A question about Bass Amps

Submitted by recycledpet on Thu, 2004-11-18 21:58

Hi,

I've been looking into upgrading my bass set up for a while now. I currently use a nice and portable combo amp - 120ish Watts into a 12" speaker, which has been good for practices though a little lacking vs a loud drummer. So I've been looking at the options of getting a head and cabinet, but have a few questions I was hoping an expert or 2 from this forum could answer.

- how many watts? I know you need alot more power to push low bass frequencys than you do for a guitar amp. My current amp is about right at 120w for jamming along with a guitarist playing through a 20 watt Marshall DSL201. When looking at a bass head what should I be after - 200watt / 300watt / 400watt ?

- ohms n stuff ... this is probably what confuses me the most. If Im looking at an amp which does 400w at 4ohms, or 2x 200w at 8 ohms, does that mean I can pick up 1 8ohm cabinet for it now, and then expand to another one later? or does it to have 4 ohms connected all the time - 2x 8ohm cabinets?

- from what I've read getting something in the 400watts with an 8ohm 4x10 cab and an 8ohm 1x 15 cab is probably the ideal, but then I read some reviews on an ampeg 410 cab which got great reviews, but is a 4ohm cabinet (SVT410), so I assume I'd lose the option to run a 15" with the 4x 10s using that cabinet unless I had an amp that could run at 2ohms? (and then would I need a 4ohm 15" cab or could I connect an 8ohm 15" and 4ohm 4x 10" ?)

so Im probably thinking about it too hard and confusing myself...

can anyone offer some advice?

Cheers,

J.

Comments

Submitted by Big_Stu on Fri, 2004-11-19 15:53.

Ideally, you want to match the output impedance of the amp to the input impedance of the speaker. Impedance is stated in ohms. For maximum transfer of power the input and output impedance need to match. Having said this the type of amp you use determines how fussy you need to be with this matching. Solid State amps tend to be a lot more tolerant of an impedance mismatch than valve amps. Most solid state amps state their rated power output as x watts into 4 ohms, x/2 watts into 8 ohms etc.

So a 400 watt amp will (probably) only supply 400 watts into a 4 ohm speaker set up. Into 8 ohms you will get 200 watts. If you want to drive, say, a 4 x 10 and a separate 15" cabinet from your amp you would connect the two cabinets in parallel. If the cabinets are both 8 ohm cabinets when connected in parallel they will provide a 4 ohm load to the amp. 4 ohms in parallel with 8 ohms will give a total impedance of about 2.5 ohms. You could probably get away with this with a solid state amp.

Valve amps tend to be less tolerant of impedance mismatches. This is due to the way the output signal is coupled from the valves to the speaker. As a general rule it is best to ensure that the speaker system and amp are well matched.

As for how much power you need ... well that is a moot point. Depends a lot on the environment you are playing in. If you are playing in a venue with a good PA and the cabinets are all miked up then you can get away with less power. Also, you might find that once you move out of the practice room the 120 watts that you have is probably enough for the bass to be heard over the drummer ... drums sound awful loud in a small space.

I guess it depends also on the style of music you are playing ... if the bass is predaminant in the mix then you will want more power. Though don't get too carried away. As a general rule to sound twice as loud you need 10 times the power. Also valve amps tend to "sound" louder for the same amount of power.

A lot of gigging bass players get away with not much more than 100 Watts into a good efficient speaker system. On the other hand a more powerful amp give you something in reserve provided that you don't run it full on all the time.

In summary, make sure the impedance of your selected speaker system matches your amps stated output impedance wherever possible.

Hope this helps

Submitted by foetusboy on Mon, 2004-11-22 08:58.

Stu always has great advice-

It sounds like you want more headroom... but as Stu has pointed out, it's not always about how powerful the amp is.

One of the coolest rock bass sounds I ever heard was an old Holden tube head going into it's own 15" PA woofer cabinet (oversized cabinet)- loaded with what I think was an Eminence driver, would've been either an 8 or 16 ohm load. Picking the right cabinet (to me at least) is the key to a good bass rig.

The Holden was rated at 60 watts, but more than held it's own on stage in a medium sized club against a very loud guitar amp and drummer, with no help from the PA. Tons of clean headroom, and the overdrive (with a pedal I think) was beautiful as well, no mosquito tone, just nice and thick. I mean, the TONE of that bass rig was just so smooth, it had a lovely natural compression and that sweet midrange that valve amps are reknown for.

You could look on trademe/cash con and pick up a set-up like this 2nd hand for $400 - $500. The most expensive part of your rig would be the speaker cabinet.

Another good thing about valves is that if a faulty tube stops your amp from working, it's relatively easy to pop in a new set of valves yourself- without a soldering iron, qualifications or any surgery to your amp (some swear by getting a technician to rebias new tubes, but if a tube fails on the night, popping in a spare will get you through your set without a problem.)

Just my £0.02!

(The link is to a Holden guitar amp on trade me... maybe not quite the right amp for a bassplayer, but my point is the price; at the time of this post the price was still under $250.00, which I reckon is a bargain.)

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