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What's a preamp for? Is it essential?


JDL

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Um, I'm not a beginner, but I was wondering what a preamp was for. I know this must be a really obvious question. Do you have one? I have a preamp out place in my amp and wanted to know what that was for. Does it add stuff to the tone, power, etc.?

 

Oh, and what is the cheapest one out there? Which one do you have(if any)?What would a good one be? I just want to know if it is essential. Thanx for the help.

 

JDL-preamphelppeace

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Paging greenboy.....

 

From my limited understanding preamps do several things.

 

They range from the little ones onboard, which beef up the signal, perhaps do some impedance magic, add tone control to the bass...

 

Every combo amp or even bass head you've ever seen had a preamp combined with a power amp...and a pre amp section and post amp section.

 

In the other post, the pre-amp is a part of an amplification component system. By using a separate power amp driving the speaker array, you can use power very efficiently...even bi amp and stuff...

 

But power amps have no tone controls at all...signal in is boosted and sent out. To use one, you have the option of using a Preamp to handle all eq stuff.

 

Among the advantages would be...you have the choice of preamps for shaping your tone, choice of power amp, ohmage, wattage, s/n level...you can mix and match.

 

With a pre-amp system, you could come closer to dialing in "your tone" and flying to a gig with just the pre-amp and performing with "your tone."

 

Pre-amps are essential. Thing is, all active basses already have 'em (that's what "active" means) and all traditional amps have them as well.

 

Do you need one? In a word, no. (Okay, there are a million reasons why you might need one.)

"Let's raise the level of this conversation" -- Jeremy Cohen, in the Picasso Thread.

 

Still spendin' that political capital far faster than I can earn it...stretched way out on a limb here and looking for a better interest rate.

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non-engineer explanation ;)

 

a combo amp has three parts in one box. A preamp, a power amp, and a speaker.

 

The preamp has the tone controls.

The power amp has the power.

And we all know what a speaker does.

 

Some people choose to buy these things as separate components, just like some people buy stereo systems as separate components. People that buy things as components are usually called audiophiles and have more money to spend than most of the rest of us.

 

If you like the sound of your amp you don't need anything else.

 

A preamp out jack lets you send your signal to another power amp. That way you could use your combo amp the way you always do while also sending your signal to a 1000 watt power amp and 16 10" speakers. (hypothetically) :eek:

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As a five-and-a-half year veteran, I was thinking of asking a similar question. What Jeremy said was along the lines of what I suspected but did not know for sure.

 

It seems like something I should know, and most other people know, and something that would prompt many mentions of the search function. But since it's already been brought up, I have some further questions...

 

(1) Are most "heads" a combination of a pre- and power amp?

 

(2) How do power ratings work? I.e., how would one match a power amp to a cabinet? How do you play through multiple cabinets? I understand series and parallel...I was a physics major and taught labs on simple circuits...but go ahead and refresh my memory and give me details from the amp-specific side of things.

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yep it is. and yep, you do.

 

Of course, it's a pretty basic pre-amp...and there is really no way to bypass it and use another on that amp.

 

On my GK I can run a seperate preamp and insert it into the amp at at the power amp stage, bypassing the onboard preamp. I've never used that feature, and doubt that I ever will.

"Let's raise the level of this conversation" -- Jeremy Cohen, in the Picasso Thread.

 

Still spendin' that political capital far faster than I can earn it...stretched way out on a limb here and looking for a better interest rate.

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Originally posted by Adamixoye:

(1) Are most "heads" a combination of a pre- and power amp?

 

(2) How do power ratings work? I.e., how would one match a power amp to a cabinet? How do you play through multiple cabinets? I understand series and parallel...I was a physics major and taught labs on simple circuits...but go ahead and refresh my memory and give me details from the amp-specific side of things.

(1)Yes. All heads and combos have a preamp in them as well as a power amp. The preamp is basically what generates the sound on the amp side and the power amp amplifies the sound. You must have both in order for an amp system to work.

 

(2)Speaker cabinets have ohm ratings, which is resistance to a signal, the signal being the sound generated from the preamp/power amp. The most common ohm loads are 2, 4, 8 and 16. The power amp has an ohm rating at which it can handle. It is essential to match this ohm load. If your amp says "500 watts @ 4 ohm/400 watts @ 8 ohms" this means that as the ohm load increases, the more power it will take to make the speakers move, thus yielding less volume.

When running an amp in series, think of it as 2 amps running side by side. The cabinets work independently of each other. This would give you a stereo sound.

When running an amp in parallel, both cabinets are running off the same signal. This produces a mono sound. When running in parallel, it's best to have 2 cabinets with the same ohm load because the load is basically cut in half. For instance, if you are using 2 8 ohm cabinets, the total ohm load would be 4 ohms. If you have 4 16 ohm cabinets or speakers, the total ohm load is 4 ohms. If you have 8 16 ohm speakers, the total ohm load is 2 ohms.

