henry@ginger.sri.com (Henry Pasternack) (01/03/91)
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.
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.
-Henrychan@uunet.UU.NET (Jeff Chan) (01/07/91)
>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.
henry@ginger.sri.com (Henry Pasternack) (01/08/91)
Jeff Chan says: >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? I don't know the difference, except that they have incredibly high capacitance density for a non-electrolytic cap. A 1uF 50V monolithic is much smaller than comparable standard ceramic disks. I have also heard that the monolithics are very low impedance, as they are used primarily for digital bypass applications. I don't know about the audio quality of these components. I have heard it aid that ceramic caps should be avoided at *all* places in audio circuits, even non-signal applications. I'm not that concerned about it. I'm using monolithics for all of the 1uF and 0.1uF locations. Some of these are bypass capacitors and some are integrators for the servo control loops. In these applications, temperature drift shouldn't be a problem. Could it be that NPO caps are more linear because they are unaffected by signal-induced micro temperature variations? Don't ask me. >As a related question to all, is a film cap a reasonable substitute for >a ceramic for RF bypassing? You'll have a hard time finding film caps small enough to fit the circuit board. Incidentally, as I was soldering on my Chater boards last week, a thought occurred to me. Some people insist on using OFHC wire even for short jumper runs on circuit boards. But who knows what the quality of the copper on the board is? Personally, I don't believe in most of the OFHC stuff, at least for short runs. Speaker cables, maybe. >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 should mention that the slew rate of the amplifier is affected by the bias current and transconductance of the input stage. It is probably no big deal to set up the NTE458's correctly, but, again, I don't believe it's worth the effort >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. I don't fully understand the rationale of using the beads. I think they are most effective when placed on component leads to kill stray capacitive coupling over distances of a half inch or less. I guess that in certain stages, the addition of a little inductance will roll off the stage response and minimize the likelihood of local oscillation, but only where a parasitic feedback path exists. In some locations, a bead seems like a bad idea. For instance, the output of an emitter follower already looks inductive. Putting on a bead might make matters worse by increasing the inductance at that point. I guess it's a matter of controlled roll-off at very high frequencies. If the oscillation occurs at a frequency much higher than the open-loop bandwidth of the amplifier, the bead is probably a good idea, provided it isn't isn't excessively inductive. There are also different grades of beads. I used a Ferroxcube "4A" material, but there is also a "3A" and "3B" bead in our lab stock. I'm not sure what's going to happen. I still don't understand why it is permissible to put a bead on each lead of the output devices, but not on the speaker leads. >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... The only reason the choke is there is to keep the amplifier from shorting out if it does go into oscillation. Most designs parallel the choke with a low-value resistor to damp out its ringing tendency. There is probably room for experimentation. I don't have the schematic here in front of me, but if you're set up to do phase margin measurements, it's probably a good idea to spend time varying the values of the compensation and feedback components to get best overshoot performance. On the other hand, the amplifier should work fine if built as designed. >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 haven't looked carefully at the power supply. National makes an LM334 which is a programmable current source requiring an external resistor to set the current. I haven't seen fixed current diodes, though. >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. I think I ought to call Old Colony and see if they can send me an update sheet. -Henry