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Amplifier Voltage Gain Explained – Matching Amp to Preamp

by April 29, 2013
Contributors: , Hadi Ebrahimi Darkhaneh
Danger! This article about voltage may be educational. Read at your own risk.

Danger! This article about voltage may be educational. Read at your own risk.

Most everyone that has ever purchased an external amplifier is at least familiar with the term voltage gain. Simply, it is the degree to which an amplifier actually amplifies the input from the preamplifier/processor. Often overlooked by those unaware of its importance, this one parameter can have significant implications on actual performance when an amplifier is introduced into an AV system. Understanding the impact that different levels of voltage gain can have in your system can very well be the difference between poor sound and getting the most out of an external amplifier.

So…what is voltage gain exactly?

When you think about it, an amplifier has a pretty straightforward job: to take an incoming voltage signal from a pre/pro and make it bigger. The amount by which the incoming signal is amplified is given in decibels (dB).  Every 6dB of gain equates to a doubling of voltage; as such, a hypothetical amplifier with a voltage gain of 30dB will increase voltage by 2^5, or by a factor of 32. For unbalanced inputs, the THX standard gain level is 29dB; utilizing balanced inputs decreases this to 23dB, though naturally the output of the preamp is boosted by 6dB under this scenario (i.e. voltage output of the preamp is doubled). For example, in the Audioholics review of the Integra RDC-7.1, the unbalanced outputs were measured to deliver 7Vrms; via the balanced outputs, the Integra delivered 15Vrms!

Sounds easy enough, but why does it matter?

Naturally too much or too little of anything can present a problem, and the ideal amount of voltage gain can vary depending on a few factors. Utilizing a receiver with poorly implemented preamplifier outputs for example can be a problem when coupled to a high powered amplifier with relatively low voltage gain and consequently a high input sensitivity, which is the amount of voltage needed from the preamp to drive the amplifier to full unclipped power. Suppose you have a receiver that can deliver 1 volt RMS from its preamplifier outputs before clipping; if you pair this receiver with a high powered amplifier expecting a huge boost in headroom, you might be sorely disappointed if its voltage gain is a below average 27dB.

Clipped Vs Unclipped Sine Wave

Fig. 1: Unclipped sine wave versus a clipped sine wave.

A gain of 27dB equates to a ~22.6x increase in voltage, meaning our amplifier will be putting out 22.6 volts RMS, or a whopping 64 watts into an 8 ohm load before our AVR’s pre-outs run out of gas. Even if the amplifier is rated to deliver 1,000 watts, all you’re going to do when you push harder is get garbage as your AVR clips the signal to the amplifier or potentially trip its protection circuits. Long story short: if you want to add power to a lower end receiver with pre-outs, you probably want something with a better than average amount and a low input sensitivity.

QSC Amplifier Specifications

Fig. 2: QSC GX Series Amplifier Datasheet

 

Above is the voltage gain and input sensitivity specification for the QSC GX series professional power amplifiers. For those mathematically inclined, you can verify the numbers with the equation:

Voltage Gain (Av) = 20 * Log (Vout/Vin)


Plugging in 48.99V for Vout (300W into 8 ohms) and 1.2V for Vin, you arrive at QSC’s 32.2dB figure for voltage gain.

OK, so barring the manufacturer of an amplifier being kind enough to provide input sensitivity, how do you calculate how much voltage is required from a preamplifier to drive an amplifier to full rated output?

First we take the power in watts that an amplifier can deliver into an 8 ohm load and convert that to voltage with the formula:

Power = Voltage^2/Load Resistance

For example an amplifier that is rated to deliver 50 watts RMS into an 8 ohm load would be 50=Voltage^2/8 or 400=Voltage^2. Solving the equation, we find that 50 watts into an 8 ohm load means our amplifier is delivering 20 volts at full power. Now we simply divide by the amount of gain that the amplifier is providing.

Going back to the earlier equation Av = 20 * Log (Vout/Vin) we can perform a bit of mathematical manipulation and say 10^(Av/20)=Vout/Vin. So if our amplifier has a gain of 28dB, we find that our amplifier is boosting the input from the preamplifier by a factor of 10^(28/20) or ~25.1. So if our amplifier rated to delivering 20 volts RMS and is amplifying the input signal by a factor of 25.1, we can know say that our preamplifier needs to deliver no less than: (20/25.1) = 0.797V RMS to drive our amplifier to full power. Isn’t math fun?

So if too little gain is a problem, we should flock to amplifiers with higher than average gain, right?

Not so fast! A very high level of gain leads to its own problem, namely noise. It makes sense when you think about it: in the previous scenario, our AVR was being asked to put out a lot of output, whereas now it is being asked to deliver relatively little voltage. As the voltage from our preamplifier output goes down, our signal will get ever closer to the noise floor of the system. Get too close, which is more likely with a higher sensitivity speaker, given that they need less output from the amplifier to begin with, and you’ll quickly learn the meaning of the saying “garbage in = garbage out”. 


