Live Sound Power Amp vs. Bass Amp

[mildbill]

Are there any fundamental internal differences?

 

It's my understanding that if you run your bass into a pre amp, or some other unit that can bring it up to line level,

that you can then run it directly into a power amp of the type used with most mixers.

These amps are usually less expensive than a bass amp of the same wattage.

Are there any drawbacks to doing this, or it it fairly common?

[isaac42]

I'm using Peavey T. B. Raxx preamp and a Pyramid PA1000X power amp. Works great. I think the bottom end is more solid with the high fidelity solid state power amp than with the tube amps I've also used.

[DeepEnd]

A bass amp is a preamp and power amp so you're doing the same thing, just a bit differently. The combination won't necessarily be tailored to your cab(s) but that's not exclusive to that situation.

[mildbill]

OK - I asked over at that other forum, about running a bass into a digital multi-fx unit, and running that into a power amp.

I was told it depended on the amp, the output of the fx unit, and to ask at the fx section.

 

Anyways, I thought I'd post my extremely limited findings here. The fx is a Zoom B1on.

I have an ancient Alesis RA100 amp that I run into a Yamaha 01V mixer, and out to equally ancient JBL 4206 monitors for general listening.

I unplugged the mixer, and ran the output from the Zoom into the amp.

 

The bass seemed quite a bit lower in volume without the mixer in the circuit, and some of the fx seemed a bit more noisy.

I use the mixer to boost the input, varying with the bass I'm using. Should have been just common sense, I guess.

But I wasn't sure until I tried it.

At this stage of the game I think I'll be better off sticking with dedicated bass stuff.

 

 

Edit: forgot to add, that while the Zoom lists lots of pre amps and such among the fx, I guess

digital emulations might not really boost the signal. I don't know.

The max. output of the unit is listed as 2dbm, but I don't know how that translates into voltage.

[DeepEnd]

Sorry, you said a "preamp" and I didn't think about an FX unit, much less a digital one. If you could get hold of a dedicated preamp for your bass you could probably do it.

[isaac42]

Right. When I think "preamp", I don't think "FX unit".

[mildbill]

I'm still fairly new to this stuff, and while not stupid, I am ignorant (lack of knowledge), about electronics.

The Zoom lists 6 pre amp models, and they all can have a pretty drastic effect on the sound.

 

I guess in the end though, they might not actually boost the level of the signal.

They are digital emulations, rather than actual physical components after all.

 

Have you guys used any separate pre amps? Any preferences.

[lug]

I use two different Ampeg pre's (SVP-PRO and SVTIIP) and 2 different power amps (Mackie 1400i and a Peaveu DCP1000) for most of my stuff for many many years, no complaints at all.

[WRGKMC]

The effects unit emulates the tone of different preamps. It does not produce the gain of those actual preamp. The Effects unit likely outputs instrument level which is lower than line level, but it still may be able to drive a power amp if you set your settings like output and gain above normal and possibly maxed out.

 

I will say your rig isn't going to come close to what you need for a live rig. First, I own an RA100. Its a studio reference amp designed to drive studio monitors. For that purpose its a good head and will produce a flat response for mixing purposes and will be fairly loud is you have good monitors. I did use it for driving some PA cabs and it was woefully underpowered for that purpose, and judging by that I'd have to say it probably wouldn't do much better then a practice amp.

 

Your effects pedal is not line level so its going to be underpowered as well. You can find rack preamps fairly cheap. Something like this often sells for around $100 https://reverb-res.cloudinary.com/im...mzkwtt4rz8.jpg I would try and find a bass preamp and not a guitar preamp because they are voiced differently for bass

 

Rocktron makes good stuff. http://www.ebay.com/itm/Rocktron-Blu...-/191537735630

 

Peavey http://www.ebay.com/itm/Peavey-Max-B...-/171727022016

 

BBE http://www.ebay.com/itm/BBE-BMax-Bas...-/261815909594

 

You would still need a better power head though. I actually damaged one channel on my Alesis RA100 driving it as a PA head. I get some crackle on one channel. Its probably a cooked cap or something so it just points towards that head not being able to take the heat. It has no fans and only has heat sinks to ventilate it. When you're pumping bass frequencies you generate allot of current and the power head really needs fans to keep it from overheating.

 

In all, you'd be better off just scoring a used bass head. I picked up two recently. A Fender 300W Rumble type head for $100 and an Ampeg 350w Portaflex for $150. Both worked great. The Ampeg was bran new and the Fender simply needed a new cord. You can find great deals like that all day long on EBay and Craigs list.

