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Old 23rd November 2013, 09:48 PM   #1
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Default DC blockers and mains filters

Possibly not strictly a PS but more about getting clean power I decided to try a DC blocker. Australia at one time had the cleanest power but since the grid has been privatized power quality has fallen. A DIY friend built a simple blocker and claimed he felt an improvement in sound quality. Because he has tube monoblocks he built two and believed a better improvement again. Also any DC on the mains will generally cause most big toroids to buzz. He had had a lot of problems with this.

Because the project is so inexpensive I decided to build one based on the Recommend Design from this link. Look toward the end of the article. To value add a little I included a cheap RFI\EMI filter in the one compact enclosure. For the last few years all my power amps have a power filter built in but not my media players or preamps. Now they have better cleaner power.

I wanted to source all components from where I worked but could only find 4700uf caps at 50V. My bridge rec. is rated 35A. Since the caps should never seed any more than a few volts this should not be a problem. I added a choke to the lead in and a 10A easy access slow blow fuse to the build.

On a short listen through my all tube system, apart from a better defined sound stage, I can hear no HUGE improvement. A more extended listen may reveal other improvements. But now I know that my gear is a no-go zone for DC and all equipment is receiving filtered power. I built this up from scratch and had it working in a short afternoon for a total cost of $35. There is no reason not to have one.
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Last edited by mhouston; 23rd November 2013 at 09:52 PM.
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Old 23rd November 2013, 09:54 PM   #2
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Unless the DC is bad I cant imagine getting a much improved sound.
The amp is not going to distort unless the power supply rail voltages are being breached by the amplifier signals.
Any other distortion should be taken care of by feedback loops.
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Old 23rd November 2013, 10:07 PM   #3
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None of my gear runs GNFB. If you read the article you will see that you only need a few mA of current to cause big problems with large toroids. DC can be caused by industrial equipment, like where my DIY friend lives, or simple devices in the home. The way mains power is managed, by default, puts DC on the grid.

EI trannies handle small amounts of DC current better but still cause partial saturation. Some of the trannies in my tube amps run hot. My 300B draws 200mA at 435V and though the tranni is massive it gets very hot. I am yet to check this amp with the blocker but hopefully there is an improvement. I know with Halcro FET amps ($40K a pair) half the amp is just power conditioning. If you don't start with clean power you will never get a clean sound.
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Old 23rd November 2013, 10:12 PM   #4
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A lot depends on how much noise gets past the smoothing capacitors and decoupling capacitors. If its getting past those you have serious problems !

I have no problems in the UK, we always have good mains.
I use loads of torroids and never had any hum problems.
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Old 23rd November 2013, 10:23 PM   #5
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Only very big toroids (500V plus) have big problems. There is a lot of RFI\EMI on mains. A lot of this will pass right through even a good PS. I noticed from my very first amp I incorporated power filters my power amps sound better, clearer. I haven't built a power amp in years that doesn't have it's own power filter. I often get comments about how clear my tube amps sound. This year alone I made four tube power amps for myself which all sold in a week. I had no intention of selling them but others liked them so much I had to let them go. I'm not saying it is solely because of the power filters but I think they have a part in it.
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Old 24th November 2013, 03:50 PM   #6
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It seems to me that you could install MOV surge supressors inside that cabinet, upstream of the EMI filter. I recall a delightful article in the #2/1991 issue of The Audio Amateur, "About Noise" by L. B. Dalzell, which included this section on page 12:

Quote:
An Audio Placebo

In medical research, when a new drug is tested on humans, who are subject to the power of suggestion, the researcher uses a placebo, an innocuous pill or capsule, and compares the results of the test group and the placebo group.

I strongly believe in the value of what I shall call an "audio placebo". I constructed four different AC filters for testing: (A) consisted only of the MOV device; (B) contained only a commercial RFI filter; (C) used a commercial RFI filter and the MOV devices; (D) was the placebo (it was wired straight through).

I used three MOVs in each of (A) and (C) just to be sure. One MOV was wired from the line to the neutral, one from the line to ground, and the last from the neutral to ground.

The four AC filters were given to audio enthusiasts for trial and were sealed in cast boxes to prevent peeking.

The resulting ratings in use, when participants were asked: choose the best of the four, were
  • A. 13% (only MOV)
  • B. 25% (only RFI filter)
  • C. 60% (both)
  • D. 2% (placebo - neither)
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Old 24th November 2013, 07:32 PM   #7
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Default DC on the mains

If you want to make a circuit for keeping mains away from the primary of the transformer please have a look in Bob Cordells excellent book "Designing Audio Power Amplifiers". The circuit can be found on page 360 and can be made from few relatively inexpensive components.

Karsten
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Old 24th November 2013, 08:08 PM   #8
Bensen is offline Bensen  Belgium
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I to have build the DC-blocker from Rod Elliot.
See the small PCB in the middle, on this PCB are also 2 safety loop breakers installed.
After reading on ESP what a huge effect even a small DC-current has on the transformer capability, it has to be there in my next design.
I'm using 63V/10000µf caps from BC Components/Vishay, rated with a ripple current at 100Hz of 5.4A. The diodes are 15A/600V devices.
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Old 24th November 2013, 08:19 PM   #9
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Quote:
Originally Posted by transistormarkj View Post
It seems to me that you could install MOV surge supressors inside that cabinet, upstream of the EMI filter. I recall a delightful article in the #2/1991 issue of The Audio Amateur, "About Noise" by L. B. Dalzell, which included this section on page 12:
This shows that a filter and protective gear actually produces better sound.
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Old 24th November 2013, 08:24 PM   #10
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Quote:
Originally Posted by Bensen View Post
I to have build the DC-blocker from Rod Elliot.
See the small PCB in the middle, on this PCB are also 2 safety loop breakers installed.
After reading on ESP what a huge effect even a small DC-current has on the transformer capability, it has to be there in my next design.
I'm using 63V/10000µf caps from BC Components/Vishay, rated with a ripple current at 100Hz of 5.4A. The diodes are 15A/600V devices.
I thought about including blockers in amps but I already include power filters in all power amps. At this point I will keep external a single shared blocker. After further listening I get the feeling the soundstage definition has improved which is the same remark a friend made who got me into them.
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