Apollon 1ET6525SA ST Amplifier Review

[Matias]

I am more of an empirical guy: put the same power supply, module, buffer boards etc in the 2 different chassis, power both with steady 500 W at 60 Hz signal into 4 ohms, and capture the 2 DUTs heat map with a thermal scanner for the first minute or so. Then all theories are validated.

Yes, music is dynamic and no steady state sine wave, I know, but the test would just measure how fast chassis temperature spreads and cools the same thermal load. Then micro apply this for every drum beat or bass note played in a song.

 

[Ropeburn]

I am more of an empirical guy: put the same power supply, module, buffer boards etc in the 2 different chassis, power both with steady 500 W at 60 Hz signal into 4 ohms, and capture the 2 DUTs heat map with a thermal scanner for the first minute or so. Then all theories are validated.

Yes, music is dynamic and no steady state sine wave, I know, but the test would just measure how fast chassis temperature spreads and cools the same thermal load. Then micro apply this for every drum beat or bass note played in a song.

And if you need a specialised test signal with power-demanding drums and bass notes, you can always ask your friendly neighbourhood Electrodoge

 

[Nachocom]

This is a review and detailed measurements of the Apollon 1ET6525SA ST Purifi based class D stereo power amplifier. It was kindly loaned to me by a member and costs 1,190.00€ (USD $1,348).
View attachment 503302
Apollon designs are unique a step up from some of its competitors. Even the power switch feels nice and has niceties such as settable brightness:
View attachment 503303
That premium extends to no less than four gain settings (although I had trouble reading the tiny fonts). Not only do we have trigger input but also output!

In use, the top of the amp was room temperature and bottom only slightly above.

Apollon 1ET6525SA ST Amp Measurements
To keep me from going crazy, I focused my testing on XLR input only. For gain, I started with the lowest gain setting of 12.X:
View attachment 503304
This performance easily lands the amp into top 20 amplifiers ever tested (out of nearly 300):
View attachment 503305
Naturally, higher gain settings rob you of some performance but not much at 20.X:
View attachment 503306
Going to 27 does cost you though in noise department:
View attachment 503307

Fortunately 20 dB gain is good enough to push the amp to max power with nominal 4 volt input over XLR connection:
View attachment 503308

If you have a professional interface that can output more than 9 volts, you can opt to use the lowest gain:
View attachment 503309

From here on, I focused on testing at 20 dB.

Frequency response is load independent as we expect in this class of class D amps:
View attachment 503310

Multitone and 19+20 kHz show superbly low intermodulation distortion:
View attachment 503311
View attachment 503312
Lower class class D amps really suffer in the above test. Not so here, leaving many class AB amps in the dust.

Channel isolation is superb as well:
View attachment 503313

Available power in the sweet spot of what I recommend:
View attachment 503314

View attachment 503315
And doesn't change much in bass either:
View attachment 503316

8 Ohm naturally has less power but most speakers are 4 ohms or lower:
View attachment 503317

I tested its ability to drive one channel with 2 ohm. While it did that superbly, it did not result in a lot more power:
View attachment 503318

Our sweeps with respect to frequency show the common rise in distortion at higher frequencies. But this is still excellent performance:
View attachment 503319

Power on noise is nonexistent but there may be noise on power off:
View attachment 503320

Finally, the amp is rock solid on power up:
View attachment 503322

Conclusions
The Apollon 1ET6525SA ST is a solid offering in premium module based amplifiers. It has done justice to the underlying Purifi module by providing superbly low noise and distortion. It has rich feature set in the form of gain setting, trigger, etc. With almost every speaker being happy with nearly 400 watts on tap, it makes for an easy and future proof choice. And a feature in the cap of our European members!

I am happy to recommend the Apollon 1ET6525SA ST amplifier.
-----------
As always, questions, comments, recommendations, etc. are welcome.

Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/

The ASR review of the Apollon 1ET6525SA is an exercise in technological asceticism that borders on the monastic.
Allow me to add some nuance here:

First, ASR glorifies a 104 dB SINAD as if it were the Holy Grail, despite their own preaching that the human ear is incapable of detecting sonic impurities above 95 dB. It’s a race to see who has the most "invisible" floor noise; it’s like paying a premium for a TV that displays colors the human eye literally cannot see.

Second, They treat the Apollon as a mere "Purifi module carrier," snubbing the power supply and internal wiring as if the chassis were just an ornament. It’s the ultimate reductionist fail: judging a three-star Michelin restaurant based solely on the chemical purity of the table salt.

Finally, Amir measures against fixed 4 and 8-ohm resistors—test subjects as compliant as a British butler. However, real-world speakers are reactive beasts with chaotic phase shifts that could make this Class D amp break a sweat. ¿ASR prefers the safety of the spreadsheet over the jungle of the living room?

ASR offers "transparency pornography." It’s fascinating to watch them obsess over noise at -120 dB while a single fly buzzing in the room generates more acoustic distortion than the entire signal chain.
Shall we raise a glass to turning music into a tax audit?
Cheers from Spain ¡¡

 

[Paul7777x]

The ASR review of the Apollon 1ET6525SA is an exercise in technological asceticism that borders on the monastic.
Allow me to add some nuance here:

First, ASR glorifies a 104 dB SINAD as if it were the Holy Grail, despite their own preaching that the human ear is incapable of detecting sonic impurities above 95 dB. It’s a race to see who has the most "invisible" floor noise; it’s like paying a premium for a TV that displays colors the human eye literally cannot see.

Second, They treat the Apollon as a mere "Purifi module carrier," snubbing the power supply and internal wiring as if the chassis were just an ornament. It’s the ultimate reductionist fail: judging a three-star Michelin restaurant based solely on the chemical purity of the table salt.

Finally, Amir measures against fixed 4 and 8-ohm resistors—test subjects as compliant as a British butler. However, real-world speakers are reactive beasts with chaotic phase shifts that could make this Class D amp break a sweat. ¿ASR prefers the safety of the spreadsheet over the jungle of the living room?

ASR offers "transparency pornography." It’s fascinating to watch them obsess over noise at -120 dB while a single fly buzzing in the room generates more acoustic distortion than the entire signal chain.
Shall we raise a glass to turning music into a tax audit?
Cheers from Spain ¡¡

I assume you have tiny ….?

 

[DMill]

I assume you have tiny dick?

 

[amirm]

First, ASR glorifies a 104 dB SINAD as if it were the Holy Grail, despite their own preaching that the human ear is incapable of detecting sonic impurities above 95 dB.

Huh? How could you claim that we say both of those? It has to be either or.

It’s a race to see who has the most "invisible" floor noise; it’s like paying a premium for a TV that displays colors the human eye literally cannot see.

No, it is a race to get to that level of *provable* transparency. 95 dB is not there. And even 104 dB is not. Put your ear next to the tweeter of an active monitor and you will hear noise due to this lack of transparency. Admittedly, they use lower fidelity amps but that is the reason we measure.

ASR offers "transparency pornography." It’s fascinating to watch them obsess over noise at -120 dB while a single fly buzzing in the room generates more acoustic distortion than the entire signal chain.

You are wrong on that as well. No one here obsesses over "-120 dB." As to fly, they can be quite noisy and annoying. For that reason, you don't want your system to make similar noise.

 

[DMill]

Not a great response IMO. But the post makes their point. Does it matter to pursue levels of distortion that are likely beyond hearing? Let’s just say for the sake of argument maybe we don’t really know. And so maybe it does? And even if it doesn’t maybe just doing it the best we can is a win.

 

[Waveform Fidelity]

This is a review and detailed measurements of the Apollon 1ET6525SA ST Purifi based class D stereo power amplifier. It was kindly loaned to me by a member and costs 1,190.00€ (USD $1,348).
View attachment 503302
Apollon designs are unique a step up from some of its competitors. Even the power switch feels nice and has niceties such as settable brightness:
View attachment 503303
That premium extends to no less than four gain settings (although I had trouble reading the tiny fonts). Not only do we have trigger input but also output!

