Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!att!tut.cis.ohio-state.edu!zaphod.mps.ohio-state.edu!wuarchive!uwm.edu!csd4.csd.uwm.edu!info-high-audio-request From: rad!CORP!chan@uunet.UU.NET (Jeff Chan) Newsgroups: rec.audio.high-end Subject: Re: More comments on the Chater amp. Message-ID: <8733@uwm.edu> Date: 7 Jan 91 13:55:07 GMT Sender: news@uwm.edu Lines: 118 Approved: tjk@csd4.csd.uwm.edu Originator: tjk@csd4.csd.uwm.edu >From: henry@ginger.sri.com (Henry Pasternack) >> [I wrote that I plan to:] >> o rf filters and surge limiters on the ac power >> o a snubber cap across the power switch > > Probably good ideas. I'll probably use a six amp grounded >socket/filter for the AC cord. I need to think a bit about shielding >the high voltage wiring to protect the output devices from surges. I >don't plan to put extra clamps on the output devices, though. I had the same thoughts. In fact, I now have a stock of such filters and right angle IEC power cords. In a reply to a letter, Chater explains why clamps on the output are not necessary. (See the letter response 2/89, p.46-49 for suggested wire routing and general RF noise control, including the switch snubber and line filter.) >> o greater use of film caps (though the signal path is DC coupled >> except for one (~optional) blocking cap at the input) > >I'm going to substitute monolithics for all of the tantalums and ceramic disks. >The input cap should be polypropylene. I'm sure all would agree on the input cap. How is a monolithic different from a plain ceramic? I always thought they were one in the same. Are you going to use a certain grade, like NPO? As a related question to all, is a film cap a reasonable substitute for a ceramic for RF bypassing? Would the impeadance of a "rolled" film cap be a problem? Or would a stacked film construction be better? >> o possibly replacing the IRFD110 MOS input transistors with NTE458 >> JFETs for lower noise >> > Noise should not be a serious problem in this circuit. The design >of the input stage is dependent upon the the transconductance of the >active devices. Changing to NTE458's might require some rethinking of >the input biasing and compensation. Chater seems like a very thorough >fellow; I suggest you stick with the devices he has recommended. Using NTE458s was a mod he approved of in the same letter response as above, and he did indicate that it would mean different biasing. > I have a question for you regarding RF beads. In one of the >letters to "TAA" someone mentioned that an RF bead on a speaker wire >can saturate and cause significant distortion. Chater agreed, but >pointed out that all of the beads in his circuit are in small-signal >locations. But I see beads on all three leads of each output device. >If a bead can cause distortion on a speaker lead, why is it acceptable >on the high-current lead of a power transistor? Any ideas? My only thoughts so far are to delete as many beads as is reasonable. The author did not indicate it, but when he found that a true source of some damaged output devices was rf noise from the AC power switch, which was cured through wire routing and the snubber cap, perhaps he would have been more amenable to getting rid of the beads on the output devices. He really likes the beads as a way of killing RF oscillation, though. >[Henry writes about using a complementary output in place >of the totem pole.] I plan to build the output "as is" for now. Perhaps until I learn enough to try what you suggest. > A couple of other things: > >1) I can't for the life of me find the text reference on the construction > of "L1", the output choke. The schematic says "see parts list", but > there is no winding information. I assume that the C25 and R40 form a > resonant circuit with L1, and that the whole lot has been tuned for > maximum stability. Variations in the winding of L1 might have some > effect on ringing, and I'd like to know what the author intended. I can't find a reference to it either except as you note in the schematic. Surprisingly, this does not appear to have come out in any letters. Perhaps people are leaving it out and still not getting oscillation... (Late update, on the p.46 letter or 2/89 L1 is listed as ~1uH. I still can't find any other details.) >2) There is a discrepancy between the schematic and the parts list for > the values of C1 and R2. The schematic shows .47 uF and 100k Ohms, > while the parts list shows .15uF and 330k Ohms. I prefer the lower > impedance values shown on the schematic. I'm not sure why Chater > chose such a high impedance input. If you're worried about noise, > you might consider using a 27k resistor at R2. Depending on the > noise figure of the transistors, it might make a small difference. > >-Henry Chater addressed this in a letter reply (2/89, p.50): "R2 is correctly 100k, but was 330k in TAA 3/88; C1 is correctly 0.47uF polypropylene, not 0.15uF." Also, TR1, 3, and 5 are 2N2222, not 2N222 and TR2 and 6 can be 2N2907A instead of the rarer 2N3799. A question from me about the power supply: R1, the "3mA current diode." What kind of device is this? Any ideas about a part or manufacturer? I definitely pays to check the letters for updates and bug fixes, even if the puzzle is still incomplete for hose of us trying to build from bare boards. Thanks for the correspondence, Jeff C. ----- I have no connection with The Audio Amateur except as a reader: Audio Amateur PO Box 576 Peterborough, NH 03458 (603) 924-9464 $20/yr, $35/two, quarterly, many recent back issues available -- Jeff C. Internet: chan@portal.com Usenet: {apple, pyramid, sun}!portal!chan I am a guest at Portal. Any opinions expressed are not necessarily Portal's.