70's 2x15" Cab Help

[Brad Crasher]

I'm having a problem with my 70's 2x15" cabinet.  Bought it a few years ago and blew the original speakers in it. The cabinet also does not have a plate on the back with the info.  I purchased 2 8 ohm eminence speakers assuming the cab was 4 ohms. They're the same speakers I have in my 4ohm 2x15" Fender Bassman cab which has no problems.  Problem is that when playing at a high volume on my amp (1200s and a Concert Lead), the cabinet cannot handle the volume and doesn't get very loud.  Almost cuts in and out at times.  I'm pretty clueless when it comes to ohms, but is it possible that the cabinet is not wired at 4 ohms and that's causing the problem?  Below is a pic of the original speakers and they don't indicate what ohms they are.

http://farm2.static.flickr.com/1098/957230165_922a5ee21c.jpg

Any help or ideas are greatly appreciated.

[po-mo preschool]

Brad, unfortunately I have no idea what's wrong with your cabinet, but I do know it has nothing to do with impedance (ohms), which depends entirely on the speakers you put in it and whether they're wired in series or parallel (almost always the latter). Two eight-ohm speakers in parallel are four ohms, which should be perfect for your concert lead, though I don't know about the 1200s.

What happens when you play those amps at high volume through your bassman cab, or play other amps through the sunn cab? What are the make and model of the speakers? Did you install them yourself?

[JoeArthur]

You may be having phase issues. This is the possibilities:

The newly installed speakers may be out of phase with each other - this means that as one moves inward, the other moves outward. This can be checked with a battery (I use a 9V) by touching the battery terminals to a plug on a cable plugged into the speaker cabinet and watching the speaker movement. Both should move the same way.

The newly installed speakers may be out of phase with the second cab. Even though both new speakers may move the same direction when tested, they may both be moving in the opposite direction of the second cab.

Even if both cabs are moving the same way, the two amps may be out of phase with each other.

[EdBass]

Yeah, that phase concept has certainly made things difficult for lots of musicians (?) in the pursuit of ridiculous SPL's!
They stack any amplifierish crap they can find up in a pile, take a picture of it to post online (to impress the other idiots who do the same thing), wire it up, turn it on and wonder why it won’t make their ears bleed.

Or, you may have the new speakers wired in series and are showing the amp 16 ohms instead of 4 ohms. That will bottle up the output potential on a SS amp.

[basiklybass]

I am not sure I quite understand the concept of; "...high volume...not very loud..." . Doesn't hihg volume mean loud?

I could see phase being a problem in a single cabinet but from what I know about it, the volume would always be less than expected and would change with frequency i.e no bass but acceptable mids and highs. If you had two cabinets, unless the cones and VC's were in the same plane, they would not be in phase anyhow. Possible at the lower frequencies to hear the effect...but any mis-alignment would effect the sound especially if the cabinets are right next to each other, in effect making one large cabinet. That's why so many companies make such a big deal about "Time-Aligned" speakers. Yes, it makes a difference, but how much?

I agree with Ed, more likely wired for 16 ohms...or even possibly...a defective speaker right out of the box. Just because something is new today....don't mean it ain't broke.

In any case, always worth the 9 volt check, easy and nothing to lose.

[JoeArthur]

I am not sure I quite understand the concept of; "...high volume...not very loud..." . Doesn't hihg volume mean loud?

I took it to mean high volume control setting giving lower volume.

I could see phase being a problem in a single cabinet but from what I know about it, the volume would always be less than expected and would change with frequency i.e no bass but acceptable mids and highs. If you had two cabinets, unless the cones and VC's were in the same plane, they would not be in phase anyhow. Possible at the lower frequencies to hear the effect...but any mis-alignment would effect the sound especially if the cabinets are right next to each other, in effect making one large cabinet. That's why so many companies make such a big deal about "Time-Aligned" speakers. Yes, it makes a difference, but how much?

Any two sound sources producing the same signal are going to have phase cancellation and reinforcement "waves". Same cabinet or not.

It is more noticeable in a single cabinet because of the distance of the speakers. With two cabinets, the closer together they are, the greater the loss of bass and volume that comes with out of phase signals. Separate them and the effect becomes less noticeable.

Exactly what frequencies get canceled or reinforced depends on the distance between the sources, and the wavelength of the tone being produced. I can't remember the exact figure, but the speed of sound in air is roughly 1000 feet per second. If you take that and divide by the frequency, you'll get the wavelength.

So for a 100hz wave, the wavelength is 10 feet. To minimize cancellation effects due to being out of phase, the speakers have to be at least 10 feet apart.

Time alignment is sort of the same animal, but different. Time alignment doesn't deal with phase directly - it attempts to insure that the different frequency components of a sound, when broken up, arrive at the same place at the same time.

This "break up" of a sound occurs when multiple speakers are used to reproduce different frequency ranges - such as a woofer and tweeter arrangement. If the sound of the fundamental frequencies from the woofer don't arrive at the same place as the higher harmonics (or vice versa) - the sound becomes "smeared" and is no longer representative of the original sound.

Obviously this is more important in hi-fi than musical instruments.

I agree with Ed, more likely wired for 16 ohms...or even possibly...a defective speaker right out of the box. Just because something is new today....don't mean it ain't broke.

In any case, always worth the 9 volt check, easy and nothing to lose.

That would explain a loss of volume with the concert alone.

[EdBass]

Any two sound sources producing the same signal are going to have phase cancellation and reinforcement "waves". Same cabinet or not.

It is more noticeable in a single cabinet because of the distance of the speakers. With two cabinets, the closer together they are, the greater the loss of bass and volume that comes with out of phase signals. Separate them and the effect becomes less noticeable.

Exactly what frequencies get canceled or reinforced depends on the distance between the sources, and the wavelength of the tone being produced. I can't remember the exact figure, but the speed of sound in air is roughly 1000 feet per second. If you take that and divide by the frequency, you'll get the wavelength.

So for a 100hz wave, the wavelength is 10 feet. To minimize cancellation effects due to being out of phase, the speakers have to be at least 10 feet apart.

Time alignment is sort of the same animal, but different. Time alignment doesn't deal with phase directly - it attempts to insure that the different frequency components of a sound, when broken up, arrive at the same place at the same time.

This "break up" of a sound occurs when multiple speakers are used to reproduce different frequency ranges - such as a woofer and tweeter arrangement. If the sound of the fundamental frequencies from the woofer don't arrive at the same place as the higher harmonics (or vice versa) - the sound becomes "smeared" and is no longer representative of the original sound.

Obviously this is more important in hi-fi than musical instruments.

Yeah, what he said.

[basiklybass]

Thanks Joe, I grow smarter.

Perhaps the "...cutting in and out...." is indeed a beating of frequencies resulting in odd standing waves and therefore nodes in the waveform. I would think you could feel this as well. If some nodes are diminished...others must be augmented. I was in place tonight that in one chair I felt uncomfortable because of the low frequencies put out by the HVAC. Move a couple chairs over and the effect was not nearly so noticable. Granted we are talking very low frequencies but the nodes were very much apparent.

 

[Brad Crasher]

Thanks everyone.  I appreciate your help.  Gonna try and test it in the next few days when I get to our practice space.