In use, the top of the amp was room temperature and bottom only slightly above.

Apollon 1ET6525SA ST Amp Measurements
To keep me from going crazy, I focused my testing on XLR input only. For gain, I started with the lowest gain setting of 12.X:
View attachment 503304
This performance easily lands the amp into top 20 amplifiers ever tested (out of nearly 300):
View attachment 503305
Naturally, higher gain settings rob you of some performance but not much at 20.X:
View attachment 503306
Going to 27 does cost you though in noise department:
View attachment 503307

Fortunately 20 dB gain is good enough to push the amp to max power with nominal 4 volt input over XLR connection:
View attachment 503308

If you have a professional interface that can output more than 9 volts, you can opt to use the lowest gain:
View attachment 503309

From here on, I focused on testing at 20 dB.

Frequency response is load independent as we expect in this class of class D amps:
View attachment 503310

Multitone and 19+20 kHz show superbly low intermodulation distortion:
View attachment 503311
View attachment 503312
Lower class class D amps really suffer in the above test. Not so here, leaving many class AB amps in the dust.

Channel isolation is superb as well:
View attachment 503313

Available power in the sweet spot of what I recommend:
View attachment 503314

View attachment 503315
And doesn't change much in bass either:
View attachment 503316

8 Ohm naturally has less power but most speakers are 4 ohms or lower:
View attachment 503317

I tested its ability to drive one channel with 2 ohm. While it did that superbly, it did not result in a lot more power:
View attachment 503318

Our sweeps with respect to frequency show the common rise in distortion at higher frequencies. But this is still excellent performance:
View attachment 503319

Power on noise is nonexistent but there may be noise on power off:
View attachment 503320

Finally, the amp is rock solid on power up:
View attachment 503322

Conclusions
The Apollon 1ET6525SA ST is a solid offering in premium module based amplifiers. It has done justice to the underlying Purifi module by providing superbly low noise and distortion. It has rich feature set in the form of gain setting, trigger, etc. With almost every speaker being happy with nearly 400 watts on tap, it makes for an easy and future proof choice. And a feature in the cap of our European members!

I am happy to recommend the Apollon 1ET6525SA ST amplifier.
-----------
As always, questions, comments, recommendations, etc. are welcome.

Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/

Any plot of phase response?

 

[Apollon Audio]

The ASR review of the Apollon 1ET6525SA is an exercise in technological asceticism that borders on the monastic.
Allow me to add some nuance here:

First, ASR glorifies a 104 dB SINAD as if it were the Holy Grail, despite their own preaching that the human ear is incapable of detecting sonic impurities above 95 dB. It’s a race to see who has the most "invisible" floor noise; it’s like paying a premium for a TV that displays colors the human eye literally cannot see.

Second, They treat the Apollon as a mere "Purifi module carrier," snubbing the power supply and internal wiring as if the chassis were just an ornament. It’s the ultimate reductionist fail: judging a three-star Michelin restaurant based solely on the chemical purity of the table salt.

Finally, Amir measures against fixed 4 and 8-ohm resistors—test subjects as compliant as a British butler. However, real-world speakers are reactive beasts with chaotic phase shifts that could make this Class D amp break a sweat. ¿ASR prefers the safety of the spreadsheet over the jungle of the living room?

ASR offers "transparency pornography." It’s fascinating to watch them obsess over noise at -120 dB while a single fly buzzing in the room generates more acoustic distortion than the entire signal chain.
Shall we raise a glass to turning music into a tax audit?
Cheers from Spain ¡¡

I have to say, this is one of the more eloquently written critiques in the thread, and I think there is actually some common ground in it.

On the SINAD point, I don’t think the intention is to suggest that 104 dB is audibly superior to 95 dB in a direct sense. The value of pushing performance that far is more about engineering margin than audibility per se. Once distortion and noise are sufficiently below the threshold of perception, the goal becomes ensuring they remain there consistently across different gain structures, loads, and system contexts.

