Amplifier Voltage Gain Explained – Matching Amp to Preamp

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.

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.

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.

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!