This review should likely be considered as a follow-on to my previous review of the PK1 and PK2 that was posted about 6 months ago, as the new OK series of phones are more “evolutionary” than “revolutionary”.

Looks 

 

At first glance, the OK series of phones look very similar to the PK series, but upon closer examination, one can see that several changes have been made to the housing.
The shell is about 2mm thicker in the area just to the outside of the housing grille when compared to the PKs.  Perhaps the most significant change is that the center of the grille is a circular opening that is threaded for tubes that allow the phones to be used as a true IEM rather than as a bud.  Other than these 2 physical differences, the cords, plugs, color, etc are very similar between the PK and OK phones.  

Tubes 

 

I will not include comments regarding the sound when using the IEM tubes, as I was unable to achieve a satisfactory seal with the standard or extended tubes.  The tips did not fit my ears well, which is a problem that I experience with many types of IEM tips.  

It is my opinion that the straight tubes will make it somewhat difficult for most users to achieve a proper seal, as most ear canals are not straight, but angled.  This makes it awkward to try to insert the IEM configured OK phone into the canal without the housing blocking the fit.  

Perhaps Yuin will make some modifications in the future to further develop this concept, but one should not consider this a negative because it is in the role of a reference-quality earbud that the Yuins truly shine!

As my PK2s are out on loan, they will not be included in the review.

Pricewise, the OK series are each priced above the corresponding PK model. For current pricing, please refer to the Head-Direct website.   
 

Methodology

 

Both the PK1s and the OK1s have over 100 hours usage, while the OK2 and 3 each have about 25 hours.  The ER4Ps have well over 300 hours use.  

The source for all phones was my iRiver H120 and Sony S618.  Music was ripped at 320kbs using the MP3 format.  I listened to each phone using amped and unamped selections of both instrumental and female vocalist tracks.  The amp used was my iQube.  


Results / Impressions

When comparing the OK3 against the OK2 and the ER4P, I found them to be very close in efficiency, with all 3 being easily driven by both of the unamped sources.  The bass response of the OK3 was slightly greater than that of the OK2 and ER4P.  Midranges were similar, with the highs much more prominent in the ER4P.  I have always found the highs in the ERs to be a bit bright for my taste, so my preference was for the sound of the OKs.  The OK3 had a warmer overall sound that did either of the other two.

The OK3 presented a closer soundstage than did the OK2, which sounded more distant, perhaps “mid row” in the the audience rather than “front row”.  I felt that, although the OK3 bass was stronger, the OK2 bass was better defined and tighter.  In this respect, the OK2 and ER4P were very similar.  When amped, the OK2 became more efficient, with slightly more volume than the other two.   There were no audible differences in signature when amped between any of the three phones.  

While the ER4s offered much greater isolation, the OKs were much more comfortable, and with the less strident highs, my preference for extended listening among all three is the OK2 as it presented a more neutral overall soundstage than did the others, and I personally prefer a more neutral sounding phone.  

It became more difficult to objectively compare the OK1 and PK1 as the OK1s have a very different sound than do any of the other phones I compared during this review.  They probably have the strongest bass of any of the phones compared, and the highs are similar to the other PK and OK phones, HOWEVER....the entire soundfield has a “surround-sound” feeling about it that the others lack. It is rather closeup and presents a very 3-dimensional feeling.  

This sounds very impressive when listening to non-vocal music, but when a vocalist is present, accurate localization seems to be difficult.  In this respect, the PK1 sound is again more neutral, with the vocalist position more accurately defined.  The OK1s sound better with higher volume levels, and I really think that one is losing much potential performance if inadequate amplification is used, more so with the OK1 than the PK1.  

There will be those who really like this sound signature in the OK1 and I am sure that there are those who will prefer the more neutral PK1. It is strictly a matter of personal preference.  For some types of music I really prefer the fullness and dynamics of the OK1.  For other types, especially vocals, I prefer the more distant and neutral PK1.  

If I had to pick just one phone from the ones in this review, I would personally choose the OK2.  It does not overemphasize any portion of the musical spectrum, it has taut, well-defined bass and smooth highs, with no sibilance or stridency.  

Unlike the ER4, it can comfortably be worn all day, and, if desired, can also be driven directly from an unamplified musical source without losing any sound quality.

Conclusion 

 

If cost is a determining factor, the OK3 is a superb choice, as it is  easily driven by any source without the necessity for additional amplification, and would clearly be the choice for bass-lovers and those who prefer a warmer sound signature.

At the higher end of the pricing scale, the OK1, PK1 and OK2 are all worthy of consideration and can easily hold their own against many higher-priced circumaural phones of audiophile quality.

These 3 phones really need to be auditioned personally before making a choice, as personal preferences will be the deciding factor.

Yuin is to be commended for engineering this type of performance into such a small phone.
    

Ron Kerlin
(HiFlight)