[fmoya]

Hi all,

I built an active preamp two months ago using a DACT selector and stepped attenuator, a Cascode Power Supply (this power supply unit is of first class!) and, of course, a Buffer Amplifier. The improvement with regards to my 300 pounds Marantz preamp was incredible: the bass, that was missing, suddenly appeared (I use a 2.7uF Mundorf MCap instead of the standard MKTs), the sound scene now has dept!!! so it's three dimensional, and not only like a picture.
I'm sooooo happy with my preamp! The thing is that last week I ordered some coax cable and resistors to Amidata and I needed to order something else to be free of delivery cost, and I remembered about the OPA 627 for the Buffer Amplifier. I should say that I was very sceptical about the improvement that this change could give to the excellent results that I was having with my preamp, but I finally ordered the 627.
Bearing in mind that I was prepared to find no difference between the OPAs I selected some cuts from different albums that I know very well. The last one that the OPA 134 played was “Lil’ Sis” from the album “Family Portrait” of Victor Lewis, recorded by Audioquest. The percussion and, specially, the metal section of this recording is absolutely superb and extremely difficult to reproduce. The result with the OPA 134 was very good (nothing new, I’ve been hearing this piece for more than two months in this preamp). The same piece “Lil’ Sis” was the first one that I auditioned with the brand new OPA 627 placed instead of the 134. Oh my god, I couldn’t believe was I was hearing: the detail was dramatically improved, to a extreme that I’VE NEVER HEARED BEFORE… the cymbals was there, it was almost impossible to think that the sound could come from two loudspeakers…(DIY loudspeaker, with Monacor SPH-165CP woofer and Scan Speak D2905/95 tweeter, 3rd passive filter at 1,7khz). It was like perfectly focussing a picture that was a bit blurred, and the tonality is now close to perfect. I’ve realised that the noise has been reduced more and now It’s almost impossible to hear anything with the ear at 1cm from the tweeter (bear in mind that I’ve used non inductive resistors and the Mundorf MCap Supreme –a non inductive capacitor- for the tweeter’s passive filter!!!).
If you’ve been reading everything until here (sorry!) maybe you are thinking the same as I’m thinking… is it possible to implement those OPAs to the MOS125? And more important, can it be implemented in the Active Crossover?.

[davidw]

The OPA627 is indeed a very fine opamp. I personally reckon that its the best one for audio currently available. If you scan the forums you'll find that there are those who think different, but even so they always put the OPA627 at number two or three. Replacing the OPA134 in a MOS125 or MOS250 with an OPA627 does indeed improve the sound quality quite markedly; that why its standard in the audiophile versions of these power amplifiers. Its a pity that the OPA627s are so expensive and that there's no dual version.
You can improve the sound quality of the White Noise active crossovers by using AD8620 dual opamps instead of OPA2134s. Analog devices position the AD8610 ( a single opamp version of the AD8620 ) in the same league as the OPA627 but I haven't done enough comparative listening to venture an opinion - but its certainly a very good opamp. The only problem with the AD8610/20 is that they're only available in surface mount packages. I sell the AD8610 or '20 soldered onto a SMT to DIL adaptor so that you can plug them into my amps, crossovers etc. Although the AD8610 is a bit cheaper than the OPA627 by the time that you've added in the cost of the adaptor there isn't much in it. Going back to the active crossovers there is no doubt that swapping the four OPA2134s for AD8620s improves the sound quality quite markedly but its a very expensive upgrade.
Glad you like the cascode power supply and buffer amp.

David

[fmoya]

I’m going to change the 134 on my MOS125 soon and make some listening test. I’ll write with my impressions, but I feel that this will make an excellent system… arguably the best price-performance ratio on DIY amps. Maybe I’ll try to add a cascode to the MOS125 to drive the 627, I’ll inform about this too… regarding this improvement…is it possible to take the power from the preamp cascode or not? Could this cause any problem?
Thanks in advance.

[fmoya]

I mean that I’m going to replace the OPA134 with the OPA627 on my MOS125 and feed those OPA627 with a Cascode… I’ve realised that my post was a bit ambiguous.
Regards.

[davidw]

Its well worth using a cascode power supply to power the opamp in a MOS125/250. With the Audiophile version of the MOS125 and the MOS250 you can leave the onboard regulator in place so that you have two stage regulation to the opamp; series followed by shunt. It is possible to use the same cascode psu as the one that you're using for a preamp but I wouldn't recommend it. Its better to have a separate cascode psu as close to the power amp pcb as possible; this doesn't preclude a separate enclosure for the cascode psu but you need to keep the connections as short as possible. If the power connections are too long this can cause the MOS125/250 to oscillate. If this happens the amp will appear to work sort of normally but it won't sound as good as usual and the mosfets will draw a quite heavy current, around 1A,  even at low listening levels.

Regards, David

[fmoya]

Thanks David for your support. I wish a Merry Christmas and happy new year to you and for all the forum users!