Thoughts about amplifiers at 3/4 volume.

Shane Presley

Shane Presley

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Jan 12, 2011
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Ottawa, Ontario, Canada
www.bssproductions.com
I was at a church install last night where they had set the amps ( yorkville AP 3400 FOH, and AP3000 MONS) at 3/4. Can anyone explain the thinking behind this, I know there are two "camps" on the subject I just forget the reasoning behind throttling back at the amplifer. The only thing I can think of, is they like the look of the master fader at unity (eyes rolling)
 
Re: Thoughts about amplifiers at 3/4 volume.

Because that's the right thing to do. Gain structure, gain structure, gain structure. The amp is not being "throttled back", the control affects the input sensitivity(voltage needed) to achieve rated power at the proper point along the curve. Amplifier gain is fixed. Having faders running around "0" is better for a few reasons, mainly the beasts are designed to operate best there.

Trust me, it took me a long time to get the gist of this back in the dark ages of no internet forums. ;>)

Best regards,

John
 
Re: Thoughts about amplifiers at 3/4 volume.

John Halliburton said:
The amp is not being "throttled back", the control affects the input sensitivity(voltage needed) to achieve rated power at the proper point along the curve. Amplifier gain is fixed.
Click to expand...

+1

As John said, the amp can still put out the same amount regardless of input sensitivity. Lower sensitivity just means you need to send a hotter signal from previous components in the chain.

If you are shooting for optimum gain structure and lowest system noise (particularly critical for installs) then you should carefully consider the optimum operating ranges for your devices and work from there.

Personally when doing installs, I like using amps with the input sensitivity knobs on the rear rather than the front panel just so that people who have always done the "wide open" thing don't get freaked out by what setting I ended up with on the amp itself. Some digital amps also have the capability of disabling/ignoring the front inputs which is also useful.
 
Re: Thoughts about amplifiers at 3/4 volume.

The main thing affected by reducing gain (moderately) at that point in a system is to attenuate output noise from the console and any noise picked up in the wiring. Since this noise is fixed, and you are now using a hotter send to get the same output, the effective S/N is improved.

The trade-off or different philosophies are;

a) throttle back the amp for lowest noise floor, often done in churches to manage noise floor in quiet ambient spaces. Of course if carried to excess you can limit maximum amplifier power output,

b) WFO (rock and roll setting), preferred for live music to insure that the amplifier will deliver full power, and if clip limiting is routinely engaged, WFO gives you the most limit range or headroom, before the audio path saturates upstream.

Both valid for different end goals.

JR

PS: As an amplifier product manager I have often pondered removing the gain trim (they cost money), but customers want them even if they don't use them much.
 
Re: Thoughts about amplifiers at 3/4 volume.

Another thing to consider, depending upon your applications is that if you run your input attenuators WFO then you don't have to worry about some schumck coming by and turning up your amps. This of course would affect the relative levels of the different bands and possibly cause damage.

I pretty much agree with what John Roberts says. In an install where you can keep stick fingers off the attenuators and need better s/n ratio then there can be advantages to using them. If you are doing a music festival in a park where anyone can get near your amps, I am in the camp of setting up your gain structure with the attenuators all the way up.
 
Re: Thoughts about amplifiers at 3/4 volume.

Jay Barracato said:
How about: If it was never meant to be used, why did the maufacturer included it in the design?
Click to expand...

I used to drive a Volkswagen Passat. Great car, roomy, and the 2 liter turbo engine got it going pretty well while returning fuel economy in the low 30MPG range (7.8 l/100Km+). The only problem is that it was front wheel drive, and the differential must have been a very generic type as in any sort of low traction environment it resorted to spinning one wheel.

Now, on the dash was a little button that allowed one to disable the traction control. By doing this, and with careful control of the throttle, brake, and steering, I could get that car through pretty much anything in short order. Enabling traction control, on the other hand, retarded the engine so extremely and applied so much braking that while the car might worm its way out eventually it was such slow going you'd have to be an idiot to put up with it. Of course if you're the sort of person who is an idiot, and when encountering a situation where the car isn't traveling at the expected speed mash the accelerator pedal to the floor, traction control is a must since it overrides your stupidity. Turn it off and mash the accelerator and you'll sit all day until either the wheel digs into so much dirt that you're truly stuck, or spins on ice long enough to catch fire.

So the traction control is there for people who don't know how to operate a motor vehicle. People who grew up driving on dirt and ice can pretty much leave it off, but the vast majority of drivers desperately need it to save them from themselves.

Or, at least, that seemed like a decent analogy in my head.
 
Re: Thoughts about amplifiers at 3/4 volume.