Besides noise configuration, an increase in amplifier gain will decrease in the bandwidth (BW) of the circuit, meaning some valuable data may get eliminated from the input signal (the amplifier works as a filter). Additionally, having a high gain amplifier may introduce DC offset at the output. In an amplifier with high input impedance, increasing the gain will introduce a DC offset which affects the operating point of the circuit (changes the balance of the amplifier).

Reading the above, it may seem that those who seek the additional output of an external amplifier are caught in a vicious catch 22. Certainly if you happen to have a combination of an AVR with a poor preamplifier output section combined with ultra-high sensitivity loudspeakers, you may want to reconsider some of your hardware choices; beyond that, careful selection can help ensure that you get the most out of your equipment. Further, it should be noted that while some low end receivers may not be the ideal starting point for adding separate amplifiers, some AVRs can do quite well; a Yamaha RX-A1010 Aventage was recently benched tested by Audioholics to deliver 2.8 volts RMS from its pre-outs, which is adequate to drive any external amplifier within reason. Meanwhile in the distant past of 2010, a Marantz SR6004 was able to deliver 7 volts pk-pk (2.49Vrms) from its pre-outs.  The preamp section of this receiver should have no problems driving any external power amplification to its full output capability.

Marantz Preamp FFT

Fig. 3: Marantz SR6004 Preamp FFT Distortion Analysis.

As part of our receiver measurement suite, we test the pre-outs to ensure they are capable of driving a wide range of amplifiers to full power.

Load Impedance

At this point, we’ve discussed voltage gain and input sensitivity, but there are a couple more potential caveats to be aware of. First is the load for which a preamp’s output voltage is rated for. There is naturally a big difference between rating voltage output on an open circuit, i.e. no load, versus 600 ohms, which is likely to be a considerably tougher task than most amplifiers you’re likely to meet, which have input impedances on the order of tens of thousands of ohms. Rating open circuit doesn’t take into account potential current limits which could bring on preamp clipping much sooner than you might expect once you introduce real world conditions such as esoteric amplifier designs with low input impedances.  In addition, some esoteric high capacitance connecting cables can cause premature high frequency roll-off.

Of course, there is also the matter of the loudspeaker load. This is old hat if you’ve read the Audioholics article on impedance.  As noted prior, adequate voltage output drive from the preamplifier to allow the power amplifier  to reach full power is critical.  The amplifier still needs a sufficiently stout current stage to deal with the loudspeakers complex load impedance, lest you run into voltage sag/clipping on the amplifier side. Ideally of course, an amplifier would act as a voltage source, maintaining output regardless of the load (i.e. it would “double down” into 4 ohms, and “double down” again into 2 ohms). However, few amplifiers are capable of accomplishing this feat at high drive levels.

Conclusion

Are you interested in purchasing a separate amplifier? If you’ve paid attention to this article, then you’re probably also interested in its voltage gain as well. It’s hard to imagine one little number that often times gets overlooked having such a big impact on overall performance.   However, this little detail can be the difference between a truckload of distortion or noise and nice clean sound. Take care in your selection, and you’ll avoid the problems outlined above. Happy listening!

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About the author:

Steve Munz is a “different” addition to Audioholics’ stable of contributors in that he is neither an engineer like Gene, nor has he worked in the industry like Cliff. In fact, Steve’s day job is network administration and accounting.

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Recent Forum Posts:

PENG posts on December 30, 2015 16:16
Irvrobinson, post: 1110941, member: 6847
As you might have expected of me, PENG, I read the specs before ordering. I suspect the problem is one or more of the following:

- The 975 doesn't have an output level problem; it has an input sensitivity problem, and has substantially less than unity gain.

- The 975 has an unusually low input impedance, perhaps because of a design defect, and is somehow stressing the cheap output devices of certain components, like the cheap Sony BD player. (I doubt this is the reason, but I don't know.)

- The 2V output spec is simply incorrect, or wasn't met in the first version of the product, which I have. (I suspect this is the real reason.)

- The ATI AT602 gain is not 28db, it is only 24db, which is woefully low. If you look at the ATI web site, which was my reference, gain is listed at 28db. If you read the owners manual, I just noticed it is listed at 24db. 24db, if correct, is the lowest gain I've ever encountered for a power amplifier. (24db of gain in conjunction with less output from the 975 than advertised could be a double whammy.)

So, I'm thinking this is a genuine case of components that are just incompatible, especially when the electronics chain is pair with a set of JBL speakers that are probably 82db/2.83v/m, sensitivity-wise. And I'm also thinking that y'all ought to consider the silliness of being so dismissive of a situation that when discussed gets an immediate match from an audience with such a small population.

I did expect you to read the specs, that's why I asked for explanation instead of doubting you. And you did not disappoint.

