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BA3 pre oscillating question.
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Old 4th December 2019, 02:03 PM   #1
ColinA123 is offline ColinA123  United Kingdom
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Default 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
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Last edited by ColinA123; 4th December 2019 at 02:20 PM.
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Old 4th December 2019, 02:44 PM   #2
Zen Mod is offline Zen Mod  Serbia
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BA3 pre oscillating question.
put 10K resistor from D point to GND
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Old 4th December 2019, 04:23 PM   #3
ColinA123 is offline ColinA123  United Kingdom
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Thanks, will try that.
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Old 4th December 2019, 07:52 PM   #4
ColinA123 is offline ColinA123  United Kingdom
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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.
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Old 4th December 2019, 07:55 PM   #5
jean-paul is offline jean-paul  Netherlands
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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.
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Last edited by jean-paul; 4th December 2019 at 08:01 PM.
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Old 5th December 2019, 03:58 PM   #6
McQuaide is offline McQuaide  United States
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Quote:
Originally Posted by jean-paul View Post
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?
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Old 5th December 2019, 04:59 PM   #7
Zen Mod is offline Zen Mod  Serbia
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BA3 pre oscillating question.
resistor in line with signal , then cap as shunt to GND
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