School me on power amp/pre-amp setups

[BurningSkies]

In terms of racks, I favor 3 space if I can get away with it (if you have a one space preamp) or 4 spaces. I hate going any more than that. You can find commonly marketed 'roto' racks in 3-5 space increments fairly inexpensively.

On how not to blow your speakers? Be careful. Start by not bridging and see what you're getting for volume and what your speakers' 'comfort' level is that way. Then bridge and turn your attenuators WAY down...bring up slowly as you test your speakers' ability to handle what you're feeding them.

 

[pedulla-2007]

All this talk of pre/power amps and only two or three pictures? Can This give the OP any ideas? This is my practice amp in my bedroom, so please forgive the clutter. Good luck with your decision.

 

[::::BASSIST::::]

Unbridged my amp puts out about 400w per side at 8 ohms. My cabs can handle 600w and probably a little more (3012LFs in my fEARful 12/6 and 12sub). Running each cab per side at 400w I can get the pre to clip with the attenuators set at max and end up with some distortion. At that point I feel the cabs are not being driven to their full potential at those settings and there is still power left on the table as far as they are concerned. My amp has the power I just need to figure out how to get it to the cabs without the magic smoke.

Then bridge and turn your attenuators WAY down...bring up slowly as you test your speakers' ability to handle what you're feeding them.

I guess this is my next/last option?

Link Removed

 

[BurningSkies]

Unbridged my amp puts out about 400w per side at 8 ohms. My cabs can handle 600w and probably a little more (3012LFs in my fEARful 12/6 and 12sub). Running each cab per side at 400w I can get the pre to clip with the attenuators set at max and end up with some distortion. At that point I feel the cabs are not being driven to their full potential at those settings and there is still power left on the table as far as they are concerned. My amp has the power I just need to figure out how to get it to the cabs without the magic smoke.



I guess this is my next/last option?

Its really the most logical. Spend some time playing, slowly bringing the bridged level up. You'll hear when the speakers start to complain.

-> coming from someone running a ~3000w amp bridged into two 3015LFs.

 

[Downunderwonder]

When you say the pre is clipping do you mean your pre or your amp input?

 

[::::BASSIST::::]

Ah, good catch.

Its actually the clip light on the qsc. What I did previously was to unplug my cab and set my pre as high as it would go til it would just clip (gain at noon, master at 3:00 on the Tone Hammer). That indicated to me the max I could go before clip and not to go any further on the pre.

 

[Downunderwonder]

Tone hammer has a mess of gain then, according to QSC it's 1.4v to get there.

As you bring up the bridged attenuated level two things can happen, the subs will go into distortion then fart, or it will stop getting much louder due to power compression, or both. Either way you got your cue to dial it back a click and call it good.

Hope you have good earplugs and noisy neighbours or it's going to get messy.

 

[::::BASSIST::::]

Yeah the Tone Hammer is quite the powerful unit. I tried the new Tech 21 VTBDI, but went back to the TH in short order.

I'll have to bring my rig to a rehearsal studio or something and mess around with it as it will be loud... and get out my earplugs too.

 

[::::BASSIST::::]

Its really the most logical. Spend some time playing, slowly bringing the bridged level up. You'll hear when the speakers start to complain.

-> coming from someone running a ~3000w amp bridged into two 3015LFs.

1500w per driver? That makes me feel better. Thats alotta guacamole!

 

[Downunderwonder]

Could set them on fire with 1500W.

 

[::::BASSIST::::]

I assume he's actually not using that much.

 

[Downunderwonder]

That was my point.

 

[BurningSkies]

1500w per driver? That makes me feel better. Thats alotta guacamole!

Could set them on fire with 1500W.

I assume he's actually not using that much.

That was my point.

Exactly. When I first got my amp, I ran it paralleled unbridged for about 500w per cab for a while. Once I was pretty solid on what the volume and capabilities were at that level, I bridged. When I first started bridging, I started with my preamp set just like I always had it for non-bridged operation. I took some time and started with the attenuators attenuating in full, then slowly adjusted them up to the same volume level I was getting without bridging. After that it was a matter of either goosing them up a little to see how comfortable with the power the cabs would be, or backing it down if I thought I was nearing their limits.

I also run a HPF with a steep roll off at 35hz, so that gives me a bit of extra protection from some of the excursion in the deep lows that might cause damage due to the fact that I DO have a lot more power to push down in that range.

Remember you can get to your MAXIMUM output without having the attenuators all the way up... so no matter where they're set you still need to be cognizant of what's going on.

 

[CL400Peavey]

1500w per driver? That makes me feel better. Thats alotta guacamole!

+1

I run my Prolite 3.0 bridged into my '66. I have never gotten to the point of hearing distress from the cab. But I am in no way using all the power from the amp. Having the head room is very nice though.

 

[bgavin]

"What do you call a guy who takes a $10,000 in gear, in $1,000 car, and hour each way for a $50 gig? A musician"

That is why we play music for free.
We only charge to move the equipment.

