G9 problem

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maxkriza

Well-known member
Joined
Sep 18, 2004
Messages
68
Location
uk
Hi to all
just finished a point to point g9 pre and I'm having some hum coming and going at about 10-20 sec cycle , between -70db and -50db . heaters are 12v dc from a seperate regulated PS. Strange thing , its not a regular cycle . will apriciate any advice
thanks
Max
 
never had anything like that...

Check proper grounding and that chassis is grounded all the way.

Check that nothing "external" disturbs it - like power transformers, computers or mobile phones..

Jakob E.
 
thank you jakob , thats a great sounding pre .
what is a reasonable hum level for a point to point version , 12v DC heaters . at the moment I'm getting about -60Db with gain and volume fully open , is there a way to reduce that hum?
thank you
max
 
Does the regulator have a good heatsink it kind of sound like a thermal circuit cutting in and out. Regulators can do strange things without heatsinks.
 
[quote author="maxkriza"]..at the moment I'm getting about -60Db with gain and volume fully open[/quote]

Hmm.. at max., the G9 has some 55-60dB gain.

.. -60dB residual noise gives you 115-120dB equivalent input noise...!

..is there a way to reduce that..
Click to expand...

Work on grounding, and you may be able to squeeze another couple of dB's out of it. Being a p-t-p wired unit, there is little I can help with regarding this..

Jakob E.
 
Hi
the heater regulator (external) is a 317t and its got a reasonable size heat sink . the HT reg doesnt get too hot and got a basic metal heatsink.
100hz hum level is -50db to-60db regardless of output level .
thanks
max
 
Is your heater supply connected to ground, or is it floating?

Best regards,

Mikkel C. Simonsen
 
Heater is conected to a star point which is at the ground of first filter cap on the power supply. I also suspect now that the fluctuating hum problem
originate in the power supply but I'm not sure.
thank you
Max
 
I got the same problem when i was testing my G9 but i haven't checked i a box yet...

Have you fixed the problem?
 
hi
Cant say I fixed it but I've changed the grounding to star ground and tidy up the heater wires , it does seem a bit better , or maybe I'm getting used to the hum , its more evident when I connect it to my pc-scope , after a few minutes I observe the 100hz level going between -55db to about -70db .
apart from that it sounds great
cheers
Max
 
Are you sure that this isn't a side-effect of having G9 close to the PC and/or grounding issues through the pc-scope?

The transformer-coupled circuits can be very sensitive to external magnetic noise fields.

Try listening to it with/without the PC and pc-scope around..

Jakob E.
 
Hi
I can still hear some fluctuating hum when I connect it to my DAT in another room, but its not too bad . now I'm having a new phantom power problem-when I connect a mic (C451), the 48volt drops to 12.5volts on XLR pins 2&3 , dont understand why . I did not use the G9 phantom circuit and at the moment it is supplying only one channel .
thanks for help
Max
 
The voltage at the pins is always pulled down by the mic. As long as you still have 48V at the other end of the 6.8K resistors, this is absolutely normal.

100Hz suggests insufficient reservoir capacitance.

Keith
 

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