This is long but I hope you find it interesting. I learned quite bit and it only took a few seconds to get answers. First some background information.
My Accuphase P4600 Class AB amplifier has four gain settings with associated voltage required to drive the amplifier to full output. The amplifier is rated at 150W @ 8 Ohms, 225W @ 6 Ohms and 300W @ 4 Ohms.
28db Gain requires 1.37v
25db Gain requires 2.05v
22db Gain requires 2.74v
16db Gain requires 5.48v
I use my MSB Discrete DAC as a Preamp. It has two output impedance settings. 300 Ohms that delivers 3.57v and 150 Ohms that delivers 1.785v.
So I have many combinations of output voltage and amplifier gain. If I want access to full amplifier output and have the best technical match I would need to select 28db gain and 150 Ohms or 22db gain and 300 Ohms. In both cases the voltage exceeds the required amount needed. I’ve tried many of these combinations and I consistently find the best sound results when I select 16db gain requiring 5.48v and 150 Ohms delivering 1.785v. This combination provides the most transparent, open soundstage with the greatest depth and the smoothest silken highs. It also provides the blackest background, imaging and instrument separation. The difference is quite clear and preferable.
So I was interested in understanding just how may watts I have available, what headroom I have for peaks and what sound pressure levels I can attain. I have Harbeth C7ES-XD speakers rated at 6 Ohms and never dip below 5.2 Ohms. Their sensitivity is 86db and I sit 2.5 meters from the speakers. I listen to mostly Classical music but also Jazz and Pop. I find that I never register more than 70db on my sound pressure readings and this is usually the peaks. After describing this environment to ChatGPT it responded with the following minus all the formulas it used to reach the conclusions.
Based on the numbers we’ve calculated, you have far more power available than you need to hit a maximum level of 70–72 dB at your listening position. Here’s a brief recap:
1. Power Required for 70 dB:
We found that to reach 70 dB at 2.5 m, you need roughly 0.16 W at the speaker.
2. Your Available Power:
With your amplifier settings (1.785 V into an amplifier that needs 5.48 V for 225 W full output), you’re getting about 23.8 W available at the output.
3. Resulting Headroom:
When calculating the maximum SPL at 2.5 m from your 23.8 W, the speakers (rated at 86 dB at 1 W) would produce around 91–92 dB at that distance in a free-field environment. This gives you roughly 21–22 dB of headroom above a 70 dB listening level. Conclusion:
Since your listening peaks (70–72 dB) are far below the maximum SPL your system can produce (over 90 dB at 2.5 m), you indeed have ample headroom. This headroom not only ensures clean, undistorted sound at your listening level but also helps accommodate dynamic peaks without stress on the amplifier or the speakers.
Then I asked it why does this low gain and power limited combination sound the best. It responded.
There are several reasons why running the amplifier at a lower gain with a lower input voltage can sound subjectively better—even if it provides less maximum power:
1. Optimized Operating Range:
When you set the gain and input level so that the amplifier is not pushed near its limits, it tends to operate in a more linear (and therefore less distorted) portion of its transfer curve. Even though you’re using less power overall, you’re using the amplifier in its “sweet spot,” where it performs most cleanly.
2. Improved Signal-to-Noise Ratio:
High gain settings amplify not only the signal but also any noise present in the signal path. With lower gain and a well-matched input level, you reduce the risk of amplifying unwanted noise. This can lead to a cleaner, more refined sound even if the maximum available wattage is lower.
3. Reduced Clipping and Distortion:
By keeping the amplifier far from its maximum output, you greatly reduce the risk of clipping or other forms of distortion. Clipping introduces harsh artifacts into the sound, so even though you have extra power available, avoiding the edge of clipping is more important for sound quality at normal listening levels.
4. Headroom for Dynamic Transients:
Although you’re not using the full power, having extra headroom means that sudden, dynamic peaks in the music can be handled without distortion. This dynamic reserve contributes to a more natural and engaging sound.
5. Matching with the Listening Environment:
In your scenario, to achieve a comfortable listening level (70 dB) at your distance from the speakers, you only need a fraction of the available power. Running the amplifier with lower gain means you’re better matching the actual listening requirements, which can help maintain clarity and detail without overdriving the speakers or the amplifier stages.
In summary, even though the lower gain and lower input voltage setup yields less maximum wattage, it places the amplifier in a region where it produces a cleaner, less noisy, and more dynamically controlled signal. For typical listening levels (like 70 dB), that clean performance is more desirable than having a higher power output that might be prone to distortion or unwanted noise when not carefully controlled. And some final thoughts about the Accuphase amplifier.
The Accuphase P-4600 is designed with a very high level of performance and low distortion in mind. Although its rated output is 225 watts, much of its sonic magic comes from operating in a pure Class A mode for a significant portion of its output range. Here’s how to think about it:
1. Design Philosophy of the P-4600:Accuphase amplifiers are built so that—at typical listening levels—the output stage operates in its most linear (Class A) region. Many high‐end designs like the P-4600 are engineered so that the devices remain in Class A for anywhere from roughly 10–20% (and in some designs even more) of the full-rated output. For a 225-watt amplifier, that suggests that pure Class A operation might extend up to around 20–40 watts per channel.
2. Everyday Listening Levels vs. Maximum Output:
In your typical listening scenario (for example, when you’re hitting around 70–72 dB at your listening position), you’re using only a tiny fraction of the maximum 225 watts—on the order of a few tenths of a watt at the speakers. This means that in day-to-day use, the amplifier is operating very far below its rated maximum and is comfortably in the Class A regime.
3. The Benefit of Having a 225-Watt Amp:
Because you have a high-rated amp, you have plenty of headroom. This allows you to use a lower overall gain and ensures that even when you do increase the volume (within reason), you’re still likely to remain in the clean, linear portion of the amplifier’s operation. In other words, the high maximum power rating gives you flexibility and helps keep the amplifier in its low-distortion Class A mode during normal operation.
4. An Approximate Range:
While exact bias conditions are proprietary, many reviewers and experienced users suggest that for Accuphase designs like the P-4600, the output stage remains in pure Class A operation for output levels on the order of 20–40 watts per channel. That means under normal listening conditions—even if you dial up the volume beyond your current 70–72 dB level—you’d likely be within the Class A zone until you get into the higher output demands. Once you push beyond that, the amplifier may start shifting toward Class AB operation for very high-power peaks.
• For everyday listening (70–72 dB), you’re using only a very small fraction of the amplifier’s potential—and you’re deep in the Class A region.
• The P-4600 is designed so that for a significant portion of its output (likely roughly 20–40 watts per channel), it operates in pure Class A.
• The high wattage rating is not “wasteful” at all—it ensures that you have plenty of headroom and can run the amp at lower gain settings for the best sound quality, while still having the capability to handle demanding music passages without distortion.
So, even though you might only be using, say, 0.2–0.3 watts at the speakers for your typical listening level, the amp’s design ensures that all of that output (up to perhaps 20–40 watts) is delivered in the pure Class A region. This is a key reason why many enthusiasts appreciate the sonic qualities of the Accuphase P-4600.