To add to your list of suspected causes, I noticed that the output impedance is stated as:

“Output Impedance (Main-RCA) <1 K?”

So it could be 999 ohms and that is not low for sure, but probably just a minor factor, as most amps, presumably including the AT602, should have input impedance >10K. Still, when you add everything up, that Outlaw is probably only good for amps with >32 dB gains.

If it has decent input sensitivity and reasonably low output impedance, even 1V can drive the 24 dB gain AT602 to its rated output but it certainly can't do anything to the ATI's larger amps.

fmw may be right about in the majority of cases, but I do think it's time manufacturers be more specific about their preamp outputs. As it is now, it seems to me most of them are vague, you just can't draw conclusions without seeing bench test data. Fortunately so far any of my AVR, preamps and prepros have no trouble driving any of my power amps. I hope that Outlaw is the only one that failed you.
slipperybidness posts on December 30, 2015 16:01
Irvrobinson, post: 1110941, member: 6847
- The ATI AT602 gain is not 28db, it is only 24db, which is woefully low. If you look at the ATI web site, which was my reference, gain is listed at 28db. If you read the owners manual, I just noticed it is listed at 24db. 24db, if correct, is the lowest gain I've ever encountered for a power amplifier. (24db of gain in conjunction with less output from the 975 than advertised could be a double whammy.)

Yeah, 24dB seems unlikely to me. More likely that it's a misprint.

Voltage Gain (Av) is pretty easy to measure with a function generator, dummy load resistors, and an O-scope.

I mentioned in another thread, when I build an amp, I typically shoot for 29dB. That seems to be about the sweet spot from what I've seen.
Irvrobinson posts on December 30, 2015 15:34
PENG, post: 1110934, member: 6097
I just checked Outlaw website and found the 975's pre outs are rated 2V, 2.7V maximum so I don't know what it can't drive your AT602 that only needs 0.9V for the rated 60W output, according to the manual and my calculations back that up too.

http://www.ati-amp.com/manuals/at1202manual.pdf

Did Outlaw give you any reason why their 2V is a problem? Assuming it is 2V rms I just can't see that being a problem.

Outlaw never gave me a technical reason for the gain problem, and I almost sent the silly thing back, but the only alternative for a prepro in that price range was the Emotiva, and I already had a bad experience with that company. And other than the remote control design being unbelievably lame, and slower than average HDMI switching times, the 975 does perform very well within its limited output. It also looks good, is easy to set up, and has been reliable.

As you might have expected of me, PENG, I read the specs before ordering. I suspect the problem is one or more of the following:

- The 975 doesn't have an output level problem; it has an input sensitivity problem, and has substantially less than unity gain.

- The 975 has an unusually low input impedance, perhaps because of a design defect, and is somehow stressing the cheap output devices of certain components, like the cheap Sony BD player. (I doubt this is the reason, but I don't know.)

- The 2V output spec is simply incorrect, or wasn't met in the first version of the product, which I have. (I suspect this is the real reason.)

- The ATI AT602 gain is not 28db, it is only 24db, which is woefully low. If you look at the ATI web site, which was my reference, gain is listed at 28db. If you read the owners manual, I just noticed it is listed at 24db. 24db, if correct, is the lowest gain I've ever encountered for a power amplifier. (24db of gain in conjunction with less output from the 975 than advertised could be a double whammy.)

So, I'm thinking this is a genuine case of components that are just incompatible, especially when the electronics chain is paired with a set of JBL speakers that are probably 82db/2.83v/m, sensitivity-wise. And I'm also thinking that y'all ought to consider the silliness of being so dismissive of a situation that when discussed gets an immediate match from an audience with such a small population.
Irvrobinson posts on December 30, 2015 15:12
fmw, post: 1110930, member: 26848
I hear you. Hopefully you will forgive my being dismissive because I still am.

I can't say that I'm surprised. As for being forgiving, the last time that topic came up I was called “demanding and uncompromising”. Make of that what you will.
PENG posts on December 30, 2015 14:43
Irvrobinson, post: 1110922, member: 6847
No, I don't. I tested the AT602 in my primary system, and that pre-amp easily drove it to clipping; the AT602 just has only 28db of gain, while most inexpensive amps these days have 32db of gain. I talked to Outlaw, and they seemed to have a procedure all ready for trying to mitigate the problem. The Sony worked fine previously with a Sony AVR. Fortunately the problem was solved by just using much more efficient speakers, which I'm guessing are over 10db more sensitive than the previous pair.

I think my case is unusual, good thing, but I also think being dismissive of the problem is incorrect.

I just checked Outlaw website and found the 975's pre outs are rated 2V, 2.7V maximum so I don't know what it can't drive your AT602 that only needs 0.9V for the rated 60W output, according to the manual and my calculations back that up too.

http://www.ati-amp.com/manuals/at1202manual.pdf

Did Outlaw give you any reason why their 2V is a problem? Assuming it is 2V rms I just can't see that being a problem.
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