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Amp clipping? huh?

M

MusicMan810

Apr 2, 2008
70
0
#1
My new bands first gig was just friday night and it was the first time my new (well new to me I should say) rig made it on stage. I have a 70's Ampeg SVT810 run by a Crest VS900 power amp and a Sansamp RBI. Ive used it for rehersals with no problem, the guitarist had been using a 20 tube watt combo and I ran my power amp bridged with both dials set at around 9 oclock (maybe around 300 watts). So we get to the gig and it was the first time I had to use my amp with his 100 watt half stack. So I needed to turn up the power amp to match his amp. So I turn up the power amp to 3 oclock (maybe 750 watts). As soon as i played anything through it i noticed the clip light was coming on from the amp. So I turn down the volume from the pre-amp and to try to set it at max power without clipping. I had to turn the the preamp to around 12 oclock.

So long story short.... why is my 1100 watt power amp clipping out at around 400 watts???.... I think I may have to buy a new one
 
#4
I have a few thoughts on this.
I use a 70's 810 cabinet, and the last guitarist uses a 100W vintage Marshall.
Using the GK 800rb (300w into 4 ohms) volume has never, ever been a problem.

You seem to be pumping alot more power than you need to. You must have a problem of some sort. I think the older cabs were rated at 400 watts. can someone verify? Be careful what you pump into that thing.

edit: I'm sure you are clipping way before 400 watts, because that would be real damn loud.
Isnt the clipping almost always caused in the front end of an amp (preamp section)
 
#5
The controls on power amps don't generally control the output power directly. They are really attenuators for the input signal to the power amp section.

The VS900 for max output requires a 1V RMS input signal. The Sansamp can deliver much more than that.

When I'm using sterero power amps, I try to set the attenuators at max (no attenuation) and control the level with the master output on the preamp. I don't try to adjust volume/clipping with the input attenuators on the power amp if I have a volume control on the preamp.
 
#6
slyjoe said:
The controls on power amps don't generally control the output power directly. They are really attenuators for the input signal to the power amp section.

The VS900 for max output requires a 1V RMS input signal. The Sansamp can deliver much more than that.

When I'm using sterero power amps, I try to set the attenuators at max (no attenuation) and control the level with the master output on the preamp. I don't try to adjust volume/clipping with the input attenuators on the power amp.
Click to expand...

Most power amps have gain controls but not input attenuators. Usually you'll find your system is less noisy (less buzz, hiss, etc.) and less sensitive to potential problems (pops from plugging and unplugging, noises from cabling issues, etc.) if you run the power amp at somewhat less than full gain, especially on amps that have a lot of gain to begin with.
 
#7
MusicMan810 said:
So I turn up the power amp to 3 oclock (maybe 750 watts).
Click to expand...

The setting of the knob has next-to-no correlation with how much power the amp is putting out, other than "higher knob setting = more power". Volume knobs aren't calibrated in watts.

If your amp is clipping, you are getting the maximum available power out of it. Ignore where the knob actually is.

Asad
 
#8
Bob Lee (QSC) said:
Most power amps have gain controls but not input attenuators. Usually you'll find your system is less noisy (less buzz, hiss, etc.) and less sensitive to potential problems (pops from plugging and unplugging, noises from cabling issues, etc.) if you run the power amp at somewhat less than full gain, especially on amps that have a lot of gain to begin with.
Click to expand...

I believe the gain control is actually an input attenuator (it seems to be the gain of the input to the power section). If you look at the VS900, the controls are actually attenuators (max is 0dB, min is -80 dB).

I'll try your suggestion about backing off the power amp settings a little, but I've been running mine at max for years.

BTW - love the QSCs - I've got 3! :)
 
#9
slyjoe said:
I believe the gain control is actually an input attenuator (it seems to be the gain of the input to the power section). If you look at the VS900, the controls are actually attenuators (max is 0dB, min is -80 dB).

I'll try your suggestion about backing off the power amp settings a little, but I've been running mine at max for years.

BTW - love the QSCs - I've got 3! :)
Click to expand...

Gain controls usually are attenuators between the input and power stages, but they're not on the inputs.

For most situations where the signal is not so hot that it'll over drive the input section, it's just a semantic issue.

Modern gear is often of good enough quality that you might not get objectionable hiss and other noise with the amp at full gain. But if you do stuff like tinkering around with your effects loop, it's wise to turn down the amp gain so that small mistakes don't get too big. ;)
 
#10
slyjoe said:
I'll try your suggestion about backing off the power amp settings a little, but I've been running mine at max for years.
Click to expand...

