Challenge: 1 MHz FSK shock test on amplifier output – What would your scope show?

[tomsoon]

None that I've used, at least. Shake tables sometimes use "normal" audio amplifiers.

Hi,
That is a very fair question, but it actually touches the core of this challenge.
From a pure semiconductor physics perspective, amplifier classifications (Class A, AB, D) are defined by the conduction angle and the operating state of the transistors. These fundamental rules of physics apply whether a circuit operates at 1 kHz or 1 MHz.
While 1 MHz is traditionally considered the lower end of the RF spectrum, the question here is explicitly focused on audio frequency topologies. I wanted to see how the output stages, feedback loops, and architectures designed for high-end audio would react if they were pushed to handle this high-frequency energy directly under a real 7.5-ohm load.
It’s exactly this overlap between audio architecture and RF-like energy where the boundaries of conventional design become fascinating. Thank you for raising this point!
Best regards,

 

[solderdude]

Can I ask if this product would be a perfectly reasonable, albeit, slightly expensive, hi-fi amp?

Well you'd need 2 for stereo.

 

[solderdude]

Hi,
Thank you for your thoughts! That Amplifier Research model is indeed a magnificent piece of laboratory equipment. You are spot on—those are exactly the machines built to handle this kind of high-frequency energy [AS INDEX].
But the real, fascinating question for an audio designer is this: is it theoretically possible to achieve this level of structural phase linearity and stability within a pure, analog audio topology, without relying on military-grade lab gear?
Thank you again for bringing that EMC perspective into the discussion, it really highlights the scale of the challenge!
Best regards,

---

Would that be needed is a better question.
What's wrong with a power bandwidth of <10Hz-30kHz -0.5dB ?
What transducer could ever do an amp justice ?
What's with the bold letters ?

 

[voodooless]

Hi ,
Thank you for your excellent and deeply technical response. I completely agree with your concluding sentence: oversimplification indeed leads to false conclusions [AS INDEX]. That is precisely why this challenge was created—to move away from the "oversimplified" standard 1 kHz sine wave tests that dominate the industry [AS INDEX].
To answer your question about why anyone would perform such an experiment:
The core intent is to evaluate structural phase linearity, bandwidth stability, and the pure transient speed of an analog architecture under extreme, dynamic conditions [AS INDEX]. Standard audio band tests are often too comfortable and mask how a topology truly handles high-frequency, real-time energy modulation [AS INDEX].
I wanted to open a discussion on whether it is theoretically possible for an analog topology to maintain an ultra-low, linear output impedance and stay perfectly stable under a 1 MHz FSK shock wave directly into a 7.5-ohm load—without losing control and without relying on heavy protective input filters [AS INDEX].
Thank you again for sharing your valuable expertise and your brilliant point about Class D filter limitations! [AS INDEX]
Best regards,

---

None of this particular explains why, though…

If 1 kHz is too comfortable, why jump to 1 MHz? What is too comfortable? You do realize that amps are tested not just at 1 kHz?

 

[fpitas]

I wanted to see how the output stages, feedback loops, and architectures designed for high-end audio would react if they were pushed to handle this high-frequency energy directly under a real 7.5-ohm load.

Right, OK. Uhm, what exactly does that simulate that one might encounter in real-world use?

 

[pma]

I do test amplifiers by pushing current into their outputs, to measure output impedance vs. frequency and output impedance non-linearity. But I will NOT participate in the topic as opened in this thread.

 

[fpitas]

I do test amplifiers by pushing current into their outputs, to measure output impedance vs. frequency and output impedance non-linearity. But I will NOT participate in the topic as opened in this thread.

View attachment 552087 View attachment 552088

I was curious why someone thought this 1MHz FSK was a good test, extreme or not. It's looking like we won't get that answer.

 

[solderdude]

Can I ask if this product would be a perfectly reasonable, albeit, slightly expensive, hi-fi amp?

Probably not as it was not designed for this goal but instead for driving all kinds test signals in all kinds of loads.

±1.0 dB, 10Hz–300kHz, (±4.0 dB, 300kHz–1MHz)
350W in 1.7ohm (10Hz–300kHz)

600ohm input is a bit low for home audio, noise and distortion is not specified either.
Distortion will be low as that would show up in measurements it were to be used in.

 

[fpitas]

Probably not (output resistance too high for one thing) as it was not designed for this goal but instead for driving all kinds test signals in all kinds of loads.

±1.0 dB, 10Hz–300kHz, (±4.0 dB, 300kHz–1MHz)
350W in 1.7ohm (10Hz–300kHz)

600ohm input is a bit low for home audio, noise and distortion is not specified either.
Distortion will be low as that would show up in measurements.

I've used amps like that to test line noise susceptibility. You drive the product's power inlet and observe the effects.

 

[solderdude]

Yep, I managed to destroy one (older type) during EMC testing.
Indeed coupled to a device for testing flicker and pulses.
They are beasts.

 

[tomsoon]

If your pets listen to supersonic tweeters, it's a good choice.

Hi,Haha, you're right, the family cat might be the only one listening directly to 1 MHz continuous tones! But jokes aside, you have a beautiful DIY horn setup there with the TAD and Altec drivers. As anyone who builds horns knows, these high-efficiency systems are incredibly unforgiving when it comes to microscopic phase delays, transient smearing, and timing issues.The core of this 1 MHz FSK stress test is not about making music for pets. It’s a pure analysis of structural phase linearity. If an analog architecture can track these brutal, high-speed phase hops perfectly into a 7.5-ohm load without phase lag, then perhaps—just perhaps—within the standard audio band (20 Hz - 20 kHz), the phase response and transient tracking might become completely flawless, delivering the kind of holographic, clean depth that ultra-sensitive horns demand.Thank you for your response, Best regards,

 

[DavidEdwinAston]

Some HEA amplifiers are purported to be "wideband" and able to pass 1 Mhz .. From the top of my head, Spectral, and some early Krell, there are many others who claim the same

from Spectral (not a safe website accoring to Chrome)

Spectral Audio DMA-300 Specifications

That Spectral looks a beast Franz!

