Say can you guys give me an idea about what is "normal" in terms of drift in the offset and bias of the F-5? Mine's just breaking in and if I set it at .59 and 0 mv and let it play for a few hours and remeasure, it seems to go up to about 15mv-(at first 25 mv) and bias at .61. Heat sinks handle it well, can touch indefinitely. After a long listen last night, it again drifted up to 14-15 mv and .60-.61 offset. I reset to .59 and 0-1 mv and powered down. This morning I turned it back on, let it set for about an hour with no music, and had 5 mv off set and .58-.59 bias. Is this normal?
Thanks for the input,
Russellc
Thanks for the input,
Russellc
I did some measurements how bias stabilizes in my amps (without thermistors): http://www.diyaudio.com/forums/group-buys/140306-f5-pcb-group-buy-15.html#post1935126
I think the thermistors help National Semi's keep a little more stable with thermistors, but importantly allow it to bias up quickly.
With no thermistors, and IRF's or Toshibas, I see rock solid offset (no drifting). It does take longer to warm up, but I say doesn't make much difference in sound after 30 minutes.
With no thermistors, and IRF's or Toshibas, I see rock solid offset (no drifting). It does take longer to warm up, but I say doesn't make much difference in sound after 30 minutes.
I find that there is a big variance between summer and winter bias and offset levels without thermistors. Basically, once you set the bias without thermistors, the amp likes to remain at the same ambient conditions that you had when you set the bias.
Else the bias goes up when the ambient temps increase, and down in winter or monsoon seasons. Thermistors do keep this more stable and reduce the amount of fiddling one has to do to keep everything nice and happy, but what good is an amp if you don't have to open up the hood once in a while anyway? And in my case, it's pretty much unusable with the 35 degree + room temperatures of a tropical summer, so for me it's a winter amp only.
However, omitting the thermistors results in much more stable bias and offset over a short-term period - personally speaking. I checked over a period of about ten days and the bias did not move at all from where I set it, and the offset was <2mV on each channel. Picture changes when the seasons change though, but I prefer that to a shifting bias point that is almost impossible to pin down with the default configuration. I end up having to keep the amp on for an hour or so to get bias to the exact point I want it at, which is fine as it is playing music at the time
Else the bias goes up when the ambient temps increase, and down in winter or monsoon seasons. Thermistors do keep this more stable and reduce the amount of fiddling one has to do to keep everything nice and happy, but what good is an amp if you don't have to open up the hood once in a while anyway? And in my case, it's pretty much unusable with the 35 degree + room temperatures of a tropical summer, so for me it's a winter amp only.
However, omitting the thermistors results in much more stable bias and offset over a short-term period - personally speaking. I checked over a period of about ten days and the bias did not move at all from where I set it, and the offset was <2mV on each channel. Picture changes when the seasons change though, but I prefer that to a shifting bias point that is almost impossible to pin down with the default configuration. I end up having to keep the amp on for an hour or so to get bias to the exact point I want it at, which is fine as it is playing music at the time
Peter, in post 300 you said once it reached .659 "It was steady as a rock."
Is it OK to leave at this higher than .59 bias point? Or did you scale it back down?
thanks,
Russellc
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