but its a complementary output stage not Single ended. sorry couldn't resist Darry,
I think you can achieve what you want with a BA-2 output stage by adding a constant current source between the output and one of the rails. Check out http://passdiy.com/pdf/leaving_class_a.pdf, specially fig 5.
I think you can achieve what you want with a BA-2 output stage by adding a constant current source between the output and one of the rails. Check out http://passdiy.com/pdf/leaving_class_a.pdf, specially fig 5.
Has any one brought P3 100 pot out to the front of the amp so the adjustment can be made while listening? If the amp was stable I could even see where you may want to adjust P3 for diferent types of music. Just a thought. I quess a similar scheme could be used on BBA3, would it require two pots or one ganged pot?
JimM
JimM
I have adjusted P3 whilst listening using a screwdriver on the pot and it would indeed have been easier with a knob on the front, however, to get the best sound the dc offset needed altering each time an adjustment was made. How you would do this with a balanced version I don't know.
I have some strange problem with the BA-3 FE DC offsets which I hope someone can help me out. On the right channel, it drifted to about 170mV from the initial 10mV when it was connected to DCB1 and with the DCB1 switched on. On the left channel, it drifted by about 40mV but it will slowly drift back to the reading that was before connecting to DCB1.
The DC offset readings for DCB1 were 3.5mV for right channel and 0.1mV for left channel.
The other problem is that the BA-3 FE DC offsets are very sensitive to heat. It keeps drifting up and down even with just a slight breeze around the case. Even putting on and taking off the top cover causes the DC offsets to drift up and down.
There is also a "TIAK" sound from the speakers when the power amp is switched off. When it is disconnected from the DCB1, then no such sound is experienced. When using my Audiolab 8000 as a preamp, no such sound is also experienced. I am confused. Where is the source of the problem?
The DC offset readings for DCB1 were 3.5mV for right channel and 0.1mV for left channel.
The other problem is that the BA-3 FE DC offsets are very sensitive to heat. It keeps drifting up and down even with just a slight breeze around the case. Even putting on and taking off the top cover causes the DC offsets to drift up and down.
There is also a "TIAK" sound from the speakers when the power amp is switched off. When it is disconnected from the DCB1, then no such sound is experienced. When using my Audiolab 8000 as a preamp, no such sound is also experienced. I am confused. Where is the source of the problem?
I have had similar problems. I set the dc offset to near zero but an hour later it had drifted some way from this (possibly as things began to heat up). I switched on the following day to see the dc offset way off zero but as things warmed up so the voltage swung towards zero only to pass it and head off in the opposite direction. The dc offset became more stable after many hours of amp use, however, it does seem to drift more than say the output dc offset of the main power board, which is rock solid.
I have run the BA-3 for long periods with the dc offset at 70mV or more (as I played around with P3) and it sounds about the same as when it is set nearer zero so I've decided that this isn't critical for good listening.
I'm not getting any of the switch-off noise you mentioned so I don't think it is caused by the offset problems.
I believe they have a more accurate tone. I am going to be rematching some at specific bias points and will be putting them up against Fairchilds and Toshibas that I have.
It seems like some of you are also having this thermal drift problem. I had already replaced the single turn pots to multi-turns type and setting the dc offset is very much easier now except that it will drift off due to thermal change. Even after a warm up of more than 1 hour, it will drift off after putting back the top cover. Looks like the only way is to have bigger heatsinks for the FETS.
I had tried that out without much success with the "tiak" sound during switching off the power amp. It will only be dead silence when the power amp is not connected to the DCB1.
With the caps on the outputs of DCB1, the BA-3 FE dc offsets did not change anymore when DCB1 was powered up. Without the caps, it drifted by about 160mV on the right channel and about 40mV on the left channel. What I am very surprised is that the DCB1 dc offset is only 3.5mV on the right channel and 0.1mV on the left channel and these readings are very stable and how they can affect the BA-3 FE dc offsets.
My BA-2 output dc offset for both channels are rock solid also, so I am not so concerned about the possible damage to the speakers.
However, I am very surprised with the drifting of the dc offsets to higher points when they were connected to DCB1 and powered up.