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Incorporating Burst Power and Slew Rate Tests for Amplifier Reviews

AdamFrandsen

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Hi Amir and ASR community,

Thank you for the outstanding work and dedication to providing detailed and objective audio measurements. I’ve learned a great deal from your reviews and deeply appreciate the scientific rigor you bring to the audiophile world.

I’d like to suggest adding burst power testing and slew rate measurements to ASR’s testing methodology for amplifiers. These tests can complement existing metrics like THD, IMD, and SNR by providing additional insights into an amplifier’s real-world performance, particularly its ability to handle transients and dynamic demands.


Why Burst Power and Slew Rate Are Important

1. Real-World Music Playback:


• Music is dynamic, with sudden transients (e.g., drum hits, orchestral crescendos) that place brief but significant demands on an amplifier’s power supply and output stages.


• Continuous power tests don’t fully capture how an amplifier performs during these short bursts, which are critical for perceived dynamics, impact, and energy.


2. Transient Response and Speed:


• Slew rate reflects an amplifier’s ability to handle rapid changes in signal amplitude, which affects high-frequency content and perceived clarity.


• While bandwidth measurements may imply sufficient slew rate, explicitly testing it can reveal subtle limitations that affect performance with highly dynamic or complex signals.


3. Dynamic Headroom and Power Delivery:


• Burst power tests can reveal an amplifier’s ability to deliver extra power for short durations without clipping or voltage sag, which is especially relevant for amplifiers with limited continuous power ratings.


How to Perform These Tests Using the APx555


1. Burst Power Testing:


• Test Signal: Generate short-duration sine wave bursts (e.g., 10ms on, 90ms off) at different frequencies (e.g., 40 Hz, 1 kHz).


• Measurement Goals:


• Peak burst power output before clipping.


• Distortion (THD+N) during the burst.


• Voltage sag and recovery during the burst.


• Benefits: Highlights the amplifier’s dynamic headroom and power supply stability under real-world transient demands.


2. Slew Rate Testing:


• Test Signal: Use a square wave or fast-rising sine wave. Ensure the rise time challenges the amplifier’s speed.


• Measurement Goals:


• Slew rate = ΔVoltage / ΔTime (V/μs).


• Evaluate consistency across loads (e.g., 4 ohms, 8 ohms).


• Benefits: Provides direct insight into how quickly the amplifier can respond to transient changes, which affects perceived detail and resolution.


Addressing Potential Concerns


1. Relevance to Audible Performance:


• While amplifiers may meet basic thresholds for transient handling, measuring burst power and slew rate explicitly can help identify subtle performance differences, particularly in demanding systems or high-dynamic-range playback scenarios.


2. Redundancy with Existing Tests:


• Bandwidth tests suggest adequate slew rate, but they don’t measure the amplifier’s actual response speed. Similarly, continuous power tests don’t reveal how an amplifier handles dynamic peaks or recovers from transient demands.


• These tests fill gaps left by existing methods, providing a more comprehensive view of real-world behavior.


3. Complexity and Standardization:


• The APx555 is well-suited for both burst power and slew rate testing, making implementation straightforward.


• Consistent testing protocols can be developed, such as fixed burst durations (e.g., 10ms) and specific rise times for slew rate evaluation, ensuring repeatability and comparability across reviews.


Why It Adds Value to ASR

Incorporating burst power and slew rate tests can:


• Provide insights into an amplifier’s transient handling that align with real-world music playback.


• Help differentiate amplifiers beyond traditional metrics, especially in systems that emphasize dynamic range and clarity.


• Maintain ASR’s reputation for thorough, science-driven evaluations by addressing both static and dynamic performance aspects.


I hope this suggestion sparks discussion and exploration. Thank you for considering this addition, and I look forward to hearing the community’s thoughts!


Best regards,
Adam
 
Slew rate is represented by the FFT graph of frequency response. The FFT is more valuable and graphically present a more readable and more applicable information set.
 
The amp reviews already include CEA 2006/490A.

I am unclear on how slew rate is of any use to consumers.

edit:
@Doodski: Slew rate is represented by the FFT graph of frequency response.
That's bandwidth, not slew rate,
 
Slew rate is represented by the FFT graph of frequency response. The FFT is more valuable and graphically present a more readable and more applicable information set.
No. The FFT graph of frequency response does not represent slew rate. Both measurements are valuable but serve different purposes. FFT analysis focuses on frequency-domain behavior, while slew rate testing evaluates the amplifier’s responsiveness to rapid transients in the time domain.
 
