Technical Questions Regarding My PA

Discussion in 'Location Recording' started by JesterMasque, Sep 6, 2007.

  1. JesterMasque

    JesterMasque Active Member

    I own a late-90's Mackie 1604-VLZ mixer, a late-90's Mackie 1400i power amp, and two Grundorf ST-152 speakers. FYI: This equipment was given to me about three years ago and I have some questions:

    1. On the power amp, the channel one input is fine; however something happens inbetween there and the output that the causes the output not to work. It got great reviews all over the web; what do you think about it? Is it worth fixing or should I buy an amp that will not die on me like that?

    I have a theory for why the output is dead. I have learned that if you have nothing on the other end of the output cable, but still run the gain up on the speaker, that the amp will fry. I ran a dead speaker for a long time, obviously unknowingly. I fixed the speaker and the amp ran great for a while, then the channel died.

    2. Is my power amp/speaker compatible? The amp is 500w RMS stereo @ 8 ohms. The speakers are only 150w @ 8 ohms a piece. I know it is bad to under-power a speaker, what about over-powering? Would my potential over-powering be causing the unusually sensitivity of the system to feedback (which is actually another great concern of mine)?

    3. Three channels on the mixer are dead, that is, no input signal. Two were already upon receipt, one happened just over a year ago. The mixer sits in a power conditioned, practically sterile control room. What would cause these channel-deaths? I am apt at soldering/working with electronics. Is there anything that I should check out that I could fix on my own?

    4. Please comment on my system. Should I trash the amp and buy one suitable for the speakers? Should I fix the amp and buy speakers suitable for the amp? Or, is my setup fine how it is and I should just get the amp fixed?

    I understand that this is a lengthy post. However, pro audio is a lengthy subject so I guess I am in the right place. Thank you so much!
     
  2. HansAm

    HansAm Active Member

    It will cost a little money. But you should have them reviewed bye a service technichian.

    Damage control.

    Very hardt to know if its worth it or not.
    Could be old woren out pots (cheapo). or something could by fried (expensive).
     
  3. JesterMasque

    JesterMasque Active Member

    Have the amp reviewed you mean?
     
  4. HansAm

    HansAm Active Member

    Hm. "have it looked at". cost estimate.
     
  5. moonbaby

    moonbaby Mmmmmm Well-Known Member

    OK. Please read through this and don't take it personally...
    Your statements are a bit confusing.
    A) How do you know that "channel 1's input is fine" if you're getting no sound from the output? BTW, the 1400i has been upgraded at least twice since that model was introduced in the 90s, and has been plagued with problems. Are there any warning LEDs on the front panel that come on when you power up the amp? How about that "HOT" lamp when you power up cold?
    B)"Nothing on the other end of the output cable, but still run the gain up on the speaker..." HUH? There is no gain control on a passive speaker. At the most a HF driver level attenuator. What did you do to "fix" the problem with the speaker?
    C) It is bad to OVER-power a speaker cabinet to the point it will burn out a driver or even the crossover. It is bad to UNDER-power a cabinet if you're constantly pushing the power amp into "clipping" distortion in an attempt to get enough volume from the cabinet. That "clipping" can kill a HF driver.Fast.
    D) If you were pushing the amp to operate into a speaker that was "dead", YES you can cook the output stage of the power amp. Transistors like to "see" a load. Correction: you can damage the protection components that are there to protect the transistors.
    E) I sit in a "almost sterile" control room. There is a power conditioner, an air conditioner, and a Glade automatic air fragrancer on the wall. One day I'M gonna die. So will cheap gear. Especially if it isn't properly operated, which brings us to...
    F) "Unusually sensitive system prone to feedback". Really. You need to learn to run the gain structure of your system PROPERLY. This is a big problem for a lot of musicians, and I suspect that it's a problem here.
    Don't take this personally!!! Mackie (as well as Yamaha and Rane and others) have published many documents addressing this. Check 'em out.
    G) You can solder, that's good. If you can open up a Mackie mixer to look at the high-density PCB, you've got one on me, dude. They are notorious for being a service tech's nightmare. Leave it to somebody who's prone to masochistical tendencies. No offense, but you'll probably aggravate the issues. Anything past a cable or accessible fuse issue, leave to the experts.
    H) I have had nothing but BAD experiences with Mackie power amps. When they first came out, I thought,"Gee, I like my 8-bus. I like my little mixers. They claim to be 'professional'.I hate spending $$ on my Crowns and Crests. What could go wrong?" But, as our esteemed moderator, Sheet, so eloquenty (and correctly) pointed out, I was trying to do a mans job with toys (I'm paraphrasing here). Now I use QSC, Crest, and God grant me another bank loan, Labgrupen (Google them for a heart attack!) to power my stuff. And some Crowns for the itsy-bitsy gigs. Buy a QSC. Or a Crown. Or even a damned Peavey.
    I) Grundorf makes heavy wooden cases that don't latch together very well. I did not know that they made speakers. Even dead ones. Are you using these for a studio? To record? Or to playback a live band? One that's good for one application, may not be suitable for the other.
    Give us some more feedback (not the 2K kind!) on what you want your system to do. I promise I'll be nicer. It's 5:30am and I've been mixing "Little Green Men" and their 80's routine all night... :lol:
     