 

[fpitas]

You ought to also study 1MHZ FSK injected into the power inlet. Perhaps you could determine why audiophiles need such expensive power cords.

 

[tomsoon]

None of this particular explains why, though…

If 1 kHz is too comfortable, why jump to 1 MHz? What is too comfortable? You do realize that amps are tested not just at 1 kHz?

Hi,
That is a perfectly reasonable question. Yes, of course, everyone knows that amplifiers are tested at frequencies other than just 1 kHz, such as 10 kHz or 20 kHz.
However, the term "too comfortable" was meant to describe the nature of the signal itself, not just the frequency number. Standard industry tests—whether at 1 kHz or 20 kHz—almost exclusively use continuous, steady-state sine waves. A continuous sine wave is structurally predictable and allows the internal loops of an amplifier to settle into a stable state.
The jump to 1 MHz with an FSK modulation is fundamentally different. It is not about forcing an audio amplifier to become a radio transmitter. It is about creating a dense, non-continuous stress test packed with violent, microscopic phase and frequency shifts.
The real question is: if a topology is robust enough to track these rapid, 1 MHz dynamic energy jumps into a real 7.5-ohm load without phase lag, then perhaps—just perhaps—it proves that its error-correction and feedback mechanisms are fast enough to handle complex musical transients and speaker back-EMF within the standard audio band with absolute perfection.
It is simply a method to explore the ultimate structural limits of analog architecture. Thank you for pushing the discussion further!
Best regards,

 

[voodooless]

As anyone who builds horns knows, these high-efficiency systems are incredibly unforgiving when it comes to microscopic phase delays, transient smearing, and timing issues.

I’d say, citation needed…

The core of this 1 MHz FSK stress test is not about making music for pets. It’s a pure analysis of structural phase linearity. If an analog architecture can track these brutal, high-speed phase hops perfectly into a 7.5-ohm load without phase lag, then perhaps—just perhaps—within the standard audio band (20 Hz - 20 kHz), the phase response and transient tracking might become completely flawless, delivering the kind of holographic, clean depth that ultra-sensitive horns demand

If you need phase response in de audio band, you measure it in de audio band…

Any music will be band limited anyway.

And please stop using AI beyond basic translations. It’s very clear you’re using it for more than just that. It is against the policy.

Official policy on use of AI

We have had recent issues come about regarding use of AI in member posts. I personally think this technology can be useful to us. And at any rate, hate having to police members left and right on what they post. That said, here are my thoughts: 1. If you are going to use AI, make it...

www.audiosciencereview.com

 

[voodooless]

The real question is: if a topology is robust enough to track these rapid, 1 MHz dynamic energy jumps into a real 7.5-ohm load without phase lag, then perhaps—just perhaps—it proves that its error-correction and feedback mechanisms are fast enough to handle complex musical transients and speaker back-EMF within the standard audio band with absolute perfection.

This doesn’t follow at all. Any proper amp design would filter out any frequencies it should not amplify. They are specifically designed to block and RFI at these frequencies.

 

[Sir Sanders Zingmore]

Are we chatting with an AI?

 

[tomsoon]

Yep, I managed to destroy one (older type) during EMC testing.
Indeed coupled to a device for testing flicker and pulses.
They are beasts.

Hi,
Wow, that must have been quite a dramatic moment in the lab! Burning out one of those beasts during an EMC pulse test definitely proves how unforgiving and intense this kind of high-frequency energy can be [AS INDEX]. Thank you for sharing that experience!
You are absolutely right that humans tend to overestimate their hearing, and our ears should primary be used to enjoy music, not as spectral analyzers. I couldn't agree more [AS INDEX].
But perhaps—just perhaps—there is a beautiful bridge between the two perspectives. The goal of designing a topology that can survive an EMC-like 1 MHz FSK stress test into a 7.5-ohm load is not about trying to hear 1 MHz [AS INDEX].
Instead, if the architecture is fast and robust enough to remain stone-cold and perfectly stable under that kind of assault, it means that within the standard audio band, all micro-oscillations, transient smearing, and phase shifts completely vanish [AS INDEX]. And that absolute technical peace is exactly what allows us to sit back and listen to the music for 16 hours straight without any brain fatigue [AS INDEX].
Thank you again for this great and honest discussion!
Best regards,

 

[fpitas]

This doesn’t follow at all. Any proper amp design would filter out any frequencies it should not amplify. They are specifically designed to block and RFI at these frequencies.

And if slew rates are being exceeded and/or things are slamming against rails, it's not very illuminating.

 

[tomsoon]

Are we chatting with an AI?

Hi Everyone,
Haha, no, I am definitely not an AI! I am a real flesh-and-blood person sitting at my workbench. However, since my native language is Hungarian, I am using a precise English translation tool specifically to ensure that the technical depth and exact logic of my thoughts are conveyed perfectly to this forum.
I want to personally thank everyone for the quick, relevant, and deeply insightful responses. Since I am completely new here, I really wanted to bring something fresh and challenging to the table instead of the usual, boring topics.
Unfortunately, finding this level of deep professional discussion is not possible on local forums in Hungary, which is exactly why I chose this highly respected, high-caliber international platform to find answers.
Thank you all once again for your incredible expertise. And trust me, this is just the beginning—there is much more to come!
Best regards,

---