School me on power amp/pre-amp setups

[BurningSkies]

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.

Its also probably important to point out that just because you're using your attenuators, doesn't mean that your amp can't generate its rated power.

Yes I have a high wattage power amp, no I don't feel like I'm missing out on using 'all its watts'. Power is cheap these days...no excuse not to have extra horsepower on hand, even if you're not planning on using it. Unless I'm getting power amp input clipping, my amp isn't distorting on the output.

 

[will33]

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.

I don't even qualify as geezer yet and I'm still ditching my big rolling rack full of PA gear and building it down into 3 smaller ones. One of those will contain only what's needed to run subs and will stay home for 90% of the little gigs we play.

I don't mind heavy stuff during load-in and setup....it's at the end of the night when it sucks.

 

[bgavin]

Yep.
My bass horns and big iron stay home 11 months out of the year.
The only gigs needing that much oomph are outdoor-types or slammin-loud dance parties in a big venue.
This becomes a big production, i.e. trailer, dollies, and all that.
They all get Ugly at load-out time.

I wish I would have opted for the lighter SKB 4RU instead of the 2x heavier ATA types.
We don't abuse our gear, nor do we tour.
That level of strength (read: weight) is overkill, and simply not needed.

My advice to the OP: go light, go simple. It is more fun and less work.

 

[BurningSkies]

Yep.
My bass horns and big iron stay home 11 months out of the year.
The only gigs needing that much oomph are outdoor-types or slammin-loud dance parties in a big venue.
This becomes a big production, i.e. trailer, dollies, and all that.
They all get Ugly at load-out time.

I wish I would have opted for the lighter SKB 4RU instead of the 2x heavier ATA types.
We don't abuse our gear, nor do we tour.
That level of strength (read: weight) is overkill, and simply not needed.

My advice to the OP: go light, go simple. It is more fun and less work.

I have one of the standard SKB racks that has been in use since about 1992 without failure. It houses my current rig these days. Even when we did a bunch of roadtrip playing it was more than enough. If I had a production crew, a gear truck or van and strangers handling my gear I'd go full ATA (which I also have), but for the same reason, I carry a good quality gig bag instead of a hard case for most gigs these days.

 

[Jim Carr]

To the OP:

I see you have a UL310. That's your 5.3 ohm load. You mentioned a 4 ohm cab, too, but didn't name it.

You don't want to bridge. The HD2000 can handle as low as 4 ohms bridged, and a 5.3 ohm and 4 ohm cabs combined give you ~2.286 ohms, which is below the minimum load your bridged amp can handle.

IMHO, you will want to hook each cab to its own channel and balance them with the gain/attenuators/whatevers on each power amp channel.

I also recommend the 4U rack. If you want to, you can use a 2U pre, can carry other items such as two 1U pre-amps, or you can leave a space for better cooling. Have fun!

 

[DiabolusInMusic]

To the OP:

I see you have a UL310. That's your 5.3 ohm load. You mentioned a 4 ohm cab, too, but didn't name it.

You don't want to bridge. The HD2000 can handle as low as 4 ohms bridged, and a 5.3 ohm and 4 ohm cabs combined give you ~2.286 ohms, which is below the minimum load your bridged amp can handle.

IMHO, you will want to hook each cab to its own channel and balance them with the gain/attenuators/whatevers on each power amp channel.

I also recommend the 4U rack. If you want to, you can use a 2U pre, can carry other items such as two 1U pre-amps, or you can leave a space for better cooling. Have fun!

The 4 ohm would be fEARful or Markbass. I do not really intend to run it as a stage rig, I know the Markbass and Epifani pair poorly from a tonal standpoint, I was just curious how to do it, since I have to try it at least once.

 

[amimbari]

I have 2 go-to SS poweramp rigs. one is 23lbs in a 6u rack and the other is almost 15 in a 2u rack.

