HE350 (Drop)

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Published: Oct-8-2016, updated: Jan-07-2022

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NO SMOOTHING is applied to the shown plots. Most measurement sites have some smoothing applied which ‘irons flat’ sharp peaks and ‘wiggles’. I do not use smoothing because some info about sound quality is lost when plots are smoothed.

Aside from a small correction of the microphone itself also some correction in the lowest frequencies is applied to the plots to compensate for the perceived loss of bass when using headphones. This is described HERE in more detail.
A ‘horizontal‘ frequency response curve on the shown frequency response plots on this website thus indicates a perceived ‘flat’ tonal signature.

ALL measurements are made with a good SEAL on a flatbed measurement rig.
shape of your head, bone structure, pad size, pad ‘softness,  (compliance), hair or no hair and or wearing glasses may (drastically) change the frequency response of some headphones, so… your personal experience may differ substantially from these plots.

Frequency response (tonal balance) is the most sound-determining aspect of headphones. A horizontal line shows audible neutral response in the plots on this website. Deviations in different severities at different frequency bands have an effect on the sound character.
The bigger the deviation the stronger the effect.

Below an aid to help determining the sound character of headphones with relation to the frequency response.


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HIFIMAN HE-350 (Drop)


The HE350 is an entry level headphone ($99.-) by Drop.
It has the looks of the higher priced (and better sounding) orthodynamic offerings.

Comfort is decent, clamping force is slightly north of normal but the large sturdy pads need quite some force to create a proper seal. Easy to bend the metal headband and adjust the clamping force though.
People with larger heads may want to look elsewhere as the height adjustment is limited.
The pads are velour but a bit on the stiff side and not as pleasant to the skin as some other velours pads.
This one doesn’t have the cumbersome SMC connectors seen on most earlier HiFiman models. Instead this uses 2 small 2.5mm TRS connectors.
It comes with a 1.5m supple cable which is not microphonic. It is terminated in an angled 3.5mm TRS connector + 6.3mm adapter.
The looks and finish appear to be of decent quality, taking the price in consideration I would even call it good.

It has been replaced by the newer HE35X.


Type: over ear (circum-aural), open
Usage: home
Driver type: planar dynamic
Pads: replaceable velour
Collapsible: no, but can fold flat.
Headphone cup connector: 2x 2.5mm TRS
Cable entry: dual sided.
Cable: 1.35m terminated in angled 3.5mm TRS jack + push-on 6.3mm adapter
Inner Pad dimensions: depth = 20mm, diameter = 57mm.
Driver size: 50mm Ø
Nom. power rating: (estimated 200mW)
Max. voltage: (estimated 2V)
Max. current: (estimated 100mA)
Max. S.P.L.: (estimated 116dB)
Impedance: 19Ω
Efficiency: 93dB @ 1mW
Sensitivity: 110dB @ 1V
Weight: 340 gr. (without cable), 390 gr. with cable
Colour: charcoal black
Clamping force: medium (5N)
Accessories: 6.3mm converter

Subjective sound description:

Sound quality has some good and some poorer qualities to it.
The bass is a slightly ‘wooly’ and definitely not super lean/tight. It does have a decent extension though and is somewhat on the lean side in bass levels.
Mids are realistic and ‘open’ sounding with excellent clarity. Neutral would be the word.
The treble can be described in one word… shrill ! I feel HiFiman really dropped the ball here. It is sharp and sibilant and has little real details, but lots of ‘fake’ detail.
This headphone doesn’t reproduce the finest details, the treble has ‘grainy/coarse’ character to it.
Separation is good, no muffled or unclear sound, good dynamics.
When you are looking for a slightly warmish/full and detailed, yet slightly subdued, treble and expect an ‘orthodynamic’ type of sound you should look elsewhere.


Below the frequency plot of the HE-350 (left, right)


Channel matching is excellent, rarely seen it this good.
From 50Hz (bass) to 1.5kHz the tonal balance is excellent.
The small dip around 2kHz is not as audible as it looks.
The +5dB peak at 5kHz is largely responsible for the ‘sharp’ sound. The +10dB peak at 11kHz also doesn’t help here either and is the biggest issue.

