Ik heb een Marantz CD67-OSE en ik lees een stukje over het biasen van de opamp in klasse A. Nou lees ik ook dat biasen met een simpele weerstand (zoals hieronder) niet echt een goede methode is (
http://tangentsoft.net/audio/opamp-bias.html).Ik benieuwd of iemand hiervan wat vindt ?
Groet, Eric
PS: op de foto is de clock modificatie (gedeeltelijk) te zien. Ik heb een Net-Audio RockClock gebruikt.
Marantz CD67 - Biasing the OPA2604's Into Class A http://home.netcarrier.com/~rstevens/cd67semods.htmlFirst remove the JRC 2114D Opamps with TI OPA2604.
OK, I got this tip from Bob Fitzgerald, from a submission dated 9/29/02 on mods he made to the analog output stage of his his Kenwood KT-8300 tuner that I found on the Yahoo FM Tuner eGroup "sister site" to the controversial Tuner Information Center--a self-righteous and needlessly damaging venture if I ever saw one--and give him full credit for it. In sharing his information, I hope to help others in the position I was in, namely liking some aspects of the CD-67SE but hating others--like the sound, to be specific. Here we go:
On the underside of the board, you will have to solder
four small 3.6K-ohm resistors--one from pin 4 to pin 1 and another from pin 4 to pin 7 on EACH of the OPA2604's (take them out of the sockets first, if you're smart). I used--gasp!--quarter-watt carbon composition 5% hand-selected resistors in mine. OK, I had them, so sue me. Here's your Roderstein, Vishay, whatever--I would have used quarter-watt carbon-films if I'd had them. All you're doing is biasing the opamps closer to Class A, so I'm not too worried about it. I hit them quickly with the soldering iron at its lowest temperature (since carbon comps are notorious for shifting value when soldered), and got through it OK. The player sounds terrific, if that's any indication.
There is one consideration one should be aware of when putting wide-bandwidth opamps into equipment that formerly had narrower bandwidth, slower, and less sophisticated opamps in it previously, and that's high-frequency oscillation. As Bob explained, it was necessary to use 5-7pf ceramic (or silver mica) caps across the feedback resistors in his KT-8300, but with the opamps in the CD-67SE removed, I measured the capacitance across the 27K (as I read it) feedback resistors, and saw what I think was slightly over 1000pf (.001mfd). So not hearing any sizzle or experiencing any deleterious effects from the switch, I left that part of the circuit alone. If I had a Spectrum Analyzer I'd look for problems, but I don't, so I can't.
