Help: Understanding Gain on Power Amp

[veeceem]

General specs:
Power Amp: NAD M22 v2
Input sensitivity (for 300W in 8 Ohm): 1.73V
Gain (3 levels): High/Medium/Low - 29dB / 26dB / 19dB

Preamp: Anthem STR preamp
Maximum Output (<0.1% THD): XLR: 6.6 Vmrs | RCA: 3.3 Vmrs

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According to specs, I know that the preamp is more than is enough to drive the amplifier even at Low Gain (19dB)
But what I don't really know is ...
1/ What does the 29 / 26 / 19 numbers mean?
2/ AFAIK, lower gain is always recommended as long as I feel that the volume is loud enough, but what does it mean by "loud enough" here? Does it mean, for example, with power amp gain set to Low, preamp at -20dB, and I feel that this volume is loud enough for all kind of music, then there's no need to set Gain to Medium or High? In other word, if the preamp is still not out of Volume at current gain on the amp, then that means current Gain level is enough?
3/ Pretty much the sum of the above questions, and what I think is the meaning of this "gain" thing: "Gain" of the power amplifier is not how loud the amp can be, regardless of Low/Med/High gain, the amp will always be as loud. As long as the preamp has enough power (Vrms) to feed it? Low gain for better THD/N if the preamp can output 1.73 Vrms or more to the amp. And set to High Gain if the preamp can't output more than 1.73 Vrms to power amp? Am I understanding it correctly?

Everytime I try to google the keywords, it comes up with all kind of formulas, which .... does not make sense to me at all T_T so a stupid-proof explanation is appreciated. I"m interested in "learning" the meaning of this "gain" thing. Not putting a question to know if X preamp is good to drive Y power amp.
Thanks.

 

[sergeauckland]

Gain in dBs defines how much the signal amplitude (measured in volts) is increased.
29dB is 28x
26dB is 20x
19dB is 9x

Full output of the amplifier is 300 watts into 8 ohms, which means a voltage level of 49 volts.

So,
at high gain, you need an input of 1.75v to achieve full output
at medium gain you need 2.45 v to achieve full output
at low gain you need 5.4v to achieve full output.

As to which is better, that depends on several factors.

Firstly, if the gain changing is done by changing the amount of feedback in the amplifier, then theoretically, the lower the gain the lower the distortion, but as the distortion will be inaudible anyway, that's of academic interest only.

If the gain change is more likely done just by an input attenuator, then there will be no difference in distortion.

What I would do is to use whichever gain setting gives you the most control of the volume on the pre-amp. In other words, excessive gain will mean that your volume control has to work at the bottom of its range, making control coarse and depending on how the preamp does its volume control, balance between channels will be worse. As long as you can play music as loud as you'll ever want to, choose the gain setting that needs more of the volume control to be advanced the furthest.

Hope this helps

S.

 

[veeceem]

@sergeauckland thanks
"29dB is 28x
26dB is 20x
19dB is 9x"

And

"Full output of the amplifier is 300 watts into 8 ohms, which means a voltage level of 49 volts"

May I ask how did you calculate these numbers?
Thanks

 

[JohnYang1997]

You are correct.

 

[MRC01]

...
"29dB is 28x
26dB is 20x
19dB is 9x"
...
May I ask how did you calculate these numbers? ...

Voltage dB: 20 * log(Vratio). Every 10:1 in voltage is 20 dB.
Power dB: 10 * log(Pratio). Every 10:1 in power is 10 dB.
For example:
28x: 20 * log(28) = 28.9
20x: 20 * log(20) = 26.02
9x: 20 * log(9) = 19.1
To do it backward, reverse the above equations algebraically.
29 dB: 10^(29/20) = 28.18
26 dB: 10^(26/20) = 19.95
19 dB: 10^(19/20) = 8.9
More info here.

 

[sergeauckland]

@sergeauckland thanks
"29dB is 28x
26dB is 20x
19dB is 9x"

And

"Full output of the amplifier is 300 watts into 8 ohms, which means a voltage level of 49 volts"

May I ask how did you calculate these numbers?
Thanks

MRC01 showed how the dB numbers were derived. I just rounded them up to the nearest whole number.

As to the voltage output of the amplifier,

watts = volts squared divided by load impedance, so

300 = V^2/8

V= sqrt 2400 = 49


S

 

[veeceem]

Thanks, everything makes sense now!

 

[cjm2077]

Also, remember that the gain of the amp will increase the amplitude of the incoming noise from the previous stage. So all things being equal, if you can turn up the input level to the amp (without increasing noise), and reduce the gain of the amp, you will have less noise at the output of the amp. This is part of how the Benchmark products work together to put out noise levels that are so low. They have very high output levels from their DACs and pre-amps, and the option to have very low amplifier gain.

 

[Cptrips]

I don't why Yamaha is then telling me to turn my power amp all the way to max when using one to drive my main speakers? I just hooked up a Behringer A 800 to drive my Kappa 7 mains. At full power the amp is absolutely quiet at my speakers but I still turned them down to 75%, not in my nature to run anything at 100%. But why would Yamaha instruct that in their manual?

 

[sergeauckland]

I don't why Yamaha is then telling me to turn my power amp all the way to max when using one to drive my main speakers? I just hooked up a Behringer A 800 to drive my Kappa 7 mains. At full power the amp is absolutely quiet at my speakers but I still turned them down to 75%, not in my nature to run anything at 100%. But why would Yamaha instruct that in their manual?

