I put my BA-3FE on the bench today to take a reading of the DC offset. It's been 5 months since I've had the cover off and to my amazement offset on both channels was ~-150ma DC (this is after warming up for 30 minutes). I've been running the the BA-3FE without output caps and using input transformers on my F5. My questions at this point:
1) What would cause the DC offset to increase by so much?
2) Have I damaged the input transformers?
My plan is to put the output caps back in, reset bias and offset, and listen to it with the input transformers in the amp. Then listen to it with the input transformers removed from the amp.
The reason I started poking around in the BA-3FE is because my system doesn't sound as full and with less bass.
1) What would cause the DC offset to increase by so much?
2) Have I damaged the input transformers?
My plan is to put the output caps back in, reset bias and offset, and listen to it with the input transformers in the amp. Then listen to it with the input transformers removed from the amp.
The reason I started poking around in the BA-3FE is because my system doesn't sound as full and with less bass.
I think this is quite normal. I don't think I have ever seen mine stay within 100ma of "0" for any prolonged period. Temp variation is the issue. Put a finger on a mosfet and watch it drift.
It's meant to be like this and it's why it has a cap on output.
I don't know if you have damaged the transformers, but if something catastrophic happened to the BA-3 you would likely see a lot more than 150ma offset.
It's meant to be like this and it's why it has a cap on output.
I don't know if you have damaged the transformers, but if something catastrophic happened to the BA-3 you would likely see a lot more than 150ma offset.
Probably wasn't winter with heat going on and off, etc? Maybe temp or air circulation was more stable?
I have umbilicals running out so I can test mine with the case covers on. The difference in DC offset between covers on and off is pretty large. The difference in DC offset when on my kitchen table vs. on my stereo rack (under a hot F5) is large.
I don't know, the heatsinks are pretty warm on mine...maybe you have better thermal conditions.
I have umbilicals running out so I can test mine with the case covers on. The difference in DC offset between covers on and off is pretty large. The difference in DC offset when on my kitchen table vs. on my stereo rack (under a hot F5) is large.
I don't know, the heatsinks are pretty warm on mine...maybe you have better thermal conditions.
Kevin, was the 150mv offset with the preamp cold or hot and at what room temp?
The offset issue has also vexed me for several months. I cant understand why a fully warmed up BA3pre in the same environment ie same room temp, no heat blowing etc displays different offset readings. Just when you think you got it right for a couple of days it shoots off by greater than 25mv suddenly a few days later. In my case being balanced each phase effects the other to compound the situation. Sure would like a better understanding as to why this happens. But it sounds wonderful!
I dont think 150mv will do any damage to the JT11P1 that you have and if slightly magnetized will demagnetize as the offset is removed. Call Dave Hill at Jensen if you still feel uncomfortable.
nash
The first BA3 FE I built gave me fits setting the offset....until I moved the work out from under a ceiling fan! Doh!
Russellc
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Why don't someone design a servo for the BA-3 or has someone already designed one?
Two things I don't like about the BA-3 are the 10uF coupling caps and the overly sensitive trimmer adjustments.
Here we go. Some progress.
The pre seems to bias nicely even with random unmatched 610/9610. I am using them for tests so that I do not blow my matched Toshibas.
BTW I really hate how twitchy the pots are (Currently using 1k). I want to try a big series resistor to ballpark bias the MOSFETs and then use a smaller trimmer to fine-tune the dc offset. It should be less sensitive and make it easier to precisely tune the offset. Plus it should be a bit more stable as the trimmer heats up or ages.
Now all I need to decide is what regulation to use...
The pre seems to bias nicely even with random unmatched 610/9610. I am using them for tests so that I do not blow my matched Toshibas.
BTW I really hate how twitchy the pots are (Currently using 1k). I want to try a big series resistor to ballpark bias the MOSFETs and then use a smaller trimmer to fine-tune the dc offset. It should be less sensitive and make it easier to precisely tune the offset. Plus it should be a bit more stable as the trimmer heats up or ages.
Now all I need to decide is what regulation to use...
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