John Curl's Blowtorch preamplifier part II

[kgrlee]

Originally Posted by Wavebourn

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.

[john curl]

For SY et al

[SY]

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.

[Bonsai]

Originally Posted by john curl

For SY et al

Why are you worried about what's going on at 2GHz in an analog pre-amp?

[marce]

14-20+layer high speeed layouts, used FR4 high reliability applications, DDR3 memory gigahertz ethernet, FPGA DSP etc etc ALL FR4 It is only occasionaly we use more exotic materials.
Usually as Bonsai has indicated in the high GHz, and FR4 top grades will go up to 10 at a touch.

[RNMarsh]

Originally Posted by kgrlee

The problem with polystyrene caps in microphones (if one ignores the reliability issues with soldering) is that they are microphonic.
Which polystyrene maker do you recommend?

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

[RNMarsh]

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.

[john curl]

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.

[Joshua_G]

Originally Posted by 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?

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.

[Joshua_G]

Originally Posted by RNMarsh

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.

Hi Richard,
What is the material of those PCBs for microwave?