Electronic Design
Bob’s Mailbox: Audio Quality,  A Crazy Rack, And The PE Exam

Bob’s Mailbox: Audio Quality, A Crazy Rack, And The PE Exam

Bob,

I just handmade a headphone amp for a good friend’s birthday. I have auditioned it favorably and I am sure she will love it. Topology: one tube gain stage (a 12AX7 clone type), then a MOSFET source follower running at 60 mA quiescent, BJT current source. \\[You don’t trust the LME49610? /rap\\]

The solid-state rail is 28 V tightly regulated. All the input stage coupling caps are 1-µF, 250-V polyester (MKT dielectric). The Zin is 100k, which means a time constant of ca 70 ms throughout. \\[Why? /rap\\] The bass response kicks \\[butt\\] completely, auditioned on my very good cans, as it should. \\[So, what if you put in 3-µF polyester? Really? What harm? /rap\\]

My good friend Trevor Lees (we graduated together), who owns the best high-end audio business in town, always says for best audio quality, all of the time constants in any circuit must be the same throughout. \\[He seems to have a fairly parochial view. I can’t imagine why he’d say that. Having one ruling time constant sounds roughly plausible, but having a couple slower taus can’t possibly do any harm. Having dc coupling can’t do any harm. So I guess I will have to go to my fallback position, “Avoid getting into arguments with \\[jerks\\].” /rap\\]

What I do notice is that this style of open-loop topology does sound marginally better than a cruddy feedback-linearized circuit. \\[You seem to think that sounds different and better. But maybe a good engineer could study the “cruddy feedback” circuit and find something wrong with it. You could carry that schematic with you when you come to the States, and I’d love to look at it. If there is any reason why feedback sounds different or worse, I would love to find it and understand why. /rap\\]

The part needing the most attention is appropriate shielded wiring and star earthing and a metal box since the 100k Zin even responds to passing your hand over the top of it. \\[That part I believe. But does it even do that when you ground yourself? Does that not apply to other circuits? Think about it. Come on. Be reasonable. /rap\\]

Watch out for the fancy “audio quality” cap scam, Bob. They are mostly just rebrands of cheap commercial film caps. The extreme nuts will lay down gold bars for special very fancy “oil and silver foil” caps that must somehow help them to sleep better or somehow convince them they are much better than their next-door neighbor. Funny ol’ world, innit Bob? Beastliness prevails! “JCD”

I’ve heard of that! /rap

Bob:

Just had to write (my first time to you) after reading your latest column, “Bob’s Mailbox: PLL Circuits, Hiking Experiences, And More Reader Reactions” (available at www.electronicdesign.com) as the subjects and comments speak to a story from my past. I also ran into some superb engineers who were not fully educated as such. \\[Yeah, I know some guys like that. /rap\\]

Take the most brilliant designer I ever know. I used to work in the main R&D labs of a huge telecom manufacturer, once (briefly) the largest in the world and now defunct, name starting with N, headquartered in a country not very far directly north of you. The fellow in question was a student intern midway through his third year, stayed on, and never went back to school. \\[The University of Hard Knocks can be a really good teacher. /rap\\]

Several years later, the core processor group was designing with the then new Motorola 68030. One manager was worried about a tight specification on the master clock duty cycle and told the young engineer to go away and solve it. Said engineer then disappeared for six months and returned with an entire rack of equipment involving multiple nested PLLs (phase-locked loops), which could control the duty cycle of this clock to within a fraction of a picosecond. \\[And he couldn’t show you a simplified version that does what you really needed? Ha! /rap\\]

I saw it, and it was impressive, to the point of involving special measurement techniques to prove the sub-picosecond control. Brilliant engineering, yes. Totally impractical, yes. (His efforts were never put onto the processor board, of course.) Had I not known the guy, I would have assumed this was the product of a PhD’s thinking.

I also loved your response to the naive young reader from India. I don’t think I would qualify for the job he wants, and I have been building circuits since public school, am a radio amateur, have a master’s degree (filer design), and have more than 30 years experience in analog, digital, and mixed-signal. But the young need to have goals and ambition, and maybe someday he will make the cut. Best wishes to you. Keep writing. -Wayne Chomik

Thanks for writing! /rap

Bob Pease:

Why haven’t you taken the PE exam, which is a hallmark of professional excellence in the U.S.? -Sunil Pedgaonkar

I’ve been too busy to take the final test. Too busy designing real circuits and helping solve people’s real problems. And writing columns to teach people how to stay out of real trouble. Taking a PE exam doesn’t help people stay out of trouble. Beast regrds. /rap

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