mike11298

Hey everyone. I jost got a kit from my local DSE store. It is an LM833 opamp based audio control board which has volume, treble, bass and balance controls. Attached are the photos of the finished product, as well as the schematic.

So far, I have tested it - and to some degree, it works. The volume, bass, trebble and balance controls work well on the LEFT chanel. They also work on the right chanel - but there is a lot of buzz. There is also buzz on the left chanel as well. There is also a bit of hash noise as well.

I have tested it so far for a couple of things. Firstly, I checked my solder joints. I resoldered most of them. It didn't fix it, so i prodded them all with a non conducting pen and couldn;t find any bad joints.

Next, I had a look at the voltages on the audio out pins. It is my understanding that they should be under 50mV or so.
When I switch the system on, the output voltages begin at 3v max, then reduce to about 8mV after a few seconds. - To my understanding, this is OK.

Next, because it is buzz, I checked the AC voltage at the outputs. A few volts at the start, as with the DC, but then goes out to zero.

Last in my thinking is the power supply. Yes, it is icky and the ground is a bit dirty, but I know that (by using the supply with 2x 20W amps) it doesn't cause too much of a problem.

So, what to do? Any help, suggestions or tips much appriciated. Thanks.

Attached Images

	top.jpg (557.7 KB, 14 views)
	solder side.jpg (910.7 KB, 10 views)

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-michael11298

mike11298

...and here's the schematic...

Attached Images

schematic.jpg (41.8 KB, 19 views)

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-michael11298

Nigel Goodwin

It's probably due to either your supply, or how you've connected it, you may have caused an earth loop, which causes the hum.

Post a picture of the complete project as you've connected it.

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mike11298

its a real super-buzz not really earth loop standard, IMHO. i'll post tomorrow anyway. goodnight for now

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k7elp60

I looked at the bottom of the board,the attached picture has some red circles on the bottom of the board. They could be solder bridges or some other areas, you might want to check them.

Attached Images

bottom.jpg (649.7 KB, 13 views)

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mike11298

thanks k7elp60, but all the bridges are intentional / just camera glare off the flux.

Nigel> I used my trusty DMM and tracked the fault to a dodgey OP AMP. I replaced it and it is working ok, the only problem to fix it the power supply, which is causing a problem with this preamp (inducing buzz - not ground loops though, IMHO). Now, I did have a look, but adding more high-value capacitors adds more buzz, but changes the tone of it to be lower. I Dont have a scope, but i may, if im really lucky, be able to get one to determine the source of the buzz.

Any suggestions?

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mike11298

Nigel > what about "capacitor ripple current'? I've been reading up on power supply design and see that this might explain it. Is that true?

Despite the adding-capacitance-makes-more-buzz problem, It seems appropriate to load up with about 20,000uF of caps for a 60V, 80W system.

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Nigel Goodwin

Like I said before, post a picture of how it's all assembled, it may be a problem there.

For an 80W amplifier 4700uF is probably enough, but the more the better.

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PIC programmer software, and PIC Tutorials at:
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mike11298

Im not too sure - heres an edited pic anyway. in real life, everything is connected exactly as marked using aligator clips. Votages at the actual device have been tested and are OK.

Note that any conncections in the picture are (to my knowledge and instructions) not needed. No label/circled connection means nothing connects here in real life.
Another note: "GND" connects directly to system star ground. The PCB also uses a star ground.
Thanks all

Attached Images

connect.jpg (377.7 KB, 6 views)

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mike11298

Also, about the capacitor hum problem - solved - I was connecting the cap's ground to star ground. Moving it to earth ground (although they are actually eventually connected) fixed the hum. (but not on the preamp.)
I also tried the new capacitor on the preamps' power lines, but the buzz is still there, so contrary to my original thoughts, it does not seem to be supply ripple or problems.

If this is confusing: My previous post's problem (buzz in preamp) is still there and does not seem to be a power supply problem. The power supply is now ripply free.

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-michael11298

Nigel Goodwin

For the third time, post a picture of the complete assembly, it's probably how it's connected up.

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PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

mneary

Is it in a metal (or screened) case? Most high gain things can pick up hum when sitting on the table.

mike11298

Thanks everyone fro your input - I have fixed it. The cause was ALL the opamps were bad for some reason. I must have fried them or something - its too big of a coincidence! anyway - the whole system is nearly dead-quiet. I'm quite happy with the results.

Thanks for everyone's input for my project.

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-michael11298