John Curl's Blowtorch preamplifier part II

[john curl]

Thanks PMA for your enlightened input.

[john curl]

I think that it should be pointed out that we were not making a military RF proof device. We were making a CONVENIENT line stage/phono preamp of the highest quality that we could. We KNEW that the shields on single ended RCA connectors are not EMP proof. So what? What we really worry about is the stuff from 150-150KHz, that is where the action is, in my opinion. If we had TVI or microwave tower problems, we would probably use specially shielded RCA cables with a separate external chassis connection, and/or an input and output transformer, but where most people live, that is not a problem.

[simon7000]

Originally Posted by PMA

... and then neglected by using insulated RCA connectors, with all the RFI getting inside along shields of signal cables. The HF shielding effect of the massive box is gone. It will work only to several hundreds of kHz and would be almost ineffective for tens of MHz and nothing special in units of MHz.

You would need to use RF connectors with body tight connected to the box, if you wanted to get HF shielding effect. As is, it is only misused overkill. Gold does not prevent against HF EMI.

Actually one of the problems meeting RFI specs is that when the connector is in the same plane as the circuitry that allows a "line of sight" RFI path. This is a big problem when using PC mounted connectors. That is why even on consumer gear the RCA connectors are mounted to a small PC board that is mounted above the main circuit board and connected with small flexible cables.

If you understand your grounding scheme you can ground these connectors and keep the Faraday shield almost intact.

[PMA]

Audio is almost never constructed to meet requirements for high frequency instruments. Try GHz frequency analyzer and measure high frequency rejection of some audio gear.

[simon7000]

Originally Posted by PMA

Audio is almost never constructed to meet requirements for high frequency instruments. Try GHz frequency analyzer and measure high frequency rejection of some audio gear.

I have been doing that! It seems that a lot of gear that measures good but sounds bad has those problems... gee what a surprise that if you don't measure something you might just be missing something.

[wayne]

After purchasing CE compliance testing equipment I played with the spectrum analyzer and found lots of interesting things going on. It really helped improve overall design from 150 K and up.

[simon7000]

Originally Posted by wayne

After purchasing CE compliance testing equipment I played with the spectrum analyzer and found lots of interesting things going on. It really helped improve overall design from 150 K and up.

Have you found anything that was oscillating and until then you didn't know it? (Would you admit to it if you did? )

Have you tried injecting noise and seeing what else comes out? Or what gets worse fast?

ES

[wayne]

Not any oscillations as they are usually audible, show up on scope or AP.
We have simple very linear circuits and they are pretty stable not 7 or 8 gain stages and all the phase shift.
Mostly power supply noise and harmonics way out there. Some of the text book methods either didn't work or made them worse. YMMV

[auplater]

Originally Posted by bcarso

But my favorite "variation on a blind testing theme" was the new piece by the artistic director of PS, the second on the program and its second performance ever (the first at another venue last night). Mark Carlson's "Cave Paintings" was a lovely work. And to my ears, quite convincingly structured, intricate, attractive without being trite in any way, an advanced harmonic language without being per se atonal.

But I felt something was not fully formed, somehow, near the end of the final movement. Nothing that sounded like outright clams, but just something slightly unsatisfying, somehow abrupt or unfinished.

I saw Mark at the end of the concert (after a spectacular Schumann Piano Quartet performance, as good as one could imagine) and after commenting on violinist Roger Wilkie's continued and ever-improving mastery, and the overall quality of the concert, said "I loved your piece. But ... I have a feeling somehow, that, if time permits, you will wind up revising slightly the last movement."

Mark said: " Um. Well. They lost their place".



So I said Well then! The movement will rewrite itself!

There will be one more public performance at the UCLA Faculty Center on the 13th. The acoustics of that room are charitably speaking, lousy, but it will still be worth attending. If you are in town I recommend it. 8 PM.

One question?? Have you ever played a musical instrument in an ensemble or orchestra??

If you have, it's pretty evident, even in music you've never heard, to tell when and if the musicians have "lost their place".

John L.

[DF96]

'Loss of ensemble' can be harder to spot with modern music, as the composer might have intended it to sound bad.