I had this venerable Nelson Pass designed passive/active preamp sitting around gathering dust for the last year or so (was in use for years previous in the main system) and wanted to upgrade it a number of times over the years and never got around to it until a month ago.......ran into all kinds of trouble during the mod (most of it caused by my own inept bumblings with new parts I'm unfamiliar with). It's primary use now is as a source switcher and pre for the head fi system...allowing flexibility to use any type of headphone amp (balanced or single ended by using the passive mode as a buffer). The features of the preamp allow so much additional flexibility and control...the addition of it is a god send for a DIY tweaker junkie like myself. I can simply add DACs, amps etc at will now without having to tear the whole system down every time....the remote control feature is also a welcome addition...if those of you that are unfamiliar with the model...look it up at Adcom for all the details.
In any event...I borrowed some suggestions from an old thread at DIY Audio.com and used some of my own concepts to complete this massive overhaul (73 parts in all).
Here are some before and after pics....please note that the Vishay caps are installed incorrectly in these pics...that was one of the bumbles ...since corrected.
Stock trim...good design...shockingly bad caps in some key sections.....Jamicon and Teapo.....need I say more ? The shortcuts to meet the price point (1.5K US) are correctable...the jacks also need to swapped for decent ones (once I get all 10 pairs of them).
The parts list for the changes....
Fairchild UF diodes
Epcos metalized poly film and Vishay/BC component Axial film caps
Vishay 1% 1W metal film resistors
Nichicon electrolytic caps (PW and VK series)
Panasonic FC series electrolytics
Soft annealed 22gage solid silver jumpers in 4 spots (out and input DC blocking caps removed as per DIY Audio mod)
NOS PIO K40Y-9 .047uf 200V and 1.0 uf 160V Russian caps (bypass CDE, Nichicon, Vishay caps)
Here are the pics......(Vishay cap polarity has been corrected, pic shows incorrect installation)
Bypass caps shown are partial completion. The CDE's have an additional set of 1000uf 100V Nichicon PW's added on the underside to the main PSU filtering bank. Secondary PSU filtering was doubled from 2200uf to 4700uf.
Preliminary Sound Quality evaluation results
Certainly a vast improvement in extension, timbre, sound staging and dynamics across the board. Simply in another league over stock. I would say this preamp now lives up to it's Stereophool Class A rating it was given years ago but left me wondering why when I first bought the pre ( in stock trim obviously).
I shall add to this post's SQ impressions as the amp burns in.....I don't want to go into too much detail yet since it needs time to settle in fully. Used examples of this amp are well worth the money IMO simply because this mod can take it well beyond what it is in stock form. The cost of the mod is less than 60US.....certainly affordable by anyone's standard.
Those that own and want to modify a great SS preamp this is your bargain ticket to true Hi end SQ on the cheap. PM me for details if you wish...I would me more than happy to discuss the finer points.
That's it for now......time to enjoy some tunes