Distortion Problem w/ my Amp/Cab setup

[GriefTroll]

I recently bought a used Ampeg cabinet for $200 with 2 15" EVM15B Pro Line speakers in them, each rated at 8 ohms & 400 watts.

The speakers look pretty new. There aren't any tears in the cones. They are wired so that the positive & negative channels from the amplifier run to the top speaker, and then a connecting wire runs from the top speaker to the bottom one.

I haven't looked at the bottom speaker to make sure it's wired positive to positive, but I'm pretty sure this is parallel wiring, which means that:
Zt = (8 x 8) / (8 + 8)
Zt = 64 / 16
Zt = 4 ohms

My Sunn Beta Bass head is rated at 4ohms, 100 watts max. I've been using it for about 6 years.

When I play thru the cab at a decent volume, I get a really nasty non-harmonic "clipping" distortion.


Alright, so I'm generally lacking in knowledge about amplification & I've got some questions:

What might be causing the distortion? (is there such a thing as "underpowering "?)

What would be the nominal wattage to send to those speakers, the way they are wired?

A fact sheet for the speakers is at the URL below, which claims that the minimal impedance for the speakers should be 6.2 ohms. does that mean I should only wire them in series?

I need to solve this issue before I kill those speakers.

.pdf is at "archives.telex.com/archives/EV/Speakers/EDS/EVM-15B%20Pro-line%20EDS.pdf"

 

[bongomania]

The minimal impedance shown is if the speaker is used by itself. Together they present a 4 ohm load, so your 4-ohm-rated amp is fine, no rewiring is needed. There is no such thing as underpowering, read the FAQ to learn more about that (and do a search for a couple of recent threads on the subject).

Unless something has gone bad--a component in the amp, or a component in the cables or speaker wiring--the distortion is probably just you clipping the amp by running it at max in order to get more volume. Amps can easily put out more wattage than they are rated for, for a variety of reasons but the main one at issue here is because the rating is for clean, undistorted signal. IOW the amp is not limited to putting out 100W, it's just that once you make it put out more than that amount, it distorts.

 

[Implosion]

There isn't a such thing as "underpowering". It's a myth that just refuses to die.

Your distortion problem is probably caused by the fact that your amp is only 100 watts. You don't have enough power to produce the volume "in a clean way" that you are targeting. Your amp starts to distort and you get the "really nasty non-harmonic "clipping" distortion". You need a more powerful amp that cab produce the volume you want in a way that doesn't make the amp distrot.

 

[GriefTroll]

Hey Bongomania! Thanks for your compressor reviews. I've got a DOD Milkbox on the way!

I'm competing with two guitarists playing sludge metal & I do overdrive my amp a bit to get more sustain (also why I ordered the compressor), but my last cab was virtually the same thing -- an Ampeg 2x15 -- & I overdrove it for a few years with no problems (until the cones ripped).

The Sunn Beta Bass doesn't provide a lot of power, but it usually gives a pretty warm overdrive. I wonder if my head is the problem.

There's a Sunn Coliseum 300 up on Craigslist for about $200, I'm wondering if I should grab that.

 

[Implosion]

I'm competing with two guitarists playing sludge metal

Sounds loud to me. You definitely need a more powerful amp.
& I do overdrive my amp a bit to get more sustain (also why I ordered the compressor), but

my last cab was virtually the same thing -- an Ampeg 2x15 -- & I overdrove it for a few years with no problems

It is possible that the speakers in your new cab aren't as sensitive (or they don't match the cabinet well) and you need to feed them more power to produce the same sound level that you had with your previous cab. Therefore you have to push your amp more and it results in a distorted sound.

until the cones ripped.

Well that shouldn't happen.

 

[cubby6708]

Hey your cab speakers are probably dostorting due to a pinch on the cone internally. At low volumes my own speaker was what they called, "pinched". I had no idea what that meant , but he showed me it when he took it out to change it. It's a bitch , but it may be the prob. You'll have to find a pro to work on it. Sorry !

 

[GriefTroll]

Hey your cab speakers are probably dostorting due to a pinch on the cone internally. At low volumes my own speaker was what they called, "pinched". I had no idea what that meant , but he showed me it when he took it out to change it. It's a bitch , but it may be the prob. You'll have to find a pro to work on it. Sorry !

Google returned no results for pinched cone or pinched speaker. I'm not sure what he meant, either.

 

[BassmanPaul]

I'm pretty sure this is parallel wiring, which means that:
Zt = (8 x 8) / (8 + 8)
Zt = 64 / 16
Zt = 4 ohms

While your math is correct you are going the long way around. If your cabinets are all the same impedance then all you have to do is divide the common impedance by the umber of cabinets.

Ie
2 x 8Ω in parallel = 4Ω
4 x 8Ω = 2Ω
3 x 8Ω = 2.67Ω

Paul

 

[KramerBassFan]

There's a Sunn Coliseum 300 up on Craigslist for about $200, I'm wondering if I should grab that.

YES. Sunn Coliseum 300's are great amps, and $200 is steal for one.
Go for it.

 

[B-string]

BTW the minimum nominal wattage to your cab with 2 15" speakers =1 watt.

 

[GriefTroll]

Everybody, thanks for your help. I ended up getting 2 QSC RMX-850 amplifiers off craigslist for $300. They're both clean (the pots & the vent needed cleaning on one) and in perfect working condition (ran a sine wave thru ‘em and it came out looking gorgeous on the other side).

I'm currently running the preamp of the Sunn Beta Bass out into one of the QSCs (the other one's just for backup or maybe reselling). I plan on putting another input jack on the back of the cab and running each channel of the preamp and the QSC into one independent speaker.

In any event, it’s already sounding great! No more breakup.