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ianc13 19th November 2009 10:03 PM

Hum in phono pre - help please
 
I have just reworked my phono pre due to some really bad hiss issues that I had, but am now suffering from hum problems - no hiss though thankfully.

The phono amp is MM at the moment with a standard 47K input impeadence and sits in front of a TVC. If I change the volume on the TVC then the volume of the hum changes in line with the volume.

Now while there is prominent hum when I have the TT connected - with and without the ground wire connected - the phono stage is totally silent when I short the inputs.

The phono amp schematic can be found here ecp.cc and the phono stage and psu are in seperate enclosures - roughly 1 meter apart. The B+ and filament supplies are both regulated, however I really do not think that the hum is coming in from the PSU as pointed out previously it is absolutely quiet when the input is shorted.

Any ideas what I can try, should be looking for to resolve this? Long term I will add MC transformers, would these help isolate the TT from the phono stage?

Thanks in advance, Ian

coldcathode 19th November 2009 11:25 PM

Noise sounds like it is coming from the turntable.

Do you have another way to amp the turntable? An integrated receiver maybe? Try listening there and see if it is the TT.

The hum sounds like noise coming in the TT cables? Or from the Turntables Transformer?

Arnulf 20th November 2009 08:05 AM

Quote:

Originally Posted by ianc13 (Post 1986741)
Now while there is prominent hum when I have the TT connected - with and without the ground wire connected - the phono stage is totally silent when I short the inputs.

Did your rework include changing the layout of cable connecting your TT and the input of your phono stage ? This includes the internal connection from input connector to the grid resistors.

I'd try as short a leg of shielded twisted pair (FTP or STP) cable between the two as possible - if hum goes down or dissapears completely you've found your culprit. Shielding (foil) should be grounded at one end.

Quote:

Any ideas what I can try, should be looking for to resolve this? Long term I will add MC transformers, would these help isolate the TT from the phono stage?
No; if you inject hum into transformer (from your TT andor cable conencting the two) it'll show up on the other side as well. Audible hum is within audio range and transformer is supposed to carry through all audible signals with as little distortion as possible, ergo it will carry through hum as well as music. You need to identify and eliminate the source of hum or inject it in counterphase to cancel it out.

ianc13 20th November 2009 10:12 AM

The strange thing is that I have been running this TT into another phone stage with no hum problems at all!

The difference being that the old phono stage is configured for MC with a step up at the front and purely for testing purposes I took out my MC cartridge and replaced with a cheap MM to make sure that the new phono stage was working properly.

So I have no problems running MC into a stepup xformer on my old phono stage but do have problems running MM directly into my new phono, however the new phono is silent with the input grounded.

If I had a balanced connection, i.e. centre tap to ground, into the step up xformer on my old phono and the hum were from the cabling between the TT and phono stage then would the not the common mode hum be cancelled out?

I will have a good look at the cabling on my TT tonight and see if moving any of the wires around changes the aplitude of the hum.

Thanks for the ideas.


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