BA3 pre oscillating question.

[ColinA123]

BA3 pre, oscillating question.

I am having a strange (to me) problem.
BA3 Pre feeding F6. both bog standard.
Genuine jfets from Spencer, BA3 using 2SJ313 & 2SK2013 (Genuine.)


There were no faults when feeding the F6 from a 10K passive.
With the BA3, there is occasionally a low level whistle in both channels.
(Not hum or mains frequency, higher pitched.)

This does not alter if the volume is altered.


Moving wires, and earth points makes no difference, but the noise does just fade in, and out on it's own.


My guess is that the BA3 is oscillating, or picking up noise from somewhere but I can not find anything with a scope when it is happening.


Any clues?
This BA3

 

[Zen Mod]

put 10K resistor from D point to GND

 

[ColinA123]

Thanks, will try that.

 

[ColinA123]

OK,
System has been playing for several hours now,
Noise very much quieter, now only audible with ear right next to the speaker.
Seems stable.
Thanks Zen Mod.

 

[jean-paul]

Pass designs generally don't have input filtering so when their bandwidth is quite wide they will amplify non audio signals they might pick up. You can add an RC filter at each input just in case. When a volume potentiometer is there it is best to put the input filter before that potentiometer otherwise a variable filter has been created.

Just connect a 1 kOhm resistor to "+in" and also a 1000 pF silver mica or film cap from "+in" to GND. This, of course, at both channels.

 

[McQuaide]

Pass designs generally don't have input filtering so when their bandwidth is quite wide they will amplify non audio signals they might pick up. You can add an RC filter at each input just in case. When a volume potentiometer is there it is best to put the input filter before that potentiometer otherwise a variable filter has been created.

Just connect a 1 kOhm resistor to "+in" and also a 1000 pF silver mica or film cap from "+in" to GND. This, of course, at both channels.

Are you referring to a Zobel with the cap and resistor in series?

 

[Zen Mod]

resistor in line with signal , then cap as shunt to GND