My guess is just to keep things simple for non-technical users.

Having volume controls on power amplifiers is very useful for gain-ranging, to maximise the dynamic range of the volume control and minimise noise. However, to get the best out of the system, it takes some understanding of gain, both in the preamp and power amp and knowledge of output voltage of the sources.

I too would set the power amp volume at 75% rotation in the absence of measurements. Having said that, in my own system with three power amps, I have them all at 100% as I need them to have identical gain, and 100% is a fixed point.

S

 

[cjm2077]

I don't why Yamaha is then telling me to turn my power amp all the way to max when using one to drive my main speakers? I just hooked up a Behringer A 800 to drive my Kappa 7 mains. At full power the amp is absolutely quiet at my speakers but I still turned them down to 75%, not in my nature to run anything at 100%. But why would Yamaha instruct that in their manual?

Which part of the system is Yamaha? My assumption is so that you can reach max power on the amp. A good way to do gain staging if you have two variable gains and want to use the source to control volume is to set the amp gain so that you are at your normal listening volume when the source gain control is roughly in the middle of it's range.

 

[Cptrips]

The Yamaha is a RVX-1400 that I am using as the pre amp for the Behringer A800 to run my mains. If I set my gains at 50% on the A800 and my Yamaha is at 50% that's the same settings for "normal" listening as before I added the A800. Does that seem right?

 

[RayDunzl]

"Full output of the amplifier is 300 watts into 8 ohms, which means a voltage level of 49 volts"

May I ask how did you calculate these numbers?

Online Calculators are helpful, at least for me.


This site has many many calculators related to Audio (and other related disciplines)

http://www.sengpielaudio.com/Searchengine.htm


Since that was totally overwhelming, try this one, and plug in what you know, i.e., 300W and 8 Ohm

http://www.sengpielaudio.com/calculator-ohm.htm

Here's one for "Gain":

http://www.sengpielaudio.com/calculator-gainloss.htm

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Everytime I try to google the keywords...

google the keyword and Sengpiel, you'll probably get an appropriate calculator.

 

[sergeauckland]

The Yamaha is a RVX-1400 that I am using as the pre amp for the Behringer A800 to run my mains. If I set my gains at 50% on the A800 and my Yamaha is at 50% that's the same settings for "normal" listening as before I added the A800. Does that seem right?

That's probably not far out. Normally, 50% rotation is an attenuation of 20dB, so if it's loud enough with both your Yamaha and A800 at 50%, that will work for you well, and give you plenty of control on the Yamaha volume control if you want to go louder.

If you're finding that the Yamaha volume control is always past 80% rotation, then just increase the volume on the A800.

S

 

[veeceem]

Just curious as I was browsing Mcintosh MEN220:
It says:
"Maximum Output Voltage4.5V Unbalanced and Balanced (Preamp-Power Amp)
2.25V Unbalanced and Balanced (Processor Loop) "

My next power amp is McIntosh Mc462, which, according to Stereophile, stated: "the 4 ohm tap's gain lower at 20.4dB "
Then if I use the 4 Ohm tap, 20.4dB is 10.47x
450watts @ 8 Ohm = 60V
60/10.47=5.7V input to drive the power amp
Does this mean this MEN220 do not have enough power to drive the power amplifier?

Or when I connect to the 4Ohm tap, I have to use the 450W @ 4 Ohm value, which is: 42V -> 42/10.47=4V

Hope someone can clarify. Thanks

 

[RayDunzl]

Amp Manual:

450W 2ohms = 30V 15A
450W 4ohms = 42.4V 10.6A
450W 8ohms = 60V 7.5A

2.1V -> 30V = 23.09db
2.1V -> 42.4V = 26.10db
2.1V -> 60V = 29.12dB

http://www.sengpielaudio.com/calculator-ohm.htm
http://www.sengpielaudio.com/calculator-gainloss.htm

 

[RayDunzl]

Stereophile, stated: "the 4 ohm tap's gain lower at 20.4dB "

20.4dB = 10.47 ratio

"Maximum Output Voltage4.5V Unbalanced and Balanced (Preamp-Power Amp)
2.25V Unbalanced and Balanced (Processor Loop) "

2.25V x 10.47 = 23.55V = 138W

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Ok, so the test and the spec don't seem to line up.

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Stereophile: "The McIntosh's voltage gain varied according to which output Autoformer tap was used and whether the balanced or unbalanced inputs were used. From the balanced inputs, the 8 ohm tap's gain into 8 ohms was a lower-than-usual 22.9dB, the 4 ohm tap's gain lower at 20.4dB, and the 2 ohm's even lower, at 17.4dB. The gains using the unbalanced input were 6dB higher rather than 6dB lower; the latter is usually the case. "

It would appear (my guess), from the measurements, that the amplifier uses the single-ended voltage, or only one leg of the balanced input, to drive the output. And, with those conditions, the output matches the spec.

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But then...

The manual says "quad balanced architecture" - in this case, two amp channels operating in differential mode, four total, for the stereo output..

If the XLR legs drive each drive an amp, or if the single ended inout is sent to one drive and and inverted signal sent to the other, then there is no difference between the two, as far as sensitivity is concerned.

In that case, either single ended or XLR result in the same output levels.

My guess, that's all I have.

 

[veeceem]

The maximum output voltages you quoted were MEN220, not the Mc462
But still, the measured 20.4dB gain at 4Ohm is much lower than the announced 26dB in the manual above :/