These are just simple illustrations of ohm loads. There are various factors that have some bearing on resistance, but it's minimal. To minimize this extra resistance, use only speaker cables to make all connections between amp components and to speaker cabinets and use the shortest cable as necessary. If you use instrument cables, you run the risk of damaging your high priced gear.

 

I hope these basic examples help you out. There is a lot more to it than I have elaborated on, but this is the basics. If you know this you should do well. But, I can't stress this enough, KNOW WHAT YOUR EQUIPMENT CAN HANDLE!!!!!!! When you go to add or switch components, you must know what you have so you can determine what is needed.

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Well lets ad some spice to this thread.

 

My Combo amp has a preamp section that amplifies the signal coming from the bass and only that not adding anything to the tone. Then I have my tone controls and amp(volume button) that gives dimension to the tone , well tries to :) not a very good amp but it works.

 

Has anyone seen this before ?

 

Oh b.t.w its a Trace Elliot Commando15.

There is only two kinds of music , good music and bad music ....oooh and drugs is bad mmmmkay :)
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Orion, sounds to me like you have a defeatable eq. Either that or you're talking about having a flat eq and the tone controls you speak of are the eq. Not all preamps will have the tone controls. I can't think of any off the top of my head, but it is theoretically possible. The power amp section is the only part of an amp that amplifies the signal. The preamp just "dresses it up" a little.
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Well ... since greenboy seems to be on vacation, I'll throw out my fauxgineering explanation. [i did teach high school physics ...]

 

All basses "translate" the vibrations of the instrument/strings into electrical impulses - in most cases by using the movement of magnets (in your pickups) to create electrical "signals." [i'll spare you the field mechanics of how that happens.] These signals are then sent through your cord to the amplifier, which takes the signal and makes it "louder."

 

But basses vary a great deal in the AMOUNT of electrical impulse they send to your amplifier. Passive basses have "smaller" signals (i.e. usually less voltage), while Active basses put out "bigger" signals (by amplifiying the signal received by the pickup before it gets sent to your amplifier).

 

And also, as noted above, many amplifiers have both preamplifier sections and power amp sections. Ideally the "power" stage in your amp makes whatever signal it receives louder without changing the sound it receives; by contrast, the preamplifier boosts, conditions, and often changes (adds EQ/effect to) the signal that gets put out of your bass, before the amplifier section on your amp makes it LOUD.

 

Anyway, you "need" an outboard preamplifier (i.e. - a preamp in your amplifier, or a box like a Sansamp) usually for two reasons - 1. because the signal coming out of your bass is too "rough," or 2. because you want more tonal variation than your bass is able to give you. The preamp boosts the signal, often cuts noise, allows you to shape the tone, etc. And then once the tone gets cleaned up, it can be amplified and pumped into your speakers ... Incidentally, one reason you need the signal boost is that it increases the signal-noise ratio, so that your power amplifier works on more bass notes and less on the "hiss" that you don't want.

 

Of course, most active basses now have onboard preamplifiers. But these are usually designed to boost/cut specific frequencies (eq) in your sound, and are usually not designed to feed a secondary-stage amp the kind of smooth/clean signal that an outboard preamp is designed to put out.

 

[Mic preamps are similar - they boost and condition the "weak" signal put out by microphones so that the power amplifier or recorder can "use" it.]

 

Others will surely give this simplistic explanation a bit more nuance ...

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Good discussion! Good replies! Jeremy summed the idea up nicely. Just a few more things to add...

 

- A preamp output on an amplifier (or a stand alone preamp) allows you to record your bass with some of the tone of they amp (drive, tone controls) without mic'ing the speakers. Run the preamp output directly to the board. Keep in mind that the speakers also modify the sound, so it may not sound exactly like your amp does.

 

- If your bass has a build in preamp, and your amp has a preamp (as all amps do), you have two preamps in series, which can increase the probability of overdriving the second preamp circuit. This is not necessarily good or bad, just something to keep in mind.

 

- Some guys with BIG RIGS use PA style amplifiers (Crown, QSC, etc.). These do NOT have a preamp. If you want to use a PA style amp - Lord knows why - you MUST plug your bass in to an outboard preamp before feeding the signal to one of these amps, because the amp will have no provision for impedance matching or boosting the signal of a bass or guitar to the input level that the amp expects.

The Black Knight always triumphs!

