DIY DC blocker? [Archive] - The Art of Sound Forum

HighFidelityGuy

12-05-2011, 12:31

I've just stumbled across THIS (http://sound.westhost.com/articles/xfmr-dc.htm) info about building a simple DC blocker to reduce mains transformer hum. I currently have some issues where my valve amp tranny runs hot and my sub trannies intermittently buzz to varying degrees through the day. So I'm thinking I may have a DC offset issue on my mains. I was hoping the Belkin PF30's I bought would get rid of this but they don't and neither does my Tacima mains block. So perhaps this may help instead. I have the parts in my spares box to build one to try.

Here's the schematic:

http://sound.westhost.com/articles/xfmr-dc-f8.gif

So I was wondering if anyone had built one of these and what your thoughts are on it's effects both positive and negative?

Cheers. :)

Reid Malenfant

12-05-2011, 12:52

Well that circuit is a waste of time, it won't block DC at all :scratch:

:lolsign:

If you look at the way the diodes are wired you''ll see that DC over 1.2V can still flow :eyebrows:

Reid Malenfant

12-05-2011, 13:00

Take out D4 & D5 & then connect the centre of the diodes to the centre of the capacitors & it'll work. The diodes are there to block DC & prevent high reverse voltages on the capacitors ;)

HighFidelityGuy

12-05-2011, 13:05

Like this?:

http://diyparadise.com/web/images/stories/dcblocker.gif

That design is from HERE (http://diyparadise.com/web/projectsto-keep-you-busy-mainmenu-26/62-da-blok). This guy mentions that putting an extra two diodes in will make it block more DC. So that's why I thought the first schematic was the way to go. So if adding two more diodes will improve it, how should I wire them?

Cheers.

Reid Malenfant

12-05-2011, 13:10

In your second diagram turn the two doides the other way round, you need them to short out any reverse voltages greater than 0.7V accross the electrolytics :) I'd suggest using at least 16V electrolytics so any reverse voltage is really low compared to the capacitors forward voltage capacity. We don't want them going bang now do we ;)

Whoever designed the first one needs to stick to his day job :lol:

Feck me, whoever designed the second one needs to stick to the day job to :mental:

Who are these people? If you are blocking DC you can't block MORE! I'd suggest you use fast soft recovery diodes to do the job, 1N5404 is only rated to 3 or 4 Amps, get some big 30A devices in there ;)

Reid Malenfant

12-05-2011, 13:18

In fact i have just the thing :) MUR3040PT & you'd only need one device as they are a dual diode package.

HighFidelityGuy

12-05-2011, 13:23

Ok you've lost me now. :scratch:

So from this diagram you're saying I need to turn the diodes the other way round but the MUR3040PT has it's diodes oriented in the way shown in the diagram, so how can I turn them round?

http://diyparadise.com/web/images/stories/dcblocker.gif

Reid Malenfant

12-05-2011, 13:25

Turn the capacitors the other way round Dave :cool:

MCRU

12-05-2011, 13:37

I have a dc blocker made and on a pcb, rated at 5 amps, want one to try?

designed and built by nick (lurcher) on my behalf and thoroughly tested both on a live mains and on a pc simulation.

HighFidelityGuy

12-05-2011, 13:44

Turn the capacitors the other way round Dave :cool:

Ah, clever. So like the attachment below then? :)

I have a dc blocker made and on a pcb, rated at 5 amps, want one to try?

designed and built by nick (lurcher) on my behalf and thoroughly tested both on a live mains and on a pc simulation.

Thanks David, that would be awesome. How would you want to work the trial? Just drop me a PM with details and we'll work something out. :cool:

Reid Malenfant

12-05-2011, 14:22

Dave, why did you reverse both the diodes & capacitors? :scratch: It's wrong...

Go back to the diagram you had previously & simply reverse both capacitors :)

HighFidelityGuy

12-05-2011, 14:26

Dave, why did you reverse both the diodes & capacitors? :scratch: It's wrong...

