Bass Amplification, Part 2

21 05 2007

In Part 1 I talked about the reason why bass players basically can’t avoid the need for lots of amplification power on stage. In this part I’ll talk about picking amplifiers and speakers and how to shop for them.

Like most people, my brain includes the “gotta get a good deal” region, so when it comes to important multi-thousand-dollar purchases, I prefer to educate myself and buy carefully.

That means that although it’s perfectly possible to spend five thousand dollars on a beautiful, sturdy, prestigious, great-sounding bass rig, I’m not really interested. There are quite a few premium brands that cost a ton, sound great, and have great endorsements (SWR, Trace Eliot, Eden, etc.), and other brands that made a name for themselves by making very good products but less prestigious and very reasonably priced (Carvin, etc.). In both cases, the default strategy is to buy a massive all-in-one amplfier with oodles of power and two massive cabinets (4×10 and 1×15, typically) from a well known brand and just turn the EQ knobs until it sounds good. This is really expensive, and leads to situations where you have to carry three bulky 60+ pound items everywhere you want to play. Nowadays the trend seems to be to replace that with a single large 90+lb combo amp from the same set of vendors. (Here’s an example from Aguilar, yours for only $1,900.00!)

Ironically, in larger, fancier venues, the house PA tends to be loud and includes stage monitors for the band, so a bassist doesn’t necessarily need to bring anything bigger than a medium sized combo amp. Sadly this is not something that the average working bassist encounters, so it’s still necessary for the rest of us to own some kind of loud rig.

One alternative that become popular in the 90s, as bass players started to be loud enough onstage to be able to experiment with tone, was the use of rackmounted components instead of an all-in-one approach. The bassist could pick a preamp for tone, a power amplifier for volume, and effects processors for fun sound experiments. Some famous artists have gone through phases in their careers where they went crazy with complicated racks and then simplified with a single vendor approach and an all-in-one amp head. Being a rebel and a nerd, and being generally dissatisfied with the flexibility and cost effectiveness of all-in-one amps, I decided to go with rackmounted components.

My theory is basically a best-of-breed, or perhaps best-deal-of-breed approach: pick individual components based on features, tone, and cost, and save big money by unbundling lame components from desirable components that would otherwise be sold in the same package.

So, my current rack includes:

  • a power conditioner (basically a rack mounted power strip)
  • an ADA MB-1 bass preamp, which is a sort of retro product from the 80’s that is tough, feature-rich, completely MIDI-controlled despite having an analog signal path, and cheap
  • a TC Electronic G Major, which is a pretty fancy mid-price-range digital effects processor intended for guitar
  • a QSC USA 400 power amplifier

The MB-1 (including a bunch of MIDI controller pedals) was $167.50; the power conditioner was $79.95; the G-Major was $285, and the power amp was $102. The rack itself was $139.95. So, for under $800 I have a super sophisticated setup with amazing tone. I absolutely love this setup, even though it took a lot of fiddling and reading to get it to do what I wanted; it is amazingly flexible.

But, it’s just not loud enough. It’s loud as hell in my apartment, but in a studio with sound dampening materials on the walls and a heavy metal drummer, I have to tweak and tweak the tone to be heard without driving the power amp to the point where its clipping limiter is kicking in. So, instead of selling $800 of gear and looking for a higher wattage version of the same thing (and probably paying for bundled features I don’t want), I went shopping for a similar power amp with more power.

It turns out that the power amp market has been fairly commodified, which is good for me. I was able to find several good brands (QSC, whose product I was very happy with; Crown; and Peavey) with suitable products. There are quite a few cheap and crummy brands too, but fortunately, community review sites identify them clearly. So, shopping boils down to specs, combined with a constrained set of known-reputable manufacturers. This is not unlike buying PC parts, really.

My speaker cabinets are Bergantino HT112s, which as far as I can tell are the best speakers money can buy at that size. Each has a single 12″ speaker and a tweeter, and are rated to handle 300W through their 8ohm load. I selected compact, loud, really great sounding (but expensive boutique-y) speakers for my own convenience, but for the sake of this technical discussion I could just as easily be talking about any decent cabinets: 2 cabs, 300W 8ohms.

They are identical because part of what I wanted to do with my fancy setup was to run stereo effects: chorus, reverb, etc. which sound dramatically better in stereo. As I mentioned in Part 1, biamping uses dissimilar speakers to get better tone for the same amount of money, but usually at the cost of punch; I didn’t want to sacrifice punch. So, I got speakers that can handle a lot of power, and I demo-ed them for about 45 minutes alongside other similar speakers at Bass and Beyond in Sacramento. (Thanks to Juan at B&B for steering me right; I was gonna buy something else but he gave me advice and let me sit there and rock out on all of the alternatives until I was convinced.) The size isn’t an issue because you can simply shove more power through them to get more bass, as compared to my previous bass rig that could handle less than half the power (but which was biamped).

