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power amp ?'s

Discussion in 'Amps and Cabs [BG]' started by lostcausebass, Dec 18, 2002.


  1. lostcausebass

    lostcausebass

    Oct 29, 2002
    okay, i'm thinking about buying a power amp on ebay. every power amp i have ever owned has had level controls on it. the one that i'm looking at has 500w per side, but no knob-based control on it. how do i get the full 500w with this amp? just juice the signal going into it so much that it HAS to be loud? help me out.
     
  2. ESP-LTD

    ESP-LTD

    Sep 9, 2001
    Idaho
    Without knowing the make/model of power amp, no one is going to be able to answer your question accurately.

    It's possible the amp runs wide open all the time and you are expected to vary your input signal.
     
  3. Lord Valve

    Lord Valve

    Dec 13, 2002
    Denver, CO.
    Many people think that the level controls on a power amplifier somehow affect the gain of the amp. Nope - what the level controls do is divide the incoming signal between the actual input of the amplifier (the "actual" input is where the signal enters the amp's circuitry, not the jack where you plug the signal cable in) and audio ground. The amp's gain (amount of amplification provided) is fixed; the volume or level control simply dumps a portion of the incoming signal to ground and lets the rest reach the amp's input. For instance, if you have an amplifier with an input sensitivity of 1 volt RMS for full output, any time the amp's input sees 1 volt RMS the amp will put out full power. If there is a level control between the incoming signal and the point in the internal wiring where the signal actually enters the amplification circuitry, this gives the amp more flexibility in dealing with signals *larger* than 1 volt. If you had 2 volts coming from your preamp/mixer/whatever and the level control was turned halfway up, then one volt would get shunted to ground and the remaining volt would drive the amp to full power. If you had TEN volts coming from your preamp and the level control was turned up only one tenth of the way, NINE volts would be shunted to ground, and the remaining volt would still drive the amp to full power. The level controls are really a convenience feature, not a way of controlling how much power the amp will put out. If you had a preamp with a really hot output (no need to worry about that in Bassland, as far as I can see...or signal-level standards of *any* sort, come to think of it...) it would be very difficult to adjust the preamp's output control, because it would be driving the amp to full power when it was just barely cracked open. The power amp's level control allows you to dump signal, giving you a wider (and not so "touchy") adjustment range on the previous (driving) device. Another plus is minimization of line hum; hum induced into the input cable going to a power amp will remain at a constant voltage; if you turn the amp's level controls down you can lower this induced hum. Of course, you'll need to compensate for the loss of level by increasing the level coming from the mixer/preamp/whatever that's providing the signal. When you have the amp's level controls wide open and you still can't get enough signal to drive the amp to full power, you need a preamp with more juice. This is a frequently encountered problem when driving pro power amps with MI-type equipment, what with people wanting to use direct boxes and whatnot as front ends for a bass rig which includes a power amplifier which is designed to receive the standard (+4dB) levels used in pro PA equipment. That's why I'm lobbying Mr. Lee at QSC to incorporate an input sensitivity switch on the PLX-series QSCs, because I'm gettin' tired of modding 'em. ;-)

    At any rate, any halfway competent tech can add level controls to your amplifier (interior layout and room permitting) or build you an external box with two input jacks, two pots, and two output jacks if you decide you really need level controls.

    Lord Valve
    *******
     
  4. FunkySpoo

    FunkySpoo Supporting Member

    Feb 6, 2002
    Hey Lord Valve, I haven't seen you post here before. If you're new to the site, welcome. Thanks for the great price and service on the PLX 1602 I got from you a while ago. You sir do indeed ROCK. It's been working great. I have a question though. Which would you recomend: Running the amp wide open and controlling the overall volume with the pre amp's master volume or running the preamp master volume wide open and controlling the overall volume with the power amp?
     
  5. Post a link to the produce.
     
  6. malibu

    malibu Guest

    Dec 26, 2001
    I bought a Crest power amp on ebay last spring -- only power switch and clipping light -- oh and I have it bridged - my gain and maybe more importantly a master volume are on the preamp I use.

    also, what does Lord Valve mean by driving the power amp with MI type equipment ? - I get alot of power line noise with this set-up that I am still learning how to use.
     
  7. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Do you know what model it is? Every Crest amp I've ever known of has had gain controls on it.
     
  8. Flight Time

    Flight Time

    Aug 29, 2002
    I'm running a QSC Powerlight 1.0 bridged (1000 watts @ 4 ohms). I have the amp running with the amp's gain up half way. By turning it up all the way, (and not driving the amp hard with my SVP-Pro), are there any thermal related issues?
     