Go back to the diagram you had previously & simply reverse both capacitors :)

Doh :doh:, I'm with you now. Cheers. :cool:

Reid Malenfant

12-05-2011, 14:28

That's the way to do it :eyebrows:

PM me your AD & i'll send you one of the MUR3040PT, I know where they are so it's not like i have to go hunting them down :)

By the way, depending on what you are going to power you'll need to take into account the current carrying capacity of the capacitors, check out there ripple current rating.

colinB

12-05-2011, 15:04

Mark,i thinks its time you started designing and making some products.:popcorn:

Reid Malenfant

12-05-2011, 15:09

:lolsign: Making my own stuff is as far as i'm willing to go Colin :) I don't mind helping out like this & i can't stand stuff that's wrong, god knows what that bloke was thinking when he posted that circuit on the net :mental:

Oh sorry, what am i on about? He wasn't thinking :lol:

HighFidelityGuy

12-05-2011, 15:47

Here's another DC blocker schematic I've just found:

http://www.htguide.com/forum/attachment.php4?attachmentid=15133

What do you think about that one Mark? Would this be any better or worse? :scratch:

Reid Malenfant

12-05-2011, 15:52

Why bother making something more complicated than it needs to be? It'll be more expensive & work no better than the two diode two capacitor version :eyebrows:

Besides which the diodes are back to front yet again!

HighFidelityGuy

12-05-2011, 16:15

Ok thanks Mark.
I can see that if you chop the right hand side of that last circuit off you're left with the circuit you recommended.

It's quite scary that both of the first to circuit's I posted are wrong and they've been in circulation for several years with many people making and using them. No wonder quite a lot of them said they didn't notice much difference in transformer buzz after installing it. :doh:

Google can be a dangerous tool at times. I'm glad I asked before I jumped in. :eek:

Cheers. :cool:

Reid Malenfant

12-05-2011, 16:29

They will probably work, in fact they will work with the diodes back to front as long as little DC voltage is present on the line. However with the diodes wrongly orientated the capacitors have zero protection against DC reverse voltages greater than 0.7V. What this means is that if there is significant DC on the line then the capacitors could well blow up :rolleyes:

HighFidelityGuy

12-05-2011, 16:39

Nice. :rolleyes:

I'll drop you a PM in a minute about the diode. Thanks for the offer btw.
On the subject of caps, I have a few of THESE (http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=191-7820) left over from my valve amp build, would they be chunky enough? Cheers.

Reid Malenfant

12-05-2011, 16:41

Yes, they'll be ok to run your amplifier :)

HighFidelityGuy

12-05-2011, 16:45

Cool, how about my pair of 250W class A/B subs? Would one of these filters run one sub ok? Cheers.

Reid Malenfant

12-05-2011, 16:48

You'd need capacitors with a higher ripple current rating to be safe. Your valve amp may well consume up to about 1Amp or maybe a tad more, a 2200mA rating is therefore OK. A 250W sub may well use 500W or more which is over 2Amps at 230V...

Got the MUR3040PT sat in front of me ;)

HighFidelityGuy

12-05-2011, 16:57

Ok, so I need to match the ripple rating of the caps to the current consumption of the connected device? That makes sense.

With your diode and my caps I can make something to compare against the filter David is sending me, so that will be interesting. Then based on what I find I'll look at making something to protect my subs and other gear. :thumbsup:

Reid Malenfant

12-05-2011, 17:03

Looky, if you bought these (http://cgi.ebay.co.uk/Electrolytic-capacitors-6800uF-25V-7A-4-pieces-/290563301019?pt=UK_BOI_Electrical_Components_Suppl ies_ET&hash=item43a6ec1e9b) you could damn near if not power your whole setup. Connect two in parallel so you get 13,600uf & you'll have a 14A ripple current rating ;)

First ones i came accross :eyebrows:

HighFidelityGuy

12-05-2011, 17:12

Thanks Mark, I'll have a look around. I've found some with over 30A ripple rating but they're rather expensive and rather overkill. :lol:

Reid Malenfant

12-05-2011, 17:58

Talking of capacitors i realised i needed some for a project (http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=110684572719&ssPageName=STRK:MEWNX:IT) :eyebrows: There are 2 x 1KVA toroidal transformers & 28 x 0.8C/W heatsinks calling their name :D

I sense a pair of balanced output class A monoblocks in the offing :cool: Looks like i'm going to stick with solid state after all..

Reid Malenfant

12-05-2011, 18:09

Well bugger me, the seller just sent me a message letting me know he'll post tomorrow. Better yet he has another 14 of these caps & he'll let me have them for £10 postage included :champagne:

:lolsign: No brainer ;)

jantheman

13-05-2011, 09:17

I currently have some issues where my valve amp tranny runs hot and my sub trannies intermittently buzz to varying degrees through the day. So I'm thinking I may have a DC offset issue on my mains.