Nice thing about these small light skinny amps is they dont take a lot of room inside, so on my "coffeehouse rig", I just put the preamp in the back. The 2x10 is 4ohms running off the left side, and if I choose to use a 4x10 cab under it, that would run off the right side.

.

 

[WretchedExcess]

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.

I don't mean for this conversation to devolve into an argument about semantics -- I certainly don't want to revive the thread where we saw two competing amp manufacturers arguing with one another about their operating classes, but it looks like this thread is being nitpicked semantically to a similar extent.

Suffice it to say that it would seem that different manufacturers like to use different words to express the same concepts. I expressed some comments previously that were in line with the terminology as used by a competing manufacturer. In fact, some of the comments were lifted from their product literature.

As I noted previously, the technicality of the circuit is that closed loop gain of an amplifier with negative feedback is fixed. A design engineer can look at the resistors in the negative feedback loop and calculate the amplifier's gain quite easily. The amplifier's gain is fixed. It never changes. What is allowed to change though, is the input sensitivity of the amplifier, which is adjusted with a knob on the front panel.

What any particular amp manufacturer might prefer to label as a "volume", "level" or "gain" control is done for the convenience of the end user. The reality remains that it is actually an input sensitivity control. It determines what input voltage will cause the amplifier to produce it's rated power.

If someone wants to look at an amplifier as a simple black box, they can say simple things like "Vin * gain = Vout". At first glance one might falsely assume that the gain of the amplifier is variable, but the reality is that it is not. The amplifier's gain is fixed, and determined by the negative feedback loop. By changing the input sensitivity of the amplifier it's easy to make it appear as if the gain is actually changing, but the reality is that what people like to refer to as variable "gain" is actually the product of input sensitivity and closed loop gain. What actually happens is that the input sensitivity control (what you're calling a "gain" control) is designed to throw away voltage, so that Vin is what gets varied, while closed loop gain of the amplification stage remains constant, and the product of these numbers changes.

I guess what really matters then, is if you're an engineer looking at the input sensitivity, along with feedback loop that defines the amp's gain, or if you're a user looking at how much the amp's output voltage changes when you tweak the sensitivity knob. Someone can call them both "gain" if they want to, but what's important to remember is that the closed loop gain remains fixed, and never changes; the "total amplifier gain" is the product of input sensitivity (which is variable) times closed loop gain (which is fixed).

Charlie posted a helpful reference earlier. It was sort of hidden at the bottom of one of his posts near the signature line, which many people train themselves to ignore.

[Invalid or Expired Link Removed]

 

[Bob Lee (QSC)]

This is not about semantics. A power amp comprises several stages: at least two with fixed gain (input and out/power) and usually one with variable gain (the gain control).

Cascading gain stages multiply. Therefore you have the fixed input section gain (usually in the range of 0.5 to 2), multiplied by the variable gain of the "attenuator" (0 to 1), multiplied by the fixed gain of the output section (10, 20, 30, 40, whatever).

In a PLX1602, for example, this would be 1 × [0 to 1] × 40. Varying that middle term varies the overall gain of the channel proportionately. That's what makes it a gain control.

You have a partial familiarity with amplifier circuitry but because it is only partial you keep making misstatements, like "The amplifier's gain is fixed." It is not.

 

[WretchedExcess]

What I said remains as true, regardless of whether someone prefers to use the term "variable gain" or "insertion loss" or "sensitivity control" or "attenuation."

Essentially you're demonstrating that we're on the same page when you make the point about cascaded gain stages multiplying. As you've noted, there are three gain stages: A, B, and C.

In your example, total amplifier gain = A * B * C; where:

A = fixed gain of input stage
B = variable insertion loss of the sensitivity control
C = fixed gain of the negative feedback amplifier.

The interesting thing about the sensitivity control is that it has a gain that's less than one. I like to think of that as insertion loss, rather than as gain, but again that leads us directly to semantics.

In the big scheme of things I think we're on the same page, but you seem rigid about the use of the term "gain" and only the term "gain". You seem unwilling to accept the use of the term "input sensitivity." It's commonly used in the industry (See the Rane tech notes, or the literature of other amp manufacturers).