I have several 4RU RoadRunner ATA racks.
These are very sturdy, beautifully made. And heavy.
On my beam balance scale, these weigh 25.0 pounds empty.
Add +21 pounds for a PLX, 9 pounds for the SVP-Pro, and 5 pounds for an active crossover.
This makes for a 60 pound package intended as a one-hand carry.

The lighter version is an SKB 3RU XRack3 at 7 pounds.
Add in a PowerLite 2.0 with built-in crossover at 12 pounds, and Sansamp RPM at 5 pounds.
This makes a one-handed carry at 24+ pounds.
For residents of Geezerville, this weight reduction makes a huge difference.

 

[drpepper]

I dipped a toe in briefly and recently decided to give it another go with a Crest ProLite 2.0.

Zoom B3 is my preamp, so that makes the amp's attenuation the most viable volume control for me.

 

[Bob Lee (QSC)]

My understanding of power amps is that they like to have their gains set at maximum and then control the volume with the volume knob on the preamp.

Power amps are in no way offended by having their gain controls set at less than maximum. If they were supposed to be run at maximum gain they wouldn't have knobs.

In fact, a system will generally have a better signal-to-noise ratio if you use power amp gain judiciously and make up for it by having a strong signal level early in the chain.

 

[Bob Lee (QSC)]

Giving it maximum output means maximum headroom. This prevents the amp from clipping when it doesn't have to. Then I use the preamp output to control total voltage output to the speakers.

Doing it this way maximizes headroom and minimizes the likelihood that your power amp will clip. IMO if you don't dime the power amp, then you're not using all of the power that's available in your amp, and you've bought a large amp for no good reason.

No, that is not true. The gain control on a power amp is not a power control that reduces the amp's power capability. It's just a gain control.

 

[Bob Lee (QSC)]

> Dialling down the amp actually doesn't limit the amp itself. It only raises the input needed to push it to full output.

Looking at this non-musically, from a tech perspective It's important not to confuse the function of an input sensitivity control with that of a volume control.

It's also important not to confuse the gain of the amplifier with it's power output.

Will's point is true -- that the gain of a closed NFB loop won't be changing. OTOH the function of a "volume" control on an amp depends entirely upon where it's been placed in the circuit. They typically work by shunting a variable portion of the input signal to ground. Shunt enough signal to ground and then you aren't ever going to drive the amp to full output, even though the closed loop gain may not be changing. Throw away enough signal and you're never going to be able to drive the amp to it's full power output, regardless of how high an input signal you might supply:

Throw away enough input signal by shunting it to ground will result in the need for an an infinite level of input signal to push the amp to full output. Obviously, that can never happen, because there is a limit to how large an input signal can be before it will overdrive / clip the input stage of the power amp. Clipping the input stage will happen long before the amp ever gets driven to full power by your high level input signal. If you turn down the power amp to the point that you have to overdrive the input stage then the gain of your 1kW power amp is accurately reproducing a clipped input signal. What's the point in that?

If you have a 1kW amp that never gets the throttle opened beyond 12:00, then it's never going to deliver 1kW. You might as well have bought a smaller amp.

If your amp has a noise problem that's a separate issue. A properly functioning power amp should have a low noise floor and can be run full-open without producing a disproportionate amount noise. (As an example of this, many HiFi amps don't even have volume controls, they run flat-out at all times and they rely entirely upon the level of input signal coming into the amp to determine their power output. That, in turn, is defined by the preamp's volume control.) Similarly, a properly functioning and properly configured set of separate components should maintain a high SNR so that volume adjustments at the last amplification stage in the signal chain won't need to be relied upon as a noise gate.

From a tech perspective, the gain control on a power amp is just a gain control--i.e., how much the amp multiplies the input signal voltage. If you want to call it a volume control, input sensitivity control, attenuator, whatever, it's still a gain control.

The noise issue with running high gain on a power amp isn't that the amp itself is noisy, but that it's boosting a weak signal and its noise floor a lot more than might be necessary. It's generally better to have a strong signal--one that's well above the noise floor--way up front in the signal chain and use less gain in the final stage than to have a weak signal, minimally above the noise floor, and then boost everything a lot. That tends to result in more hiss, buzz, and other noise.

 

[Bob Lee (QSC)]

This may well be one of the most important, but most easily overlooked points in the thread. As a practical application, it's worth revisiting this point -- by turning down the input attenuator on the power amp, you'll end up increasing the amount of rotation of the preamp's MV control that corresponds to the desired change in volume. In other words, decreasing the amplifier's "sensitivity" or "volume" allows you to have a finer degree of granularity in the preamp's MV control. For some people this might be important.

The vast majority of pro power amps have gain controls but not input attenuators. A general rule of thumb is that if the amp has balanced inputs, input attenuators would be highly impractical; if the amp has only unbalanced inputs, input attenuators are at least feasible.