[matheprat]

I've finally put everything together. All the voltages are fine (+-12, 12.6, 40, 40), but when the volume pot is at anything other than full volume there is a really load buzzing in both channels. The buzzing doesn't change volume with the music volume (it's always the same level no matter if the source is loud or silent). The only time it disappears is at full volume.
I've had a search here and on Headwize but I can't find anything to do with my problem. Does anyone have any ideas as to what I can do about it?
Cheers, Mat.

 

[ericj]

that's kinda odd. you sure the pot is wired correctly?

100k pot? does it have a metal chassy that needs grounded?

 

[tomb]

How did you connect the transformer? If you wired the house ground into the center tap it can cause a lot of problems, perhaps similar to what you're reporting.

 

[matheprat]

I'm pretty sure the pot is in correctly. It does change the volume of the music. My case is plastic so it doesn't need insulating. It's a 47k pot, rather than 100k. I thought that was an acceptable substitution?
I did wire the centre tap of the transformer to the house ground, should i not have done? The mains earth is also connected to ground through a resistor and 2 diode network.
Many thanks, Mat.

 

[tomb]

Wiring house ground to the center tap is a no-no. The SOHA output is a virtual ground created by the two regulators. If this is referenced to the house ground through the input/output jacks, you will have a big buzz.

 

[matheprat]

Thanks for the reply. I built it following this schematic:
http://headwize.com/projects/images/cavalli2_fig5.png
which shows the centre of the transformer connected to ground. You're saying this should just be left not connected to anything?
Many thanks.

 

[Yen]

Quote:

Originally Posted by matheprat /img/forum/go_quote.gif Thanks for the reply. I built it following this schematic: http://headwize.com/projects/images/cavalli2_fig5.png which shows the centre of the transformer connected to ground. You're saying this should just be left not connected to anything? Many thanks.

That's not the center tap what is connected to the ground, but secondaries' ground windings...

Please take some pics of your build?

 

[matheprat]

My transformer has 4 wires out, 0, 15, 0, 15. I connected the middle 0, to the middle 15, and that's what I connected to ground in the circuit. The other 0 and 15 are going into the first bridge. Was this not correct?
Some rather dark and out of focus photos, the best I can get for now:

http://img90.imageshack.us/my.php?im...ge00030cp3.jpg
http://img153.imageshack.us/my.php?i...ge00031wl8.jpg
http://img442.imageshack.us/my.php?i...ge00035kf3.jpg

I built it on stripboard rather than the proper one. You can't see anything in the pictures, and even if you could I think it would be quite hard to work out how it's all connected together.
Many thanks.

 

[tomb]

That's not what we're talking about. If you connected the middle 0 and the middle 15 together with the ground plug in your wall outlet - that's bad and will buzz.

However, your post may have revealed a different source for the problem:
The two zero's should be connected together and to the Gnd on the board. The two 15's should be kept separate and connected to AC1 and AC2, respectively. Some transformer's windings do not have both coil's beginning turns in the middle - sounds like yours is one of those.

Agreed - photos will help.

 

[matheprat]

The diagram of the output of my transformer is as follows:

|----- black
| sec 1 15v~
|----- red

|----- orange
| sec 2 15v~
|----- yellow

The datasheet is here: http://www.rapidonline.com/netalogue/specs/88-2500e.pdf

I connected the red and orange together, and then connected them to the ground of the circuit. The black and yellow are going into the bridge. The earth from the wall is connected to the ground of the circuit by a 10 ohm 2 watt reisitor, and 2 diodes in opposite directions, all in parallel. The buzzing is the same whether the earth is connected to the ground in this way, or not connected at all.

 

[Yen]

Quote:

Originally Posted by matheprat /img/forum/go_quote.gif The diagram of the output of my transformer is as follows: |----- black | sec 1 15v~ |----- red |----- orange | sec 2 15v~ |----- yellow The datasheet is here: http://www.rapidonline.com/netalogue/specs/88-2500e.pdf I connected the red and orange together, and then connected them to the ground of the circuit. The black and yellow are going into the bridge. The earth from the wall is connected to the ground of the circuit by a 10 ohm 2 watt reisitor, and 2 diodes in opposite directions, all in parallel. The buzzing is the same whether the earth is connected to the ground in this way, or not connected at all.

You have got it all wrong. You should connect the both 0s to the ground, and 15V lines like the schematic to the bridges. That schematic is for 15-0-15, and that's why there are only 1 ground. For 15-0, 15-0, connect both 0 to ground and separate 15Vs like the schem.

 

[matheprat]

Thanks for your reply.
So the yellow and red lines should be connected together and to ground, and the black and orange to the bridge?
I'll give it a go and then let you know what happens

Thanks for all the replys, much appreciated.

 

[Yen]

Quote:

Originally Posted by matheprat /img/forum/go_quote.gif Thanks for your reply. So the yellow and red lines should be connected together and to ground, and the black and orange to the bridge?

No.

My old pic was wrong. I was too lazy to look at Datasheets... My mistake. New picture added according to datasheet.

GJ FallenAngel.

 

[FallenAngel]

No no no buddy

Black + Orange to ground, Red to AC1 and Yellow to AC2

Black = 0
Red = V
Orange = 0
Yellow = V

Black & Orange are 0V grounds, Red and Yellow are voltage.

 

[matheprat]

Cheers for the help! I wired up the transformer as you suggested and the hum is still there as before, except now it's still there when the pot is at full volume. For some reason, I now can no longer get 40v on the tube, I can dial the trimmers to a maximum of about 35v. The heater voltage, and the chip voltage are both still correct.
Many thanks.