Below the FR plot of another HE350. Also perfect channel matching and no obvious production spread as well. Different behaviour around 1.5kHz, and above 4kHz compared to the other HE350. Not nearly as shrill but still too ‘sharp’.


Below the difference between both HE350’s


The HE350 has less presence around 1.5kHz and doesn’t have a peak around 5kHz that peeps out above the 1kHz level. The HE350 has a +5dB peak around 5kHz. Both headphones show about the same behaviour above 6kHz. The peak of the HE350 around 11kHz, however, is about 3dB higher than that of the HE350.
Given the more than excellent channel matching the differences between these headphones are audible and either HIFIMAN has loads of driver and matches them very carefully or changes were made over the production period.
In any case it would appear as though the HE350 has noticeably more treble energy than the HE350. Perhaps this (partly) explains why some people find the treble objectionably high and others feel it just a tad elevated. This may not all be down to taste…

Below the phase response of the HE350 (Left, Right)phaseSlow phase shifts are not very audible. Sharp changes in a narrow frequency bands may well be audible.
Only at 7.5kHz and 11kHz there are some potentially audible phase shifts.

Below the distortion plot of the HE350 (Left channel)dist
The plot above shows the level differences between the signal (upper trace) and the harmonics.
Most people prefer to see percentages instead of level differences so below the exact same plot except ‘normalised’ to the actual signal and level differences given in percentages.dist percentWhat is obvious here is the 3rd harmonic distortion which reaches 8%. This gives some colouration in the bass but isn’t audible as ‘distortion’. Together with the increased higher harmonics this points to the driver becoming limited in excursion (compression of lower frequencies, a kind of soft clipping)
Above 200Hz the distortion remains below the 0.2% which is decent. Above 10kHz the 2nd harmonic rises to 0.5%

The actual 2nd harmonic distortion may well be lower than 0.2% . A shortcoming of my measurement rig.

Below the CSD (Waterfall) plot of the HE350 (left and right overlaid)


The elevated treble doesn’t seem to be caused by resonances. Perhaps the dense paper in front of the driver already damps the treble sufficiently.
The Spectrum plot below shows some resonance alike behaviour around 750Hz and 1.8kHz.
Again nothing to really worry about.


Step response

Below the step response of the HE350. (Left, Right)stepThe initial rise reaches 0dB so the driver reacts fast. After 500μs the level starts to drop downwards showing the ‘lean’ sound signature and lack of ‘body’ to the sound.

square-wave and impulse response

The ‘time domain plots’ above don’t seem to point towards problems in the treble area. Only the ‘amplitude domain’ plots show the ‘sharp’ sound.


These plots do show the difficulties the driver has in the treble area. The 440Hz square-wave shows lots of overshoot and ringing. The needle pulse shows overshoot and ringing at high frequencies. After the actual pulse a second pulse is there which almost reaches the amplitude of the actual signal.


HIFIMAN sells leather pads as well as hybrid pads (Focus pads). The original velours pads are very cheap. For slightly more you can buy the leather pads which will physically fit on this headphone but they will mess up the tonal balance. Velours pads, Leather pads.


A lot of headphones absolutely need a good seal around the ears to get a good bass. This one is no exception but fortunately doesn’t react as strongly to a seal breach (think hairs, bone structure, glasses) as some others do.
The effect of this seal is shown below. Perfect seal, wearing glasses, 5mm rod between pad and test fixture (pretty big leak) and 10mm (ballpoint) between pads and fixture.


Fortunately there is a rather easy fix for this headphone. In this case felt and toilet paper are not the answer though. The effect of these material is marginal and removes much of its better sonic properties as well. Of course you can stuff a lot of paper towel, toilet paper or felt in there but this doesn’t address the problem area only… it affects the entire treble range and extension. There is more information about the experiments done with these headphones HERE in the forum

Treble modification

The best solution requires some soldering but fortunately it is a very simple and not risky endeavour. The needed schematic and mounting instructions are shown below.

