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« on: June 26, 2009, 11:10:06 PM »
Ken,
This is never ending story and you know well what I mean.
I went over so many different caps in power supply/decoupling that it became in fact more and more apparent to me that this is the key in Revox design. I have been eyeballing thier other pro designes wishing to grab some ideas but there is no way, this is one way street. The objective in my case was the noise and distortions and resolution together and while it is no brainer to swap those OK Philips signal caps with nice BG/N you are getting straight into the trap, you are loosing the meat obtaining in return uninvolving texture-less with twisted dynamics sound. Attack/decay are so off and the weight behind the notes is so pale that you have no other option that proceed deeper and deeper hehe. So this innocent logical step hey lets swap those old and dry caps is very deceiving, particularly in multistage device like this deck. I have found all caps to hold perfect BTW. But like you have said and emphasized a few times the grain, that is the most funky part. You change one cap and disaster strikes 'the grain is roaring'. Well you wait wait and it is not going away until to tackle the supply gang, the end of story. I tried usual suspects and BG has won hands down, but you know that you dont know how it will go until you have tried all options before you are sure. You never know what is the source of the problem, the better cap or remaining ones maybe combination, go figure. So it takes time and money at the end but it can be done. While I was lucky with tantalum resistors as a replacement, the most pesky in fact are mica pF range comp caps. What they have used is grain and nothing but the grain generator, but they bring authority and a kick hard to get from very limited offering at present. 20pF-200pF is available as styrene and some silver mica (mostly guitar blend) and russian monsters. Silver mica like CDM available all over the web could not cut it, they were too electronic with a grain but otherwise with good dynamics and drive, good styrene are very hard to get unless you like thin super thin sound of industrial sort. I have got tons Philips/Rohm/Noble/TRW/ etc but only Mial NOS 600V worked very well. There is also very very good Russian silver-mica but only in 120pF 350V (good for 150pF and 100pF spots) difficult to mount but superb. The rest nF is RTX and RTE, they work fine. In all cases what you have to watch out for is a constant sanitation of the sound, every time you go for better cap you will loose some harmonics and fatness of the sound and this is why I had to deal with resistors and use PIO caps in power supply (remember these are vintage PC PP MMK PEPT caps replaced with styrene). Opamps are the simplest, there is only a handfull to try and you would prefer to use the same type all over the boards of course. Their sound is very distinct you would know in 1 minute which way to go /love and hate. You will never go back.
Repro decoupling I think is good, each stage has in close proximity sufficient storage. Don't forget about C1 , it is a key to the sound, you don't need more than 68n but the best you can afford. Because of the distance it feeds output amp too.. BG/FK sound better than BG/N for 22uF and 100uF local decoupling in my tests. For 4.7uF i used BG/N. Cerafine caps are unbearably thin sounding, FM will about match what is already there less grain. Ground planes on the cross connect board are quite thoughfull to better overall delivery considering local decoupling. You know you could mod it to use tube stage?
Best
Andrew
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