Subwoofer hookup without dedicated sub output on amp

[Boomzilla]

I'm switching from an AV receiver to a stereo integrated amp. I'd like to run my Heresies with a sub to extend the bass. The integrated amp has 8-ohm speaker outputs only - no 4-ohm capacity and no preamp outs.

The subwoofer, a Definitive Technology Supercube, has speaker-level inputs and an internal crossover. Using that crossover, however, will probably degrade the sound.

How can I use the subwoofer without using the internal crossover? If I hook the speakers and subwoofer in parallel, the amp will see two 8-ohm loads in parallel, equivalently 4-ohms, and will overheat.

I'd prefer not to build a speaker-level to line-level converter if I can help it, although I can do so if I must.

Creative suggestions?

[CECAA850]

You're assuming that the speaker level connectors will present your amp with an 8 Ohm load which is incorrect. If your integrated amp has a speaker A and speaker B outputs, put your speakers on one and your sub on the other. If not, just run your wires to the sub and from the sub to the speakers. Using those connectors will not degrade the sound to your speakers, it's more of a pass through.

[Boomzilla]

Thank you for the response, but I believe you may not have a full picture of how the amp and the subwoofer crossover work...

My amplifier has but one pair of speaker terminals. Even if it had "A" and "B" terminals, the both of them are driven by the same output transistors. Therefore, if you connect an 8-ohm load on the "A" terminals and another 8-ohm load on the "B" terminals, the load presented to the output transistors will be 4 ohms. Only by selecting one or the other of the speaker outputs, but not both, does the load equal the impedance of the speakers attached.

Secondly, the Definitive Technology sub does not have a "pass through" for the main speakers. Instead, it has a passive, adjustable network designed for 8-ohm speakers that can set both the main speaker high-pass frequency / level and the subwoofer low-pass frequency / level. This additional (and inexpensive) set of crossover components would interact in unpredictable ways with the speaker's own internal crossover providing audible differences. This is not only true theoretically, but I can hear the difference with my own ears.

The only idea that I've come up with is to build an "impedance adapter box." By inserting resistors in the hot wires of the integrated amplifier's speaker outputs, I could create line-level variable sources to drive the subwoofer's right and left "preamp inputs." The resistors would preserve separation between the right and left channels, and the high resistance would interfere neither with the main loudspeakers' crossover components nor with the amplifier's output section. In fact, Paul Klipsch, himself designed such a network to provide a common "center channel" driver for a center power amplifier.

Thank you again for your response, and if you have additional ideas, I welcome your feedback.

Cordially - Boomzilla

[Chris A]

The only idea that I've come up with is to build an "impedance adapter box."

I agree - this is probably your best approach...unless you can find a common channel tap out of the preamp section of your receiver (good luck on that... [8-)] )

PWK articles on center channel box (for reference):

http://forums.klipsch.com/forums/storage/3/1519036/Dope%20From%20Hope_Bridged%20Center.pdf

http://forums.klipsch.com/forums/storage/3/1519047/Stereophonic%20Sound%20With%20Two%20Tracks%2c%20Three%20Channels.pdf

http://forums.klipsch.com/forums/storage/3/1519044/CIRCUITS%20FOR%20DERIVING%20SUM%20SIGNAL%20(Dope%20from%20Hope).pdf

Chris

[CECAA850]

Which supercube model do you have?

[Boomzilla]

Thank you, Chris - I appreciate the links. Because I have variable volume already built into the sub, I should be able to design for greater channel separation on the main speakers. Assuming that I need 2.5 volts drive at full 100 watt amplifier output, I need to attenuate the 28.8 volts at the speaker terminals by a factor of 10. That should allow me to drive the sub to full volume at full speaker volume.

Of course, I don't plan on using either full sub or full speaker volume, but the adjustable volume on the sub should allow fine adjustment. Since I'm using HWO model Heresy speakers, I expect to cut in the sub at the same 50 Hz that the speakers roll off at? If I remember, though, Uncle Paul spec'd the heritage speakers at -5dB for their frequency response, so maybe a bit higher? Phase matching I can do by ear or with a Stereophile test CD.

Of course, if the "speaker level" inputs on the Definitive Technology Supercube are NOT eight ohms, then I may be able to get away with running the speakers and sub in parallel off the amplifier speaker taps. Because the Heresies are high efficiency, and because I typically don't listen at high volumes, the amp may not care about a low impedance? I think I'll try this option first. Should the amp run too hot, then I'll consider taking the time to build the impedance matcher. I appreciate your response - Cordially - Boomzilla

[Chris A]

Here is the only Heresy FR curve that I could find (thanks to Mike Bentz)...The lf rolloff is typical for acoustic suspension systems--about 6 dB/octave.

Here is an impedance plot:

Looks like you'll be okay if your sub isn't much below ~5 Ohms (min.), or you have a pretty good receiver amplifer section that can handle the low impedance.