  6. Boswell

    Boswell Moderator Distinguished Member

    You are describing this from the point of view of what you think you know about the problem and not what we need to know to help you.

    What do you mean when you say "the channel one input is fine" but there is no output? Do you mean that you know you are putting a good signal into channel one? What about channel two - does that work OK? What happens if you interchange the inputs?

    This would be true for valve (tube) amps, but is not likely to be the cause in this case.

    With that amount of power available, you are in danger of blowing the speakers. You may be able to restrain yourself in terms of how loud you run it with music, but it only takes a phantom power pop or a guitar lead being plugged into a battery-powered pickup to put a damaging surge through the system. Trying to get too much out of an underpowered amp can give rise to harmonic distortion that can fry tweeters, but most main speaker damage is caused by excessive power.

    The power mismatch has nothing whatever to do with sensitivity to feedback. You get feedback when excess gain is used for the system as set up, and is due to poor microphone and loudspeaker positioning, inappropriate microphone polar patterns, unfortunate venue acoustic properties or, most often, deficiencies in technique.

    What do you mean by "no input signal"? There will be an input signal there if you give it one. Do you mean that some of the channels do not sum into the mix? If so, do the auxes work on those channels? Do you get any send signal on the channel inserts?

    Get a quote for fixing the amp and also the mixer. Consider getting new speakers that will handle the power from this amp or a different amp if you decide to get one in the future.
     
  7. JesterMasque

    JesterMasque Active Member

    First off I would like to thank both of your for your speedy and thourough responses! I love this forum so much. No for the rebuttle...

    Moonbaby:
    A) There are LED's on the front that tell you the level of the signal POST gain in the amp. They light up and adjust accordingly when I turn up/down the gain, but no sound comes out. The hot lamp does not come on when I power up cold.
    B) I thought that everything was ok until one day I realized that one speaker was not working. All the while before, I was running the amp at normal gain settings. After testing I discovered it was the speaker. So I was running power into a source that was not recieving it(?). Anyway, Boswell knew what I meant and told me about the difference between tube/ss amps. I learned that it damages the amp by reading the manual for my Mesa 50:50 btw... On the speaker it turned out that an old wire on the back panel just had a bad/loose connection. That is how I fixed the speaker.
    C) Now I understand; thank you for clarifying for me.
    D) That is my theory for why the channel is dead. Could that possibly be a big money fix?
    E) My point was that it was not used in a dirty or electrically unstable environment. With all due respect, I do know how to operate most of my gear properly. Power amps/speakers are the only things in my posession that I have never done enough research on because I can hardly understand the physics that goes along with the explainations and no one has been able to explain them in lay-man's terms CLEARLY. Except you guys... you all rock with knowing your audience.
    F) That probably IS my problem more specifically regarding how much gain to run from the amp to the speaker. I know how to run the mixer accordingly, but not the amplifier. I tried to read Mackie's explaination the other day but again, it just confused me with all of the physics. Honestly, the only science that I am not totally apt at learning and it holds me back from doing the things I love the most! *shakes fist* I will check out the Yamaha and Rane docs. Thanks for the reference.
    G) I was thinking that it might have been a common problem solved by an experienced solderist(?). I will go so far as to say "most," but a lot of select problems can be fixed by getting your hands dirty a bit.
    H) My dad has a Peavey CS-800. That thing is seriously incredible! It is about twenty years old and only needed service for the on/off switch. The thing seriously sounds amazing too! He powers a pair of Peavey SB-4's with the Black Widow drivers. Kick ass stereo setup...
    I) I had no idea either. (lol at your comment) I use it just a general PA for my practice space. I use it to jam to music on my guitar, run my singer's vocals, or his electric-acoustic guitar. I have on occasion needed to bring it along for a small gig at a crappy venue, which turned out to be ok running drums and vocals through. The drivers held their load quite well when put under that kind of pressure.

