Both are desktop amplifiers, $375 against $499. This page puts them side by side on the only three things a maker can be held to — power, output impedance and noise — and then asks the question that actually matters: which headphones do they treat differently?
The Topping A90 reaches 100 dB on 567 of the 567 models here, the Bottlehead Crack (OTL kit) on 565.
| Specification | Bottlehead Crack (OTL kit) | Topping A90 |
|---|---|---|
| Power into 32Ω | 271 mW | 2.0 W |
| Power into 150Ω | 403 mW | 427 mW |
| Power into 300Ω | 333 mW | 213 mW |
| Power into 600Ω | 227 mW | 107 mW |
| Output impedance | 120.0 Ω | 0.1 Ω |
| Output noise | not published | 0.2 µV |
| Price | $375 | $499 |
| Headphones it reaches 100 dB on out of 567 | 565 | 567 |
A figure marked not published is missing from the maker’s own material, which is not the same as being good or bad. Where one of these two publishes a number and the other does not, the comparison on that axis cannot be made at all — and saying so is more useful than guessing.
| Load | Bottlehead Crack (OTL kit) | Topping A90 |
|---|---|---|
| 32 Ω | Derived | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Published | Derived |
| 600 Ω | Derived | Derived |
Both makers publish a power rating, which is less common than the table above makes it look: of the 280 sources here, 209 publish a figure at at least one load and only 2 publish one at all four. Bottlehead Crack (OTL kit) publishes at 300 Ω; Topping A90 at 32 Ω. Every other row is worked out from those — between two published loads, or beyond them holding voltage constant, which understates rather than flatters.
So the two columns are not equally firm row by row: at 300 Ω only the Bottlehead Crack (OTL kit) figure is the maker’s own and at 32 Ω only the Topping A90 figure is the maker’s own. On those rows the derived number is the one to doubt first.
What each of these labels means, and why a derived figure is still worth printing, is set out in how this data was built.
2 headphones reach 100 dB on the Topping A90 but not on the Bottlehead Crack (OTL kit) — Moondrop PARA, Symphonium Titan.
567 headphones are left alone by the Topping A90 under the one-eighth rule but coloured by the Bottlehead Crack (OTL kit) (64 Audio Aspire 1, 64 Audio Fourte, 64 Audio Nio, 564 more).
565 of the 567 models here reach 100 dB on both. That is a power count and nothing more; whether either source suits a given headphone also depends on the two lines above.
The counts above say how many models the two treat differently. This says by how much. The same 8 headphones appear on every page of this kind, ordered from the easiest to drive to the hardest, so a row can be read across pages as well as across columns. Headroom is at a 100 dB SPL peak.
The widest gap on the list is the HiFiMAN HE400se: Topping A90 has +23 dB of headroom there against +14 dB. 9 dB is about 7.4 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for Bottlehead Crack (OTL kit) because that maker publishes no output noise figure. That is a gap in the specification, not a quiet amplifier, and it is left blank here rather than filled with a plausible number.
| Headphone | Impedance | Needs | Bottlehead Crack (OTL kit) headroom | Topping A90 headroom | Bottlehead Crack (OTL kit) hiss | Topping A90 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +33 dB damping | +42 dB | not published | 0 dB |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +13 dB damping | +22 dB | not published | -20 dB |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +26 dB damping | +31 dB | not published | -21 dB |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +27 dB damping | +26 dB | not published | -26 dB |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +14 dB damping | +23 dB | not published | -27 dB |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +22 dB damping | +19 dB | not published | -33 dB |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +17 dB damping | +15 dB | not published | -37 dB |
| HiFiMAN Susvara | 60 Ω | 50 mW | +7 dB damping | +13 dB | not published | -39 dB |
The Topping A90 costs $124 more than the Bottlehead Crack (OTL kit). Out of the 567 models here, that difference buys 2 more models driven to 100 dB and 567 more models left alone under the one-eighth rule.
Nothing runs the other way: there is no axis here on which the cheaper of the two does better.
Everything in this paragraph counts only figures the two makers published. An advantage on an axis one of them leaves blank cannot be counted, and is not counted — how often that happens is its own page.
Each has a page of its own with every power figure traced to what the maker published and a full headphone list: Bottlehead Crack (OTL kit) and Topping A90. Or put either against your own headphones in the source comparison tool.