Car Stereo Amp = Cheap PA?

[ashcatlt]

This is a half-baked (*cough**cough*) idea I've had rattling around.

A quick froogle search seems to show that these things seem to have an extremely low $/watt ratio. I'm seeing things like 1200W into 4Ω for < $100! I happen to be in the market for a stereo power amp for this band I'm in, and I'm wondering what it might take to press one of these into service.

Question the First:
What sort of nominal input voltage do these things expect? I haven't messed around with auto audio, but something tells me that the stand-alone power amps are generally patched in between the output of a reciever (or something similar) and the speakers. The reciever's outputs are generally speaker level already though, aren't they?

Would I be able to get any meaningful output if I feed this a line-level signal? Would a headphone output drive it better? Or do I need some smallish power amp before I get here?

B:
These things obviously run from a (nominal) 12VDC power supply. I'm pretty sure it's going to need something a little heftier than a wallwart to get the sort of power necessary, though. I'm getting a little confused trying to figure the current requirements. Any ideas in this area are greatly appreciated.

I suspect that the solutions to these questions will end up putting the pricetag somewhere in the neighborhood of a used Peavey "workhorse" or similar, but it's worth a shot, no?

Thanks for your input.

[newey]

Ash-

We had some discussion a while back about using a car audio amp as a guitar power amp stage:

guitarnuts2.proboards.com/index.cgi?board=Amps&action=display&thread=2796&page=1

This did also delve into the issue of what size of a power supply would be needed.

As far as the input specs, most modern car audio amps are designed to take a "line level" input (which I put in quotes because I don't want to imply that this is the same as what "line level" means in pro audio gear). Most modern "head units"- the part that goes in your dashboard and has all the controls- have a separate output for use with an external power amp, so it's essentially just like a preamp/power amp setup.

I imagine the specs for input voltage would be available in the manuals of these things.

I suspect the real problem is going to be distortion. They hang a big wattage number on these things to entice teenage boys who don't look at anything but the watts. It may well be 1200 watts at 2-3% distortion- and it might be barely a hundred watts at any usable distortion level. And because of ambient noise levels in a moving car, people will tolerate a lot more distortion in a car audio signal than they would ever listen to on a home unit.

[ashcatlt]

I remember that! Thanks for helping me dust of those synapses. To paraphrase one of my favorite songs (a Lorenzo's Tractor classic) which quotes an episode of Friends:

"There's an old saying: 'Sometimes brain cells die.' It's not a great saying, but it certainly is appropriate today."

I actually tried this way back when I was first starting down this dark and treacherous path. Bought a cheap car stereo amp in hopes that I could use it as a guitar amp. I don't remember all the specifics (remember that thing about the brain cells?), but I'm pretty sure I just hacked a guitar cable into the inputs and used some 12VDC wall wart for power. The results were (predictably) disappointing.

Now I know enough more than I did then to be dangerous.


In my situation, I'm not sure that I need much more than a couple hundred watts per side to be happy. How much distortion (%THD) can I tolerate? Well, okay, how much can my band mates tolerate? I can take plenty. (Remember that thing about Lorenzo's Tractor?)

Can it be worse than a Peavey?

[newey]

The problem is being able to do a true "apples-to-apples" comparison. Reputable manufacturers will quote a distortion rating at the full rated RMS power, the cheaper ones usually either don't disclose the distortion rating, or they give a number without telling you what power was being run to produce that number.

Sony, Kenwood, etc. all make power amps with less than 1% distortion at full power, but these are going to be in the $300 range. You'll see cheaper amps that tout a "0.05%" distortion rating, but they seldom disclose at what power that was measured.

I've heard the statement that less than 1% distortion is inaudible, but I assume that means "in a car". I know I can certainly hear the difference between a home stereo at 50 watts with 1% distortion versus one with 0.01% distortion, which is what my Yamaha receiver is rated.

In a live setting, I would think 1% distortion would be OK for vocals, but that's just a guess. You also have to consider the noise floor- signal-to-noise ratio. You can tolerate a considerable amount of hiss in a moving car.

I think that, once you factor in the added costs of a suitable power supply, and all the connectors you'll need to convert the RCA inputs on a car amp (unless your mixer/preamp/whatever has RCA outs), a decent used Peavey will start to look pretty good.

Most car amps use binding posts, or more RCA jacks, for the speaker connections, so some conversion there is likely to be needed as well. And these sort of connections are not the most stage-worthy, either.

[D2o]

Aug 7, 2009 1:22:51 GMT -5 ashcatlt said:

I remember that! Thanks for helping me dust of those synapses.

Ditto from me, newey. I searched unsuccessfully for some time for just that thread to refer Ash to.

Can it be worse than a Peavey?

Hey! ... I think almost all things Peavey are good - and, factoring bang for the buck in, even very good to great.

I suspect you feel the same, though, or you wouldn't have bothered mentioning it.

Am I right?
Can I get an Amen?
Fo shizzle my nizzle?

(hmph ... I am now all out luke-warm / quasi-cool sayings)

D2o

[ashcatlt]

Aug 7, 2009 8:35:58 GMT -5 D2o said:

Can it be worse than a Peavey?

Hey! ... I think almost all things Peavey are good - and, factoring bang for the buck in, even very good to great.

I suspect you feel the same, though, or you wouldn't have bothered mentioning it.

Am I right?
Can I get an Amen?
Fo shizzle my nizzle?

(hmph ... I am now all out luke-warm / quasi-cool sayings)

D2o

Well, to be honest, most things Peavey to which I've been exposed have been barely tolerable and all around pretty crappy. They are relatively cheap. They are also generally good at taking large amounts of abuse. They are also pretty ubiquitous and therefore pretty easy to get repaired and/or replaced.