    I want to have a PA system good for practice as well as an emergency system in case we need it for a show. Given that, would a 150w stereo amp be loud enough to use in a live situation? If not, I may just buy new speakers and keep the amp I have, for now anyways. That is until I can afford to buy a new one before this one dies again. BTW moon, that sounds like sheer torture.....

    Boswell:
    1. What information do you need for you to possibly help me?
    2. Addressed in answer A to moonbaby
    3. As said before, thank you so much for clarifying that for me. I learned that by reading the manual for my Mesa 50:50 tube power amp. However, review moonbaby's D answer that he wrote to me please.
    4. At practice the system is run very quietly, so I believe that I will be ok running it with one speaker for now until I either decided to get a new amp for the speakers or new speakers and get the amp fixed.
    5. Would not all of those things be deficiencies in technique? Well, for me it is probably a combination of the gain dysfunction and acoustics because I can try all different positions for hours and still get feedback. I admit to not knowing how to set-up a live PA accordingly. Mic technique, tone, stage setup I am A level. PA gain and leveling, about C to C+ only because I am not blowing speakers left and right and things are still audible.
    6. I meant that the mixer does not read the input signal, specifically 4, 11, and 12. 4 and 12 were already not working when I got it. 11 died a little over a year ago. When I plug in a source and turn up the trim I get zero sound nor a reading on the VU meter. I have never used the auxes or sends. I will try that out when I get home tonight. That will tell me if the problem is either on the input side or something deeper in the strip, right? I guess that is a good thing to know. I do not want to get ripped off IF I get it fixed.

    Thanks again for your awesome responses! I hope I was able to clarify to your needs.
     
  8. Boswell

    Boswell Moderator Distinguished Member

    But yes for the rebuttal? Moonbaby was composing his excellent reply at the same time as I was composing mine, so I didn't have a chance to see his before posting.

    I think the information has leaked out that one channel of your power amp works OK. So, you just have a dead channel. Wire both speakers to the working output and run it as mono until you can get it fixed or buy another one.

    For the mixer, the shortest path that you can test easily is from the line input on a channel to the insert point output (tip of the insert jack). If a signal input at the line in appears at the insert output, try it then at the direct out (channel 4 only). If it's OK there, the mute, fader or assign switches are the problem. You haven't got the faulty channels muted or the L-R button up, have you?
     
  9. JesterMasque

    JesterMasque Active Member

    My apologies. I meant: "Now for the rebuttle:" :p

    I only have one 1/4" output for channel two. However I do have a banana cable out put. Can I use both of the outputs at the same time? I will pick one up from Guitar Center right now just in case, since I am going there.
     
  10. moonbaby

    moonbaby Mmmmmm Well-Known Member

    I looked at the Grundorf ST-152 speakers on their website. Actually, they rate the power capacity as 250watts RMS, 500 watts "music power". A pair of these should work fine without getting blown up by the 1400i, especially for the type of use you described.
     
  11. JesterMasque

    JesterMasque Active Member

    Maybe the new ones, however mine specifically say 150w on the back.
     

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