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Yet another power amp question...


groover

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Just purchased a used SWR Goliath III cab and am looking for a suitable power amp. In my research, there seems to be two different camps on this issue. Some believe that you shouldn't get an amp that puts out more power than the cab can handle for obvious reasons. The other camp thinks that underpowering a cab is just as likely to damage the speakers from the resulting distortion of pushing the amp too hard, and that by getting an amp with more power than the cab, will allow you to maximize on headroom.

 

I know I have some additional fundage coming my way for Christmas so I'm going to be seriously shopping next week. I've already asked about brands and models. I'd like to hear from some of you that have more experience using an amp and cab set up and what you think on this subject. Thanks!

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Though it's marketing for the upcoming powered cabs really, I recommend reading the personal message on the Bergantino site. There is a lot of insight displayed, and it somewhat encapsulates the whys of bass players chasing their own tails.

 

I'm of the camp of having more power - and QUALITY power from an SR power amp - for several reasons:

  • upgrading a system later comes easily then
  • more power allows one to use cabs with greater low end response and/or less efficiency without running out of wattage before the speakers have fully done their jobs. It also assures that cab-corrective EQ has some space to work with
  • 3dB or so of headroom is available which provides better audio and assures that power amps are not clipping. It's pretty much the standard among professionals in the SR sector, for good reason.

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Originally posted by groover:

Some believe that you shouldn't get an amp that puts out more power than the cab can handle for obvious reasons.

And they're wrong.

 

Bass cab power ratings mean next to nothing - all they're concerned with is thermal power handling whilst woofers tend to be blown by a lack of mechanical (excursion) power handling.

 

To put this in perspective, last night I was running 900W (from a QSC PLX - a quality PA/SR power amp) into an Acme Low-B2 rated at a mere 350W. And the clip LEDs (or more accurately limiter LEDs because the clip limiter was engaged, as ever) were occasionally flashing on loud peaks. Did the cab complain, did it make farting noises, did any magic smoke escape? No.

 

However, if I really crank the bass knob I can make the cab really grumble without the amp running out of power. In fact, with foolish use of EQ you can probably over-excurse most commercial bass cabs with way less than their rated power.

 

My recommendations, then:

 

Get lots of power - twice your cab's rating is a nice amount, more wouldn't hurt.

Get high pass / low cut filters - you can get way more useful bottom out of a cab if you stop it trying to produce the useless ultra-lows.

Get clip limiters - that's another 3dB for nothing.

Get something you can move on your own.

Get something that other bassists are already using with success - some of the cheaper DJ oriented brands may not perform as well as their specs suggest.

 

Alex

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Originally posted by C.Alexander Claber:

Get low pass filters - you can get way more useful bottom out of a cab if you stop it trying to produce the useless ultra-lows.

oops!

 

what CAC meant to say is "high pass filters". what they do is stop the output below a certain frequency before it hits the power amp. frequency response above that is flat and doesn't change your tone. the benefit is that your amp doesn't waste power damaging your cabinet by trying to reproduce frequencies the cabinet can't make anyway.

 

robb.

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Thanks for the clarification robb.. I did know what he meant even though I didn't catch the error.

 

greenboy, I agree with you point about future upgrades, that's one of the reasons why I'm looking at a power amp and not a head. Thanks for your advice and it is well heeded.

 

Alex, I appreciate your input of gigging experience. That's the kind of stuff I wanted to hear. I very rarely boost bass, in fact I rarely boost anything except mids, and only if I feel that I'm not cutting through the din. So I don't believe I'm going to have a problem overpowering the amp with lows. And thanks for the clarification on cabinet power ratings.

 

You guys have given me lots of good stuff to think about and consider. Thanks!

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I personally prefer steep high pass filtering no higher than 30 Hz or so. My main cab shows the difference especially when using some of the V-Bass synth sounds and I use at least a low B (fundamental at 30.87 Hz is audible).

 

If I had a cab like many out there I'd probably want to filter everything below 50 Hz or so.

