional insight.

Best regards,
-- Al