henry@ginger.sri.com (Henry Pasternack) (12/26/90)
"Audio Amateur" is a lot like "Speaker Builder." You can't count on each issue containing a project you want to build, but over time a number of excellent, professional designs have been presented. One of the contributing editors is Erno Borbely, formerly of Dynaco and Hafler. He is a prolific designer, and quite thorough. He has written a large number of useful articles for the magazine. Check out his "Third Generation MOSFETs" series in 1/2:84. A relative newcomer to the publication is William Chater. Chater has a very systematic approach to his designs, and presents a lot of decent measurements. He has developed an all-MOSFET power amp and preamp. Look for the 40W amplifier in 2/3:88 and the preamp in 1/2:90 It's remarkable that between "Audio Amateur" and "Speaker Builder", we have the opportunity to learn from the likes of Borbely, Chater, D'Appolito and Bullock. -Henry
chan@uunet.UU.NET (Jeff Chan) (12/31/90)
In article <8546@uwm.edu> henry@ginger.sri.com (Henry Pasternack) writes: > > [Henry speaks about Audio Amateur & Speaker Builder...] > > [...] A relative newcomer to >the publication is William Chater. Chater has a very systematic >approach to his designs, and presents a lot of decent measurements. >He has developed an all-MOSFET power amp and preamp. Look for the >40W amplifier in 2/3:88 and the preamp in 1/2:90 Has anyone on the net built or heard the Chater amp? I've got a pair of the boards and plan to build them as mono amps with regulated power supplies. I'll probably make some of the mods suggested in the follow-up letters of Chater and others, such as: o rf filters and surge limiters on the ac power o a snubber cap across the power switch o greater use of film caps (though the signal path is DC coupled except for one (~optional) blocking cap at the input) o possibly deleting or mod'ing some of the safety things like bypassing with caps the power supply fuses to the boards o possibly replacing the IRFD110 MOS input transistors with NTE458 JFETs for lower noise o power supply regulation using a Ryan/Staggs-like circuit I have little doubt that this combination will sound pretty good. (Nothing like a little golden-ear self-fulfilling prophecy. ;^) ;^) ;^) 'Course my cobbled together Magnavox mods did turn out rather well according to others who have heard them (and me).) Does anyone have any comments on construction or sound of this amp ("stock" configuration or otherwise)? -- 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) (01/03/91)
Jeff Chan writes: >Has anyone on the net built or heard the Chater amp? Does anyone have any >comments on construction or sound of this amp ("stock" configuration or >otherwise)? I've got four boards and am in the process of stuffing them now. >I'll probably make some of the mods suggested in the follow-up letters of Chater >and others, such as: > > 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. > 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. > o possibly deleting or mod'ing some of the safety things like > bypassing with caps the power supply fuses to the boards The low-level circuitry is isolated from the regulator by 100 Ohm resistors and heavy bypassing. I don't really think that the line fuses will have much effect on the performance of the output stage. Consider, if you will, that the voltage swing on the output devices due to the output signal itself is far greater than what might be caused by the in-line fuses. If you really want to linearize the output stage, why don't you boost the supply voltage and build a fully cascode output? > 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. > o power supply regulation using a Ryan/Staggs-like circuit Probably can't hurt. 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? I am also somewhat skeptical about Chater's argument that the totem-pole arrangement is superior to the complementary-symmetry topology. A letter writer brought up the same concern, and Chater dismissed him by saying that the current source drive renders both halves of the output functionally identical. Nevertheless, the top half of the totem pole is a source follower and has unity gain, while the lower half is common source and has a gain of roughly eight or nine (depends on the transconductance). Still, when you look at the supposedly "complementary" devices in the IR catalog, you'll see that the matching P and N transistors don't match well at all. Even IR admits that fact. I recall that the Hitachi power MOSFETs used by Borbely have much better maching and lower input capacitance than the HEXFETs in the Chater amp. I wonder if the amp would be improved by redesigning the driver stage slightly to accommodate a complementary pair in the output. -Henry