Apollon 1ET6525SA ST Amplifier Review

[Descartes]

As has been discussed before: No, not really, comparing apples with apples.

The cases are gorgeous but expensive

 

[harkpabst]

The cases are gorgeous but expensive

Give an example in how the product is more expensive than a comparable one from another make.

 

[Sokel]

Regardless of this case, if done right, bigger case/more mass = better thermals if the (passively cooled) application asks for it.
Nothing to do with looks.

 

[Apollon Audio]

The cases are gorgeous but expensive

I’ve noticed that you’ve raised concerns about the pricing of our premium and lux enclosures several times throughout this thread, so I thought it would be appropriate for me to step in and provide some clarification.

You’re missing a key point: enclosure cost is not linear, it scales dramatically with material thickness, machining complexity, and production time.

A 10-20 mm aluminum enclosure is not even remotely comparable to a basic 2-3 mm one:

• Material cost alone is several times higher (10-20 mm billet vs thin sheet)
• Machining time increases significantly (CNC time is one of the biggest cost drivers)
• Tool wear is much higher when cutting thick aluminum
• Weight increases → higher shipping and handling costs
• Finishing (anodizing thick parts evenly, polishing, tolerances) is more demanding
• Often these are low-volume, precision builds, not mass-produced stamped cases

A 2-3 mm enclosure is typically bent sheet metal, often stamped in large batches. A 10-20 mm enclosure is closer to a machined block product, completely different manufacturing class.

So saying “it’s just a case” is like comparing plastic trim to a solid milled chassis.

And honestly, this is the same logic as cars:

Some people buy a VW, some buy a Mercedes or BMW. Both will get you from point A to B, some even share the same engines, but questioning why the Mercedes costs more misses the entire point of what you’re paying for: materials, engineering, refinement, and build quality.

If you don’t value those things, that’s perfectly fine, but then you’re simply not the target customer for that product category.

 

[Mort]

I’ve noticed that you’ve raised concerns about the pricing of our premium and lux enclosures several times throughout this thread, so I thought it would be appropriate for me to step in and provide some clarification.

You’re missing a key point: enclosure cost is not linear, it scales dramatically with material thickness, machining complexity, and production time.

A 10-20 mm aluminum enclosure is not even remotely comparable to a basic 2-3 mm one:

• Material cost alone is several times higher (10-20 mm billet vs thin sheet)
• Machining time increases significantly (CNC time is one of the biggest cost drivers)
• Tool wear is much higher when cutting thick aluminum
• Weight increases → higher shipping and handling costs
• Finishing (anodizing thick parts evenly, polishing, tolerances) is more demanding
• Often these are low-volume, precision builds, not mass-produced stamped cases

A 2-3 mm enclosure is typically bent sheet metal, often stamped in large batches. A 10-20 mm enclosure is closer to a machined block product, completely different manufacturing class.

So saying “it’s just a case” is like comparing plastic trim to a solid milled chassis.

And honestly, this is the same logic as cars:

Some people buy a VW, some buy a Mercedes or BMW. Both will get you from point A to B, some even share the same engines, but questioning why the Mercedes costs more misses the entire point of what you’re paying for: materials, engineering, refinement, and build quality.

If you don’t value those things, that’s perfectly fine, but then you’re simply not the target customer for that product category.

9/10 I'm deducting a point for the car analogy.

 

[Apollon Audio]

9/10 I'm deducting a point for the car analogy.

Fair point on the analogy, it was just a simplified way to illustrate value perception.

To keep it strictly technical:

The price difference is not arbitrary or branding-driven; it’s a direct result of manufacturing method and cost structure.

So when comparing pricing, the relevant question isn’t “why is it more expensive,” but rather whether the added material, rigidity, thermal mass, and build quality are worth it to the end user.

For some customers they are, for others they aren’t, and both positions are perfectly valid.

 

[Matias]

And even better is to offer these choices to the consumer inside the same brand, even same build and parts, and let each one decide the value.

 

[hikkiDO]

The Lux’s extra weight — 6.8 kg vs 2.9 kg for the ST, and roughly 3.6 kg (8lbs) for a similar Buckeye-style build — is not just cosmetic. More aluminum mass can improve rigidity and thermal behavior by helping absorb and spread heat. It may not change the sound dramatically, but from a thermodynamic and build-quality standpoint, the heavier chassis has real value.

 

[Jukka]

Mass alone affects only the time it takes to reach a specific temperature, it doesn't actually increase cooling capacity. Shape has way more effect.

