crossover frequencies

Jeffw69

I have the Kenwood AVR 507
If you recall about a week or two ago you helped me out with some "humming" when hooking up a second sub-thanks again.
I am now trying to fine tune the subs.
I have Center large, fronts large, sub-on , running subs out of sub-out. On each sub, I have the crossovers set at around 80 and the sub vol. around 12'oclock. I have been adjusting the receiver's sub level between 0 or +2, depending on what I am viewing, although when I watch any music dvd's, it is way too boomy and I have to put it at -8 or more. Suggestions???

HBombToo

Do you have the AVR 507 crossover set? I don't have your model but my AVR auto turns sub on when the speakers are set to large then I have an option of frequencies to choose from. The typical setting on the AVR is 80 Hz then you should crank the crossover to max Hz on the sub itself. This method will prevent double filtering. Why double filtering is bad would be best in a different topic.

HBomb

Dr. Spec

The filter in the AVR 507 is fixed at 100 Hz.

Set all your speakers to small.

Set the sub to on/yes.

Set the sub level to -3 on the AVR.

If you don't have an LFE input on the subs, use line level and (as HBomb says) set the sub filter to its highest setting.

Stack the subs in one corner.

Set the phase switch to whatever setting (0/180) gives you better results.

Make sure both subs have the same phase setting.

Get a SPL meter and calibrate the system using test tones and 75 dB all around for your surround speakers.

Calibrate each sub separately, and adjust volume at the plate amp.

Calibrate each sub 3 dB "cool" compared to the surround speaks.

If the subs still sound boomy, experiment with placement. Try to keep them together so they don't interfere with each other, since you don't have a variable phase control for either sub.

One way to experiment with placement is to actually place the subs at the key listening position and walk around the room and listen to the bass quality. Wherever it sounds best, that's where you want your subs to go.

Doc

Tour2ma

Originally posted by Dr. Spec
One way to experiment with placement is to actually place the subs at the key listening position and walk around the room and listen to the bass quality. Wherever it sounds best, that's where you want your subs to go.

You get this from gonzo, or did he get it from you some time ago?

Jeffw69

Thanks, I followed your advice Doc and HBomb.
I'm just wondering why if my fronts and center are capable(per their specs) down to 60 hz, why I would set them as small?
Regarding the SPL meter, that is the most useful thing I own.
I will just need to experiment with the placement now.
Thanks again.

Tour2ma

The benefits of “Small”:

1. Eliminates the possibility of standing wave interactions due to multiple bass radiators. This can happen all the way up to 100 Hz with ease. You still have room reflections from your sub to be concerned about, but small simplifies the management of this. You just need to move your sub stack around until the problem goes away.

2. Unloads your main amp from having to produce the low stuff. This is where a big chunk of the power requirement resides. If you're set to "large", even if your speakers can't reproduce below 60 Hz, the amp is going to "ask" them to by sending them the lowest frequency signal it's being fed.
Moving this load over to the sub frees up the amp to make the rest of your speakers "sing" better (and louder if you want). You should hear the difference.
Why ask your speakers to do more than they can?

And with the stacked subs you will not go wanting for enough bass... Everything will be doing what it is best suited to do.

Capice?.

Dr. Spec

Phew - good deal Tour - can't emphasize that enough - large sends a FULL RANGE signal to that channel.

Full range contains signals down to 20 Hz and even below. If your speakers can't play it, don't send it there - it's that simple.

If they are rated down to 60 Hz, I wouldn't high pass them any lower than 80 Hz, since the typical high pass filter rate of 12 dB/octave will still ask them to play well down towards 60 Hz, albeit at a steadily diminishing level as the frequency drops.

At 60 Hz, speakers high passed at 80 Hz are only -6 dB from the 80 Hz level, and that assumes they are naturally flat to 60 Hz. If they start trailing off at 60Hz due to limitations of their design, then they might be as much as -9 or -10 dB at 60 Hz and this will create a hole in the FR at that point.

This is the danger of high passing at too low of a frequency - drop out in the FR curve. My rule of thumb is to high pass about 1/2 octave above the F3 point of the speakers in question.

HBombToo

I really like Tour's first point. I never actually considered the impact of constructive/deconstructive interference from multiple sources. That is a good 1 and I think I further understand some of the advice the Doc was sending my way....

I have a question regarding point 2 and a speakers limitation on how low can it go. The physical size of the woofer is a consideration that I can deal with. The inductive reactance of the voice coil on the other hand is where my question lies. I would imagine that at a certain point in the band the voice coil begins to look like a short??? If this is the case that is a further reason to keep low frequencies off of a speaker cause the heat caused by the high current could, "melt?", the voice coil. I'm basing this only on knowledge of how an inductor reacts to dc... its a short.