John Halliburton said:
Because that's the right thing to do. Gain structure, gain structure, gain structure. The amp is not being "throttled back", the control affects the input sensitivity(voltage needed) to achieve rated power at the proper point along the curve. Amplifier gain is fixed. Having faders running around "0" is better for a few reasons, mainly the beasts are designed to operate best there.

Trust me, it took me a long time to get the gist of this back in the dark ages of no internet forums. ;>)

Best regards,

John
Click to expand...

While *technically* correct, there is practically no reason to turn down the input attenuators of a modern day amplifier.

Why?

Because they have superior S/N that the rest of the audio electronics, especially once mics are open.

Most mixers can barely muster 80-90 dB of SNR with the channel gain trims all down. Fahghettabatit when more than a few are up. Basically, what boils down to is nearly every amplifier on the market has a 20-30dB S/N advantage on the rest of the system. Turning those trims down is just masking the noise.

If you have a DSP, turn down the crossover outputs, if not... bring down the master fader by the same amount. You'll see that the same result is gained, plus like another poster mentioned, you don't have to worry about someone fiddling with the knobs "on accident."

Another bad thing that can happen that Bob Lee of QSC used to mention is that when you turn those attenuators down, you make it easier to overload the input stage of the amplifier. I.e. if the amp clips at +8dBu and you turn it down 10 dB you have to drive the line that much harder with 18dBu to clip the amp. It' only gets progressively worse the more you turn it down.

If you really don't believe me, the next time you fire up a system, and you notice some "hiss" from the speakers, unplug the drive lines from the amplifiers. I guarantee you the hiss will go away. Amps are not hiss makers. Mic gain stages and electronics are.

I also agree with JR on the WOT for Rock and Roll too. At most live gigs, your going to be dealing with a 80 dB (A) SPL noise floor (maybe worse). What ever gain the system has is only going to amplify that further. Hiss from the speakers is your least worry here.

If there is a lot of hiss with the mics open, where it's litterly distracting, find the source and antennuate it. More than likely it's an FX return (from an old FX unit) or a wonky channel gain. I've also found this to be a reason to always start a show with all pads out. I once mixed a gig where all the pads were engaged (programming error on a digital console). There was so much hiss it was unbelievable because each gain knob had to be turned up more, relative to the input and it seems digitally controlled pre-amps seem to be exponentially more noisy at the top end of there gain. I've also found that systems that do have a lot of hiss are more unstable feedback wise.
 
Re: Thoughts about amplifiers at 3/4 volume.

Jeff Babcock said:
+Personally when doing installs, I like using amps with the input sensitivity knobs on the rear rather than the front panel just so that people who have always done the "wide open" thing don't get freaked out by what setting I ended up with on the amp itself. Some digital amps also have the capability of disabling/ignoring the front inputs which is also useful.
Click to expand...
What we do on installs is to install pads between the DSP output and the amp input (unless the DSP has a digitally controlled analog pad), then run the levels all the way up. That has the same effect as turning down the amp levels and lowering the noise floor. Again it does nothing to the max output capability of the system.

That way it is real easy to remember where they were set at origionally during the alignment and most "normal people" never turn anything down, they only turn it up-which not only throws off the alignment of the system, but also the gain structure.
 
Re: Thoughts about amplifiers at 3/4 volume.

Maybe I don't drive full out all the time, but I have plenty of shows where if I don't need the full output of the box, it is easier to turn back the gain a bit and keep the rest of my gain structure through the system the same, especially if I am also doing IEM monitors from FOH. I seem to have often heard that gain is gain. Take it away from the place that affects the least other things.

That doesn't mean I also haven't done plenty of shows with the master fader at half mast.

Spending most of my time in the bluegrass/acoustic world, I can't think of a single time in years I had a problem with anything in a system clipping frequently. In 4 years of ownership and a couple hundred shows, I have never seen the clip lights on my QSC's. I guess I am not the guy out in a high performance car, I am the guy out for a slow sunday drive with cruise control.
 
Re: Thoughts about amplifiers at 3/4 volume.

Ryan Lantzy said:
While *technically* correct, there is practically no reason to turn down the input attenuators of a modern day amplifier.

Why?

Because they have superior S/N that the rest of the audio electronics, especially once mics are open.

Most mixers can barely muster 80-90 dB of SNR with the channel gain trims all down. Fahghettabatit when more than a few are up. Basically, what boils down to is nearly every amplifier on the market has a 20-30dB S/N advantage on the rest of the system. Turning those trims down is just masking the noise.

If you have a DSP, turn down the crossover outputs, if not... bring down the master fader by the same amount. You'll see that the same result is gained, plus like another poster mentioned, you don't have to worry about someone fiddling with the knobs "on accident."

Another bad thing that can happen that Bob Lee of QSC used to mention is that when you turn those attenuators down, you make it easier to overload the input stage of the amplifier. I.e. if the amp clips at +8dBu and you turn it down 10 dB you have to drive the line that much harder with 18dBu to clip the amp. It' only gets progressively worse the more you turn it down.