 

[Apollon Audio]

Mass alone affects only the time it takes to reach a specific temperature, it doesn't actually increase cooling capacity. Shape has way more effect.

Your statement is only partially correct, but it misses a key point about how real-world thermal systems behave.

It’s true that mass alone does not increase steady-state cooling capacity, it primarily affects thermal inertia (i.e., how quickly a system heats up). However, in a properly engineered amplifier enclosure, mass is not acting in isolation.

A thick (10–20 mm) aluminum enclosure fundamentally changes the thermal behavior because it becomes an active part of the heat dissipation system, not just a passive container.

1. The enclosure acts as a distributed heatsink

When the chassis is machined from thick aluminum:

- It has high thermal conductivity (~205 W/m·K)
- Heat from internal components is rapidly spread across the entire enclosure surface

This is critical. Instead of having localized hot spots (as in thin sheet metal), the heat is distributed over a much larger effective area. That directly increases the effective radiating and convective surface.

2. Surface area + conductivity = real cooling capacity

Cooling capacity is governed by:

- Surface area
- Temperature gradient (ΔT)
- Heat transfer coefficient (natural convection + radiation)

A thick aluminum enclosure improves all three indirectly:

- Better heat spreading → more uniform surface temperature
- Higher average surface temperature → stronger natural convection
- Entire chassis participates in heat exchange, not just a small heatsink

So while “shape matters” is true, thermal spreading is what enables the shape to actually work.

3. No heat accumulation in steady state

In steady-state conditions:

- Heat input = heat dissipated
- The system stabilizes at an equilibrium temperature

Because the enclosure continuously transfers heat to ambient air via:

- Natural convection
- Thermal radiation

…the heat does not accumulate indefinitely. A properly dimensioned enclosure simply reaches a stable operating temperature.

4. Why thick aluminum outperforms thin enclosures

Thin enclosures:

- Have poor thermal spreading
- Create internal hot spots
- Rely on small, localized heatsinks

Thick aluminum enclosures:

- Turn the entire chassis into a heatsink
- Eliminate thermal bottlenecks
- Improve long-term reliability (lower component junction temps)

5. Mass still plays a secondary role

While mass doesn’t increase steady-state dissipation directly, it:

- Reduces thermal cycling amplitude
- Slows down temperature spikes
- Improves overall thermal stability

Which is beneficial for electronics longevity.

Bottom line

You’re right that geometry matters, but in a high-end amplifier, geometry + high thermal conductivity + distributed mass work together.

A properly designed thick aluminum enclosure is not just “more mass”, it is effectively a large, fully integrated passive heatsink, continuously dissipating heat into the surrounding air without accumulation.

 

[Ropeburn]

Bad car analogy time: same power and aerodynamics assumed, a heavier car reaches the same top speed as a lighter one, it just takes way longer. The lighter one will be more usable and fun both, because it actually reaches those high speeds under practical conditions while the heavy one simply takes too long.

(This is arsebackwards relative to heatsink mass effects of course, a beloved tradition in bad car analogies)

 

[Ruediger]

For class D with heat production in line with activity/load the extra kilos of the lux case should do a very good job.

With f.e. a class A inside the effect would be that the high temperature will equally be reached, just later. As with the heavy car.

Top explanation @Apollon Audio

 

[Apollon Audio]

Bad car analogy time: same power and aerodynamics assumed, a heavier car reaches the same top speed as a lighter one, it just takes way longer. The lighter one will be more usable and fun both, because it actually reaches those high speeds under practical conditions while the heavy one simply takes too long.

(This is arsebackwards relative to heatsink mass effects of course, a beloved tradition in bad car analogies)

The car analogy doesn’t translate to thermal systems the way you think it does.

You are treating the enclosure as “just added mass”, which would only affect how long it takes to heat up. That would be true if the material had low thermal conductivity or if the heat path to ambient was limited. That is not the case with a thick aluminum chassis.

Aluminum is highly conductive, so the enclosure does not behave like a lump of mass. It behaves like a heat spreader. Heat is rapidly distributed across the entire chassis, which means you are not dealing with a small hot source anymore, but with a large effective radiating surface.

That directly changes steady-state behavior, not just warm-up time.

Ask yourself this. If you take the same heat source and couple it to a tiny heatsink versus coupling it to a 15 kg aluminum block with large external surface area, will both stabilize at the same temperature. Obviously not. The equilibrium temperature is defined by how effectively heat is transferred to ambient, and that depends on usable surface area and temperature distribution, not just shape in isolation.