All in all, I think we're in violent agreement conceptually, with only a few words getting in the way. I think we're making progress. At least now the term "attenuator" is back on the table.

 

[Bob Lee (QSC)]

What you had said is partially true but it is misleading because it omits some important concepts. And I think that's why you misunderstand it.

"Gain" is the proper term: Vout/Vin = gain. As I stated, cascading gain stages multiply, even if they are less than 1. Multiplication works quite well with numbers less than 1 as well as those equal to or greater than 1.

Input "sensitivity" is a commonly used term in power amp marketing specs, but it does not mean what you think it does. It refers to the input signal voltage that, at maximum gain or some stated reference gain, produces the stated output power rating into a particular resistive load impedance. I think it's an unfortunate choice of words, because a higher input "sensitivity" might sound synonymous with "higher gain" to someone unfamiliar with what the term actually means. But traditionally used terms tend to be hard to dislodge even when they're misleading or technically inaccurate, like so-called "RMS power."

 

[WretchedExcess]

You have a partial familiarity with amplifier circuitry but because it is only partial you keep making misstatements, like "The amplifier's gain is fixed." It is not.

What we have here is failure to communicate.

You misunderstood something very specific that I said, and the result is that you keep accusing me of having partial familiarity and of making misstatements. You're wrong to do that.

I have more than partial familiarity, and I said something quite specific that was no misstatement. Unfortunately, you didn't comprehend what I had said, and the result is that you're applying my statement out of context and falsely accusing me of unfamiliarity and of misspeaking.

I made specific reference to a specific subset of the amplifier circuitry which operates within the confines of a closed negative feedback loop. My reference to "closed loop gain" can only be correctly interpreted to refer to the gain of a particular segment of the amplifier that operates under the control of negative feedback; it is a gross oversimplification to read my use of that term and to mistakenly interpret that I was referring to the net gain of the entire amplifier. This is where the problem lies. I used that term in a specific context that can only refer to one sub-segment of the amplifier, and you're mistakenly generalizing my statement as if I had applied it to the entire amplifier, when I did not. What's ironic is that as you do that, you're claiming that I'm the one who is wrong!

"Closed loop gain" can not be applied correctly as a reference to the net gain of entire amplifier. I never used the term "fixed gain" it in that context, even though it appears that you are misreading my statement to have been made in that context. The result of your misunderstanding of what I had actually said has led us to where we are now, where you're accusing me of partial familiarity and of making misstatements. Let's be fair -- the real problem is failed comprehension on your part.

To clarify:

I never said that the amplifier's total gain is fixed so please stop saying that I did, and please stop saying that I'm wrong because of something that I never said. What I actually said was that the closed loop gain of the negative feedback circuit is fixed. Just to make it clear for people who are reading along and have only partial understanding of amplifier circuitry: any competent design engineer understands that the "closed loop gain of the NFB circuit" refers ONLY to the segment of the amplifier that falls under control of the closed NFB loop; this is the output section of the amplifier, which I referred to as "C" in the preceding post. Any circuit designer will understand which portion of the circuit operates within the confines and control of the closed negative feedback loop. In unknowing agreement with my claims, your previous post even asserted that this portion of the amplifier had fixed gain that never changes:

Cascading gain stages multiply. Therefore you have the fixed input section gain (usually in the range of 0.5 to 2), multiplied by the variable gain of the "attenuator" (0 to 1), multiplied by the fixed gain of the output section. [emphasis added]

So even though we're in agreement that the gain of the closed loop is invariant, I think you failed to appreciate that my reference to "closed loop gain" can only refer to that section of the amplifier controlled by the NFB loop, and you've mistakenly interpreted my statement to refer to the entire amplifier. The result is that you're now accusing me of incomplete comprehension. That's not accurate.

