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In Reply to: Where to Find Better Polystyrene Caps? posted by Triode_Kingdom on April 16, 2007 at 20:33:04:
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Better DA and DF than Polycarbonate, the third best but PC is usually only found in metal "canned" caps with glass to metal hermetic seals, ie military and critical industrail applications like process control systems. Polypropylene follows along with PPS, especially good at high temps.
At my day job, I am now using these caps in earnest for new RF amplifier/filter designs. They're ideal for this due to their extremely high Q, overall stability and freedom from low-level nonlinearities. I also use them extensively in tube audio EQ circuits, such as tone controls and phono EQ, as well as NFB loops. They are accurate and in my opinion sonically superior to most of the alternatives. Please note that I'm referring only to the small, clear/silver tubular polystyrene caps. I don't use the other varieties of these that have been introduced more recently into the audio market.
Hi.Different dielectric materials, e.g. tantalum (oxide), mica, PP, PE, teflon etc etc yield different dielectric absorption. Of course, lower the better.
That said, lower dielectric constant does NOT warrant better linearity.
Let me put it in perspective, Ic=C.dV/dt. We can plot a linearity chart of various caps on this formula.
PP film cap (dielectric constant K=2.1) is lower than polystyrene film (PS, K=2.56). But it is not as linear as PS though the curvature difference is very very small. Why? Maybe due to the aromatic structure of PS being more easily ELECTRONICALLY oriented than PP resulting less mechanical change of polymer structure.
In simple language, the interpolarity charges in PS 'relaxes' faster than that in PP.So your choice of PS caps is good for phonostage coupling & RIAA EQ.
Incidentally, I just completed built a very basic stereo phonostage using discrete SS biplars (sorry, not tube this time) in dual-monaural stereo format with battery power supplies. It is now singing like a nightingale after a 7-day non-stop 2.5KHz wideband white noise breaking in.
Obviously, thorough breaking is needed for new caps & new hook-up wires particularly for phono amps. For low cost, I used non-polarized small polyester (PE) film box caps, ying-yang parallel to build up the required uF values for I/P & O/P coupling. Also PP & PE film caps for the simple loop FB RIAA EQ. Ag plated oxygen free high conductivity (OFHC) wires in teflon dielectric, without shielding, are used for all signal paths.
I used PE (K=3.2) simply because of the uF values required that I can add up cheaply rather than resorting costly exotic large film caps. Yet, it sounds so gratifying, proving my choice is proper.
You don't want to know the tolerance those caps & so called 1% metal film resistors when measured. That's why trimming is needed.
c-J
PS: I built it in a small black plastic equipment box. Guess how I handle the EMI/RFI/hum issues?
> Guess how I handle the EMI/RFI/hum issues?I'm guessing balanced wiring consisting of twisted pairs. Yes? No?
Hi.I said the whole phonostage is housed in a PLASTIC case as contrary to conventional metal housing.
Why? I want to prove something. A very expensive line of audio amps, made in Germany, are all housed in cases built with acrylic type plastic sheets. Per the owner/designer, non-metal housings sound better than metal housing. He used various methods of layouts of the PCBs to provide the shielding needed.
So I borrow his idea but the small plastic equipment box does not provide any room to play around with the PCB layouts. I did it the tedeous way - I laminated ALL the interior surfaces of the box with
aluminum foil wrap (cheapie from any grocery stores), & then
insulated it with clear box tape.The tricky part is how to ground each of the two mono phono amps to the aluminum foil shield of the housing, & then to the external grounding of the turntable.
Mind you, the electrical reference 'ground' of each channel is separated from each other to achieve true monaural format as each channel is powered with its own battery supply.
> I did it the tedeous way - I laminated ALL the interior surfaces of the box with aluminum foil wrap (cheapie from any grocery stores), & then insulated it with clear box tape.I 've seen lots of wood/plastic-based gear over the years that used foil for shielding. It's often glued onto something like fishpaper, which is then creased and folded to fit into the chassis properly. It's a very workable solution when the cost of custom metal fab can't be justified.
Hi.It took me a while to figure out how to ground the dual mono phonstage channels inside the plastic box to the aluminum foil shield, & then to external turntable chassis ground wire & as well as to my tube phonostage chassis grounding post (where this SS phonostage hooked up to its 'AUX' input).
It works with pretty low low hum (with ear stick to the loudspeaker woofer cone) on amp volume set to max AND turntable running.
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