Chris

[Boomzilla]

Thank you, Cask05 - It looks as if a slightly higher crossover frequency would do the Heresy some good - 80 Hz looks like a good target, should I opt for the impedance matcher. The impedance curve is also a relief - I'm pretty sure that the amp will tolerate a 10 and an 8 ohm load in parallel (4.44 ohms) without too much problem provided I don't ask for high output levels. If the input of the sub is also greater than eight ohms (a possibility I'm waiting on Definitive Technology customer support to confirm), then the amp will have an even easier drive. For the gentleman who asked, my sub is a Supercube One. Thanks again to all who have responded. Boomzilla

[CECAA850]

Looks like you'll be okay if your sub isn't much below ~5 Ohms (min.), or you have a pretty good receiver amplifer section that can handle the low impedance.

Chris

Chris, you realize that he's using a powered sub correct? Am I missing something?

[CECAA850]

. For the gentleman who asked, my sub is a Supercube One.

According to DefTech (online manual), if you use the high level inputs, the crossover in the sub sends everything over 80 Hz to your mains. The high level inputs are made to drive a normal speaker load without taxing your amplifier section. Again, am I missing something?

[Chris A]

Chris, you realize that he's using a powered sub correct? Am I missing something?

I'm not sure how the hook-up is actually going to occur, so I hedged my bets by stating a lower limit. I would think that these types of subwoofer amplifiers have high input impedance and low output impedance in order to maintain something like linear system response.

Chris

[CECAA850]

I'm not sure how the hook-up is actually going to occur, so I hedged my bets by stating a lower limit.

What I would think should happen is that L and R speaker wires will come from the amp section to the sub. The sub will high pass the signal and send everything over 80 Hz to the speakers through the high level out terminals on the sub amp. The sub will then take everything below 80 Hz and run through it's own amplifier section and power the driver in the sub.

I'm not sure how a high pass filter would tax his amp if anything, just the opposite would occur. A large load would be lifted from his amp.

[Boomzilla]

I think we're all getting confused over hookup. If I use the speaker-level crossover in the sub, then I've placed a variable passive crossover between the speaker (which is NOT an 8-ohm resistance) and the amplifier. The high-pass speaker-level crossover in the sub is a simple RC network with a variable resistor to set the 6-dB per octave rolloff on the speakers. Since this variable passive crossover is in series with the speaker's internal crossover, unintended changes to the speaker's crossover frequencies/slopes AND significantly cruddier sound will result. Yes, it would produce an "easier" load to my amp because the sub would be handling all the high-amperage frequencies below about 80 Hz, but the degradation to the rest of the music isn't worth the "benefit" of making life easier on the amplifier. Being an acoustic suspension design, the Heresy speakers roll off naturally at about 6dB per octave. I'd prefer to run the speakers "full range" from the amplifier. I currently plan to run speaker wires to the subwoofer's speaker inputs in parallel with the Heresy speakers. This arrangement provides two advantages - the speakers have a direct coupling with the amplifier, producing correct crossover performance and superior sound, and the subwoofer can be kicked in at any phase, frequency, and amplitude that works best in the room. Essentially, I'm giving up the "advantage" of not working the speakers in their bass range for the better accuracy and sound of having the speakers NOT go through an additional passive crossover. Clear as mud? Boomzilla...

[CECAA850]

I think we're all getting confused over hookup.

I don't think WE are getting confused about anything....................it's most likely just me. Bear with me if you will.

Am I correct that you are referring to the high level filter in the sub amp as a variable passive crossover? I thought it was a fixed 80Hz high pass filter for the mains with a variable low pass filter for the sub driver.

Next stupid question. Have you taken an Ohm meter to the high level connectors on the sub amp and taken a reading? If the resistance is minimal, I can't see the harm in hooking it up parallel to your mains. (Oops, I see you've thought of that).

[russ69]

I didn't read all the replies but if you are hooking up a powered sub, there is not much of a load to the amp, it's just reading the signal. So you can parallel from the speaker without any problem.

Now, if it's not a powered sub and you are worried the impedance might be too low for your amp, you can just run the subs in series with the phase flipped to match the main speakers. Either way, you are good to go.

Thanx, Russ

[Boomzilla]

NEW INFORMATION: Per a tech support e-mail from Mr. Chet Pelkowski of Definitive Technology, the speaker level inputs have an impedance (to the amplifier input) of 2,000 ohms. This means if I run them in parallel with the 10-ohm Heresy speakers, the equivalent impedance to the amplifier will be 9.95 ohms - well within the comfort zone of the amplifier. Mr CECAA850, you were correct; the Super Cube subwoofer's speaker-level crossover (high-pass side) is 6dB per octave, set at 80 Hz for an 8-ohm speaker, and is NOT adjustable. That means that if I hook up the (nominally) 10-ohm Heresy speakers to that crossover, the actual crossover point will be 64 Hz rather than 80. It also means that I'll have a 25 microfarad capacitor in series with the speaker's internal crossovers (not a good thing). As to bearing with you, I'd rather beer with you, but we can't do that over the internet, LOL... Thanks again to all who have participated in this thread! Cordially - Boomzilla

[CECAA850]

[Y]

Hope you like the sound.