So it’s less about chasing “invisible improvements” and more about ensuring the amplifier never becomes the limiting factor.

Regarding the idea of a “module carrier,” I understand where that perception comes from, especially with highly integrated Class D designs. But in practice, implementations using the same core module can and do differ measurably, as is also often seen in independent measurements across different builds. Input buffer design, gain structure, power supply behavior under load, grounding topology, layout, wiring, and thermal management all influence how consistently the amplifier maintains its performance envelope over time and under varying conditions.

The module defines the potential, but the implementation determines how reliably that potential is realized.

On the topic of resistive measurements versus real-world speakers, it’s true that a resistor is a simplified load. At the same time, one of the key design goals of modern Purifi-based amplifiers is load invariance, very low output impedance and stable behavior even under complex phase angles, including cases with complex speaker loads that have been measured and discussed here and elsewhere.

So the “British butler” tends to stay remarkably composed, even in less polite environments.

And finally, on the broader point about “transparency culture”: there is always a balance to strike. Measurements provide a controlled, repeatable baseline that allows meaningful comparison. They are not the entirety of the listening experience, but they are a very effective way to ensure that the amplifier is not adding its own character.

If anything, the goal is not to reduce music to numbers, but to remove uncertainty from one part of the chain.

In that sense, it’s less asceticism and more engineering discipline.

Cheers.

 

[Ruediger]

I find the sinad argument rather subjective. Who decides how much sinad anyone needs. What rooms people have, how sensitive and big the speakers are, what is demanded from the audio system and how far anyone likes these amps to be turned up.

This line of argument could be made for many parameters. Let's just take power. The amp discussed here has a lot. Who listens with that much output. I know no one, so that's unnecessary overengineering too?

This is a hobby and everyone is free to like overengineering even to large degrees. For whatever reasons, practical ones included. And I don't see this specific amp as a prime example of that. Did you even look at the price, Nachocom? There is a lot of hardware out there for much more money that does not even achieve these levels of headroom/reserves. And people are free to like that too.

 

[DavidEdwinAston]

The ASR review of the Apollon 1ET6525SA is an exercise in technological asceticism that borders on the monastic.
Allow me to add some nuance here:

First, ASR glorifies a 104 dB SINAD as if it were the Holy Grail, despite their own preaching that the human ear is incapable of detecting sonic impurities above 95 dB. It’s a race to see who has the most "invisible" floor noise; it’s like paying a premium for a TV that displays colors the human eye literally cannot see.

Second, They treat the Apollon as a mere "Purifi module carrier," snubbing the power supply and internal wiring as if the chassis were just an ornament. It’s the ultimate reductionist fail: judging a three-star Michelin restaurant based solely on the chemical purity of the table salt.

Finally, Amir measures against fixed 4 and 8-ohm resistors—test subjects as compliant as a British butler. However, real-world speakers are reactive beasts with chaotic phase shifts that could make this Class D amp break a sweat. ¿ASR prefers the safety of the spreadsheet over the jungle of the living room?

ASR offers "transparency pornography." It’s fascinating to watch them obsess over noise at -120 dB while a single fly buzzing in the room generates more acoustic distortion than the entire signal chain.
Shall we raise a glass to turning music into a tax audit?
Cheers from Spain ¡¡

If you are a Spaniard, your written English is excellent!

 

[Waveform Fidelity]

If you are a Spaniard, your written English is excellent!

Its ^^ always been that way in Hifi… and peculiar, I would say to Audiophiles! Rarely is there a systems engineering approach… a simple noise or THD budget for the complete replay chain which shows the dominant elements for say distortion. Folks are chasing 4+ noughts of distortion in the amp but the speakers generate no better than 0.5% at best, but somehow they are “separate”

On the plus side, it is very good for business and many folks get great satisfaction from owning distortionless amps.

As long as they are intellectually honest and don’t claim they can hear a difference, then no harm done!

Ps I really would like to see the phase response of one of these distortionless amplifiers.

 

[NTK]

Ps I really would like to see the phase response of one of these distortionless amplifiers.

