John Curl's Blowtorch preamplifier part II

[john curl]

You never know, Joshua. It did NOT hurt, except my pocketbook, to use Teflon boards for this test vehicle. It is always better to try the 'best', at least once. Maybe, it is too much, but then, what is too little?

[bcarso]

Originally Posted by morinix

Wash your hands!

Yes, Victoreen makes it clear that one is not to touch the parts other than by the leads.

[john curl]

Clean those parts!

[elektroj]

Originally Posted by SY

........
Teflon PCB is a good choice for some very specialized applications, but audio isn't one of them- except for the fashion and hype factor, of course.
........

Oh yes, tell me about it, SY
I have a Bruel&Kjaer 2429 Psophometer here (that's audio range, btw) and its balanced input stage (600 Ohm or >10k) is built on TEFLON board! So much for the hype

[scott wurcer]

Originally Posted by elektroj

Oh yes, tell me about it, SY
I have a Bruel&Kjaer 2429 Psophometer here (that's audio range, btw) and its balanced input stage (600 Ohm or >10k) is built on TEFLON board! So much for the hype

At B&K's prices who knows and who cares. The expense would be adsorbed in the price and some engineer's idea of what had to be would be realized weather it mattered or not.

[magnoman]

If your chassis is thick enough then you should also pull a vacuum and use silver plated traces.

-Antonio

[kgrlee]

Originally Posted by bcarso

A Victoreen "HI-MEG" on teflon feedthroughs, 2000 megohms in this case. I actually needed it for a measurement of JFET gate leakage recently.

In da late 70's, I made a concerted effort to find small better HiZ resistors for Calrec. Most didn't really obey Ohm's Law but were more like cr*p diodes.

http://www.saturn-sound.com/images%20-%20microphones/calrec%20pcb's.jpg

The top 3 boards show the huge blue 500M resistors we used up to then. The bottom 3 have 2 little blue beads which were 1G and very nice indeed. You can get good 1/8W size resistors up to 10G these days from Digikey etc but I'd love to know if my little blue beads are still available.

Small size helps good layout, which as marce points out, is more important than whether your PCB is hand carved from solid Unobtainium by virgins rather than FR4.

You can immediately tell if a maker understands LN design by how he lays out an input circuit.

The Calrec Soundfield Mk4 used 2G and the afore-mentioned Unobtainium PCBs. OK it was just FR4 with guard rings.

Didn't trust any conformal coatings in dem days but I'm told they are much better now. Then, as now, scrupulous cleaning was vital.

Teflon standoffs are a good innovation for DIYs and nearly as good as skyhooks[*]. Also useful is to minimize stuff at HiZ. If you can isolate both sides of a capacitor mike capsule, you get rid of one coupling cap, one HiZ resistor & one Teflon standoff.

COG/NPO ceramics are better than polystyrene and certainly better than silver mica at this critical point ... despite what the loony .. I mean Golden Pinnae brigade claim.

[*] High tech device that supports anything in any position without any visible or actual signs of support.

[RNMarsh]

There are several formulations of "Teflon" as there are polysytrene. For caps, there is only one mfr left that makes a superiour ps dielectric for the cap industry - they are in China. Reliable Cap brand sources them exclusively to their specs. The PS cap fell out of favor when the PP type had wider temp range and low cost and almost as good properties. The low temp range of the PS is what killed it off - almost. For many cap dielectrics, the thinner ones (low voltage) are more problematic -- often with microscopic pin holes which reduce reliability. True of all low voltage (thin) films. Otherwise, it is a superiour dielectric... even to npo/cog types. Just not as flexible in apps and a limited values and large size. Now more costly due to limited volume. Just like certain transistors, they may be great but cant get them if the volume of use isnt maintained.

The Best pcb material is that used for microwave circuits. Bold statement? Not really. But its also expensive by the sq inch. Anything to be gained for audio... ? Dunno but if the design is the best, might use the best associated materials to go with it.

I look at high end products as more than just numbers as well. I once had a tailored suit made from a cloth blend of silk, cashmire and mink. The feel of it is out of this world. Now would you wear a Casio watch with that? I didnt think so. Thx-RNM

[kgrlee]

Originally Posted by RNMarsh

For caps, there is only one mfr left that makes a superiour ps dielectric for the cap industry .... Otherwise, it is a superior dielectric... even to npo/cog types.

The problem with polystyrene caps in microphones (if one ignores the reliability issues with soldering) is that they are microphonic. This isn't a Golden Pinnae effect needing $50,000 analysers to find .. but can be seen if you do a noise plot in one of Guru Wurcers very low noise HiZ FET microphone circuits with the cap replacing the capsule and compare this with a COG/NPO ceramic of the same value .. or just bang this test set up a bit.

I'd be interested if you have links to evidence that polystyrene caps are superior to NPO/COG ceramics. The studies I've seen including Bateman etc all show NPO/COG ceramics are probably the best caps you can buy within their value/voltage ratings. My only criticism is that voltage ratings greater than 100V for valve amps are relatively Unobtainium.

BTW, my criteria for a good cap is that I = C dV/dt and nothing else.

You may also like to measure loadsa NPO/COG ceramics & 'precision' polystyrenes and plot distributions of their values. Very illuminating.

In this case, the coupling cap between capacitor mike capsule & grid/gate, the 'cheep' component is better than the High End Golden Pinnae item.

Which polystyrene maker do you recommend?

[Wavebourn]

Originally Posted by kgrlee

If you can isolate both sides of a capacitor mike capsule, you get rid of one coupling cap, one HiZ resistor & one Teflon standoff.

But don't you attract dust and moisture particles to the capsule such a way?