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PostPosted: 15 May 2010, 14:52 
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Joined: 13 Apr 2010, 10:39
Posts: 37
Location: Dallas, Texas, USA
Sounds like you're most of the way there...

You didn't say what you were using to power the amps - perhaps a recycled / reused power adapter from some now defunct gear.

Try it with a 12V gel battery: that should eliminate any possibility of hum. If it does, then you're amp is just fine: you'll just probably have to build a small, approximately 12.6 volt *regulated* power supply.

It's simple enough - a couple of 1000 uf capacitors on input, an LM317 and a few resistors (probably one of them will be a 2k or 5k trimpot so you can adjust to the right voltage you want out). You'll probably need a heatsink - as it'll draw about 400ma or so. And bypass the output with a .1uf capacitor (mylar, ceramic - doesn't matter). This should give you a CLEAN, regulated output. I built one and it makes the amp completely silent with the input audio muted.

Many of the wall-warts (A/C adaptors) are filtered, but unregulated, and will pass some form of 'ripple' which is probably the source of your 50hz Hum. Even some of the regulated laptop-type (switch-mode) supplies which ARE regulated will pass some line-frequency ripple or even some switching ripple which will show up at a higher frequency.

If it still picks up hum while ON a battery - then you might be picking up some strong, nearby A/C hum via induction. Some sources might be fluorescent lighting, a (HUGE!) transformer nearby, nearby power lines, or some other electrical source.

Mike Y
Dallas, Texas
J&B wrote:
Me and a friend built this amplifier and it worked first time, but there is a horrible hum. To me it sounds like a 50Hz hum (we live in Europe) but we're not feeding 50Hz into the amp. Anyway, the volume control controls the level of the music but the hum is constant. It's the same on both our amps and on both channels. Anybody's got any idea of what's wrong? I have tried two different 12V sources and it's the same with both, I haven't tried a battery so far though.


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PostPosted: 18 May 2010, 16:08 
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Joined: 18 May 2010, 14:57
Posts: 1
can you explain how you made volume control?


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PostPosted: 18 May 2010, 23:29 
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Joined: 15 May 2010, 10:01
Posts: 6
Hey

I bought a new 12V supply that the guy in the electronics store said would be good, but still the hum. My friend got his working with a better power supply so I will test mine with it tomorrow. I'm starting to suspect it might be induced after all. I think I'm going to try with 8 flash light batteries as well, just to make sure.

paere wrote:
can you explain how you made volume control?

The volume control is simply put on the in signal. One pin from the jack, one pin to the PCB and one pin to ground. I have a 2 channel pot so I control left and right side at the same time. That's basically it.

Cheers

/Jonas


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PostPosted: 19 May 2010, 09:51 
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Joined: 03 Aug 2008, 20:22
Posts: 386
Location: Denver, Colorado
Paere,

Welcome, an "audio taper" volume potentiometer is best for use in this amplifier. You can find them in different sizes and values. Do not use a linear taper potentiometer as it will not track well. Here is a great tutorial on potentiometers. http://sound.westhost.com/pots.htm


Jonas, A gel cel battery (like that from a computer ups) will be best as you can recharge it. If not you can use two 6 volt lantern batteries in series to test the unit. It will last a few hours but when the batteries are depleted you will have to throw them away.

Also most thrift stores have wallwarts, you must look for a switch mode type. These are usually for printers or other business machines. They usually have the power supply in the middle of the cord, vs at the wall.


Keep up the great work everyone, thanks for all the responses. Dont forget to post pics when you are ready.

Thanks

Rogers


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PostPosted: 20 May 2010, 12:37 
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Joined: 15 May 2010, 10:01
Posts: 6
Hi everybody!

I'm extremely happy today because the amp works! It turned out I have crappy power supplies, even the one the elecronics guy recomended. I tested with my friends power supply and it works great. Both chanels work and no magic smoke. :) (We got some smoke from my friend's amp when he mixed up + and - on the power. :) )

Anyway, I will finish the mechanics and then if you're interested we can post some pics.

Cheers!

/Jonas


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PostPosted: 20 May 2010, 12:46 
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Joined: 13 Apr 2010, 10:39
Posts: 37
Location: Dallas, Texas, USA
Heh - very cool.
Well, they're probably good supplies; they're just not clean and regulated.

I almost always include a Schottky diode in series with the supply to prevent misapplication of power (-/+ becoming +/-).
Like a 1N5817, a 20V, 1A diode. I keep a few on hand...

Mike Y
J&B wrote:
I'm extremely happy today because the amp works! It turned out I have crappy power supplies, ...


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PostPosted: 21 May 2010, 13:46 
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Joined: 21 May 2010, 13:43
Posts: 5
Hello,

I am looking at building this as my first tube amp, but I dont like the idea of using a pre made psu and I also dont like the idea of the battery from a ups. I would like to learn about power supply building for tube amps as I would like to learn as much as possible. Is there a good power supply for this amp? DIY style?


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PostPosted: 21 May 2010, 18:50 
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Joined: 28 May 2008, 21:53
Posts: 4579
Location: Winnipeg, CANADA
Yes, it could be as simple as DC wallwart, 7812 regulator and a couple of caps. Or you can use a LM317 voltage regulator and set the output somewhere between 12 and 12.6V.

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PostPosted: 21 May 2010, 21:32 
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Joined: 03 Aug 2008, 20:22
Posts: 386
Location: Denver, Colorado
CL

Welcome to the forum,

This amp design is in quite a niche of tube amps, as it is low voltage. You get some tube sound and satisfaction of building and learning tube circuits. The power supply can be as easy as Gio stated, as we are only making 12.6 volts. A standard wallwart or transformer from radio shack.

Here is a link to building power supplies and a program to download. http://www.duncanamps.com/psud2/ . Tube amps generally have a B+ of over 200 volts so RC RLC circuits are very important.

If your heart is set on building a 12v ps , tangent has one http://tangentsoft.net/elec/teps/ and so does amb http://www.amb.org/audio/sigma11/ the amb shop has the pcb for sale.


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PostPosted: 25 May 2010, 22:22 
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Joined: 21 May 2010, 13:43
Posts: 5
So I finished the build tonight and powered up the amp and the 10ohm resistor on the right channel immediately started smoking. I checked all my solder joints and the circuit nothing looks out of place, any ideas on what could cause this?


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