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 Post subject: simple 12v power supply
PostPosted: 07 Mar 2018, 22:48 
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Joined: 14 Nov 2008, 09:09
Posts: 26
Hi Group -
I'm interested in making this simple Velleman kit - pre amp K8084
https://www.vellemanusa.com/downloads/0 ... _k8084.pdf
I'm a bit fuzzy on the power supply -- do I just need to add the 12v - 0 - 12v transformer?


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PostPosted: 08 Mar 2018, 15:01 
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Joined: 04 Jun 2008, 20:59
Posts: 3854
Location: Arizona, USA
Hi, It appears all you need is a transformer that puts out 12-0-12 vac at 100 ma (AKA 24vct). It should be easy to source. I would be sure to hooke it up with a fuse and power switch. Personally I would use an IEC power entry connector with built in EMI filtering and the fuse. If you search around they are not expensive. Use a metal case for the project for best noise and hum rejection.

Good listening
Bruce

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PostPosted: 08 Mar 2018, 20:09 
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Joined: 14 Nov 2008, 09:09
Posts: 26
cool! Thank you -
from looking at the schem, I see there is a rectifier and filter caps - but the cap values seem a bit low - would it be worth putting a full power supply (transformer, bridge rectifier, regulator and filter caps) in front of the kit pcb?
Reed


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PostPosted: 09 Mar 2018, 01:36 
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Joined: 09 Oct 2012, 19:43
Posts: 318
Location: Vancouver Canada
I would not. At least not before it is built and listened to. You can always do that later if need be. If you can't find a 24vCT / 100mA trans but find one larger great as you could use the extra later to drive relays etc for input source selection. It just depends on your plans. The rec diodes look fine. Standard 1N4000 type good for 1 Amp. C21 and C22 could be greatly increased in size to 1,000 or maybe 2,000uF / 35v by mounting them laying horizontal on the board. C22 to the left and C21 downwards to the front. Some hot melt glue and you are secure. Nothing on that board will be producing any heat so that is not a problem.
Buy the kit and a trans, build it and experiment. How close can you place the trans before hum starts getting injected. Which physical orientation of trans and board is the quietest. What else are you going to do with the kit? Expand your input capability to many more by using relays. What is the "sound" of the amp. Do you like it. Then put it in a box that suits what you have come up with, and yes, use an IEC connector with EMI filter. They come with sw / fuse / filter / and the standard IEC connector in one package. And a bit of looking and they are now very reasonable. Being able to unplug the cord if it is going to be portable is often helpful for packaging.


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PostPosted: 09 Mar 2018, 08:18 
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Joined: 14 Nov 2008, 09:09
Posts: 26
excellent! Thanks for the input! I will get started with the kit and 12 - 0 -12 transformer and let you know how I fare --
Thanks guys!
Reed


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PostPosted: 21 Mar 2018, 07:20 
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Joined: 14 Nov 2008, 09:09
Posts: 26
got the kit yesterday in the mail - along with a 12 - 0 - 12 transformer --
really nice kit --

for an enclosure, I'm thinking of using a cigar box -- if I cut pieces of thin flexible metal, would that prevent hum from nearby emfs?

the manual recommends keeping the transformer as far away as possible -- would it make sense to enclose the transformer in another small electrical work box (like what you would use for household wiring) and just run a power cable from that to the preamp enclosure?

I like the idea of using male EIC connector on the preamp enclosure --

if the cigar box with metal shielding isn't a good idea, I could use the cabinet from another shielded device that already has a EIC connector in it --

What do you think?

Thanks guys!
Reed


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PostPosted: 22 Mar 2018, 05:42 
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Joined: 09 Oct 2012, 19:43
Posts: 318
Location: Vancouver Canada
You have the board, parts, and power transformer. So build it, listen to it, and experiment with how close the trans could be to the board before you actually select and build the cabinet. You may want to add some features into the cabinet like extra inputs and their selector switch etc.


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PostPosted: 22 Mar 2018, 19:26 
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Joined: 14 Nov 2008, 09:09
Posts: 26
yes - unfortunately, I assembled and tested but had a very bad humm - I'm guessing it was because the device was not grounded --

I didn't realize that it was a ground issue until later -- my initial though was that the IC chips were in backwards - so I flipped them which had the anticipated results...

now I'm trying to source 2 new NE5532P chips and BC547/BC557 transistors


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PostPosted: 14 Jul 2018, 23:52 
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Joined: 14 Jul 2018, 21:06
Posts: 2
Doing the same thing as you. Building the K8084 preamp and K4003 amp. What transformer did you buy and can I buy one transformer to drive both amp and pre-amp or do I need two. Also where did you get the fuse assembly for power cord. Thanks


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