If you really don't believe me, the next time you fire up a system, and you notice some "hiss" from the speakers, unplug the drive lines from the amplifiers. I guarantee you the hiss will go away. Amps are not hiss makers. Mic gain stages and electronics are.

I also agree with JR on the WOT for Rock and Roll too. At most live gigs, your going to be dealing with a 80 dB (A) SPL noise floor (maybe worse). What ever gain the system has is only going to amplify that further. Hiss from the speakers is your least worry here.

If there is a lot of hiss with the mics open, where it's litterly distracting, find the source and antennuate it. More than likely it's an FX return (from an old FX unit) or a wonky channel gain. I've also found this to be a reason to always start a show with all pads out. I once mixed a gig where all the pads were engaged (programming error on a digital console). There was so much hiss it was unbelievable because each gain knob had to be turned up more, relative to the input and it seems digitally controlled pre-amps seem to be exponentially more noisy at the top end of there gain. I've also found that systems that do have a lot of hiss are more unstable feedback wise.
Click to expand...

The whole idea is NOT that the amp has hiss. The idea is to reduce the hiss that the rest of the system is producing. That is why turning it down reduces the noise/hiss.

You cannot make the system any louder-that is based on the amplifier power and the sensitivity of the loudspeaker.

BUt you can reduce the system noise-yielding a greater dynamic range (by not having the noise be louder than the signal on quiet passages).

This is not important in many cases-but is very important in others.
 
Re: Thoughts about amplifiers at 3/4 volume.

Ryan Lantzy said:
While *technically* correct, there is practically no reason to turn down the input attenuators of a modern day amplifier.

Why?

Because they have superior S/N that the rest of the audio electronics, especially once mics are open.

Most mixers can barely muster 80-90 dB of SNR with the channel gain trims all down. Fahghettabatit when more than a few are up. Basically, what boils down to is nearly every amplifier on the market has a 20-30dB S/N advantage on the rest of the system. Turning those trims down is just masking the noise.

If you have a DSP, turn down the crossover outputs, if not... bring down the master fader by the same amount. You'll see that the same result is gained, plus like another poster mentioned, you don't have to worry about someone fiddling with the knobs "on accident."

Another bad thing that can happen that Bob Lee of QSC used to mention is that when you turn those attenuators down, you make it easier to overload the input stage of the amplifier. I.e. if the amp clips at +8dBu and you turn it down 10 dB you have to drive the line that much harder with 18dBu to clip the amp. It' only gets progressively worse the more you turn it down.

If you really don't believe me, the next time you fire up a system, and you notice some "hiss" from the speakers, unplug the drive lines from the amplifiers. I guarantee you the hiss will go away. Amps are not hiss makers. Mic gain stages and electronics are.

I also agree with JR on the WOT for Rock and Roll too. At most live gigs, your going to be dealing with a 80 dB (A) SPL noise floor (maybe worse). What ever gain the system has is only going to amplify that further. Hiss from the speakers is your least worry here.

If there is a lot of hiss with the mics open, where it's litterly distracting, find the source and antennuate it. More than likely it's an FX return (from an old FX unit) or a wonky channel gain. I've also found this to be a reason to always start a show with all pads out. I once mixed a gig where all the pads were engaged (programming error on a digital console). There was so much hiss it was unbelievable because each gain knob had to be turned up more, relative to the input and it seems digitally controlled pre-amps seem to be exponentially more noisy at the top end of there gain. I've also found that systems that do have a lot of hiss are more unstable feedback wise.
Click to expand...

Ryan,

I agree with your statements-I think a gave the wrong impression. On my gear, with processors, we've typically done all the settings using the amps wide open,for the reasons you've pointed out and quoted. I also tend to run my trims a bit hot. One thing about leaving the amps "up", it is easier if you have to swap one out, and there's no having to remember what each amp was set at. Adjust things in the DSP gains.

Best regards,

John
 
Re: Thoughts about amplifiers at 3/4 volume.

Ivan Beaver said:
The whole idea is NOT that the amp has hiss. The idea is to reduce the hiss that the rest of the system is producing. That is why turning it down reduces the noise/hiss.

You cannot make the system any louder-that is based on the amplifier power and the sensitivity of the loudspeaker.

BUt you can reduce the system noise-yielding a greater dynamic range (by not having the noise be louder than the signal on quiet passages).

This is not important in many cases-but is very important in others.
Click to expand...

I never implied that amplifiers would be causing hiss or that was the whole idea.

There is one, and only one reason to turn down the input stage of a device (such as an amplifier) instead of doing it in a previous stage. That reason is if the noise floor of said device is higher than the system connected before it. This would be done to maximize SNR.