Note there appear to be different HE350’s with a different amount of treble peaks around.
The biggest issue, however, is the 11 kHz peak which has to be lowered.
This filter works O.K. with both versions.

220uhWhen mounted inside the cups (very easy and quite risk free) you only need 1 inductor (220μH), one resistor (33Ω) and one capacitor (470nF) all in parallel (per side) which has to be soldered in series with the blank wire. Thus between the connection on the 2.5mm socket (where the wire was connected originally) and the wire itself.

The described filter is a notch filter very similar to a ‘slider’ on an equalizer. The filter thus does not roll-off the upper treble extension like paper in front of the driver thus.
The filter thus has better treble extension than an acoustical solution and only addresses the problem area.
The treble peak is completely removed and sounds great now. No more sibilance or sharp treble.
The distortion around the problem area (was 0.5%) drops below 0.2% as well as a bonus.

The second HE350 I measured actually needs a different amount of correction. This second headphone needs less correction around 5kHz.
This means this headphone could easily be ‘corrected’ using a single sheet of (3 layer) soft toilet paper. The brighter version can’t be effectively tamed in the upper treble without causing too much upper treble roll-off.
The toilet paper, however, also lowers the treble energy above 18kHz which is less desirable. The electronic filter does a slightly better job here. Could be placebo but feel the filtered HE350 sounds slightly better than the same headphone with toilet paper over the driver. The plot below shows the difference. The filtered version has 5dB more treble above 18kHz. The toilet paper version, while lower compared to stock, still has about 3dB too much energy around 15kHz.


Below the frequency response of the second (stock) HE350 fitted compared to the ‘B-filter’ version.


Schematic diagram for the filter is shown below.


The square-wave and needle pulse plots below also clearly show the improvement this filter makes. On the left ‘stock’ on the right the filtered version.


The 440Hz and 100μs pulse show substantial differences. Especially the sharp over and undershoot are gone. The ‘extra’ pulse is still there but the high frequency ringing is gone.

Below a short tutorial… Remove the retaining ring on the back. It is clamped in there with 4 small notches. Just put your finger nail between the ring and the cup and pry it out.
The backplate is loose in there.


Desolder the copper coloured wire from the socket. do NOT desolder the wire from the driver itself !!


Solder the filter (shown above) in series with the wire and the now open connection of the socket. (this filter shows 33Ω resistor and 220μH inductor)


When you are really anal about this use film type capacitors and hot-meld glue or use silicone stuff or other materials to secure the filter to the cup. I just kept it as it is and used a ceramic multilayer (X7R/50V) capacitor.
Then put the back cover back and mount the retaining ring… Notice the small prongs and the small slots they have to fit in…
Enjoy a more enjoyable HE350 !!

For those looking for the specifics on the parts:

Inductor: Look for an inductor with 220μH (220 micro Henry = 0.22mH) value. It must be able to handle at least 80mA but no need to go much higher than 200mA. Its resistance should be between and . You can use axial or radial inductors. Just don’t buy any that are physically too big to fit.
Capacitor: Film type 470nF is best, voltage rating >25V. Mind the physical size !
I used ceramic multilayer X7R in 50V rating which works fine for me.
Resistor: 33Ω. A power rating between 0.3W and 0.6W is more than sufficient.

Below the frequency response with filter (left, right)


Impressive 30Hz – 18kHz +/-3dB (disregarding the notch @ 8kHz).
The plot below shows the frequency response of the less trebly HE350 with a filter installed.


Sounds speaker-like full and ‘gentle’ without a trace of sharpness or sibilance.
I can recommend this headphone WITH the filter.
Without the filter the treble part is the biggest deal-breaker.
The treble quality, while improved, will still leave you with the desire of wanting a bit more extension and more realism.
Slightly subdued poorer quality treble is always better than elevated poor treble.

