[Yen]

Quote:

Originally Posted by matheprat /img/forum/go_quote.gif 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.

I have got the hum with the pot at max position (actually turning over 13-14 will hum and gets louder until max). I think that's ok and normal. Within the normal usage range (7-13), my SOHA is dead silent. However, I am using the digi's PCB so depending on your choises, I dont how much it differs with your build.

 

[matheprat]

Sorry for not being clear. I meant the hum is the same as before, but now occurs all the time, regardless of the position of the pot.

 

[Yen]

Quote:

Originally Posted by matheprat /img/forum/go_quote.gif Sorry for not being clear. I meant the hum is the same as before, but now occurs all the time, regardless of the position of the pot.

Do you still have your wall earth connected with the circuit? Try disconnecting it.

As you can see from my build, I left the earth pad of my IEC connector float.

 

[tomb]

Confirm your CCS resistors if you are using the LND150 - R7 must be switched with R8, R17 with R18. Your earlier mis-wiring of the transformer may have produced more voltage and masked an underlying problem with the CCS. If you don't have them switched, the bias will be too low and cannot be adjusted high enough.

Again, photos would help.

 

[matheprat]

I'm using J112 JFETS, with a resistor value of 2.2k. However, before the JFETS the B+ value was about 40v, not the 55-60 suggested by the schematic.
Removing the earth connection has no effect on anything.

 

[tomb]

If that's true, then you may still have the transformer wiring messed up. What is your voltage at AC1 and AC2?

 

[matheprat]

Before the bridge they are 17.6v AC on each rail. After the bridge, 19.6 and -23v DC. B+ before the JFETS is 35.5v. The transformer is deffinately wired up as per your previous suggestion.

 

[tomb]

Well, the AC1 and AC2 sound right. However, I'm unclear of those voltages after the rectifier - where exactly did you measure them. Is the 35.5V what you measure at the "+60V" position on the board?

I wondering if you have some bad caps. That would cause the noise, too.

 

[matheprat]

The voltages after the rectifier were measured between the + pin of the rectifier and ground (19.6v DC), and between the negative pin and ground (-23v DC). I measured the 35.5v where from the schematic says it should be +55-60v, basically between the 2.2k resistor and the CCS.
How do I know if I have a bad cap? Which are likely to be bad to cause the hum?

 

[Yen]

Quote:

Originally Posted by matheprat /img/forum/go_quote.gif The voltages after the rectifier were measured between the + pin of the rectifier and ground (19.6v DC), and between the negative pin and ground (-23v DC). I measured the 35.5v where from the schematic says it should be +55-60v, basically between the 2.2k resistor and the CCS.

So you got the -12,6V right for the heaters? Also 12V and -12V for the opamp after VR1 & C7 and VR2 & C8 are correct?

You should recheck, and rerecheck the positioning of the bridge rectifiers and caps' polarities. There is definately something wrong in your, PS if you are getting 35Vs only...

35.5V is measured after C3 and before the JFet?

 

[matheprat]

Yes, 12.6 and +-12 are fine, measured from the places you suggested. 35v is measured after C3 and before the JFETS.
I'm going away for a couple of days today, so I'll have another check when I get back with a fresh head. Hopefully I've just put something in wrong. Is the transformer deffinately wired up correctly now? I just seemed to have the correct voltages before, and hum, where as now I don't have the correct voltages, but I do still have the hum.

 

[Yen]

Hmm, the datasheet you provide suggest that you have dual primary toroid? How have you hook it up?

 

[matheprat]

Blue and brown connected to AC, and grey and violet connected together (but not to anything else). I'm in the UK, so 240v AC.

 

[tomb]

Quote:

Originally Posted by matheprat /img/forum/go_quote.gif Blue and brown connected to AC, and grey and violet connected together (but not to anything else). I'm in the UK, so 240v AC.

Well, I don't use the Amveco Toroids, so someone else will need to confirm this. Your other voltages are OK - but they go through regulators anyway. Not much to confirm there unless the voltages are too low, or you have too much heat.

Yen has a good point about the bridge rectifiers, but it could be your caps, too. Seems to me that you have to have a rectifier turned wrong, a bad cap, a reversed cap, or too small a cap somewhere. BR1 and C4 (the bootstrap) are the really important ones for B+, I think.

 

[ferencvaros]

I assembled my soha and I have similar problem: but, when I use source, which jacks are grounded (between RCA and AC ground is ~0 ohm), there is not buzz, my soha is dead quiet, but when jacks are not grounded, soha is buzzing loud...

case is alluminium hammond with transformer
to the enclosure's body are connected: pot, AC ground, other parts are isolated

can somebody help me ?