So I set out to find a power amp that would deliver 300W per channel, stereo, into 8 ohms. There are unfortunately several ways to measure power output of an amp — a 1KHz sine wave vs. 20Hz-2KHz pink noise — and really what you’re measuring is how much power it delivers with a certain amount of maximum distortion (in a power amp, distortion is bad bad bad because it can damage speakers, and generally sounds terrible). So you have to kind of match up the fudging and exaggeration that vendors use to pump up their numbers. QSC’s web site has a power amp selector that suggests that your power amp should be able to drive 1.5x to 2x as many watts into the speaker as the speaker is rated for, and once I started looking for that I found several people who agreed with that rule of thumb.

So, now I was looking for up to 600W per side into 8 ohms, which is… how loud is that, actually? Well, it’s pretty expensive, I know that: $1000 or more retail for just a power amp, compared to $100 for what I had, which was not quite loud enough. So maybe there was a lower power level than 600W that was plenty loud and affordable. The question was, how loud is loud enough, in watts (per channel into 8ohm speakers)?

Well, yesterday in the studio, I figured out that my rig combined with the old, kinda crummy practice rig in the studio, were more than loud enough. Both claim “400 watts” but mean something slightly different, and not at all the same rating I was looking for. The QSC USA 400 delivers 200W per channel into 4 ohms, for 400W total, but that’s with the 1Khz sine wave measurement, and my speakers are an 8ohm load. So for my speakers and a 20Hz-20KHz signal, it’s delivering… drum roll… 110 watts per channel.

So, assuming for my purposes “watt” means “watts per channel stereo, with a 20Hz-20KHz signal into 8ohm load”, I bought 110W of amp for $102.50. That’s pretty close to $1.1/W.

$660 (600W power limit for my speakers, at $1.1/W) was out of my desired budget. But did I need 600W? Nope.

The other amp at the studio that made the combined setup loud enough was also “400 watts”, and I had a hard time finding specs because it’s so old, but it’s a Peavey Mark IV 400, which according to many sources I found delivers 400W into one channel at 2 ohms. The QSC 400 can deliver 250W per channel of a 1KHz sine wave into 2 ohms, therefore I figure that the Mark IV is probably less powerful than the QSC 400 given any particular load. That means I probably need a little less than 220W per channel for my setup, or $232 worth of power amp, to achieve “loud enough” status.

Now, how loud is that? Well, I don’t have the equipment to measure nor the engineering education to compute from the specs the decibel level of the sound I’m calling “loud enough.” But I can say that standing in front of the studio’s amp or my current rig while I’m playing will cause pain (during treble-y spikes) and probably hearing damage given prolonged exposure; we always practice with earplugs. So compared to a home stereo, this is really, really loud. Based on decibel charts I’ve seen, and the fact that you can feel the air moving and the vibration when you’re a couple of feet away, this is probably about 110dB.

A bit about units: decibels are logarithmic while watts are linear. That means that if my amp/speaker setup uses 220 watts of power and produces 110 dB of sound, and I want it to be twice as loud (120dB), I need ten times as much power.

Instead, what I learned by combining both amps together was that I really wanted less than twice as much power, which means about 3dB of additional volume. 3 dB is about the amount of difference in volume that is noticeable as “yeah that’s a little louder”.

So, I went looking for gear. Fortunately my needs are apparently common, and there’s a lot out there that would produce from 200-350W for my setup, and most of it is in the $1.1/W price range if bought in used condition. I ended up getting lucky, and ordering a new-in-box Crown XS 500 from eBay; it even comes with a warranty. The XS 500 delivers 400W per channel for my setup, and this one is costing me $338 including shipping (which was $40-$50 for similar items). So that’s about $298 without shipping for 400W, or $.745/W, and it comes with a warranty. Sweet!

That pushes the price of my rack up to almost $1000 total. Contrast that with, say, the Ampeg B4R, which has half the power of my rig, and nowhere near the features, and I feel like I’m definitely winning via the rack approach. By my $1.1/W metric, the Ampeg B4R delivers similar power to a $220 used power amp, but it costs $999 new. $5/W, ouch! Even with the preamp added in, that’s spendy.

Your choices about rack vs. all-in-one head may be different, but the rules still apply:

  • Don’t underbuy power just because the manufacturer uses the most favorable situation to claim a high power rating. Woo hoo, 1200W! But that’s really for a 1Khz sine wave, using a bridged mono configuration into a 2ohm speaker load. Make sure you’re looking at the right numbers for your speakers (what signal, how many speakers, how many ohms?).
  • Don’t underbuy power based on how many watts the vocal PA or guitar amp you’re playing onstage with has. You need much a more powerful amp to get the same perceived volume than your bandmates do.
  • Don’t overbuy power due to a lack of understanding of how loud you need to be. Make sure you understand how the linear W / logarithmic dB relationship can tell you how much more power you want in order to be louder by a given amount.
  • Don’t overbuy speakers (or underbuy an amp for speakers you already own) due to an overly conservative interpretation of power handling ratings. It’s not necessary to just match amp watts to speaker watts; a ratio of 1.5x-2x amp watts to speaker watts is safe, and as long as the signal coming out of the amp is clean (i.e. the amp isn’t clipping), the speakers will be fine. For more info, read this article.

I hope this helps you to put together a bass rig that sounds great and doesn’t break the bank, or at least to understand how to pick these components. Good luck!

P.S. Remember to practice! Everybody’s gonna hear your mistakes much more clearly when you have a clear, loud bass rig. :)



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