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where would a high pass filter go in the chain of things?

Do you specificall seek a power amp that has a high pass filter built in?

do you open up the amp and figure out the last peice of circuitry involved in the chain and then throw a high pass filter after everything?

 

For my information...a resistor is a type of passive high pass filter...right? Similar to how capacitors and inductors are passive low pass filters.

 

jason

2cor5:21

Soli Deo Gloria

 

"it's the beauty of a community. it takes a village to raise a[n] [LLroomtempJ]." -robb

 

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Originally posted by robb.:

oops!

 

what CAC meant to say is "high pass filters".

Thank you robb! Half the time I write 'low cut' and half the time I write 'high pass'. And then sometimes I let the two phrases breed to create an unsuitable mongrel...

 

I might add that my approach to high pass filtering goes beyond merely getting rid of the frequencies that my cab can't produce effectively (i.e. those below the tuning frequency for a ported cab) - I actually get rid of the fundamentals of my lowest notes despite my cab being able to reproduce them easily. I can hear the difference soloed but it sounds better in the live mix and allows the cab to play that much louder.

 

Alex

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so, greenboy uses a v-bass that has high pass filter circuitry.

 

what does calex use? what does robb use?

 

Does this vbass go in the signal chain right after your bass? does it go right after the preamp? does it go right after the power amp?

 

For the guy who only plays clean..please enlighten me.

 

jason

2cor5:21

Soli Deo Gloria

 

"it's the beauty of a community. it takes a village to raise a[n] [LLroomtempJ]." -robb

 

My YouTube Channel

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well, if i played lower than a 4, especially if i had a penchant for boosting lows (which i rarely do), i'd make it a point to use some sort of EQ unit that had a lowcut filter -- not (underlined) a low shelving control.

 

the practical difference is that with a low shelving control, not only do you cut the really low, but you also cut the sort-of low, which can thin out your tone. with a highpass/lowcut filter, you only cut the really low, leaving you with a girthy, full tone that is safe for speakers.

 

if i weren't looking to spend a ton of money, i'd probably look to buy an inexpensive PA crossover (like a 1U kind of thing) to put in my effects loop. the dbx cheapie only goes down to 45Hz, which is a little higher than i'd want to go. but i can't find anything that goes lower.

 

i have seen pictures of a bergantino line driver (that's snob for "gain-only preamp in a stompbox form factor") that has a frequency variable highpass filter. i'm sure that'll be $$$.

 

i don't know. i think alex uses the lowcut built into his QSC PLX power amp. anyone else have any ideas?

 

robb.

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I have a high pass filter @ 30Hz on my SWR Grand Prix, and high pass filters @ 30/50Hz (switchable) each channel of my QSC PLX. It doesn't matter where it goes as long as it's an active filter with a steep slope before the power amp.

 

I think the QSC PLX line have a great balance of size (2U but fit in a shallow rack), weight (21lbs), features (switchable: limiters, high pass filters, bridge/parallel/stereo, decent metering, good protection) and power (from 1600W to 3400W bridged @ 4 ohms) for the gigging non-roadied bassist. They're also readily available used, QSC offer great support, and seem to have a good reliability record.

 

I'd also consider going the head route with an EA iAmp 800. Hardly heard a bad word about this amp - light, reliable, clean, powerful. More EQ than most would like but you can switch it off!

 

Alex

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I posted this a while back, but I feel it bears repeating. I found a device that handles the highpass function very nicely. Harrison Labs (www.hlabs.com) has several filters available. Designed for autosound, these devices are inexpensive, passive, insert inline where active devices usually go, and they work. I have an HMOD-Sub plugged into my preamp's effects loop. This particular device is stereo, and you can set the filter via the included jumper plug. I set mine at 30, and just plug it in.. Makes a huge difference, folks.. Sound is punchier, tighter, and im not chasing tone all night.. Something to consider They have an ebay store where you can get one at a modest discount.
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