@pma measured the original Purifi 1ET400A.

PURIFI EVAL2 Eigentakt Mono Amplifier Kit

PURIFI EVAL2 Eigentakt Mono Amplifier Kit I bought this kit from Soundimports https://www.soundimports.eu/en/purifi-eval2.html I had some troubles with the first kit, it would not start switching. However, Soundimports customer support were very helpful and they sent me after return a...

www.audiosciencereview.com

 

[Waveform Fidelity]

thank you

 

[HiFiHelp]

At just 2.9 kg with a built in power supply, this seems like the highest performance amplifier on the market for portable use cases.

Each Topping B200 monoblock is 2.7 kg and that doesn't include the heavy power supplies (each similar in weight to a single B200 amplifier). Despite not being class D, the Topping B200 seems to have a less flat frequency response and more variability with load impedance than the Apollon 1ET6525SA ST. Not to mention that two Topping B200s outputs 83 W less into 4 ohms than a single Apollon 1ET6525SA ST.

ASR Topping B200:

ASR Apollon 1ET6525SA ST:

Of the pre-built 1ET6525SA stereo amplifiers on the market, I believe the Apollon version is the lightest weight and the best featured (more subjective).

The Buckeye 1ET6525SA 2-Channel's lowest gain with these modules is 15 dB versus 12.8 dB with the Apollon 1ET6525SA models, most likely letting the Apollon models achieve a slightly higher SINAD rating in the lowest gain settings versus the Buckeye implementations. Also, the LED on the front of the Buckeye amplifiers cannot be defeated while the amplifier is on, and would be distracting for dark room A/V experiences, unless dimmed with tape or fully out of sight. The Apollon on the other hand lets you control the front LED brightness and even defeat it altogether.

One thing I am curious about is that the frequency response variance with load seems slightly higher with the Apollon 1ET6525SA ST than Purifi's spec for the module. I wonder if Apollon's 1ET6525SA Lux model, or other implementations of this module, are closer to the Purifi spec:

Apollon rates the distortion of their 1ET6525SA Lux model as:
THD+N – – 0.00017 % 20Hz-20kHz

and 1ET6525SA ST as:
THD+N – – 0.0002 % 20Hz-20kHz

I wonder what the reason for the slightly higher THD+N in the ST model is and if it has anything to do with the slight high frequency variability with load impedance. The Apollon 1ET6525SA Lux model weighs 6.8 kg, 1 kg more than double the weight of the Apollon 1ET6525SA ST.

I placed an order for an Apollon 1ET6525SA ST.

 

[Cpopovic]

Prior to purchasing my new Nord Acoustics 9040 MK2 monos, I had eyed the premium version of the Apollon amplifier and compared the specifications in detail. While I acknowledged the minor improvements it offered, the 6.2K euro price of the premium model and its long production times were enough to put me off, especially when the standard Nord 9040 MK2 came in at around 3.2K GBP (roughly 3.8K euros, depending on conversion and taxes). I’ve since been very happy with the Nord’s build quality and particularly enjoy the gain‑setting switch added to the 9040 MK2 model, as it helps me optimise the signal path and maintain better gain integrity across different sources and preamp setups. My monos have been set to the lowest gain (14.4 dB), which helps preserve the soundstage and detail when listening at low to moderate volume. You cannot go wrong with either amplifier.

 

[cEbNVDyfMy]

The ASR review of the Apollon 1ET6525SA is an exercise in technological asceticism that borders on the monastic.
Allow me to add some nuance here:

First, ASR glorifies a 104 dB SINAD as if it were the Holy Grail, despite their own preaching that the human ear is incapable of detecting sonic impurities above 95 dB. It’s a race to see who has the most "invisible" floor noise; it’s like paying a premium for a TV that displays colors the human eye literally cannot see.

Second, They treat the Apollon as a mere "Purifi module carrier," snubbing the power supply and internal wiring as if the chassis were just an ornament. It’s the ultimate reductionist fail: judging a three-star Michelin restaurant based solely on the chemical purity of the table salt.