Bass modification

There is a video on youtube describing a bass modification.
This requires opening up the backside (very easy to do) and removing the ‘paper’ sticker covering the center hole in the magnet. (do this careful so you can stick it back on)
Of course I explored some other possibilities too.
Note this HE350 is fitted with a treble filter modification.
center hole covered with the paper sticker, center hole open (bass mod described in the video thus sticker removed), center hole covered with paper sticker & tiny hole in the paper covered part around the driver magnet sealed, center hole open and the small hole in the paper covered part around the driver sealed.


Normal tonal balance, ‘fat’ sounding with wooly bass, analytic/bass shy/midrangy, tastefully warmer slightly bassier.
Pick your poison..

Pad change

I do find the stock pads not the most comfortable pads around. They are a bit stiff and slightly ‘prickly’ to the touch. They are dirt cheap as well.
Have tried a bunch of different pads and it would seem like HIFIMAN made the best possible pads for this headphone.
Only one pad did make little change and combined with the filter I would give this modification the ‘nod of approval’. The comfort of these pads is higher and can easily be mounted on the rings. These pads are oval by nature but become almost round when on the rings.
The pads I am talking about are velours Shure SRH1840 pads with the order code HPAEC1840.
Below the HE350 with stock pads versus HE350 with Shure SRH1840 padshe350-stock-vs-1840-pads

Slightly… very slightly more bass and warmth. slightly more ‘presence’ as well (which I find welcome) when using the filter below. The small peak around 5kHz is just +2dB opposite 150Hz and sort-of compensates for the dip around 2.5kHz. The 11kHz peak is the most offensive part and is effectively removed by the filter.

To me the headphone sounds better in the treble (less coarse/grainy) and has excellent bass/mids with somewhat better clarity. To me a worthwhile mod…
If you want even more bass warmth you can combine these pads (+ filter) with the bass modification above (open the hole in the magnet and tape the small hole in the paper around the magnet off).


Another option is to use a Kameleon (in-line, desktop or portable) which can also extend bass as a bonus. Ofcourse, the Kameleon costs 2-3x the price of the HE350 and the difference with the filter described above is small. Kameleon, stock (without the passive filter)


The dip around 2.5kHz is audible though as a (too) ‘lush’ sound which is lacking clarity.
Because the Kameleon filter can only address one peak the above filter is designed so it only extends the bass and removes the 11kHz peak.

Those that have installed the passive filter in their headphones have the possibility to use another HE350 filter module which fills in the dip. This increases the clarity.
It doesn’t look like a big improvement but the 5dB extra presence is quite audible.


I dare say it lifts the sound quality to another level. The bass extension is excellent.

Below the resulting FR (left, right)


Even more sound quality improvement can be had when using the earlier mentioned Shure HPAEC1840 pads. Together with the passive filter the treble quality improves with a leap.
The sharp notch isn’t audible as such. The -10dB dip is reduced to only -5dB and the +3dB peak around 5kHz audibly ‘compensates’ for the small dip around 2.5kHz.
Filtered HE350 with SRH-1840 pads and the HE350 + 1840pads but with a passive filter installed (470μH, 470nF, 33Ω) and the HE350 + passive filter via Kameleon with a passive filter installed inside.


Below the frequency response of left and right channel of the filtered HE350 fitted with Shure SRH1840 pads and a filter installed in the headphone.


Below the distortion plot of this combination.


Below the CSD, Left and Right are superimposed.


Also the square-wave and impulse response has greatly improved.
On the left a stock HE350, on the right with passive filter + 1840 pads + Kameleon.


This headphone (on Kameleon) embarasses many times more expensive headphones and is a very enjoyable listen. It still lacks a bit in the upper treble quality though. The upper treble lacks sparkle somewhat and sounds very smooth and polite.

No hyper details but at the same time a very relaxed listen. One of the very few headphones (filter + Kameleon + SRH1840 pads) that I have difficulties with to stop listening to. Despite the ‘wish’ for more ‘detail’.

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