[isaac42]

The effects unit emulates the tone of different preamps. It does not produce the gain of those actual preamp. The Effects unit likely outputs instrument level which is lower than line level' date=' but it still may be able to drive a power amp if you set your settings like output and gain above normal and possibly maxed out.[/quote']

 

This.

[mildbill]

Like I said in my earlier post, I just use the RA100 for general listening. I have a small Roland Cube bass amp (30 watts),

and a small Eden outfit: http://www.edenamps.com/products/wtx/EGRW264.html

for bass.

The Eden might be OK for small gigs, but I'm not currently even in a band. I use both for home use/practice only.

 

About the topic though: Are stand alone pre amps made to be used specifically with power amps that have no pre amp.

 

I guess the Zoom unit I got has a lower output because it's supposed to be used with a bass amp that already has a pre amp.

 

What happens if you run a pre amp into a pre amp (combo unit with built in pre amp)?

[isaac42]

Like I said in my earlier post, I just use the RA100 for general listening. I have a small Roland Cube bass amp (30 watts),

and a small Eden outfit: http://www.edenamps.com/products/wtx/EGRW264.html

for bass.

The Eden might be OK for small gigs, but I'm not currently even in a band. I use both for home use/practice only.

 

About the topic though: Are stand alone pre amps made to be used specifically with power amps that have no pre amp.

 

I guess the Zoom unit I got has a lower output because it's supposed to be used with a bass amp that already has a pre amp.

 

What happens if you run a pre amp into a pre amp (combo unit with built in pre amp)?

 

Yup. Preamps are made to be used with power amps.

 

Yup. The Zoom is supposed to be used as an effects unit, with the signal then fed into a preamp.

 

If you cascade preamps, you add gain stages. That increases hiss and will likely result in distortion.

[mildbill]

OK - Thanks.

I think I'm starting to get it a little.

 

Unless I'm off track, I found this db to voltage chart:http://www.sengpielaudio.com/dB-chart.htm

It says 2 db = .794 volts. That's the max. output of the Zoom unit.

And I think most power amps expect to see somewhere around 1.25volts (+4db).

 

That's why it sounds so much quieter directly into the amp,

compared to running through a mixer (with pre amps), or one of the bass amps (with pre amp) first.

[isaac42]

Not quite. .794 volts is -2dB relative to 1 volt. Decibels are a way of expressing relative power. There always has to be a reference. The equation is 10log(x), where x is power over reference power. So a 200w amplifier compared to a 100 watt amplifier would be 200w/100w. In dB, 10log(200w/100w)=3.01dB. Going the other direction, comparing the 100w amp to the 200w amp, gives -3.01dB.

 

With voltages, the equation is 20log(v), where v is voltage divided by reference voltage. The chart you were looking at was using 1 volt for its voltage reference, so 20log(.794/1)=-2.00dB. Note that when the measurement to be compared is larger than the reference, the resulting dB will be positive, and when the measurement is smaller, the result will be negative.

 

There are some standards for reference values. dBV is a voltage referenced to 1v. dBm is power referenced to 1mW (also dBmW). So .794 volts is -2dBV.

[WRGKMC]

Preamps can be great for recording direct. I own several just for that purpose. Some even have built in effects and cabinet/mic emulation and some have midi pedals that will switch effects and adjust volumes. They were the precursor to the multi effects boards Boss and others capitalized on. My buddy had one of the bigger Boss boards and those had built in settings where it could be set to work like a preamp and drive a driving a power amp directly. When he was deciding on a new amp he bought a Boogie power head and a pair of 2X12 cabs with Celestions.

 

He was then able to place a cab on either side of the stage and get full stereo effects happening. It wasn't exactly my cup of tea. I tried it and his particular tones were pretty dark, especially his cab choices but he got some decent sounds out of it and nailed cover tunes very well which is all that mattered.

 

I still choose to use a full head. Its allot easier to move and set up as well as get the tones you need without a lot of tinkering. If I had a need for more power, then using the preamp out on a regular bass head would be my option. The tone and volume controls give you easy access to control what's feeding the power head and you can simply set the mains volume on the PA head to match.

 

With older tube heads we used to use speaker jack DI boxes which are very simple to build. You could then run a cable right into a power amp or PA mixer if needed to fill the room with sound. Doesn't help your stage volume unless you crank it through the monitors too and since bass consumes allot of the woofer power you need a pretty good setup to handle the bass piped through the PA system.