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Old 1st October 2004, 10:05 AM   #1
shyfx is offline shyfx  Australia
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Default bass guitar amplifier

Hey guys, I am thikning about building an AV400 from http://aussieamplifiers.com for use as a bass amplifier but I am unsure of whether this amplifier would be suitable for driving such a signal. Is the "live" nature of a bass guitars output not suitable for an amplifier like that? If it can be used, is there anything that should be changed? Or can I just use it as it is?

Cheers
-Mikey
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Old 1st October 2004, 12:08 PM   #2
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It should be fine. Just use more heat sinking than you think you need and maybe add a fan (on a thermal switch if you want). An amp in a sound production role tends to run at much higher average power than a home amp and heat the enemy of reliability.

I assume that you realize you'll need a preamp to bring your bass' output to a level that will drive the power amp. There are several preamp designs available on the web. I am currently running one based on a dual op amp that is just gain - no tone controls or anything else. I think I'll build one of the jfet designs when I find some time.
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Old 4th October 2004, 07:35 AM   #3
shyfx is offline shyfx  Australia
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Awesome thanks. I am guessing the amp will be run pretty hard probably as loud as it will go without distorting most of the time HEHEHE.

I think I better sort out a pre-amp too then!

Cheers
-Mikey
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Old 4th October 2004, 05:59 PM   #4
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The only thing i have seen in schematics for bass amps VS audio amps is a driver stage limiter on some. Like the Hartke amps have a limiter that starts to short out the drive current in a gross over drive situation.



Zero
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Old 5th October 2004, 07:12 AM   #5
Enzo is offline Enzo  United States
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Actually output stage limiters are quite common in pro audio power amps. ( the shunts you refer to) I shudder when I see circuits without them. SWR amps for example lack them. If the speakr cord pulls half out of the speaker or the load otherwise shorts, BOOM the output stage burns up.

Many bass players use plain old commercial power amps for their rig along with a Bass preamp. QSC, Mackie, Peavey, Crown being common popular brands. Bass heads per se don't usually go over a few hundred watts. You want 1500 watts, you steal a power amp from the PA system.
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Old 5th October 2004, 08:17 AM   #6
shyfx is offline shyfx  Australia
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So the output stage limiter, instead of being a fuse will just limit the current to a safe value for the output stage in case of a short or an incorrect load being attached?
How does one limit current on an already existing amplifier?
Enzo mentioned a "shunt". Is that like a current shunt that you use with a multilmeter ?

-Mikey
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Old 5th October 2004, 08:54 AM   #7
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Quote:
Originally posted by Enzo
Many bass players use plain old commercial power amps for their rig along with a Bass preamp. QSC, Mackie, Peavey, Crown being common popular brands. Bass heads per se don't usually go over a few hundred watts. You want 1500 watts, you steal a power amp from the PA system.
I use a big ol' Perreaux 6000 for most of my bass rig and either a Yamaha PB1 or my own custom preamp. The actual amount of power you need is dependent on
- the size of the room you need to fill
- whether you're using PA for support
- how loud you want/need to go, ie are there two guitarists with 100W Marshalls and a heavy hitting drummer, or a girl-and-acoustic-guitar to compete with
- the efficiency of your speakers
- how much size and weight you're willing to hump around
- headroom is usually always a good thing (providing the weight isn't an issue)

If by some chance this is for your bedroom, you'll need a max of 30W; My Alembic into a K140 is neighbour-complaint-inducing with about 3W.
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Old 5th October 2004, 09:07 AM   #8
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It looks as if Enzo and Zero are talking about different things: Short circuit protection and limiting.

Short circuit protection is always a good thing for amps that are moved around and plugged and unplugged a lot.

The other thing is avoiding overdrive (i.e. clipping) since it sounds annoying, kills speakers while making inefficient usage of output power. One possibility is to design the amp in a way that it clips soft. This feature is already intrinsic with many tube amps. But it can be done with SS amps as well. More common is the use of a VCA based limiter.

Me personally I prefer uncompressed bass-sound and use a generously dimensioned PA amp instead (1kW bridged into 8 Ohms) and do not even play very loud usually (i.e. I am one of the inefficient power users).

Regards

Charles
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Old 6th October 2004, 04:50 AM   #9
Enzo is offline Enzo  United States
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Yes, the reason to use a 1500 watt amplifier is not so much to be REAL LOUD, but it allows headroom for the peaks which are way over the bulk of the signal. I like the effortless sound that results from a power amp that doesn't have to crank for all it is worth to drive the speakers.

I think zero and I are on the same page. The things I refer to are the small transistotrs that sample the voltage across the output transistor ballast resistors. This voltage is applied to the base of the small transistor and if it gets large enough it turns on the transistor. The emitter of the little guy goes to the output buss, and the collector goes to the base of the output transistor - with some isolation by a diode. This liimits current in the outputs by shunting excess drive away from the out put device base. it protects the amp from over loading. It is not there to protect the load really.

Sorry if "shunt" confuses. It just means a bypass. If the little transistor is across the base-emitter junction of the output device, then if you turn it on, it shorts the B-E, thus preventing it from turning on further.

A good commercial power amp should have lots of protection anyway. it doesn't matter what you use it for, it should protect itself from blowing up.
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Old 6th October 2004, 06:57 AM   #10
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Quote:
Yes, the reason to use a 1500 watt amplifier is not so much to be REAL LOUD, but it allows headroom for the peaks which are way over the bulk of the signal. I like the effortless sound that results from a power amp that doesn't have to crank for all it is worth to drive the speakers
A G R E E D !!!


Quote:
I think zero and I are on the same page. The things I refer to are the small transistotrs that sample the voltage across the output transistor ballast resistors. This
In this case YES. The usage of the word limiter was confusing me since the thing that you describe is usually called short-circuit protection or CURRENT - limiter. When only the word limiter is used then one is most often talking of a dynamic processing device (which are very often part of instrument amps, specially bass-amps).

Regards

Charles
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