Capacitor tolerance

[EC8010]

Originally posted by boxerboxer
Where do you suggest looking for these? Searching Mouser, Allied, and Digikey failed to yield anything in 1% tolerance at the voltage spec. I need.

In the UK, Farnell do polystyrene and silvered mica in 1%. The fact that valves/tubes have tolerances of +/-20% is not necessarily an issue. It will all depend on the design.

I would agree with SY regarding accuracy. A function generator has an inherently flat response. Or you could search the second-hand electronic instrument shops for an old component bridge.

[boxerboxer]

Originally posted by EC8010


In the UK, Farnell do polystyrene and silvered mica in 1%. The fact that valves/tubes have tolerances of +/-20% is not necessarily an issue. It will all depend on the design.

I would agree with SY regarding accuracy. A function generator has an inherently flat response. Or you could search the second-hand electronic instrument shops for an old component bridge.

It's not a matter of finding the values in 1% silver mica, I've got that on mouser, it's just the bigger ones are almost 15USD a pop.

[Johan Potgieter]

Ouch.

Perhaps I should , I really do not normally differ from EC8010!

But when I take a 'holographic look' (such a nice, cultured term to use ...), I come up against worse ups-downs in loudspeaker response a.o., with steeper rise times - and these differ from model to model. My point is, it is not going to be the steepness of the characteristic that is going to suffer with tolerances of a few%, just the position of the curve. There is still basically one R.C, thus 6dB/octave in the limit - not that steep. (Then what about equalisers? One gets 12 - 18dB/octave there.) I have not simulated - it might broaden my horizons to do so! - between different 'curves' resulting from a few % deviation in C. (... there, EC8010! Now you have done it! I am now curious and will go to bed even later after having added some 'Spice' to my life. All your fault. ).

In fact, I am known to be naughty. I mostly use RIAA in a feedback topology, and I find it easier to come right with some sub-sonic attenuation, if I allow a slight rise from about 80 Hz, peaking to +2 - 3dB above correct RIAA at 30 Hz odd. What with everything else usually starting to drop in that region, I had nothing but praise for 'maintained solid bass' resulting from such (not revealed, of course!).

Edit: spelling

[EC8010]

Originally posted by Johan Potgieter
My point is, it is not going to be the steepness of the characteristic that is going to suffer with tolerances of a few %, just the position of the curve.

That's exactly right, and that's why I fret about tolerances. Mispositioning one curve relative to another causes a shelf error, and although loudspeakers typically have peaks and troughs of 2dB or more, it's important that their general trend is correct. Worst of all, typical RIAA errors tend to cause shelf changes across the ear's most sensitive region - but I expect you've seen that now that you've played with SPICE.

[oshifis]

An alternative would be to multiply the RIAA resistances by 5 or 10, and divide the capacitors by the same value. Don't forget to take into account he source ad termination resistances in this case.

[glorocks]

Mouser sells Wima polypropylene film (FKP2) caps in 2 and 5% tolerances at very reasonable prices. They are cheap enough that you can buy a bunch of them and hand match them.
Very important that you match the caps so they are as close as possible channel to channel as well.
B&K used to sell a capacitor meter for about $80, and a lot of modern DVMs have the ability to measure capacitance. If you don't own one of these, make the investment.

[jackinnj]

Originally posted by Geek
Hi,



A trick for that is have a larger value DC blocking capacitor (0.1uF to 0.47uF @ 630V is easier to find than RIAA valued ones) in series with your accurate cap. 50V is fine for the accurate cap because there's no way any AC value in an RIAA stage will (or should) exceed the RMS rating.

Cheers!

Yes, the polystyrene caps are rarely available in voltages above 100VDC -- you can find some 160VDC, 500VDC but these are comparatively expensive. Mouser sells PS Caps, so does Nebraska Surplus.

WRT an inverse RIAA network -- this a scope and sweep generator are all you need, but you can do it by hand. You might find that your hand-held DMM is not very accurate over a few kHz, however.

I wrote a routine for an HP3577 network analyzer does the analysis on the fly (remembering that you have to change the input impedance of the analyzer!).

You can drive yourself batty designing a tube RIAA network, and I am exempla par gratia. Gotta remember not to overdrive the darn thing!

[Geek]

Originally posted by jackinnj
...so does Nebraska Surplus.

Oooh, forgot about them. Thanks!

Originally posted by jackinnj
.You can drive yourself batty designing a tube RIAA network,...

Yes!

I measure the cost of phono R&D in qty. of cigs and wine, not parts, LOL!

Cheers!

[Conrad Hoffman]

IMHO, a properly designed circuit, ss or tube, will not change its response much with age or as tube characteristics change. You want the RIAA curve to be accurate and the best way is buying a number of less expensive polypropylene caps and measuring them. I like polystyrene in ss circuits, but the difference between the two is minimal. Silver mica sounds good to me, but prices are high and it has a high dissipation factor. Go with the film. I try to select my RIAA parts to 1%, but match the channels to half that or better. It's pretty trivial to make a single range bridge for the values you're talking about, if you have one known value cap and a good DVM to measure resistance. But, a used 1650 or a new cap meter is a lot easier.