 

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Dan South: If you want to use a PA style amp - Lord knows why - you MUST plug your bass in to an outboard preamp before feeding the signal to one of these amps, because the amp will have no provision for impedance matching or boosting the signal of a bass or guitar to the input level that the amp expects.
The Lord told me a couple of reasons to use real power amps. One is for higher power and headroom with honest ratings, another is because the tonal ideal you seek requires amps with better s/n, slew rate and damping specs. In my case it was a little of each - plus I could have a singel rack space amp that weighs 14+ pounds and makes up to 1400 conservative watts. I like mix-n-match AND fidelity ; }

 

Heads are basically preamps in one box with a power amp usually designed on one main PCB. They are convenient for bassists but often have some cost-cutting compromises built into them. But they offer good bang for the buck and some of them are really well designed and don't cut many corners that impact tone or full wattage delivery.

 

Combos are basically heads jammed into a cabinet that includes a speaker or two.

 

Preamps are in all of these and simply buffer input levels, give good impedance matching, and allow pregain control to match the incoming signal for best s/n. Then they raise the signal to line level at proper impedance, which is what ANY power amp expects to see - whether it is inside a head or is a separate component.

 

Instrument-specific preamps may also throw in some tailored voicing, default EQ contours, and some hands-on EQ as well. They may also incorporate cheaper crossover and compressor circuits (even some type of noise gate), DI, several types of outputs, and maybe some effects (like the not shabby Ashdown suboctaver).

 

Tubes or tube emulation front ends (and blending with solid state front ends) can also be involved. Here you can get into religious territory, but bassists are ignorant enough not to be the PITAs that guitarists are almost certain to be ; }

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I never want anyone to think I don't believe in acknowledging the quality of others' posts. I hate that when people won't pay respect for others' contributions. Lots of good replies above mine. Why anyone would think I'm needed to describe the basics when these people do that and more, so well, is beyond me.

 

Props to all!

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I pulled the avater from the Spinal Tap website. I was watching the outtakes on the DVD and was, basically, ate up with it. I've been going around quoting Spinal Tap for the last few days.

 

I forgot about some preamps sending a little power. This isn't enough to do any damage to a power amp, though. It is a bad idea to send power to a preamp and to another power amp. The only place you should send power is to a speaker cabinet. I know this didn't come up on this thread, but I have seen it rear it's head once or twice.

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Me too, cornbread! When I first saw the movie I wouldn't stop quoting it :D But when I mention it to people that I know, they don't get it because they haven't seen it! Anyone on this forum who has not seen Spinal Tap needs to see it now!(By the way, I'm going to post to see who has seen and it and then we can talk about it)

 

JDL($

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To minimize this extra resistance, use only speaker cables to make all connections between amp components and to speaker cabinets and use the shortest cable as necessary. If you use instrument cables, you run the risk of damaging your high priced gear.

 

Hi cornbread,

Just out of curiosity what would you classify as a instrument cable? Something that has shielding outside the signal wires?

 

I am terribily curious about the FX of using the wrong cable, since the cable I am using to run the guitar/bass speaker is also what I use for my instruments. The only difference I can find between the instrument cable and the speaker cable I bought to run my talkbox is shielding, besides that everything is identical.....

 

Anythoughts? :wave:

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The resistance of an instrument lead's center conductor makes it exceptionally lossy - inefficient and heat-producing - and negates good power amp damping (control of overhang) for power to speakers where lots of coppper is needed. In fact, longer runs at higher wattages can damage power amp output sections.

 

Speaker cables should have no shield (the capacitance is also a problem), and consist of pairs, either twisted or side-by-side (zip or lamp cord). There are lots of jive-ass "audiophile" companies selling junk science right now, but the bottom line is lots of copper, finer strands, and running the shortest cable lengths practical.

 

Instument leads are shielded, and use the shield as one of the two conductors. The best shield for this is braided as it is nearly total in shielding and doesn't separate strands from repeated flexing like spiral-wrapped, which also shields incompletely. It's also good to have a conductive layer under the braid to keep audible crackling from being an issue. Foil is a great total shield for patch cables but has a tendency to disintegrate when flexed too much and is less flexible for a given mass. So it isn't good for bass and guitar cables.

 

For passive instruments the capacitance should be low so that "loading" won't roll off the treble. Here's actually the main reason cables can sound different - on PASSIVE instruments. For active instruments the onboard preamp's buffering makes this a non-issue. Under 50 picofarads a foot is a decent figure. These are not issues for patch cables since they are not carrying such high-impedance signals.

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The speaker cables are the fatter cables. As greenboy indicated, you need more mass to move the current, so, more copper means fatter cables. Instrument cables aren't designed to handle the load as well. The two different cables look similar, but they're vastly different. The shielding is needed for instruments, but not speakers. I'm not an electronics engineer, so I don't know all the why's and what for's. My advice is to stop using the instrument cables to connect the amp to the speaker cable. It's perfect for going from the instrument to the amp, but only that.
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Originally posted by greenboy:

the bottom line is lots of copper, finer strands, and running the shortest cable lengths practical.

