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Mono vs Stereo Power Amplifiers - why bother with two mono amps?

Mono vs Stereo Power Amplifiers
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Here’s another question we get quite often, ‘why should I consider a mono amplifier over a stereo amplifier?’

To date, we only build mono power amplifiers (also called monoblocks) and the reason is simple:

The sonic benefits of mono amplifiers cannot be ignored.

288 Mono Amplifier Stereo Pair
288 Mono Amplifier Stereo Pair


1. Stereo Amps have Shared Power Supplies

In stereo amplifiers, the power supply is almost always shared between both channels. Therefore, if one channel demands a lot of power, it tends to steal the available power from the other channel. There is crosstalk in the speakers through the inevitable power supply impedance common to both channels.

While power supplies impedance is undesired and designed to be low, it is never zero. If we have a stereo amplifier that draws 1A of supply current for the right channel’s output stage during a transient with a supply impedance of 500mΩ (0.500Ω), then the power supply will dip by 0.5V (0.5 x 1). If either the current demand or the supply impedance is higher, the problem gets much worse.

With tube amplifiers, the power transformer’s high voltage secondary winding resistance alone is often 50Ω-100Ω! Due to the economic design goal of stereo tube amplifiers, the power transformer is always shared between channels. Then add additional filtering components such as chokes or resistors to create a quiet noise free supply, and the resistance of the power supply only gets higher!

Furthermore, high-power amplifiers using Class-AB power stages show a markable increase in the output stage’s average current consumption at higher volume levels. The occurs as the output stage swings out of its Class-A region and into the Class-B portion of the operating range. Therefore, any voltage droop during a transient, as demanded by the audio program material, is superimposed on the droop caused by the higher average power consumption. This leads to even more sagging of the power supply voltage.

Don’t believe me, take a look at the analog bias meters on our 288 amplifiers and you will see the average output current through the KT88 tubes increasing at higher and higher output power levels. In many Class-AB tube amplifiers, the average supply voltage droops by 50V or more at full output power!

For those running AVRs (Audio Video Receivers) with many channels such as a 5.1 surround sound setup, the audio specifications often only list the output power for any one channel, not for all channels running combined.

Looking at a specification for the Denon AVR-X1700H 7.2 AV Receiver, it lists 175W for 1-channel and 120W for 2-channels. This receiver has 7 power amplifiers! What happens when they are all running under a typical scenario?

The reason for this loss of power is simply the power supply voltage sag caused by one output stage effecting the other power amplifier stages.


2. Perfect Stereo Imaging at any Volume

First and foremost, mono amplifiers maintain far superior stereo imaging across all listing volumes. It can’t be understated that transients (large crescendos, drum hits, or bass thumps) at otherwise normal listening levels also applies here.

Since there is no shared power supply impedance, the audio content on one channel does not cause the other to collapse sympathetically. This keeps the stereo image exactly where is should be paced for every beat of a kick drum or womp of a loud and low cello passage.

This tends to help maintain an open, effortless, and consistent sound field with less blurring or collapse of directionality especially at higher volumes or for load transients and passages.

This is a particular problem for tube stereo amplifiers when more instruments are added into the mix. The sound field tends to narrow and close in as average volume increases due to the mixing of all the different instruments.


3. More Dynamic Rage

Because each channel is independent, we can get the rated output power of the amplifier to each speaker at any instant as demanded by the audio program material. Ignoring the stereo image benefits for a moment, this alone allows mono amplifiers have more apparent headroom and dynamic capability for the same power rating.

Combine this with the better stereo imaging and it’s no wonder why mono amplifiers are best choice where space and cost permit. If you spend your hard-earned cash a quality set of stereo speakers, then why squash their imaging and dynamic performance potential with a stereo amplifier?

Why compromise performance if you don’t have to?

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