Tube DIY Asylum

Do It Yourself (DIY) paradise for tube and SET project builders.

Return to Tube DIY Asylum

Message Sort: Post Order or Asylum Reverse Threaded

replaced diodes/caps in power supply?

74.103.75.241
Posted on October 25, 2014 at 06:07:10
Thom
Audiophile

Posts: 232
Joined: March 6, 2000
 I would appreciate any help with the new development of a buzzing power transformer.

So, I've replaced diodes and filter caps as follows:

(3) 1N5408 with (1) Cree Schottky C2D10120A 10A 1200V per leg of the full-wave center-tapped rectifier circuit.

(6) 540uf 450V computer grade lytics with same qty of 470uF 450V UCC KHM 'lytic caps.

The secondary is 150-0-150 center-tapped and I get about 225 VDC out of this circuit.

All-in-all, a very impressive change for the better, but the power tranny now buzzes ... as in can hear from across the room.

When Cary does this mod, they use Hexfred diodes, 15A 1200V and use two in series. Is there some sort of imbalance possible in just using one diode per leg? When Cary strings three in series, I would think they would use voltage balancing resistors across each diode, but they do not. I figured a 10A 1200V device was plenty large enough to replace (3) 3A devices. The total draw on this preamp is less than 20 mA sheesh.

Any thoughts most appreciated!

 

Hide full thread outline!
    ...
RE: replaced diodes/caps in power supply?, posted on October 25, 2014 at 06:25:05
Chip647
Audiophile

Posts: 2653
Location: The South
Joined: December 24, 2012
 Well, your Crees have a lower impedance than the 1N5408 which causes the transformer to have higher current spikes. Adding a 50 ohm resistor right after the diode will fix it.

I don't usually like using 500uf caps as they are kind of like band aids. A better power supply can be made with a couple of 60 to 100 uf caps and a decent choke. I can understand why the manufacturer used the big caps as they are a cheaper way to make the background noise lower.

 

RE: replaced diodes/caps in power supply?, posted on October 25, 2014 at 14:44:34
Thom
Audiophile

Posts: 232
Joined: March 6, 2000
 Thanks, Chip! I'll give that a go.

I'm committed to film caps in the PS, these were just easy replacements and from what I'd read they would sound much better than existing ... and they did. The outboard PS will take some good size films, but the preamp chassis itself has zero real estate for anything like that so I'd like to find radial-lead, 1 3/8" films that could mount through the existing holes/clamps, but have not seen any candidates yet. I could also probably use round axial types with something close to that dia, but then I'd have to put the ground end up in the air out of the chassis with a wire running back down inside. Kind of kludgy.

 

Radial-lead film caps, posted on October 26, 2014 at 11:17:35
JJ Triode
Audiophile

Posts: 716
Location: Northwest
Joined: December 2, 2004
 Take a look here:

http://www.mouser.com/ds/2/88/UNL-9847.pdf

Mouser stocks the 30µF/400V, 35µF/500V and 30µF/600V Cornell-Dubilier radial cylindrical caps in this series. All are 35mm in diameter which is very close to what you need (1.375" = 34.9mm) so if your clamps have a screw adjustment they should fit perfectly.

 

RE: Radial-lead film caps, posted on October 26, 2014 at 18:53:22
Thom
Audiophile

Posts: 232
Joined: March 6, 2000
 Yes, they will fit fine. Thanks!

 

RE: replaced diodes/caps in power supply?, posted on October 26, 2014 at 18:55:49
Thom
Audiophile

Posts: 232
Joined: March 6, 2000
 Didn't work, Chip. Under the load of the filters and just bleeders, the transformer is silent. Once I apply the full preamp load, the buzz starts :( Would a bit more resistance be wise?

 

RE: replaced diodes/caps in power supply?, posted on October 29, 2014 at 14:32:39
Sherwood Forest
Audiophile

Posts: 240
Joined: October 7, 2014
 I was trying to explain something that could be causing this, but found a much better explanation here posted by Al Sekela: http://www.audioasylum.com/cgi/t.mpl?f=tweaks&m=123567


" In Reply to: It's time for someone experienced to say Schottkys are not suitable for audio. posted by gads on October 23, 2005 at 05:39:16:

As long as the devices are robust and used within their dissipation ratings, Schottky rectifiers should sound better than anything short of vacuum tubes.

The problem with all solid-state junction rectifiers is minority carrier storage in the depletion region during forward conduction. When the input waveform reverses polarity, this charge is swept out in the form of a current pulse. These pulses have steep trailing edges, with lots of HF content.

The HF content in the reverse pulses sets the power supply transformer and associated wiring to resonating, with audible consequences to both the involved equipment and any other equipment sharing the power circuit or just located nearby.

Switch-mode power supplies lose significant efficiency if these pulses are large, as they are with conventional rectifiers. This is why the HexFRED and similar rectifiers were developed: they minimize the stored charge to improve the efficiency of switchers. Some audio people recognized HexFREDs' ability to reduce the magnitude of the input noise to the power supply issue and adopted them more than a decade ago. "Minimize" does not mean eliminate, however.

Schottky rectifiers are majority carrier devices. They do not store charge, so there is essentially no reverse pulse when the input waveform reverses polarity. Silicon Schottky rectifiers are limited to low peak reverse voltages, so they traditionally have been used only for filament DC supplies and similar low-voltage applications. Their lower forward voltage drop compared to silicon junction rectifiers is the main reason for their use.

CREE developed silicon carbide Schottky rectifiers a few years ago. Silicon carbide is a semiconductor with a larger band-gap than silicon, so the Schottky diodes can be made with higher peak reverse voltage ratings. The trouble with silicon carbide is that it is a refractory material, and difficult to grow into defect-free single crystals. If there is a crystal defect within the rectifier, it may fail under normal service. I recall hearing about some failures when these devices first became available, but I don't know if CREE has found test procedures that would guarantee reliable operation of parts sold today.

Is this technical enough for you? "

 

well, that's interesting .., posted on October 29, 2014 at 18:03:34
Thom
Audiophile

Posts: 232
Joined: March 6, 2000
 Thanks!

Seems Cree's sonics are questionable? I certainly have reason enough to try FREDS just due to the buzzing.

 

RE: well, that's interesting .., posted on October 31, 2014 at 10:48:09
Sherwood Forest
Audiophile

Posts: 240
Joined: October 7, 2014
 I have no idea what is going on in your case, since I can neither examine nor measure anything. I honestly do not know if what I quoted is applicable in your case.

I posted it because it sounds like it *might* be related to the switching noise of those devices, or really, the current pulse they generate when polarity reverses.

I think the only way you're going to find out is by either trying other rectifier components there, or by looking for those pulses with a scope.

 

Page processed in 0.019 seconds.

[ Top Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2024, Audio Asylum, all rights reserved ]
[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]