 

Hi greenboy! :wave:

Thanks for your advise! :thu:

 

I was under the impression that the width(crosssection) and length of the wire & heat determins the resistance value of the current flow, so wouldn't it be better to use fatter wires like cornbread suggested? :confused:

 

Yes, I'll have to go and buy some fat unshielded cables soon to make up some speaker cables. As soon as I've finished all my exams at tech......

 

At the moment I have having some problem getting a "good"/the right sound for my bass from the JCM 900 amp, it sound terrible...... when the bassman slaps the bass, instead of ping! pling! It sounds more like pinmm! plonmmm! just can't seem to get a good clean/clear sound out of it! :(

 

I've raise the issue with my lecturer at tech, he has suggested that may be it could be an impedance issue. Where some of the freuquencies have been reflected after being sent into the amp, that's why it sounds messy and not being able to be translated/amplified by the amp.

It is exactly what the bass sound like when being played on that amp(lack of punch and low end, while it sounds messy where you can actually hear the slap sound better off the string itself rather than through the speakers, the sound/vibration/frequencies are not fully being translated/amplified).

 

I've being advised to use a transformer to match the impedance but it sounds a huge pain in the butt....... Instead, I might try to use a preamp and see if it'll works/sound better this weekend. It ain't exactly a preamp, it is a Lax SE1200 Multi-band Sound Enhancer. http://www.laxaudio.com/productmain.htm

 

Anythoughts?

 

Thanks in advance! :thu:

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Well, you're using a guitar amp for bass. While it's not "wrong," I've found that using guitar amps, especially a JCM 900, tend to sound thin for bass. Perhaps your bass player is playing too hard for that amp.

 

Going back to what greenboy was saying, speaker cables are fatter because they have more copper in them. I'm not sure how it is where you are, but you can buy speaker cables already made in reasonable lengths. I think they come in 18 inch lengths and even shorter. Make sure and ask for speaker cables.

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interesting - hadn't ever thought of the effect of capacitance on the high end, but it makes sense.

 

GB alluded to this - but I'd really avoid wasting your money on super-expensive speaker cabling unless you're doing long runs from amp to speaker. Copper lamp cord is cheap and plenty heavy enough for the typical 4-foot journey from speaker to cab. And as far as I know, claims by certain manufacturers that super-oxidization/ionization lowers impedance are unproven (marketing hooey). Here in NYC where people put all kinds of useful stuff out on the street as garbage, I've been seen whipping out my pocket knife and hacking off the cord to some poor, abandoned lamp, sitting on the curb ...

 

DanSouth - Here's one reason I for one have been thinking about buying a QSC or other preamp-less hi-power amplifier: They're stereo, and can easily push two high-watt cabinets. My 300W mono amp is just ducky driving EITHER of my two cabinets. But when I use it to drive BOTH of them, I get telltale "underpower" noises. Then again, as I've posted elsewhere, I haven't yet played a room where I've needed more than a 4x10 (at least one that didn't already have a rig to play through), so for now I'm fine with what I've got ...

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Real 2-channel power amps have the advantage of redundancy. They also allow one to choose a stand-alone preamp if ones available in heads are not considered satisfactory.

 

Wire: just because strands should be thin (read up on Skin Effect) does not mean the cable is or needs to be thin. For higher power and /or longer runs it's good to use larger thicker cables (with fine strands) for speakers. I use 12-gauge or better wherever possible.

 

I notice ProCo now has some 8-gauge, which had to have special connectors built to accomodate. Because solder is not especially a good conductor, and many people don't know how to do a good job soldering, laser welding is being used in some sectors to increase reliability and conductivity.

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There is little difference in the essential qualities of a guitar preamp or a bass preamp. But the default vocing or EQ facilites of the two can be different. But guitar cabs are crappy for bass use, and many guitarists use way more tube gain than sounds tight with bass.
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Hey thanks greenboy! :wave: For the reading recommandation! :thu:

 

But guitar cabs are crappy for bass use, and many guitarists use way more tube gain than sounds tight with bass.

 

Heh! you know for me it is actually vice versa, I've got a bigass 15' bass cab and it sounds beautiful with the JCM900 for guitars.

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Hi everyone,

Just finished soldering my speaker cables and about to make some more to hook up the Lax 1200 to the JCM900 from the bass guitar, yet facing another delimma the input for the Lax is balanced! argh........ which means, I can't plug my guitars into it! :mad:

 

It just doens't seem to end or may be it is just not designed for that kind of use I guess...... :wave:

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Yeah, it takes a little bit to "bond" with your gear, although, I only buy stuff that "speaks to me." If it doesn't grab me within the first few minutes of playing, I move along. I played 7 Stingrays, all virtually identical, before I found mine. All 7 were at the same place and I just picked one up and kept moving around the store. I was actually about to give up on the 'rays, but, as I was walking out, I noticed it sitting there. I hadn't noticed it before, so I picked it up and instantly knew it was my bass. You just know it when you find it.
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