Bottlehead Forum
General Category => Technical topics => Topic started by: audioclass on July 12, 2012, 08:22:25 AM
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Not sure how popular or common it is to build your own power supply, but I'd like to get some opinions or maybe a lead in the right direction on how to accomplish this.
I'll start this off by saying that I've got a cMoyBB from JDS labs that I use as a portable amplifier for my DT880/600s when I can't be at home listening to my beautiful Crack. As the Beyers are a bit 'heavy' for the stock amplifier, it has been modded to run at 18V using 2x9V batteries. This is great for on the road, and it sounds wonderful for an Altoids cMoy kit... Unfortunately, I've been having a bit of a tough time locating a good 24v linear power supply that I could use to power the system in my cube at work. Carrying the Crack back and forth just isn't going to happen, and I hate to blast through 9V batteries to listen at work as they get pricey when you chew through them listening to music for 8-10 hours a day.
Now, the JDS manual has some suggestions, recommending a linear(but not offering or suggesting one) rather than a simple switching PS, noting that switching supplies may cause some audible hum through the cans. Maybe I'm going overboard here wanting to build one, but I'd like something that doesn't take up a lot of room, is very portable and supplies enough power without going overboard (some of the supplies I WAS able to find are huge and rated at over 5A and I just don't need that kind of power for a simple cMoy. I just want clean, portable power than I can either buy somewhere fairly cheap, or build myself for that sense of accomplishment.
I basically need a portable, small 24Vdc linear regulated power supply that I can slap a 3.5mm male jack onto to mate up to my cMoyBB.
I hope I put this in the right place, I know this isn't Bottlehead-related but it seems like this place is a helpful bunch of peers and I'm hoping that leads me in the right direction :) Please let me know if this is unrealistic or if you think a normal switching PS is more than quiet enough for my purposes, I'm just trying to avoid any jitter that could induce noise to the system.
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I'm sure you've done this but Google it. I thought I just read about one on this sitediyaudioprojects.com (http://diyaudioprojects.com) but can't seem to find it just now.
John
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Yes, this is the right place.
What you want to assemble is a plus and minus 12V power supply. I suggest this over 24V. You can use three pin regulators made to produce a positive and negative 12V supply. The LM 7812 and LM7912 are two examples.
You buy a transformer with 12-0-12 volt output. Then rectify them both, separately and add a capacitor to it. The rectifier output center tap is grounded. Then the three pin regulator and a small, and nice low ESR cap on the output. These caps can be rated for 36V if you want to be very safe. Or 16V if you are sure the full 24V won't ever get to them.
You tie the negative of the +12V supply to the positive of the -12V supply together making three connections.
Can you work from a schematic or are you looking for a kit?
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Hey thanks for the replies. John, I have indeed googled it but I haven't found much of anything on a small-scale.
Grainger, I can work from a schematic if you feel so inclined. Greatly appreciate the assistance and any suggestions on quality components you can provide. The only work I've done is on avionics for Army Helos, and everything is already spec'd out in the manual with part numbers so I haven't had to do a lot of sourcing on my own!
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I'll sketch one and get it to you. These are awfully simple. Much better, lower noise, less ripple than switching supplies of the same cost.
See page two for a sketched schematic.
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Can't thank you enough! I think I may need to start getting my feet wet in the pool of knowledge to appease this addiction of mine, haha.
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dont worry - you got lots of company here!
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-->Now, the JDS manual has some suggestions, recommending a linear(but not offering or suggesting one) rather than a simple switching PS, noting that switching supplies may cause some audible hum through the cans. <---
One of the quietest supplies I've designed from scratch was a switcher. It was quieter than even the fancy HP linear supply we had in the lab.
To roll a quiet hobbiest switcher from digikey parts you need a reasonable switcher with a filter on its output: So get
..a medical grade, switching, two prong wall wart rated for 1.5X to 3X the current you need. Both over sizing and under sizing the switcher is bad.
...a 1 ohm wire wound resistor. This resistor should be rated for the larger of 3X actual current used or 2X the rating of the wall wart. (1W for 0.58A load current or a 0.7A wall wart, 5W for 1.29A load or a 1.58A wall wart.)
....three 35V ~330uF to 1000uF -->low ESR<--- 3000 to 4000 hour aluminum caps and
.....a clamp on Ferrite bead ( LFB material)
Take the cord of the wall wart and wrap it three times through the clamp on bead. Put the bead a couple inches from the body of the wall wart.
Run the wall wart hot to the 1 ohm resistor then to the caps. Put the 3 caps in a "line" like tanker cars on rail road tracks where the tracks are the leads on the caps. Power from the 1 ohm feeds into the left most cap. The first cap feeds cap #2. Cap #2 feeds cap #3. Cap #3 feeds your device. The leads on the caps can be 1/2 to 1 inch long. Make all solder joints 1/4 to 1/8 inch from the rubber seal on the cap.
The 1 ohm resistor keeps the wall wart from oscillating and provides ripple filtering.
The daisy chain line of caps uses the wiring inductance of the leads as a filter inductor.
The clamp on bead makes a common mode choke.
You may be very pleasantly surprised.
There is one step more you can do to make it better. Put heatshrink on six ~3/8 inch diameter LFB beads and put one bead on each lead going between the caps (one on + and one on ground) and one bead on each wire going to your amp. I doubt you'll need that, but it's cheap to try.
There has to be tape/ heatshrink on the outside of the 3/8 beads. The beads can go conductive after a while and short things out. I learned this the hard way. The beads supported 24V for weeks and burst into flames just as the boss walked by the lab. . .He laughed. . .I said something worse than "Ouch."
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VoltSecond,
Are these caps in series or parallel?
What is, where can you buy, medical wall warts? Did you say Digi-Key?
