Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on September 24, 2012, 02:30:24 pm

Title: source of noise
Post by: Leevi on September 24, 2012, 02:30:24 pm

I have built an amp using using the schematic below.
There is a weird voltage divider (470k/470k) between 1st and 2nd amplification stage which causes
noise even if the volume is off. I tried to place the divider close to the tube pin but it is
still very microphonic and sensitive for disturbances. I also tried to connect it with a shielded cable but it didn't help.
Is it better to change the values little bit in order to get rid of the noise? Do I loose
something soundwise if I change the values?

http://ampgarage.com/forum/files/x10_schematic_486.pdf (http://ampgarage.com/forum/files/x10_schematic_486.pdf)

/Leevi

Title: Re: source of noise
Post by: phsyconoodler on September 24, 2012, 02:46:56 pm

There really is no way for that attenuated voltage divider to make noise.However,if the preamp make noise it may SEEM like you have isolated the noise there.
  And also inpossible for it to be the source of the nosie with the volume off.You have noise in the preamp that needs to be addressed.

Title: Re: source of noise
Post by: HotBluePlates on September 24, 2012, 08:58:58 pm

Quote from: Leevi on September 24, 2012, 02:30:24 pm

... I tried to place the divider close to the tube pin but it is still very microphonic and sensitive for disturbances. ...

Resistors can be microphonic. So can caps. And tubes...

I'm assuming you noticed the noise while prodding the area, and it seems to isolate to the resistors. They could be bad, but the mechanical action of prodding those resistors could be flexing other things in the area (even possibly a wire running to the tube grid).

You might try replacing those resistors, or the tubes nearby or caps. Or even using stranded wire instead of solid wire in the preamp.

Title: Re: source of noise
Post by: Leevi on September 25, 2012, 12:33:52 am

Quote

There really is no way for that attenuated voltage divider to make noise.However,if the preamp make noise it may SEEM like you have isolated the noise there.

The voltage divider works here like a volume pot which is open e.g. 12 o'clock position.
The circuit is then very sensitive for all kind of dirsturbances. The situation is same if
you use an empty plug in input jack and increase the volume.

You are right, it's not the source but it makes the admittance possible.

Quote

I'm assuming you noticed the noise while prodding the area, and it seems to isolate to the resistors.

Exactly!

/Leevi