  9. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    No, turning up the gain controls does not hinder cooling. If the cooling was okay with the gain controls turned up just halfway, then it's likely to be fine if you turn it up.

    Does your preamp have trouble putting out enough signal?
     
  10. Like Bob said, it shouldn'g make any difference in temp if the overall volume and power output remains the same. What I have found is that the tone will change depending on your pre-amp/power-amp gain settings. I like to run my gain on my tube pre-amp at about 60% and turn the power amp gain up and down as needed. Running the gain on my pre-amp and 60% gives me a ting of over-drive that I like. For the cleanest sound I turn the power amp up full and turn down the gain on the pre-amp.
     
  11. Flight Time

    Flight Time

    Aug 29, 2002
    Bob:

    I've had no trouble with my preamp though I have not used any other brand. I just started to bridge the QSC and noticed that there is more heat now - I expected this. A nationally known musician I worked with told me that in a PA setting that not running a power amp all the way up sometimes makes the sound somewhat compressed. I don't mind running the QSC all the way up bridged. I just did not want to end up having a thermal problem.

    GreyBeard:

    Thats how I've been running my preamp (60%) and power amp (halfway up) also.
     
  12. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    No, that's not true, unless there's somehow something defective with the amp. But even with a defect, there's no mechanism I can think of in an amp's circuitry that could accidentally or inadvertently compress the signal.
     
  13. Lord Valve

    Lord Valve

    Dec 13, 2002
    Denver, CO.
    Whichever way sounds the best and produces the lowest noise floor. Some preamps sound better when they're working harder (i.e., turned way up) and if you have one of those and the noise level isn't objectionable when it's operated that way, then that's the way to go. However, there are simply no rules on this - whatever sounds best to *you* will be just fine.

    Lord Valve
    *******
     
  14. Lord Valve

    Lord Valve

    Dec 13, 2002
    Denver, CO.
    MI or "Musical Instrument" gear isn't required to conform to any industry standards as far as operating levels (input-output signal voltages) are concerned. Because of this, we have a slew of preamps (or DIs, or whatever) that are all over the map, signal-wise. It's a total crapshoot as to whether or not your chosen preamp will have the moxie to properly drive a professional power amplifier...it may work, but you may not be able to drive the amp to full power. I've seen preamps ranging all the way from instrument- to broadcast- level outputs. If the standard bass rig is going to become a rack system, with a separate preamp, maybe an EQ and a compressor, and a power amplifier (and the flexibility this imparts) then the folks who manufacture bass (or guitar, or whatever) preamps had better pull their heads out of the sand and address the output level issue. It's a pain in the ass for the people who *sell* this gear, not to mention the users. When I sell a PA system, I have total felxibility - I can pick a mixer from manufacturer A, an EQ from manufacturer B, FX from manufacturer C, a limiter from manufacturer D, a power amp from manufacturer E,
    and speakers from manufacturer F, and the whole mess will work just fine. When it comes to bass preamps, it's anybody's guess.

    Lord Valve
    *******
     
  15. Lord Valve

    Lord Valve

    Dec 13, 2002
    Denver, CO.
    There's a smattering of truth to this, but it's rare - it has to do with the loading the level control imparts to the driving device. If the next piece of gear upstream (an EQ, for instance) has a relatively high output impedance, the resistive loading placed on the device by the amp's level control may cause it to lack headroom (voltage-related clipping, or at least, a softening of transients) and some folks may hear this as "compression." It isn't usually a consideration if good gear is being used.

    Lord Valve
    *******
     
  16. Flight Time

    Flight Time

    Aug 29, 2002
    To tell you the truth, I couldn't hear a difference when he told me that. I chocked it up to a trained ear versus an untrained ear.
     
  17. Bob Lee and Lord
    I know this has been asked before, but please indulge me. I have the Demeter VTBP201S single rack pre amp going into the QSC2402. Is the Dem powerful enought to bring the amp to full power. I usually run in parallel not bridged. Hope you have the spec on this pre. Don't remember what it is.
    Thanks
     
  18. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Exactly. Well said.
     
  19. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    The power amp would have to have the pots right on the inputs, before any input buffer. I've seen a couple amps like that, but not in the past decade and a half.
     
  20. malibu

    malibu Guest

    Dec 26, 2001
    for what it's worth, Crest Audio LA-901