Capacitor as Crossover for High Pass?


Please excuse my ignorance here. I have read various opinions on the use of either passive or electronic (active) crossovers, or both, in the forum. Hoping I can get some advice on the following and if what I'm planning makes sense.

I have a pair of satellites and my thoughts are to add a pair of bass panels for the lower frequencies. I will be bi-amping (SET tube for sats and SS for bass panels). My thoughts are to use a passive crossover (high quality capacitor) for the high pass to the satellites and an electronic crossover for the low pass to the bass panels.

If someone could please tell me, if I do install a capacitor as the high pass, where is the capacitor installed? Between the amp and speaker, between the pre and amp? Also, how do I affix the capacitior to either the speaker cable or the amplifier? Is the capicitor housed within an enclosure of some type?

Thank you very much for any help you could give and if I'm off my rocker please let me know.

Best Regards,

Lee V.
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.

-- 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,

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.

Next crossover Slopes?

Could I get some thoughts on slopes to use on both active (for the low pass) and passive (for the high pass)?

Is there a reason you wouldn't want to use 8th order slopes? Complexity, noise? It would seem that one would want to use the greatest slope and place the two crossover frequencies within exactly an octave of each other (say 125hz for the low, 250hz for the high)? Would not the drivers then be either positive or negative the same "amount" throught the slopes.

If so, would the object be to have both drivers, in this case the mid and the woofers, carry some of the frequency range with one driver negative and the other driver positive through the slope as they approach a crossover point exactly between the two crossover frequencies? In essence the frequency ranges between these two points are flat without any (+/-) overlap?

Or would it be more of a blend of slopes with one of the driver types "carrying more" of say the bass frequencies towards a point between the two crossovers?

I understand I am being very simplistic here. I imagine differences between speaker types, volumes, phasing and type/wattage of amps come into play.

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

Thanks again!
I just picked up on this thread today. I haven't been posting on Audiogon for long, but I've been in or around speakers and building for 32 years. A Simple capacitor circuit should not cause any damage to your speakers or amplifier. What you will find is that your panels will swing in impedence at given frequencies and your capacitor will not properly cross your speakers at a given point like your are probably after. If you can get impedence readings, you can put a simple impedence correction circuit in your speaker leads at your speaker to help. This is common and works, but when you say thay you are using panels, I assume that you mean planars, ribbons or electrostatics. Electro statics will not like a cap, planars and ribbons will handle the cap, but correction circuitry will most likely stabilize them at a lower impedence. With all this confusion said, it is a much better idea to go active filters... Unfortunately a poor filter can really contaminate the sound of a good amplifier and a great filter is a lot of money. My best advice is to use your panels full range and adjust your subwoofer to blend or spend the money on a good high pass active filter. Some panels do have some pre made crossovers for passive high pass (maggies). Good luck
Sorry, I just re read the thread and mis read a bit. I would not use panels for bass. I gave you info for panels as your main speakers and traditional drivers in a subwoofer. This should make my posting make more since.
Other mentioned phasing and frequency ripples. At the low frequencies that you are talking about using, there should not be any issues here. 36 or 48 db per octave @ 100 hz will do a good job, I would not cross the woofer at 125hz, even with steep slopes, on occasion it will become directional, you will hear where it is in the room. Try to keep your crossover at 100 or lower. The higher the frequency the more phasing will be a problem in imaging. In the lower frequencies, it will can affect bass output if the drivers become too far out of phase, but this should not be a big problem with active crossovers. Good luck
Thank you Timlub.