Sennheiser is one of the most recognized headphone brands in the world. Among their most well-known models are the HD 600, HD 650, and HD 660S. They are all so good that they deserve special attention. Recently, we had all three models at our measurement lab, and in this article, we will talk a little about each of them and compare their sound before and after calibration with Realphones.

The Sennheiser HD 600 were released over 20 years ago, but are still considered some of the best headphones in their class. They offer high sound accuracy, thanks to the driver with a diaphragm made of kevlar and a low distortion magnetic system. The HD 600 have a very flat and smooth response in the mid and high frequencies, which allows you to hear every detail of the sound. However, some users feel that the low frequencies are slightly restrained. The HD 600 also have a wide soundstage, making them ideal for listening to classical music and jazz.

The HD 650 were released in 2003 and became even more popular than the HD 600. These headphones also have an open-back design and a similar sound signature to the HD 600, but have some differences. The HD 650 use a different diaphragm material - a thin layer of aluminum with a copper coating. This material gives the sound warmth and softness that may be lacking in the HD 600. The HD 650 have more pronounced low frequencies than the HD 600, making them ideal for listening to bass-heavy music, such as rock or electronic music.

The Sennheiser HD 660S are a newer model in this series, released in 2017. They were developed using more modern technologies and have slightly improved characteristics compared to the HD 600 and HD 650. For example, they have a wider frequency range (10 Hz - 41 kHz), which makes the sound more detailed and accurate. Also, the HD 660S use a modern diaphragm material - a special polymer that gives the sound high clarity and transparency. These headphones have a high level of sound accuracy and detail, making them some of the best on the market.

The sound of the headphones before Realphones calibration:

The HD 600 originally sound less colored than the higher-end models. In terms of tonal balance, they are more balanced. However, their level of detail is not as good as that of the HD 650 and HD 660S. This means that they do not color the sound in any direction and display the sound image in a more accurate and natural form. HD 600 are perfect for critical listening and monitoring recordings in the studio, as they are capable of transmitting every detail of the sound material. The bass in the HD 600 headphones is strong enough but not as dominant as in other models. The mid-range, which includes vocals and main instruments, also sounds very clean and transparent. High frequencies sound detailed and clear but may seem overly bright to some users.

The HD 650 are gave more natural high frequencies, even if they are less exciting. Comparing the sound of the HD 650 with the HD 660S and HD 600, it can be said that the HD 650 have a warmer and softer sound. Low frequencies sound more deep with excellent depth. The midrange is very transparent and detailed, without any obvious coloration. High frequencies also fit well into the overall picture and do not have any sharpness or hardness. Compared to the HD 600, the HD 650 have a more refined sound. They offer greater detail and a wide soundstage, making them ideal for listening to classical music and vocals. At the same time, the HD 660S have a more balanced and accurate sound than the HD 650, reproducing music more clearly. The low frequencies in the HD 660S are more dense and powerful, and the highs are more detailed and bright than with the HD 650.

The HD 660S, sounds beautiful and balanced, but slightly "closed" and "rounded". Compared to HD 650, the HD 660S are subjectively perceived as headphones of a higher class. They have a deeper bass and lower midrange, and the mid frequencies are more "velvety". The high frequencies of the HD 660S are also more velvety, but slightly sharp and "grainy". On the one hand, the sound of the Sennheiser HD 660S is more transparent and open than that of previous models. In the upper range, they are slightly brighter and more transparent than the HD 650 and HD 600. The sound is detailed, with good articulation of each note, and has excellent spaciousness. The soundstage is deep and wide, allowing every detail of the recording to be heard. The bass in HD 660S headphones is more accurate and faithfully reproduces the original recording, but not as warm as in the HD 650. In addition, the HD 660S are more transparent in the upper midrange, which can make some recordings sound a bit brighter than they should. They are ideal for use in the studio, as well as for listening to high-quality music at home.

Headphone sound after Realphones calibration:
The overall character of the HD 660S is preserved, but some "nasality" in the sound disappears. The sound becomes more open and transparent, with previously indistinguishable details now audible in the high midrange. Spatial reproduction also improves.

After calibration, the HD 650 gets rid of the bump in the lower midrange, and the mid frequencies become less colored. Clarity is increased, but not to the extent of the HD 660S.

Similar improvements occur in the sound of the HD 600, except that the sound of this model is more "calm". In comparison, it is subjectively perceived as less detailed and "distant".

Conclusions:

These models have a similar sound character and are perfect for mixing and arranging. They are particularly suitable for working with "live" music and acoustic musical instruments in conjunction with Realphones correction and emulation.

The HD 600 is the most "natural" of the three, but details are less audible than in the other models.

The HD 660S has a more exciting and velvety sound and is better in detail than the other models.

The HD 650 is somewhere in between the HD 660S and HD 600. They sound "natural" enough, but detail is not sacrificed as with the HD 600.

All three models are definitely worth considering. Yes, the 600s were released more than 20 years ago. Yes, the 660S is the freshest model where mistakes were fixed. Yes, the 650s can be considered a "golden mean" in some sense. Overall, all three models have high sound quality and are intended for professional music work.

However, each model has its own characteristics that should be taken into account when choosing. If you prefer a more neutral and uncolored sound, the HD 600 is a good choice. If you are looking for more versatile headphones with deeper bass, then the HD 650 will suit you. And if you want a more transparent and detailed sound, as well as a deeper and wider soundstage, then the HD 660S is the right choice. Each model with Realphones will help you make the right decisions during mixing.