Your analogy assumes identical cooling conditions, which is the core flaw. In reality, the thicker enclosure increases the effective area that actually participates in convection and radiation because thermal spreading is far more efficient. A thin case cannot utilize its full surface because heat remains localized. A thick aluminum chassis can.

So no, it is not “same top speed reached later”. It is a different equilibrium point because the heat rejection mechanism itself is improved.

What you are describing would only be true if both enclosures had identical thermal conductivity and identical effective heat spreading, which they do not.

That is why a properly designed thick aluminum enclosure is not just delaying heat buildup, it is acting as a fully integrated passive heatsink and lowering operating temperatures in steady state.

 

[Ropeburn]

The car analogy doesn’t translate to thermal systems the way you think it does.

You are treating the enclosure as “just added mass”, which would only affect how long it takes to heat up. That would be true if the material had low thermal conductivity or if the heat path to ambient was limited. That is not the case with a thick aluminum chassis.

Aluminum is highly conductive, so the enclosure does not behave like a lump of mass. It behaves like a heat spreader. Heat is rapidly distributed across the entire chassis, which means you are not dealing with a small hot source anymore, but with a large effective radiating surface.

That directly changes steady-state behavior, not just warm-up time.

Ask yourself this. If you take the same heat source and couple it to a tiny heatsink versus coupling it to a 15 kg aluminum block with large external surface area, will both stabilize at the same temperature. Obviously not. The equilibrium temperature is defined by how effectively heat is transferred to ambient, and that depends on usable surface area and temperature distribution, not just shape in isolation.

Your analogy assumes identical cooling conditions, which is the core flaw. In reality, the thicker enclosure increases the effective area that actually participates in convection and radiation because thermal spreading is far more efficient. A thin case cannot utilize its full surface because heat remains localized. A thick aluminum chassis can.

So no, it is not “same top speed reached later”. It is a different equilibrium point because the heat rejection mechanism itself is improved.

What you are describing would only be true if both enclosures had identical thermal conductivity and identical effective heat spreading, which they do not.

That is why a properly designed thick aluminum enclosure is not just delaying heat buildup, it is acting as a fully integrated passive heatsink and lowering operating temperatures in steady state.

Oh come on, I appreciate your detailed explanations but you didn't seriously expect a short, deliberately cheeky comment to be accurate?

 

[hikkiDO]

I agree that shape, surface area, airflow, and heat path matter more for steady-state cooling. Mass alone does not magically increase heat dissipation.
But in a real aluminum amplifier chassis, extra mass is not meaningless either. Aluminum has high thermal conductivity, roughly 200+ W/m·K, so if the modules/PSU are thermally coupled to the chassis, the added aluminum can spread heat more evenly instead of allowing small hot spots. It also has meaningful heat capacity, about 900 J/kg·K, so a heavier chassis acts as a thermal buffer and reduces short-term temperature fluctuation.

So I would put it this way: mass mainly improves thermal inertia, while geometry controls dissipation. A heavier aluminum chassis is not automatically a better heatsink, but if it also has good heat coupling, surface area, and ventilation, it can have real thermal value beyond cosmetics.

 

[Descartes]

Wondering if there is a mechanical and electronic engineer who can confirm what was said?

 

[harkpabst]

Everything ever said here is true.

Was this helpful?

 

[Sokel]

Wondering if there is a mechanical and electronic engineer who can confirm what was said?

Here's everything about it, including the math, etc:

How to Select a Suitable Heat Sink

Laptop computer designers, audio amplifier makers, and power supply manufacturers can keep their products cooler by following these fundamental heat-sink

www.designworldonline.com

In short: real estate is the main contributor, and of course all the rest, Apollon is right about it.

 

[NTK]

Wondering if there is a mechanical and electronic engineer who can confirm what was said?

Done my share of thermal analyses when I was working, but not enough to tell just by looking at the design. Qualitative hand waving is easy but gives no details to the story. Let's put some actual numbers in and run some simple FEA's to see what are the differences between beefy and skinny heat sinks.

I ran my model using the TI TPA 3255 instead of the Purifi because the parameters are easier to find. The heat sink is modeled is the one used in the TI eval kit. I did the FEA in 2D since it is easier and runs faster, and I am not after high accuracy. I modeled the heat sink (cross-section) as-is and one with fins and the base half thickness. For the skinny case, the fins are 1 mm thick and the base in about 3 mm thick. The thermal contact area between the TPA chip thermal pad and the heat sink is 4 mm wide.