Heres a quote from the Kemp electronics website which may help you.

'To determine, if you actually suffer from DC on your mains, act as follows:Listen carefully close to your equipment, if you have any hum at all. If the answer is yes, then stay there for a while, and listen carefully. Does the hum alter in time, you can be sure, that DC on the line is the cause of this hum. Do you hear a very constant hum, always present and always the same loudness, the transformer itself may well be the cause'.

And a link to the page.
http://www.kempelektroniks.com/getdoc/0f25cfe4-9965-40f2-8a8a-bf4e2fcf7de7/Power-DC-X-terminator.aspx#pdcxcplus

Not sure if you want to buy one..they are over 300 squids.......

HighFidelityGuy

13-05-2011, 09:29

Thanks Ray, that's exactly what I hear, a transformer hum that changes in volume. Most of the time it's very quiet and constant, then it suddenly swells to a level where you can hear it across the room and starts going up and down every few seconds. Then after a few minutes it dies down again. I haven't been able to attribute it to anything plugged in in my house, so it must be coming from an external source. I live fairly close to some steel works and other heavy industry, so I guess it must be them firing up some big machines.

Until reading about mains DC offset I'd just assumed it was the mains voltage changing but after monitoring this for a couple of weeks I began to doubt that theory. I was starting to think I'd either have to live with it or buy an expensive mains regenerator but hopefully one of the solutions mentioned here will help for a lot less money.

Cheers. :cool:

jantheman

13-05-2011, 09:43

Not tempted by the Kemp one then.....:lol:

lurcher

13-05-2011, 09:52

I did some work identifying ways of detecting DC on the supply so the action of the DC remover could be verified objectivly.

I think David is planning on posting it on the Mains Cables R Us site some time soon. It was done for him to use, so I won't post it directly.

HighFidelityGuy

13-05-2011, 10:06

Not tempted by the Kemp one then.....:lol:

Nah, think I'll give it a miss for now. ;)

I did some work identifying ways of detecting DC on the supply so the action of the DC remover could be verified objectivly.

I think David is planning on posting it on the Mains Cables R Us site some time soon. It was done for him to use, so I won't post it directly.

Interesting stuff, thanks Nick. So will this be a little box that checks for DC and provides some kind of simple display of the results?

lurcher

13-05-2011, 11:10

No, I did some looking into how to detect DC on the bench, and then showed how to create it on demand, and then how the use of the blocker fixed the problem. the document details what I found. And in the process I noticed it happening on my real mains supply, and as you mention it occured as the TX's buzzed along with the event.

It would be possible to have a box as you mention, but it would be more complex than you describe. The best indication I found involves a XY scope display. Could be done, but its simpiler to make as a detector that then gets connected to a scope as a display.

Rare Bird

13-05-2011, 13:47

Reid Malenfant

13-05-2011, 13:51

Indeed, just the way i like it :eyebrows:

Glen B

13-05-2011, 16:51

I've just stumbled across THIS (http://sound.westhost.com/articles/xfmr-dc.htm) info about building a simple DC blocker to reduce mains transformer hum. I currently have some issues where my valve amp tranny runs hot and my sub trannies intermittently buzz to varying degrees through the day. So I'm thinking I may have a DC offset issue on my mains. I was hoping the Belkin PF30's I bought would get rid of this but they don't and neither does my Tacima mains block. So perhaps this may help instead. I have the parts in my spares box to build one to try.

Here's the schematic:

So I was wondering if anyone had built one of these and what your thoughts are on it's effects both positive and negative?

Cheers. :)

I've built a number of DC blockers for people with no DIY skills, as a favor. None have reported any negative effects, only a thank you for solving their hum problem.

http://img.villagephotos.com/p/2003-5/191557/DC%20Blocking%20Circuit_2.jpg

Reid Malenfant

13-05-2011, 16:57

The problem is Glen is that the first circuit you quoted in your post simply won't work ;)

The second one won't protect the capacitors due to the diodes being the wrong orientation. Either the caps need changing round or the diodes need rearanging :(

Whoever put those to a drawing simply didn't think about the consequences :rolleyes: The capacitors will experience whatever DC is on the mains but with the voltage on them reversed. If things were otherwise they could only experience a maximum of 0.7V of reverse voltage & any DC would be forward charging them so it'd be safe :)

