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BA312 replacements vrs original IC - volume linearity

429cobraman

429cobraman

New Member
Hi All, Wanted to see if anyone had noticed this characteristic before. I had refurbished a 2275 for a friend and had to replace the BA312's on the PE01 board with the mini board with 3 discrete transistors and 4 resistors. He's been noticing that he has to turn the volume up more than before and it seems to hit a steep takeoff just past center. By co-incidence I had another 2275 on the bench with good original BA312's in it so did some data taking. With a scope I can see a pretty consistent increase in amplitude on Pin 2 input of the BA312 as I increase the volume. But on the output Pin 6, it tracks up to just past mid point of the volume and then seems to hesitate, or hit a flat spot for a minute, and then takes off to the moon. Frequency response and channel to channel is on the mark. Its definitely the BA312 replacement board and although not a failure per sey does create a user issue unless you are used to the control. I've ordered a set of Sparkos Labs replacements to check them out. Any thoughts?2275 BA312 data.jpg
 
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When it comes to the BA312 I first try replacing its bias resistor (there is a pinned thread about it), and if if fails I replace with a new BA312 - they are still out there. I never used discrete components.
 
The volume pots do have a "dead" zone at midpoint where the resistance doesn't change much. I have no clue as to if that is intended or coincidental. It makes the midpoint not as critical for balance and tone controls, not that this is an issue!
 
volume pots are "audio taper / Log" controls so I expect the curve to match the blue one. The dead spot is thru the discrete BA312 replacement. I can monitor the input and see it going up but the output stalls for a second around mid range and then is a very steep curve - like its trying to catch up. Again nothing is out of spec but if you weren't expecting the sudden surge it would blow your ears. Just wondering if anyone else has experienced this.
I also looked at 4 discreet pots for resistance over travel. No flat spots250k volume pot resistance.jpg
 
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