Likely they'd be just fine for what I want to do, but I'm just looking around for options. I personally have no desire to own another PA. The band needs it, but the band doesn't have any money yet.

[D2o]

Ubiquitous for sure - which means low cost of acquisition, which may have colored my opinion ... but not much.

I realize that you may be talking PAs, specifically, but let me give you some examples of Peavey stuff I have acquired and what I paid for it:

A 1992 Peavey Predator "strat-style"guitar and a 1980s Peavey Studio Pro 50 112 amp ... got them both for $125.

A Delta Blues tube amp (like a Classic 30, but with a 15" speaker) that I got for $250.

A (nearly mint) 1982 Peavey T-60 guitar that I got for $250.

So, yeah, I have what I would consider a decent "strat", a really good "LP-style" guitar, a (surprisingly) good and versatile practice amp and a pretty killer tube blues amp ... all for $625 ... I can't really complain.

Again, I realize that you may be talking about certain of Peavey's offerings with which I have no familiarity, which means our mileage may vary.

D2o

(P.S. sorry to take this thread off-point, Ash)

[sumgai]

Another couple of points to ponder......

In that other thread, I jacked around with RJB, and upon re-reading it, I realize that all of us failed to mention the real culprit here, the wattage ratings.

In essence, when a manufacturer quotes a rating, that number is worthless without some sort of reference point. In this case, as newey as alluded to above, it's the wattage at some distortion level, as measured with a steady-state input - it doesn't vary over time, so the output shouldn't either. This is, usually, the RMS value. It can be diddled with, but in general, if a company says "XYZ watts RMS at so-many ohms", you can probably rely on that figure.

Note though, that nothing was mentioned about distortion. This is notoriously hard to pin down, and not because it's subjective. In fact, there are at least 4 kinds of distortion I can name off the top of my head, and some of my synapses fired a moment ago, trying to tell me that there are more than a dozen kinds. Sadly, I can't even start to remember their names, and I perceive that it doesn't really matter, so I'm gonna go all lazy on ya, and not bother to Google for the answers to that non-burning question.

The bottom line for us is harmonic distortion. This is defined in various ways, but I like "when the amplifying device introduces harmonic artifacts that were not present in the original signal". How that happens is not germane to this topic, so we'll leave it at that. But the fact is, if you apply too much operating power to a device, or if you apply too strong of an input signal to that device, then harmonic distortion will occur. Note: not may, but will occur. The only question left to answer is "how much of that distortion can a listener withstand?".

And that's also not really a concern for us.... we're after tone, and as everyone who's played an electric gutar for more than 10 minutes can tell you, an electric guitar should sound distorted, or else it "doesn't sound very good at all". This is an "AHA!" moment folks, take note, please.

Now we're at a cross-roads of 'how much power' versus 'how much distortion' versus 'how much do we like the sound', aren't we? Well, if it's any consolation, newey's correct about the auto bit where distortion and hiss are of less concern (until you get to the real competitions, where they put your rig in a sound-proof booth and bring in all kinds of meters), but for reference, Leo started with the Williamson circuit straight out of the 1946 RCA Tube Reference Manual, which featured the "low distortion factor of only 5%"!!! You read that right, five percent was considered good enough, in the early post-war era.

Today, Fender, and most other amp companies, are down around 1%. This helps to explain why, after CBS bought the company in 1964 (and started monkeying with the engineering in 1965), the power ratings were reduced, even with almost the exact same power amp circuits. The distortion factor was factored into the equation, and the output rating was adjusted accordingly.


Now, getting back on topic.....

Not only does distortion play an integral part here, but actual power requirements do too. IOW, the power rating is for a steady-state input signal (aka RMS), but how often does that occur for a guitar player, eh? Not very, I'm inclined to answer. When we pick a string, even without a pick, just our skin, the attack transient is very sharp, and in order to reproduce that accurately, there needs to be a high amount of power delivery available. After that, the sustain portion of the signal (ADSR - look it up) takes over, and thats where we usually operate, about 95% of the time.

To boil it down, we really need, and use, only 2-5% of the total power of which our amps are capable. Only during the initial pick attack do we ever exceed that low power level. (Well, if you use echo, then the attack may occur repeatedly in quick succession, but you get my drift here.) For that, we have available, and may well use, all 50 watts or all 100 watts, or whatever.

But wait, what if we don't need "clean" sound during the pick attack, can't we just use a smaller-wattage amp? Sure can, Homer. However, the problems that arise from that are real gotchas. First, we need to worry about the speakers and how they respond to distortion products from the power amp. Ooooh, let's stop right there, that's a headache all by itself. No, better to keep things matched up as known, accepted, and mostly loved, by the whole industry. Are we cool with that, Jethro?


Nitty gritty time....

Long story short, you can tap a 1200 watt amp for duty, but you really should read the fine print first. Is that rating in RMS watts? And at what distortion level, please? And of course, at what output impedance. (As an aside, did you know that Bose now makes a new line of LifeStyle speakers called the LM-48, which needs it's own special Bose-made amplifier? They operate at 0.75Ω of impedance, something that would absolutely guarantee a steady income for fuse makers, if you tried hooking them up to any "normal" power amp. 'Struth!)


Bottom line.....

In short, power is cheap, good clean power is not. And that's before we start talking about reliability, size and weight, connectivity, etc.

But hey, experiment away.... Who am I to fly in the face of the next Lee de Forest?!

HTH



sumgai


p.s.

ash,

I don't know your/the band's budget, but I picked up a cheap QSC RMX850 for $150 off of craigslist. That and eBay should be your top Bookmarks/Favorites, until you finally get what you need/want. Just a thought.