His Master's Noise: A Thoroughly Modern Tube Phono Preamp - Page 38

21st April 2010, 12:00 PM
SY On Hiatus Join Date: Oct 2002 Location: Chicagoland	His Master's Noise: A Thoroughly Modern Tube Phono Preamp After more than 25 years of faithful service, it seemed that it might be time to redo my phono system. After all, I like to think that I've picked up a few tricks in the intervening years... The old system consisted of a VPI HW17-II, a Linn Ittok LVII tonearm, and a Troika cartridge. The Troika was... Click here to read the entire article Last edited by Variac; 2nd April 2011 at 12:17 AM.

2nd June 2017

mrdave45

diyAudio Member

Hi peafarmer. I think the cap you are refering to in the psu is across AC mains. I see why that needs to be X2 rated.

C5 on the output board is decoupling the 160v dc rail across the last tube stage. But potentially may see more until stabilized hence 400v rating as raw B+ will be around 350v.

C4 is the output blocking capacitor of the audio. Doesnt an X2 rated cap seem a bit industrial in this position?
Arent film caps self healing as well, with a tendency to fail open?
Thanks

2nd June 2017

lehmanhill

diyAudio Member

I admit up front, this is an apples and oranges comment, but at least the design was a phono pre by SY in both cases.

In the EO MM pre, SY uses caps in the same way, decoupling the rail and output blocking cap. In the EO BOM and schematic higher than rail voltages are specified for the caps (but only 125 V B+ rail). However, no mention of X caps. I checked because I know I used an audio cap for the output blocking caps and I think I did for the rail cap as well.

I will leave it to you to infer what you want for that, but I'm glad I used audio caps for blocking because I tried a couple and came away with a preference.

Jac

2nd June 2017

mrdave45

diyAudio Member

Hmm. Neither article specified x rated caps. Just was printed on the pcb I received. I can definitely see why you would not want them to fail. It would be ugly. Maybe just covering himself?
The impasse has been fine with a wima. Although that's rated at 630v but it does have a 350v rail. It's only 0.1uf.
Does anyone know how you decide what value to use for decoupling the rails across tubes?