Capacitor tolerance

[boxerboxer]

I'm working on the steve bench phono pre circuit and am trying to figure out sourcing for the riaa network caps. The values to go for according to salas are 6500pf (1800||4700) and 11400 pf (1000||2200||8200). What I'm wondering is, how tight do the tolerances need to be for these. I found them on mouser.com in silver mica 1% (except the 8200pf which is non-stocked - 2% is in stock). However, it would be over 50 bucks just for those caps, so I'm wondering if I can go wider on the tolerance and/or find a better deal elsewhere.

Ben

[sawreyrw]

I'm not an expert on this, but I think it is OK to go with +/- 20% parts. They should be a lot less expensive.

[EC8010]

For RIAA you need the closest tolerance parts you can get. RIAA is incredibly sensitive to errors because you are trying to cancel out one steep curve with another to get a flat response. 2% tolerance is not good enough. Try looking for polystyrene instead of silvered mica.

[454Casull]

Might be cheaper to invest in a good LC/LCR meter and get a bunch of high-tolerance caps.

[Geek]

IMO, 2% is just fine.

I have measured tubes drifting spec. over their lifetime more than that, making any cap tighter than 2%, moot.

Now a SS RIAA, yeah, go with 0.5% or whatever you can afford.

Cheers!

[boxerboxer]

Originally posted by Geek
IMO, 2% is just fine.

I have measured tubes drifting spec. over their lifetime more than that, making any cap tighter than 2%, moot.

Now a SS RIAA, yeah, go with 0.5% or whatever you can afford.

Cheers!

Right, if I didn't need 500V caps this would be a whole lot easier.

[boxerboxer]

Originally posted by EC8010
For RIAA you need the closest tolerance parts you can get. RIAA is incredibly sensitive to errors because you are trying to cancel out one steep curve with another to get a flat response. 2% tolerance is not good enough. Try looking for polystyrene instead of silvered mica.

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.

[DaveInVA]

As someone else mentioned get a LC/LCR meter and hand pick your own. I feel its as if not more important for the caps and resistors in each channel to match each other so you don't notice a difference in tonal balance between channels. ie; if one cap is at one end of its 10% tolerance in one channel and the same cap in the other channel is at the other end of its 10% tolerance you will likely hear it..

Dave

[SY]

There's two schools of thought on RIAA accuracy. One is, "make each channel be as tight to RIAA as possible." The latter is, "absolutel conformance is nice but channel-to-channel matching is most important." My own views tilt toward the latter. I want each side matched to better than 0.1dB.

You can use an LCR meter, but a better way (considering tube and stray tolerances) is an inverse network (same network used in both channels' measurements) fed from a very flat generator or flat input and accurate AC voltmeter measurement of the output. Undersize the caps a bit, then trim to spec with smaller caps.

[Geek]

Hi,

Originally posted by boxerboxer
Right, if I didn't need 500V caps this would be a whole lot easier.

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!