Here are the meshes.

The TPA heat dissipation (heat load = 22 W) is for 1/8 rated power for 4 Ω load with BTL configuration. The 1/8 rated power figure is used as it represents the typical crest factor of dynamic music type signals. I used a convection heat transfer coefficient of 7.7 W/m²·K, based on these class notes. This number is reasonably suitable for natural convection in a well ventilated area (i.e. not enclosed). The physics of convection heat transfer is highly complex and analyses results aren't going to be highly accurate. There is no need for highly precise parameters.

The top graph is for the beefier heat sink, and the one below is the skinnier. The steady temperatures are very close to each other, as expected. The faster temperature rise of the skinnier heat skin is due to it lower thermal mass, also as expected. The solid black lines are for a lumped mass of the same mass as the heat sink, and without any heat transfer to the ambient. It is for a quick check to see if the FEA results are reasonable (the initial slopes should match).

Here are the detailed temperature distribution plots for the first 5 minutes (300 seconds). At steady state, the max-min temperature delta for the skinny heat sink is about 6.5 °C whereas for the beefy one it is 3.8 °C. Not a significant enough difference, and no hot spot problem with this skinny heat sink (for this application).

 

[Apollon Audio]

Done my share of thermal analyses when I was working, but not enough to tell just by looking at the design. Qualitative hand waving is easy but gives no details to the story. Let's put some actual numbers in and run some simple FEA's to see what are the differences between beefy and skinny heat sinks.

I ran my model using the TI TPA 3255 instead of the Purifi because the parameters are easier to find. The heat sink is modeled is the one used in the TI eval kit. I did the FEA in 2D since it is easier and runs faster, and I am not after high accuracy. I modeled the heat sink (cross-section) as-is and one with fins and the base half thickness. For the skinny case, the fins are 1 mm thick and the base in about 3 mm thick. The thermal contact area between the TPA chip thermal pad and the heat sink is 4 mm wide.

Here are the meshes.
View attachment 528798
View attachment 528799

The TPA heat dissipation (heat load = 22 W) is for 1/8 rated power for 4 Ω load with BTL configuration. The 1/8 rated power figure is used as it represents the typical crest factor of dynamic music type signals. I used a convection heat transfer coefficient of 7.7 W/m²·K, based on these class notes. This number is reasonably suitable for natural convection in a well ventilated area (i.e. not enclosed). The physics of convection heat transfer is highly complex and analyses results aren't going to be highly accurate. There is no need for highly precise parameters.

The top graph is for the beefier heat sink, and the one below is the skinnier. The steady temperatures are very close to each other, as expected. The faster temperature rise of the skinnier heat skin is due to it lower thermal mass, also as expected. The solid black lines are for a lumped mass of the same mass as the heat sink, and without any heat transfer to the ambient. It is for a quick check to see if the FEA results are reasonable (the initial slopes should match).
View attachment 528800
View attachment 528801

Here are the detailed temperature distribution plots for the first 5 minutes (300 seconds). At steady state, the max-min temperature delta for the skinny heat sink is about 6.5 °C whereas for the beefy one it is 3.8 °C. Not a significant enough difference, and no hot spot problem with this skinny heat sink (for this application).
View attachment 528802
View attachment 528803

Nice effort putting numbers behind it, always good to see actual modelling instead of just opinions.

One thing to keep in mind though is that your setup is essentially comparing two already efficient heatsinks with exposed fins under natural convection. In that scenario, as expected, steady-state is dominated by external surface area and convection coefficient, so the difference ends up small.

Where it diverges from a real amplifier chassis is that you are not really modeling the enclosure as a distributed heat spreader with multiple internal sources and imperfect coupling paths. In a full chassis, a big part of the benefit comes from reducing local thermal resistance and spreading heat away from concentrated sources before it even reaches the outer surface.

So the question becomes a bit different, not just how well a heatsink dumps heat to air, but how effectively the structure prevents hotspots and utilises the entire available surface.

That’s where higher conductivity and mass start to matter more than your simplified case suggests.

Also worth noting that using a fixed convection coefficient kind of “locks” the result, while in reality better heat spreading tends to increase effective ΔT across more surface, which slightly improves real convection as well.

So I’d say your result is correct for the specific model, but the model itself is a bit too idealized to capture what happens in a full enclosure design.