It’s funny how your standards and expectations increase as you follow the upgrade trail, what was almost acceptable yesterday is not good enough today.

My stock NS-1000M sounded nice, but renewing the caps raised the bar, changing the brand of caps took it a little further and going active cured some of the minor niggles that had bothered me. Putting the active system under scrutiny now has me questioning whether the op amp’s used are the best match for squeezing the last bit out of the Yamaha’s?

Time for a bit of op amp rolling.

The KMTech crossover boards come with Burr Brown OPA2134 fitted as standard. I had been browsing through Rod Elliott’s site (ESP) in particular the Forum section relating to his 09 active crossover. This is also a Linkwitz Riley circuit using op amps, so has a lot in common with the KMTech design. A poster had used OPA2134 in his build and described the sound as……

“somehow the crossover was "muffling". Maybe that’s what "they" mean by the "laid back Burr Brown sound" – muffled”

He dropped in some LME49720 from Texas Instruments and commented that it brought back what he had been missing from his passive circuit……

“there seems to me to be a marked improvement over the OPA2134s in the HP (LP seems unchanged. Everything has smoothly sharpened up and subtle underlying detail and "space" has returned around the vocals.” 

Generally the OPA2134 has no major vices but is reported around the forums as having a good warm bass and lower mids, if not the deepest bass available. An ok but not brilliant top end and a so so midrange that is detailed but lacks dynamics and has a poor sound stage and maybe a touch grainy. You have to be careful when reading comments for different applications, many of the views expressed are for use in headphone amps, but I must admit this is describing what I am hearing. The mids do seem to be the weak link and to me sound a little shut in and slightly grainy.

The comments by the guy on the ESD Forum, regarding the OPA2134 struck a note with me. Therefore his views on the LME49720 in a similar application to mine seemed quite relevant and the spec sheets do show very low figures for distortion and noise level. So I got a set of x10 from RS, they are not expensive, just £21 for the set including next day delivery. There are more exotic parts around but for the time being I will keep it simple and use a like for like dual channel in a dip8 packaged chip. Adaptors for mounting SMD’s and pairs of single channel devices opens up the options (and price point) somewhat and may be looked at along the way, if need be. 

I changed all five op amps on each board, as much to make sure they were all burnt in as for any other reason, these devices tend to settle quite quickly, 5/10 hours over a couple of days in my experience. Using a mix of different types may be beneficial and could be tried later. 

How do they sound?

I have to agree the mid range is much better, with space around instruments/voices and very transparent. Bass/lower mids is different but I will have to consider this further as to whether it is a better different, same goes for the top end, so more listening required.