My experience working with sound in radio and TV since 1968 is that anytime you run ANYTHING in a signal chain at 100%, you generate noise....and the need to do so usually indicates a problem in the signal path.

I make it a practice never to run amplifier and audio signal controls at more than about 75% if there's any way to avoid it. I find that with preamp controls it's often easy to over-modulate, so I'd rather keep preamp controls well below 75%.

Result: cleaner signal, less tendency to clip, better sound.
 
#11
Bob Lee (QSC) said:
Gain controls usually are attenuators between the input and power stages, but they're not on the inputs.

For most situations where the signal is not so hot that it'll over drive the input section, it's just a semantic issue.

Modern gear is often of good enough quality that you might not get objectionable hiss and other noise with the amp at full gain. But if you do stuff like tinkering around with your effects loop, it's wise to turn down the amp gain so that small mistakes don't get too big. ;)
Click to expand...

I think we are saying the same thing - when I said input to the power stage. You're right it's a semantic issue; I see a lot of posts here that seem to think the gain controls on power amps are on the OUTPUT.

yea - small mistakes getting too big - that's what amps are for, right? ;)
 
#13
slyjoe said:
I think we are saying the same thing - when I said input to the power stage. You're right it's a semantic issue; I see a lot of posts here that seem to think the gain controls on power amps are on the OUTPUT.

yea - small mistakes getting too big - that's what amps are for, right? ;)
Click to expand...

"Input attenuator" to most users would imply that the attenuator is on the amp input. That would be rather expensive to do with a balanced input! The misnomer bites some people who have really hot signal levels that overdrive the amp input stage and mistakenly believe that trimming the so-called "input attenuator" will cure the problem.
 
#14
I ended up having the power amp set at 75% and the pre amp at 50% and that was about the max i could get without the clip light coming on (bridged at 4ohm). I even had to turn down the bass and drive down. The sound of it was fine, that didnt scare me at all. Just the light was making me nervous. All I run in my rack is the Crest,Sansamp, and a furman. I am pretty cautious of not putting too much power to the old cab because i know its rated for 350 watts, thats the reason why I normally run the power amp at 25% but I needed some more juice for the gig. Maybe running the amp stereo would help. Obviously Im no expert on power amps, this is my first rack setup. Does it sound like theres a problem with the power amp or is it just different input levels from the pre amp? The amp is kind of old, it has the bananna style outputs. Maybe I might be better off upgrading this.
 
#15
Though it can take a bit more than its rating, a 70's SVT cab was rated at 250w back in the day. Also, turning the dial 25% doesn't necessarily mean you're only putting 1/4 of the wattage into a cab.
 
#16
MusicMan810 said:
I ended up having the power amp set at 75% and the pre amp at 50% and that was about the max i could get without the clip light coming on (bridged at 4ohm). I even had to turn down the bass and drive down. The sound of it was fine, that didnt scare me at all. Just the light was making me nervous. All I run in my rack is the Crest,Sansamp, and a furman. I am pretty cautious of not putting too much power to the old cab because i know its rated for 350 watts, thats the reason why I normally run the power amp at 25% but I needed some more juice for the gig. Maybe running the amp stereo would help. Obviously Im no expert on power amps, this is my first rack setup. Does it sound like theres a problem with the power amp or is it just different input levels from the pre amp? The amp is kind of old, it has the bananna style outputs. Maybe I might be better off upgrading this.
Click to expand...

Doesn't sound like there's a problem, you've just reached the maximum you can get out of your amp. If you feel you need a little more power you'll just have to get another poweramp like you suggested, either that or play about with your EQ/gain stage in general, so your getting the most effeciency out of it possible.

You'll notice if your cab starts to complain though, putting too much power through a speaker makes come very nasty noises that you'll probably notice very quickly.
 
#17
I'd watch the amount of "sweetening" the RBI was giving you. My BDDI will turn the sound to goo if I even approach 50% on the blend knob. It has taught me to really hate mid-scooped tones. I think that the RBI is probably the culprit. It's probably fine if you don't use it to make the sound as Sans Ampy (new adverb here) as possible.

I haven't sold my BDDI and I wouldn't preclude ever owning an RBI, but they really do have to be approached with caution or they'll eat up all of your headroom and you'll be thinking "Why can't I hear myself?"

KO
 

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