Glen B

13-05-2011, 17:00

Reid Malenfant

13-05-2011, 17:04

I can't figure :scratch: Just look at the way the circuit works & you can plainly see that the caps will be reverse charged & only a maximum of 0.7V could be charged forward :lol:

HighFidelityGuy

14-05-2011, 12:22

I've had a very slight change of plan. I decided that seen as the caps I have at the moment are only good for about 2A and I already have some nice 4A diodes that I'd build a small test version of the circuit that Mark recommended. The DC blocker that David from Mains Cables R Us is sending me is good for 5A and the diode Mark is sending me is good for 35A, so I'll use the latter to build a 13A version with some new caps. I'll then have a nice range of power capabilities to play with.

I've built the 2A version and tested it on an old 65W amp and it didn't blow up, so I now plan to try it on my valve amp. The transformer on the valve amp runs at a constant 55 degrees C, so I mainly want to see if the DC blocker reduces this at all. The transformer on this amp also hums a bit but only slightly now that I've mounted it on silicon bushes. So I'll see if that changes at all. I first need to let the amp cool down though, so I'll post my finding later on today.

I'll be honest and say that I don't think it will make much difference to the temperature as I think these amps just run hot but I wanted to try this first out of interest.

lurcher

14-05-2011, 12:36

The problem is most pronounced with toroids, EI and C core TX's have a natural air gap that makes them more tolerant.

HighFidelityGuy

14-05-2011, 16:10

The problem is most pronounced with toroids, EI and C core TX's have a natural air gap that makes them more tolerant.

Thanks Nick, that explaines why my valve amp doesn't hum as much as the toroid based equipment I have.

Well I let my amp cool down so that the transformer was 30 degrees C. I then hooked it up the the baby DC blocker and switched it back on. So far so good, it all works. I'll leave it to warm up for a good 30-60 minutes and then measure it again. BTW, I'm using a non-contact infra red thermometer to take the measurements. Back later...

EDIT: I forgot to mention, the slight hum the valve amp made does seem to have been reduced slightly by the filter but it was already pretty quiet so it's difficult to judge it accurately but it's a good start.

HighFidelityGuy

14-05-2011, 17:33

I've just measured the transformer again and it's now reading 54 degrees. Oh well, I guess it wasn't a DC offset issue that's causing the heat. At least I know now. So I'll try the filter on some toroid based equipment next to see if it works any better at reducing hum. That's a job for tomorrow though, I'm off out for some bevvies now. :cool:

Reid Malenfant

14-05-2011, 17:44

Dave, i have some rather large (20,000uf+) 20V caps here that i'm not really likely to use. If you'd like them i can send them to you with the diode as long as you cover the postage.

If you decide to go for it i'll get the capacitors down from the loft & reform them before sending them to you :)

Let me know chap ;)

HighFidelityGuy

15-05-2011, 09:04

Reid Malenfant

15-05-2011, 10:42

No, but take it from me that it'll be at least 10Amps if not more. They are computer grade types with screw terminals, made by Mallory if i remember correctly :scratch:

MCRU

15-05-2011, 13:18

i don't think there is any need for me to send you the dc blocker nick made me is there now as you guys seem to have sorted it between you all?

HighFidelityGuy

15-05-2011, 13:38

No, but take it from me that it'll be at least 10Amps if not more. They are computer grade types with screw terminals, made by Mallory if i remember correctly :scratch:

Sounds good, thanks. PM me payment details and I'll sort it out. Cheers. :)

Reid Malenfant

15-05-2011, 13:43

Ah, i'm glad you didn't leave things too late :eyebrows: Stick about or pop in after say 7PM & i'll have them sorted & weighed in packaging. I'll send you a PM with detail on the postage costs (won't be a lot), paypal gift will be fine :cool:

I'll go find em as soon as i have had a meal & let it settle.

HighFidelityGuy

15-05-2011, 14:23

i don't think there is any need for me to send you the dc blocker nick made me is there now as you guys seem to have sorted it between you all?

Hi David, I see what you're saying and I guess not. Thanks for the offer though. :) I've been quite overwhelmed with generous offers in this thread. Many thanks to everyone. :cool:

HighFidelityGuy

15-05-2011, 14:24