Ok, so here's the score for those who are interested in my quest to "Hot-Rod" my Digi-125, or their own. You are also no doubt aware of another thread I started in the quest to find out the sound quality differences between devices, (and many thanks to all those who contributed advice and tips on that forum) so I guess this is pertinent to that in some way.
I'm not very good at elaborating but here is how my quest turned out so far;
The first devices that I changed were the outputs to the Toshiba 2SC5200/2SA1943 as I reported previously. This brought about a substantial change that was noticable immediately and a definite improvement in sound quality compared with the previous op devices. At this point the driver transistor Q3 (ref to schematic posted by Palesha on prev. page) was a BF469 as I had changed it some time back due to the fact that the little BC546 that was designated for that spot constantly ran hot due to the current going through it and in danger of it going pop eventually as it has once in the past. I have tried several other devices in this spot (MJ340/2N3019) but they all sounded indifferent and the amp lost it's transparent and sweet sound quality with these when used with the Toshiba op's.
I had thought of trying the MJE1530/1531 devices in place of the BC556 & BC546 specified for Q4 & Q5, but in the end I used the Toshiba driver transistors 2SA1837/2SC4793 instead, which was a total waste of time as the soundstage just collapsed along with the openess and clarity which I found so beguiling and revealing. Back went the BC546/556 devices and everything was as before, sweet detailed and transparent.
In hindsight perhaps I should have tried MJE 15030/1531 devices, but because I'm a cheapskate I opted instead for the Toshiba ones... Perhaps at a later date...
In any event the sound quality to my ears is exceptional, transparent, detailed and sweet, though there was a mild disconcerting something in the sound (a little harshness/grain perhaps) which I could not put my finger on. In the event this problem disapeared when on a whim, which proved fortuitous, I changed the compensation cap on Q3 (BF469) from 150pF to 68pF to see what would happen. Well, that touch of grain, or whatever it was disapeared. The sound quality was more ballanced and if possible even more open. For instance on KD Lang's CD "All You Can Eat" on the first track, "If I were you", you can clearly hear the backing chorus which was almost indistingiushable from Lang's main vocals previously, not just on the Digi but other amps as well...
In conclusion anyone contemplating building a Digi-125, replacing the output devices Q6 & 7 with the Toshiba 2SC5200/2SA1943 and the driver Q3 with the BF469 along with the compenstion cap to 68pF is very worthwhile and effective. Oh and I almost forgot, put an extra diode in with the existing two shown on the circuit, and you will have an amp that is remarkably fast, transparent, and one that will make your speakers sound like electrostatics...
cheers,
tomcat
