Double Bass Low wattage amp with high wattage cab... issues?

[McSalterson]

I'm currently piecing together a rig using what I have and picking up some things along the way.

I'm currently running a SansAmp BDDI to an old 75w poweramp (rated at 150w, but that's in bridge mode) that I've had for 20 years, then into an 4 ohm 800w 4x10 cab. My goal is to ultimately get something like a 300-500w head. I like the tone currently, so I haven't been in a rush to get a new head.

I've been noticing some unwanted distortion lately, and I was thinking that it could be because the poweramp is very underpowered compared to the cab, there could just be a lack of headroom. Is this a thing?

I also saw a thread on here from about 10 years ago that said an under powered amp is more likely to cause damage to a cab than an over powered amp. Is this true? Has me worried that I may have damaged the cab already.

 

[Stevorebob]

Underpowering a cab is not a thing. Any distortion you experience has another source.

 

[Francois Blais]

Just FYI, you've posted this on the DB side of Talkbass.
But of course, amp/cab issues are common to both worlds.

 

[Wasnex]

I'm currently piecing together a rig using what I have and picking up some things along the way.

I'm currently running a SansAmp BDDI to an old 75w poweramp (rated at 150w, but that's in bridge mode) that I've had for 20 years, then into an 4 ohm 800w 4x10 cab. My goal is to ultimately get something like a 300-500w head. I like the tone currently, so I haven't been in a rush to get a new head.

I've been noticing some unwanted distortion lately, and I was thinking that it could be because the poweramp is very underpowered compared to the cab, there could just be a lack of headroom. Is this a thing?

I also saw a thread on here from about 10 years ago that said an under powered amp is more likely to cause damage to a cab than an over powered amp. Is this true? Has me worried that I may have damaged the cab already.

75W is not a lot of power. So I would not be surprised if the amp is the source of the distortion.

If the cab has a tweeter, clipping the amp could potentially put the tweeter at risk. The tweeters in bass cabs are typically rated for 25-35W, and their RMS rating may actually be lower. With clean bass, this is usually sufficient unless you really crank the treble.

With distortion the tweeters power rating may not be sufficient. Most of a bass's energy is concentrated in the second harmonic. So if you play the open E string, the fundamental is 40hz and the 2nd harmonic is 80hz. Many tweeters are crossed over around 3.5hkz, and bass does not typically generate a lot of energy in this range.

Distortion causes harmonics. This means it shifts power up in the frequency spectrum. If the distortion causes enough power to shift up into the tweeters pass band, you blow the tweeter.

Shouldn't be a problem for the woofers. The amp is most likely rated for 75W RMS. If you push the amp into clipping it can make more than the rated power, but I can't see the power ever approaching the 800W rating of the 10s in your cab.

 

[McSalterson]

Just FYI, you've posted this on the DB side of Talkbass.
But of course, amp/cab issues are common to both worlds.

Ah, thanks for the heads up. I’m new to TB and saw this forum pop up when I tried doing a search, so I figured it would be the right place to post. The other reply here has enough information so I don’t need to repost elsewhere, but thanks for letting me know.

 

[McSalterson]

75W is not a lot of power. So I would not be surprised if the amp is the source of the distortion.

If the cab has a tweeter, clipping the amp could potentially put the tweeter at risk. The tweeters in bass cabs are typically rated for 25-35W, and their RMS rating may actually be lower. With clean bass, this is usually sufficient unless you really crank the treble.

With distortion the tweeters power rating may not be sufficient. Most of a bass's energy is concentrated in the second harmonic. So if you play the open E string, the fundamental is 40hz and the 2nd harmonic is 80hz. Many tweeters are crossed over around 3.5hkz, and bass does not typically generate a lot of energy in this range.

Distortion causes harmonics. This means it shifts power up in the frequency spectrum. If the distortion causes enough power to shift up into the tweeters pass band, you blow the tweeter.

Shouldn't be a problem for the woofers. The amp is most likely rated for 75W RMS. If you push the amp into clipping it can make more than the rated power, but I can't see the power ever approaching the 800W rating of the 10s in your cab.

Thanks for the information! Very helpful.

 

[Jefenator]

I've fried a few woofers from too much power.
I've never wrecked a cab yet from not enough power - but I have gotten some ugly distortion and in one case, a rather unsettling "hot electronics" smell.
For me it all boils down to: not taking a knife to a gun fight. Using one side of an old 150W power amp for bass is almost certainly an example of that.
FWIW the amp I nearly melted was running 440 watts into 4 ohms! (I was hitting super hard in a punk solo kind of way. A good 50 or 100 watt tube amp will sound freaking awesome when I do that without smelling like burning asbestos. Apparently all amps are not created equal.)