Again, I think the reality is that we're on the same page, but there's just a communication failure. I'm more inclined to place the blame on semantics, or perhaps quick skimming of the thread, rather than trying to accuse someone unfairly of not knowing what he's talking about.

 

[bgavin]

 

[Bob Lee (QSC)]

Here's the thing: you said that an amp has fixed gain because of a closed NFB loop. The truth is that parts of an amp have fixed gain because of a closed loop--not the entire amp. The input stage is one of those closed-loop, fixed-gain circuits; the output stage is another. I'm pleased that you now recognize that the attenuator in between, whose gain is user-variable between 0 and 1, them alters the overall gain. I'm also pleased that you now recognize that the gain control does not regulate how much power the amp can put out, and that turning the gain control down does not in fact reduce the amp's power, only its gain. I urge you to continue learning about amplifiers, and stop blaming your errors on semantics.

For example, here are some main points of your misstatements:

several truths remain:

* dialing down the amp does indeed limit the amp's output -- it's why people do it
* closed loop gain may never change, but
* input attenuators do change the available voltage out by decreasing the amount of signal swing available for amplification
* if you never open up a power amp, then you've bought more amp than you need

> "An attenuator does nothing to the available voltage out."

I guess that statement would be true if you were talking about a load attenuation device, but in the context of this thread haven't we all been talking about input level attenuation?

 

[Downunderwonder]

Wretched, you made gross misstatements to the nature of noise and headroom. I'm no expert but I picked up a few basic concepts from guys like Bob who are an absolute treasure to the community. I'm glad he picked up on the nonsense. Please give up the semantic gymnastics.

 

[will33]

I've always taken it to mean a poweramp multiplies the voltage it's fed "X" times....and by turning those knobs, I am not changing how much power the amplifier can make. It means if it's clipping, I can reduce the level of the signal it's being fed (that it will multiply "X" times) further toward the beginning of the chain, or, if that stuff happens to sound good set where it is for whatever reason, I can reduce it using the poweramps knobs on the front.

I use this way of thinking to also make tube amps with a master volume control behave more like non-master volume tube amps by setting the master volume wide open, or at least quite high. Some don't sound that great completely cranked, I assume because there are a couple more parts in the signal path instead of being wired "straight through" so to speak.

Works for me whether I'm setting gain structure in a PA system, using a poweramp as part of a bass rig, wondering if some preamp has enough "juice" to drive some poweramp, or manipulating a tube amp to act how I like...but I may've easily used a little incorrect vocabulary or explanations from a technical perspective.

 

[::::BASSIST::::]

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.

So essentially what you are saying is to crank up the preamp's master volume to give maximum signal to the power amp (without clipping) and then use the power amps' gain control(s) to set the actual volume level from the cab?

 

[Downunderwonder]

That wasn't what he was saying, and has it's own potential drawbacks.

The pre could get noisy at high gain or it could overdrive the other pre in the scenario of Passin'wind's diagram.

Gain staging is essentially having each unit in the chain operating in its sweetspot to achieve maximum clean headroom, ramping up and attenuating as necessary. The Rane note is great.

 

[Bob Lee (QSC)]

So essentially what you are saying is to crank up the preamp's master volume to give maximum signal to the power amp (without clipping) and then use the power amps' gain control(s) to set the actual volume level from the cab?

No, not necessarily to crank the preamp's master volume.

• Get maximum signal from your bass.
  
• Set the input gain of the preamp high enough to put a decently strong signal level through without clipping any of the preamp processing stages (EQ, filtering, FX, etc.).
  
• Set the preamp master high enough to drive the amp input strongly without clipping the preamp output or the amp input.
  
• Dial in your volume with the power amp gain control.

 

[bucephylus]

So essentially what you are saying is to crank up the preamp's master volume to give maximum signal to the power amp (without clipping) and then use the power amps' gain control(s) to set the actual volume level from the cab?

No. DUW summarized it correctly in Post #10 and which I quoted in Post #14. I recall Bob clarifying way back when I was initially running QSC amps (great amps). Sorry I started the debacle again.