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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: Re: Audio Analogue Donizetti Modifications posted by dst on November 23, 2000 at 22:40:03:
Following Sig.Paoletti step by step (remember I didn't tweak the original - making a clone).1. Increasing the interstage caps: Do it, just keep the wires short. Maybe a Solen with long flex leads? A Solen larger than 2.5uF won't fit under the board. Or as an alternative, a cheap 4.7uF-63V film is only about 10*25*25 mm and 4 of these per channel could fit under the board, as Mr.Paoletti suggests. I'd keep the originals in place.
2. Ground: Do it. Is the noise really bad? This fix looks so easy it might be done at factory already.
3. Opamp replacement: Think twice. Opamp replacement may require re-tuning all the feedback. The guys who know better on this board voted for OPA604, OPA132 second. But they were speaking of fixed power voltage applications, while in this amp the power rails float. (For good or bad, TI-made 5534's are as unavailable in Russia as they are in Italy, that's why I discarded this option from the start). How about soldering in IC sockets and simply swapping and testing?
And, by the way: if you look at the distortion chart on his site, ask yourself, why does distortion of Philips opamp start rising at 20W? I'm not completely satisfied with Mr.Paoletti's explanation. The chart says nothing about 'sonic pattern' at low levels. So what is the 20W/8Ohm? That's 18V peak output. That's when the Upper supply rail is supposed to float to 18+18V=36V, and Lower to 18-18=0V. It doesn't get to 0V, but quite close. So I'm afraid this 20W thing is related to driving the opamp with input common-mode too close to the floating power rail. And maybe, inadequate filtering in the opamp supply. Try to drive the amp to full power, even with no load, and look at the actual voltage at each power rail. How close it gets to ground (and input)? Then why the same amp from TI doesn't behave so badly? Well, I suspect the TI can either work at wide common-mode input range, or can swing output closer to power rail, or both.
4. Idle Current: As Mr.Paoletti says, increase it. Check that the bias doesn't float up too high on a hot amp.
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Follow Ups
- Re: Audio Analogue Donizetti Modifications - klaus 04:13:29 11/24/00 (1)
- Re: Audio Analogue Donizetti Modifications - jameshin 07:33:32 11/24/00 (0)
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