The amp reviews already include CEA 2006/490A.

I am unclear on how slew rate is of any use to consumers.

edit:

That's bandwidth, not slew rate,
The CEA 2006/490A standards focus on continuous power, distortion, and SNR, which are essential metrics, but they don’t address slew rate or transient response. These are key to understanding an amplifier’s ability to handle fast transients, like sharp drum hits or sudden orchestral peaks, which are an important and audible aspect of music playback.

While slew rate itself may not be directly meaningful to every consumer, its impact is highly relevant for reproducing high-frequency content and maintaining clarity during rapid signal changes. An amplifier with insufficient slew rate can introduce audible smearing, distortion, or loss of detail in fast, dynamic passages, even if it performs well under continuous power tests.

Incorporating slew rate testing alongside CEA standards provides a more complete picture of real-world performance, helping consumers better understand how an amplifier handles complex and dynamic musical signals.
 
Slew Rate Testing:


• Test Signal: Use a square wave or fast-rising sine wave. Ensure the rise time challenges the amplifier’s speed.


• Measurement Goals:


• Slew rate = ΔVoltage / ΔTime (V/μs).
The usual misunderstanding of slew rate. Slew rate (in most class a/b amps) measures the amps recovery speed from front end overload, and has little to do with its linear operation, the only place you want the amp to be. If the amp can put out 30khz at full power with rated distortion it has enough slew rate. And very few modern amplifiers dont have a fast enough slew rate.
 
The CEA 2006/490A standards focus on continuous power, distortion, and SNR, which are essential metrics
They prescribe 20ms tone bursts. And those data are included in the reviews here. You might want to actually read a few of them.
An amplifier with insufficient slew rate can introduce audible smearing, distortion, or loss of detail in fast, dynamic passages, even if it performs well under continuous power tests.
1971 called.
 
And very few modern amplifiers dont have a fast enough slew rate.
I can't think of any, much less very few. :D Though it's possible that some ultra-expensive fashion audio product might be an exception.

Slewing is indeed vastly misunderstood, and it is clearly the case here.
 
While bandwidth measurements may imply sufficient slew rate, explicitly testing it can reveal subtle limitations that affect performance with highly dynamic or complex signals.
Testing it with what? A single that is way outside of the bandwidth of any playback system? If so, what would it tell us?
 
No. The FFT graph of frequency response does not represent slew rate. Both measurements are valuable but serve different purposes. FFT analysis focuses on frequency-domain behavior, while slew rate testing evaluates the amplifier’s responsiveness to rapid transients in the time domain.
I thought slew rate is the ratio of slope to the voltage which can be used to determine max frequency response. The FFT shows that in the form of max frequency response. Am I mistaken, kinda close or outrageously wrong... LoL. I am thinking the rise time of the slew rate slope can be used to calculate max frequency. So they are inter-related.
 
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Add-on to my last comment. I found some details in a old textbook about manually calculating the upper frequency but I did not find the formula for calculating the upper critical frequency from the slew rate. I'll keep searching the old textbooks for the procedure for slew rate to upper critical frequency calculation to show how they are related. I skimmed and scanned 2 ~1500 page textbooks and am unable to find the details explaining the relationship of slew rate to upper frequency response. There is simply such a large amount of information there it makes finding little tidbits like that difficult. It could even be part of the OP Amp study too and that's a whole other section. The idea I am trying to convey is that the slew rate does not give a actual upper frequency limit and to replace that dated spec the FFT does that and more.
Screenshot 2025-01-01 215656.png
 
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Testing it with what? A single that is way outside of the bandwidth of any playback system? If so, what would it tell us?

Thank you for the clarification. You are right that bandwidth measurements from an FFT provide strong indications of an amplifier’s ability to handle signals within its operational range. However, the relationship between bandwidth and slew rate isn’t always linear or sufficient to fully assess real-world performance, as I understand it...

1. Slew rate is a time-domain measurement, not just frequency-domain. Bandwidth measures how well an amplifier reproduces steady-state sine waves but doesn’t capture its ability to respond to sharp, transient signals. Slew rate directly measures this, focusing on how quickly voltage changes over time, even within the audible range.

2. Slew rate and bandwidth are related, but not identical. While sufficient bandwidth often implies a decent slew rate, it’s not always guaranteed due to differences in amplifier design:

Feedback loops: High feedback designs may have excellent frequency response but introduce time-domain limitations like phase shifts.

Transient limiting: Some amplifiers perform well with continuous signals but struggle with short transients, leading to slew rate limiting.

