You could raise bias in two 5mV steps up to 20 mV using the amp before and after each adjustment, at reasonable power. Then test Vbias again after removing the input, so that even low level signal is not affecting the tiny reading. There is not much point in checking bias without having the amplifier at working temperature. That's why you use it for a while first, with the cover on and preferably test with the cover on too, or smartly after you open it!
Just remember that though you may be able to set bias anywhere, having stable bias that returns to the preset value after hard use is another matter.
It would also help to know if you heard any difference in SQ after increasing bias setting from 3 to 10 mV. What did you think?
If you said 60s, I could agree but we were firmly into silicon, even for DIY, by the end of the 60s - in Oz at least.
Got it set on 15mv now seems cool still and sound seems beefier or am I imagining it? lovely sounding amp though at any setting. I will keep it on test
many thanks
many thanks
Put it up to 20mv, now getting a bit of hiss on the speakers (doesn't increase with volume) perhaps back it off a bit?
Many thanks
Many thanks
Sorry I missed your last 2 posts.
First, are the output transistors and heatsink noticeably warmer?
I'm not sure about that hiss. Being only a trickle of DC, The bias current itself should not introduce any noise but if it did, it would most likely be a 100 Hz power supply buzz.
There is no amplification in the output stage to make a small noise more audible so I'm thinking there may be a need to look at why the noise at all. There are a couple of measures that may help but backing off the bias shouldn't need to be part of it. Consider that if this noise increases with DC current, what would happen when the signal AC was at that level and above?....the noise would rise with the signal peaks and appear as distortion. I don't think that would be too interesting but it will likely be there.
On the sound quality, I'm looking for whether you notice a change in smoothness between zero bias and 3 mV, then 10mV but by 20mV you may not notice much further improvement. If you can't hear any differences, you may need some help from other speakers plus another family member or friend with a sensitive ear for the top register.
The important issue is that the current remains stable when tested at intervals. There's no problem to back bias off to perhaps 15 mV as you say - a half-way setting that was often seen on Japanese amplifiers. A little sharp-edged sounding occasionally but not a problem for many people.
First, are the output transistors and heatsink noticeably warmer?
I'm not sure about that hiss. Being only a trickle of DC, The bias current itself should not introduce any noise but if it did, it would most likely be a 100 Hz power supply buzz.
There is no amplification in the output stage to make a small noise more audible so I'm thinking there may be a need to look at why the noise at all. There are a couple of measures that may help but backing off the bias shouldn't need to be part of it. Consider that if this noise increases with DC current, what would happen when the signal AC was at that level and above?....the noise would rise with the signal peaks and appear as distortion. I don't think that would be too interesting but it will likely be there.
On the sound quality, I'm looking for whether you notice a change in smoothness between zero bias and 3 mV, then 10mV but by 20mV you may not notice much further improvement. If you can't hear any differences, you may need some help from other speakers plus another family member or friend with a sensitive ear for the top register.
The important issue is that the current remains stable when tested at intervals. There's no problem to back bias off to perhaps 15 mV as you say - a half-way setting that was often seen on Japanese amplifiers. A little sharp-edged sounding occasionally but not a problem for many people.
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