Sennheiser HD 600 Series Mod – A Practical Evaluation Of The CNC Copper Mass Loading DIY Kit By Custom Cans

The 29 GBP CNC Copper Mass Loading Mod DIY Kit By Custom Cans allows for an easy Mod of the Sennheiser HD 600 series headphones that improves their notoriously limp bass response.

With an addendum by Biodegraded.

Introduction

The Sennheiser HD 600 and HD 650 have been standard staples with audiophiles since 1997 and 2004, respectively. I own a pair of the HD 600, the leaner sounding model of the two that is preferred by many over the thicker sounding HD 650.

The HD 600 has been popular because of its midrange delivering natural, organic, and intimate vocals – not to forget its well extended treble. But its bass is superseded by speedier and more articulate headphone models. A HD 600 series mod to improve the bass quantity was long overdue.

Custom Cans are a British company that specialize in headphone modifications.

Purpose of Mod

Straight forward: to improve bass response by adding detail and sub-bass quantity – and keep the quality of midrange and treble. Vary the low end level according to your personal preference by selecting from four different tuning foams.

Physicals

Two kits exist, the “regular” 29 GBP copper version for HD 580, HD 600, and HD 650, which is used here. A 35 GBP “nickel-plated” copper version is optimized for the HD 6XX issued by drop.com.

In the bag are the two copper weights, these stickies, and four pairs of different tuning foams. I did not have to use the stickies as the copper weights stuck firmly to the ear of the driver without.

The Modding Process

In order to apply the mod, you have to partially take the headphone apart. Disassembly and reassembly of the Sennheiser HD 600 series is easy as well documented in videos such as this one.

For disassembly, we remove…

1. cable from earcups
2. earcups from headphone
3. grille from earcup to expose rear of driver
4. black stock foams from rear driver

We then add…

1. copper weight to back of driver
2. tuning foam of choice into the hole of copper weight

..

Now we re-assemble the headphone by

1. clicking the grilled back on the driver
2. re-attaching the earcup to the headphone
3. plugging the cable back in

Too fast? Just watch this video:

Result

I settled for the bassiest “bored” foam (sample A in the image above). The sonic result of my efforts is a more detailed bass and a bit more sub-bass…and some interesting optics. Midrange and treble remain unaffected. Biodegraded measured the frequency responses of his HD 6XX with the different foam inserts, which yield different frequency responses below 1 kHz.

For the unlikely case, you don’t like the result and would like to reverse the mod you may have to develop some patience to get the black foams back into place. The rest is easy.

If you don’t dare to remove these black stock foams, Custom Cans recommend just plugging the copper weight on top of them. This yielded zero sonic difference to my ears (I A/B-ed by sticking and removing the weight on the fly while playing music).

Concluding Remarks

The CNC Copper Mass Loading DIY Kit By Custom Cans improves the low end response of the Sennheiser HD 600 series. If you know what you are doing, the mod is performed within 5 minutes. If you don’t know what you are doing, some easily comprehensible instructions videos will help.

If you are not a modder and lack self confidence, talent, and have two left hands, you are like me. Rest assured, you cannot screw this one up.

I like the result of this mod and will keep the kit on my HD 600s for good. Try it, too, if you are not 100% satisfied with the bass of your HD 650 or HD 600.

Until next time…keep on listening!

Addendum: Some Amateur Measurements and Musings

By Biodegraded

Intrigued by mentions on internet forums, I bought this kit to try with my 2017 (Ireland) HD6XX. Like Jürgen I noticed increased bass extension and mid-bass quality, and also a bit more smoothness in the texture of upper mids and highs.

As shown by the frequency-response measurements above, mid-bass level can differ by more than 5 dB depending on which foam plug is used (my home-made flat-plate coupler unfortunately rolls off below ~70 Hz so doesn’t show relative differences in the lower bass very well). 

I was curious what might be the reason for the perceived increased bass tautness and smoother mid/treble texture. To investigate distortion in the bass, I made measurements for different mod configurations with SPL for each being 96 dB @ 100 Hz (a convenient mid-bass reference) rather than leaving them independent as in the graph above.

Comparing distortion measurements (behind the spoiler below) it’s possible to see changes particularly in how the 3^rd harmonic behaves. Ignore the big spikes in the bass, they’re because of electrical noise nearby in my building: my measurement space is right on top of the main transformer, so there’s hum at 60 Hz and multiples which the measurement program assumes is distortion – meaning 60 Hz hum appears as a spike at 30 Hz in the 2^nd harmonic, at 20 Hz in the 3^rd harmonic, and so on.

This is easily visually removed, though – let your eye follow the curves’ baselines and note how with more/denser foam, H3 is higher in the bass & lower mids compared to the other harmonics.

Distortion Measurements

Stock configuration, for reference:

[L distortion stock pic]

Copper ring only, no foam:

[L distortion Cu B pic]

Bored foam:

[L distortion Cu 5 B pic]

‘3’ foam:

[L distortion Cu 3 B pic]

Yellow foam:

[L distortion Cu 2 B pic]

Blue (densest) foam:

[L distortion Cu 1 B pic]

[collapse]

Overlaying just the third harmonics from each measurement, you can see how they vary in the bass when SPL is similar there (same colour scheme as in the FR graph):

[L H3 overlays B pic]

Notice between about 40 & 200 Hz how the tan and green curves (stock and ‘3’ foam, respectively) are about the same but purple and blue (no foam, bored foam) are lower and orange (yellow foam) higher; and the red curve (blue, densest foam) is considerably higher from the bass up to about 400 Hz.

Also, when measured at the same SPL in mids & highs (for brevity not shown here), the stock H3 curve (tan) is up to ~5 dB higher than the rest above ~500 Hz. The curves represent the levels of the 3^rd harmonic at the excitation frequency, so would manifest at ~120~600 (mid-bass to lower mids) and >~1500 Hz (upper mids & higher), respectively. 

If these measurements reflect reality (have to admit they’re pretty noisy), they indicate that adding the copper ring slightly reduces 3rd harmonic distortion in the upper mids & treble and that more/denser foam plugs markedly increases it in the bass.

The curves are all below -45 dB down from the signal level (horizontal line at 0 dB), so whether a few dB of difference down there would be audible might be argued; but the ear is sensitive to distortion, and the measured differences might cause or correlate with some other effect to generate the perception of more textured, better quality bass (don’t know about the blue foam – as the FR shows, bass is kind of absent with that one) and smoother upper mids & treble.

Regardless, I enjoy the subjective effects. My favorite configurations are the bored foam and the ‘3’ foam, with ‘3’ seeming tonally the most balanced but bored seeming timbrally the most correct across the range (and sometimes I don’t mind a bit of extra bass). Like Jürgen, I’ll be leaving this mod installed.