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VoltSecond,
Thanks for the suggestion. I'll certainly consider it. I think I'm leaning more towards Grainger's suggestion at the moment. I'm hoping to avoid having to use a wallwart to keep the 'looks' clean as well.
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One of the quietest supplies I've designed from scratch was a switcher. It was quieter than even the fancy HP linear supply we had in the lab.
This idea coming from you means that we will be building one in the lab next week. I'll let you know how it turns out.
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here's an interesting site on the subject:
http://sound.westhost.com/power-supplies.htm (http://sound.westhost.com/power-supplies.htm)
john
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VoltSecond,
Are these caps in series or parallel?
What is, where can you buy, medical wall warts? Did you say Digi-Key?
The caps are electrically in parallel but hooked up in a straight line. Imagine barrels hanging from two clothes lines.
I got my wall wart from Digikey. It's powering a headphone amp of my own design.
If the wall wart is oversized (1 amp rating used at 0.1 amp), the output will motor boat and make low frequency noise. If the output is undersized (0.10A rating for 0.10 amp load), it will run hot and have high output ripple.
The two prong ones won't make a dc ground loop through chassis. Safety regs limit the capacitor from line to output to be ~3300pF. So that's a reasonably low cap value for high frequency ground loops; however, we can easily filter against that 3300pF and make it better.
Note: I've measured more capacitance primary to secondary in some 60Hz transformers than 3300pF.
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Thanks, I have got it now.
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I don't know much about this but would a quality computer power supply (atx) make a good starting point?
PC enthusiasts can be just as discriminating as audiophiles about parts selection. An atx supply has lots of outputs, some of which go to the CPU, which I would think need to be high quality.
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I was thinking that too. I have a t-amp that comes with a 30V laser printer switcher. It sounded pretty darn good compared to battery. I bit more of an 'edge'. Kind of like good redbook vs. the same in 24/96, although about half the difference.
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Ok, I finished the sketch and scanned it. Hopefully it is not too big for this page.
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi244.photobucket.com%2Falbums%2Fgg7%2FGrainger49%2FFor%2520Posts%2520Private%2F24voltsupply.jpg&hash=95f7913c69c93442d18fecf3c609a0b1298e9cf2)
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Thanks a bunch for the schematic. I'll be trying this in the next couple of weeks once I get back from vacation!
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The capacitors can be anything from 220uF to 22,000uF. The caps after the regulators smaller (I would go with a nice low ESR 220uF maybe just 100uF) because the regulator smooths.
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Perfect. Also one question on schematic. I can't quite tell if there is a connection from the center-tap on the transformer to the center of the rectifier circuit or not.
Should it be this?
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi1248.photobucket.com%2Falbums%2Fhh493%2Fbeaugagne%2Frectifiercircuit1.jpg&hash=f384894721db3dd79760bef7bae40676fcd69289)
Or this?
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi1248.photobucket.com%2Falbums%2Fhh493%2Fbeaugagne%2Frectifiercircuit2.jpg&hash=926b4438744633aa5a6ad4b4452cc465bed721f3)
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The first one.
You are right, the schematic is wrong. I should have the line to the center of the two bridges eliminated. I'll correct it and change the link. (OOOoooo I get to use my erasing shield again after all these years, I was trained on a drawing board, then Intergraph, Cadam and AutoCad).
Thanks!
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Well I think you have it on there correctly, but looks like there might be some eraser marks there and it made it difficult to tell. No biggy thanks for the quick reply and of course thank you for drawing up the schematic for me in the first place!
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Is this along the lines of what I'm looking for in a transformer?
http://www.digikey.com/product-detail/en/241-5-24/595-1056-ND/952926
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You only need one bridge (four diodes).
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I'm no EE so could you elaborate on that a bit? I'm going off of Grainger's design here using a 12-0-12 transformer and my knowledge bank is pretty empty beyond understanding some BASIC (just enough to troubleshoot simple circuits) DC theory and reading schematics.
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Paul is saying you can use a 24V transformer and create a virtual ground. A pair of matched 1/2 Meg ohm resistors, in series between the two DC legs will create a virtual ground. Then attach the capacitors and regulators to the positive and negative rails, ground is between them.
Your transformer is 24V rated at 1/2 amp. So if that is enough current for the load it is fine.
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(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi1248.photobucket.com%2Falbums%2Fhh493%2Fbeaugagne%2Frectifierfullwave.jpg&hash=5a3e7159e42f6172db1e814e2b7241f483a3da82)
This is the design then? Are there any disadvantages to using this method to rectify instead of the two bridges?
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Ok, I looked up the unit and it uses a chip that draws 4.2mA quiescent. So any power supply you build will be more than it needs. The above transformer you linked will do well.
What you have drawn will be fine. The dual rectifiers might have more separation, might not.
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Oh wow, so I could go to a smaller transformer in an attempt to save some space then? Say 100mA or less? The 500ma would be considered overkill then most likely?
As for the bridges, maybe I'll build both and see which sounds better. I'm sure I could find a buyer for the second one :D
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The dual bridges will go up in smoke. You don't need a "virtual ground" - the center tap is the ground, assuming you are looking for +12v and -12v.
I can't believe it - I googled a half dozen different sets of keywords and couldn't find the simple design - and found lots of incorrect stuff. It's crazy.
Here's the circuit.
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Do you even need a bipolar supply? I thought the cmoy had a chip that provided a virtual ground and plus/minus rails internally to the board. I know my omega micro dac has one of these Ti split rail chips in it because the current draw is so low and it simplifies the external power supply design.
Just asking because it's been a long time since I even looked at the cmoy.
-- Jim
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I just need a 24v linear regulated PS. The specs don't specify +24v vs +12v/-12v.
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That's what I figured.
-- Jim