HBomb

Tour2ma

Originally posted by HBombToo
I really like Tour's first point. I never actually considered the impact of constructive/deconstructive interference from multiple sources. That is a good 1 and I think I further understand some of the advice the Doc was sending my way....

Why thank you, but TBPH, it's something I picked up from Doc a month or so ago...

I have a question regarding point 2 and a speakers limitation on how low can it go. The physical size of the woofer is a consideration that I can deal with. The inductive reactance of the voice coil on the other hand is where my question lies. I would imagine that at a certain point in the band the voice coil begins to look like a short??? If this is the case that is a further reason to keep low frequencies off of a speaker cause the heat caused by the high current could, "melt?", the voice coil. I'm basing this only on knowledge of how an inductor reacts to dc... its a short. HBomb

First, the short answer is, yes, reducing the low frequency reproduction demands on a speaker reduces the amount of work it has to do dramtatically.
But if a VC does overheat, where the damage occurs is problematic. It could just as likely occur at the amp as current draw increases or at the tweeter as clipping is induced.

The way I understand it, and I may be wrong, is that there is a deadly spiral associated with over-worked voice coils. It’s something like:
Work always releases heat as a by-product (it's called Carno's (sp?) Cycle... it's a thermodynamic thing), but at low levels heat can be dissipated as fast as it is generated.
As work increases, heat generation increases and a point is reached where its generation exceeds the rate at which it can be shed, and this excess heat must be retained (now we’re into your inductor reaction HBomb).
As heat is retained, the temperature of the body retaining it rises. In an inductor, here the VC, that means its impedance begins to fall.
As impedance falls, current demands to produce the same level of work rise. As the current rises, heat generation increases and therefore temperature increases and impedance falls further causing current demands to rise and on and on. All this time the VC, as you put it, gradually takes on the appearance of a “short”… until something gives… or a protection circuit steps in to save the day… or the CD ends.

As for what “gives”, again there are three choices I mentioned above. I would think an amp would go into clipping before the VC “melts” down, and that puts the tweeters, the innocent bystanders in all this, at risk.

Dr. Spec

Ripped off from one of my many bookmarked sites:

The resistance of the VC increases as it heats and the amount of power the amp is able to deliver through it decreases and the output of the driver drops considerably - sometimes as much as 3-6 dB over the "cold" driver. This phenomenon is known as Power Compression, and it affects all woofers to some extent. Good VC design and venting of the VC gap help to mitigate.

Signal burn is a failure mode where the voice coil is burnt across its entire width, indicating uniform voice coil travel with respect to the stationary magnet structure. Such a burn pattern is not indicative of amplifier malfunction but instead is due to excessive signal or program level. The cause is simply trying to get more from the speaker than it is capable of delivering.

DC (direct-current) burn is a failure mode where the voice coil is burnt only at one end. This is an indication that it has been traveling in one direction more than the other. Since the transfer of heat is from the voice coil to the adjacent magnet and metal parts, the voice coil will be burnt on the end that stays the farthest away from the top plate.

Woofers: When a DC burn pattern appears on the voice coil of a woofer, the problem will be due to a fault in the associated electronic equipment. Most likely, the power amplifier has leaky or shorted transistors that are allowing its internal power supply voltages to be applied directly to the loudspeaker or loudspeaker system.

Midrange and Tweeter: When a DC burn pattern is observed on the voice coil of such devices, it DOES NOT always mean that the amplifier is faulty. In systems with passive crossovers, mid and high frequency drivers are protected from DC by the cross-over. The most likely cause of DC-like burns is an overdriven amplifier.
When an amplifier receives an input signal capable of driving it beyond its power rating, the result is clipping. This means that the negative and positive peaks of the amplifier's output signal are "clipped" off. The amplifier may also clip in an asymmetrical fashion, meaning that the positive side of the signal is clipped more than the negative (or vice versa). When subjected to an asymmetrical clipped waveform, one end of the loudspeaker's voice coil is "on average" spending more time outside of the gap (corresponding to the direction that is clipped) than the other. The end of the coil that is spending more time outside of the gap has poor heat transfer to the magnet structure. As a result, it overheats and burns.

Jeffw69

Thanks everyone, I didn't even think about the demand of setting speakers to large would have on the amp, I was only thinking of the speakers and sub. My poor neglected amp. I have followed all of the advice and now am calibrating the 2 subs separately.
I have that Stereophile Test CD with the sine waves and warble tones which I had used previously when I had just one sub hooked up. Thanks again!