Finally, Amir measures against fixed 4 and 8-ohm resistors—test subjects as compliant as a British butler. However, real-world speakers are reactive beasts with chaotic phase shifts that could make this Class D amp break a sweat. ¿ASR prefers the safety of the spreadsheet over the jungle of the living room?

ASR offers "transparency pornography." It’s fascinating to watch them obsess over noise at -120 dB while a single fly buzzing in the room generates more acoustic distortion than the entire signal chain.
Shall we raise a glass to turning music into a tax audit?
Cheers from Spain ¡¡

Thank you Claude

 

[LucreciaBorgia]

Prior to purchasing my new Nord Acoustics 9040 MK2 monos, I had eyed the premium version of the Apollon amplifier and compared the specifications in detail. While I acknowledged the minor improvements it offered, the 6.2K euro price of the premium model and its long production times were enough to put me off, especially when the standard Nord 9040 MK2 came in at around 3.2K GBP (roughly 3.8K euros, depending on conversion and taxes). I’ve since been very happy with the Nord’s build quality and particularly enjoy the gain‑setting switch added to the 9040 MK2 model, as it helps me optimise the signal path and maintain better gain integrity across different sources and preamp setups. My monos have been set to the lowest gain (14.4 dB), which helps preserve the soundstage and detail when listening at low to moderate volume. You cannot go wrong with either amplifier.

There are other models at Apollon than the "premium" that make the bill less expensive...and (to my taste) looking much nicier than North ! As for the gain setting, it is present (to my knowledge) in every Purifi model (9040/7040/6525...), so not a North specificity

 

[Jukka]

The ASR review of the Apollon 1ET6525SA is an exercise in technological asceticism that borders on the monastic.
Allow me to add some nuance here:

First, ASR glorifies a 104 dB SINAD as if it were the Holy Grail, despite their own preaching that the human ear is incapable of detecting sonic impurities above 95 dB. It’s a race to see who has the most "invisible" floor noise; it’s like paying a premium for a TV that displays colors the human eye literally cannot see.

Second, They treat the Apollon as a mere "Purifi module carrier," snubbing the power supply and internal wiring as if the chassis were just an ornament. It’s the ultimate reductionist fail: judging a three-star Michelin restaurant based solely on the chemical purity of the table salt.

Finally, Amir measures against fixed 4 and 8-ohm resistors—test subjects as compliant as a British butler. However, real-world speakers are reactive beasts with chaotic phase shifts that could make this Class D amp break a sweat. ¿ASR prefers the safety of the spreadsheet over the jungle of the living room?

ASR offers "transparency pornography." It’s fascinating to watch them obsess over noise at -120 dB while a single fly buzzing in the room generates more acoustic distortion than the entire signal chain.
Shall we raise a glass to turning music into a tax audit?
Cheers from Spain ¡¡

While I agree that it would be nice to see additional measurement against a speaker-like load, there are plenty of errors in your statements.

First point, an amplifier with a better single-tone SINAD tends to have better multi-tone performance as well, which is always behind single-tone performance and has more real world meaning. However, if you cannot interpret the many measurements that are available in reviews, you should educate yourself or just fallback to the SINAD figure (MORE be BETTER). This is also one reason why SINAD past 100 dB matters.

Second point, Amirs test method treats the device under test as a "black box". It gets signal into its inputs and what comes out from the outputs is measured. That accounts for everything in between, not just a single module.

 

[Waveform Fidelity]

On the pure resistance topic, in the mid seventies, there were lots of DIY power amp designs. One very well respected designer was John Linsley Hood. He published a simple 30W amp design in Hifi news. Anyway, he commonly did the load measurements of a square wave with a resistive load and one with a R&C in series (don’t recall the values off hand) and it was clear how the Amp had small ringing when driving a capacitive load, caused by the crossover and an indication of amp stability. It seems no-one does this anymore. Also it not common to see Gain and phase margins - the classic metric for stability of any amplifier. If measurements down to 4 noughts of distortion are going to be taken seriously, then the load impedance should be realistic too!