However, since amplifiers most always have more SNR than the system before it, there is no advantage in doing this versus turning down the output of the thing immediately previous to it (or some other piece of gear which is post noise source).

If on the the other hand, the noise floor for the amp *was higher* than the system before it, then the *only* way to improve the SNR would be to turn down the input stage of the amp.
 
Re: Thoughts about amplifiers at 3/4 volume.

Ryan Lantzy said:
I never implied that amplifiers would be causing hiss or that was the whole idea.

There is one, and only one reason to turn down the input stage of a device (such as an amplifier) instead of doing it in a previous stage. That reason is if the noise floor of said device is higher than the system connected before it. This would be done to maximize SNR.

However, since amplifiers most always have more SNR than the system before it, there is no advantage in doing this versus turning down the output of the thing immediately previous to it (or some other piece of gear which is post noise source).

If on the the other hand, the noise floor for the amp *was higher* than the system before it, then the *only* way to improve the SNR would be to turn down the input stage of the amp.
Click to expand...

Hello,

Errr....no. You may need to "turn down" the input stage to prevent over-loading the input which would cause distortion of the signal.

Hammer
 
Re: Thoughts about amplifiers at 3/4 volume.

You guys are confusing me.
Ryan Lantzy said:
I never implied that amplifiers would be causing hiss or that was the whole idea.

There is one, and only one reason to turn down the input stage of a device (such as an amplifier) instead of doing it in a previous stage. That reason is if the noise floor of said device is higher than the system connected before it. This would be done to maximize SNR.
Click to expand...
If the noise floor is referenced to the output, turning down the input will not change that output noise, but will reduce the level of the input, making the signal part of S/N less. This would logically reduce S/N.

If the noise floor is dominated by noise in the wiring/interface, or noise floor of the source, padding down the amp input would improve the combined result.

The best case for a noisy amp (with noise referenced to it's output) is to operate WFO, but as noted most amps are designed with adequate S/N to not be the limiting factor in a system noise floor.
However, since amplifiers most always have more SNR than the system before it, there is no advantage in doing this versus turning down the output of the thing immediately previous to it (or some other piece of gear which is post noise source).

If on the the other hand, the noise floor for the amp *was higher* than the system before it, then the *only* way to improve the SNR would be to turn down the input stage of the amp.
Click to expand...

In a typical power amp the balanced input stage will be running at a noise gain of only 2x or so, while the amp proper will be running at tens of dB voltage gain. ASSuming similar low noise components the amplifier proper should dominate the amp's internal noise.

The next consideration is how the amp trim is performed. Your advice makes sense if the amp trim is actually a fully variable gain controlling the actual amplifier stage, but in practice most amps use a variable pad placed in front of the fixed gain amp. With a fully variable gain amp, you could turn down the amp's own noise while you turned down it's gain, but for stability reasons amps are typically operated at fixed gain** with a variable pad in front of the amp, between amp stage and input receiver/buffer.

To the operator (and readers here) this is TMI and inside baseball since the net effect of how the trim is performed inside the amp is transparent to the end user, when looking at the amp as a black box. That said it does make a difference for this esoteric noise analysis. In my experience most amplifiers internal noise will be output referenced, but this noise level is not large enough to be a concern, so debating this is not really productive.

JR


** some amps may have two position switched gain, but not fully variable. Note: Low gain is the most difficult to stabilize and prevent oscillation with reactive loads when using negative feedback.
 
Re: Thoughts about amplifiers at 3/4 volume.

JR,

Taking into consideration what you said, then there is really no reason (even the one I stated) to use the gain knobs to improve SNR vs. doing so on a peice of gear connected earlier in the chain. Correct?
 
Re: Thoughts about amplifiers at 3/4 volume.

Let's face it, it is so loud and noisy in the rooms most of us work in that the S/N ratio of our equipment is not a factor.
 
Re: Thoughts about amplifiers at 3/4 volume.

Ryan Lantzy said:
JR,

Taking into consideration what you said, then there is really no reason (even the one I stated) to use the gain knobs to improve SNR vs. doing so on a peice of gear connected earlier in the chain. Correct?
Click to expand...

Sending a hot signal and padding down the input at the amp, will reduce noise picked up in that interface between amp and the sending unit. This is less of an issue with good modern balanced inputs, balanced sends. and balanced (3 circuit XLR) wiring. In the not so good old days, using unbalanced interfaces and wiring, it was a very valid technique to manage noise in quiet ambient venues (like HOW). I can imagine a modern venue with distro or whatever issues and hum in the amps, that might benefit from hot level and pad, but with clean interfaces, not so much.

So arguably with modern gear you "should" be able to get by without amp trims, but there is a lot of inertia in the marketplace so customers will still want their amp trims for some time to come, and there could be the occasional problem gig where this technique could help.

JR
 
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