3. Testing doesn’t require signals outside bandwidth. Slew rate testing can use fast-rising square waves or sine waves near the upper edge of the amplifier’s range (e.g., 10-20 kHz), staying relevant to audible signals while assessing transient handling.

What slew rate testing adds beyond bandwidth:

Verifying real-world behavior: Confirms how the amplifier handles rapid voltage changes under load, which bandwidth alone doesn’t reveal.

Revealing limitations: Identifies transient smearing, overshoot, or phase artifacts that may not appear in an FFT analysis.

Bandwidth measurements are valuable, but slew rate testing complements them by focusing on time-domain behavior within the same range. It’s not about testing outside the amplifier’s bandwidth but ensuring it can handle real-world transients with precision.
 
They prescribe 20ms tone bursts. And those data are included in the reviews here. You might want to actually read a few of them.

1971 called.
My main concern is that while 20ms bursts might provide insight into short-term power output, they don’t fully address other transient-related performance metrics, such as slew rate or the amplifier’s ability to handle rapid voltage changes over shorter durations. These are important for real-world music playback, especially for signals with steep transients or complex dynamics.

To better evaluate transient performance, burst testing could be extended as follows:

1. Vary Burst Duration: Test with shorter bursts (e.g., 5ms or 10ms on) combined with off-times (e.g., 90ms). This simulates the kind of short, sharp dynamic peaks common in music, such as drum hits or plucked strings.

2. Test Across Frequencies: Perform burst tests at different frequencies (e.g., 40 Hz for bass, 1 kHz for midrange, and 10 kHz for treble) to see how the amplifier handles transients across its operating range.

3. Measure Recovery Time: Monitor how quickly the amplifier stabilizes after each burst. Slow recovery could indicate limitations in the power supply or feedback network.

4. Compare Distortion During Bursts: Measure THD+N and any clipping artifacts during and immediately after the bursts to identify performance differences under dynamic conditions.

5. Voltage Sag Observation: Observe whether the amplifier’s power rails sag under burst conditions, which could affect its ability to sustain transients.
 
Thank you for the clarification. You are right that bandwidth measurements from an FFT provide strong indications of an amplifier’s ability to handle signals within its operational range. However, the relationship between bandwidth and slew rate isn’t always linear or sufficient to fully assess real-world performance, as I understand it...

1. Slew rate is a time-domain measurement, not just frequency-domain. Bandwidth measures how well an amplifier reproduces steady-state sine waves but doesn’t capture its ability to respond to sharp, transient signals. Slew rate directly measures this, focusing on how quickly voltage changes over time, even within the audible range.
No it isn't! Slew rate is measured in volts/uS, and whether adequate is easily tested by a 20kHz sine wave at full power. If the amp can do that at its rated distortion, then the slew rate is sufficient. In any bandwidth limited system, it's not possible for a transient to exceed the amplifier's slew rate as then the bandwidth would be inadequate.

S
 
Revealing limitations: Identifies transient smearing, overshoot, or phase artifacts that may not appear in an FFT analysis.
So the Fourier theorem is wrong? Do tell. The Fields Medal awaits. (NB: the two almost certain "tells" that someone is regurgitating audiophile nonsense are the words "quantum" and "smear")

My main concern is that while 20ms bursts might provide insight into short-term power output, they don’t fully address other transient-related performance metrics, such as slew rate or the amplifier’s ability to handle rapid voltage changes over shorter durations.
Gasp! You mean a single measurement doesn't automatically provide every other measurement? Where's my fainting couch? Point remains, the peak power you claimed wasn't measured actually is if you bother to read the reviews. Throwing irrelevant new stuff onto that changes nothing.

This stuff reads like AI.
 
No it isn't! Slew rate is measured in volts/uS, and whether adequate is easily tested by a 20kHz sine wave at full power. If the amp can do that at its rated distortion, then the slew rate is sufficient. In any bandwidth limited system, it's not possible for a transient to exceed the amplifier's slew rate as then the bandwidth would be inadequate.

S
Got there before I did. :D There's no indication that this concept is understood at all beyond buzz-wording.
 
So the Fourier theorem is wrong? Do tell. The Fields Medal awaits. (NB: the two almost certain "tells" that someone is regurgitating audiophile nonsense are the words "quantum" and "smear")


Gasp! You mean a single measurement doesn't automatically provide every other measurement? Where's my fainting couch? Point remains, the peak power you claimed wasn't measured actually is if you bother to read the reviews. Throwing irrelevant new stuff onto that changes nothing.

This stuff reads like AI.
I know that the peak power is measured
 
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