Yes, Victoreen makes it clear that one is not to touch the parts other than by the leads.
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.
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.
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various comments -
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
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
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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?
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But don't you attract dust and moisture particles to the capsule such a way?
Only as much as a large diaphragm variable pattern mike which will have both diaphragms and backplate isolated.
Thanks, John. Yes, DK and DF are very important in multilayer boards carrying high speed digital signals (note the frequency range of those graphs!). But that's not what you're doing- your boards are one or two layer, carrying low frequency signals, and (unless they've changed) using principally through hole components. In fact, I'd think that a bit of loss at gigahertz frequencies (though admittedly negligible for one or two layer through hole boards) would be a good thing for audio circuits.
The film itself isnt microphonic. However, how it is made will determine if it will be microphonic. The better parts are annealed. With PS they are heated to the point where the film begins to soften and it fuses together to make a solid, hard piece. If you have seen oval shaped caps, this is what the mfr was doing -- heating then pressing out the voids and air trapped between layers. It makes for a solid piece and improves the voltage reliability and the rating. They are Not microphonic when made this way. They are a solid block of film and foil. Cheaply made caps which are wound at high speed (instead of slow and tight) will also be microphonic by traping air between windings. It depends on the mfr of the part and not on the film material itself.
Now you can move a cap in a very high z circuit and affect C to ground et al and this will have an affect by modulating the c from the body of the cap to close-by ground or other parts. And a loosely wired cap will vibrate more easily, too, if not 'staked'.
As far as harmonic distortion is concerned the npo et al are very good. Ditto the ps type. The DA of the PS is sometimes better than the Teflon film formula.
I feel confident to recommend Rel Cap out of so calif for the most reliable and well made parts - they are a specialty cap maker for communications industry and military... where high reliability and long operating life is foremost.
-Richard
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The use of the best pcb microwave material is not for its GHz properties when used for audio freqs; It is extremely stable dimensionally against wide temp and humidity and has a consistant, stable, defined low DF vs freq. It isnt easy to get and is costly but it is the best - a good fit for the best OPA. But yes FR4 will work instead and is easy to get and cheap.
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Interesting input, Richard.
Also, I would like to point out that the Vendetta Research and the CTC Blowtorch preamps use double ground planes tightly spaced.
I believe that you, Richard, was the first person to recommend that to me, 30 years ago.
Yes, it works, BUT it allows much more surface area to be seen by the circuit as parasitic capacitance to ground. I think that Scott's comment on what lousy capacitors FR-4 makes, and noisy, and that too should be considered.
Also, I would like to point out that the Vendetta Research and the CTC Blowtorch preamps use double ground planes tightly spaced.
I believe that you, Richard, was the first person to recommend that to me, 30 years ago.
Yes, it works, BUT it allows much more surface area to be seen by the circuit as parasitic capacitance to ground. I think that Scott's comment on what lousy capacitors FR-4 makes, and noisy, and that too should be considered.
Hi John,
It seems like the only way to know is to build the same circuit on 2 PCBs, one made of Teflon and the other made of cheaper material.
As for P2P wiring on Teflon standoffs, it seems to be more practical for tube amps. Yet, one may well try it on SS circuits. If and when practical, it may be better than any PCB.
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