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. Both variations are connected to the "big" rails and they are doing their +-33V by themself.Thanks Chalky.....highly appreciate your contribution to my input board issue.....will try with your new changes.
Amplitude, I would guess that any modern low noise, low distortion, integrated opamp would do a creditable job of driving the output board. Probably cheaper too; a decent opamp and a suitable pcb from fleabay should cost less than buying and populating a Bryston clone input board. Good to know that discrete and integrated options are available.
Incidentally it occurs to me that the schematic I posted in my "Mirror VAS" thread would also make a good driver for the output board. Alas I no longer have the breadboard to try it out as I developed it into a full power amplifier prototype. Perhaps I'll build another one or maybe do a pcb ( slow process as I'm using very old cad tools ).
Yes I saw your mirror vas thread, you could give it a try.
Using the lme49600 lme49720 sound was crystal clear, I haven't got around to do a actual spectrum check, I'm busy with the classic jlh amplifier, a real nice little amp that seems to grow on me.
Have you finished the output boards yet?
Using the lme49600 lme49720 sound was crystal clear, I haven't got around to do a actual spectrum check, I'm busy with the classic jlh amplifier, a real nice little amp that seems to grow on me.
Have you finished the output boards yet?
Yes hfe 455 PN100As with hfe 480 PN200As is not a bad match. Much, much better than the match I got from 100 of each of the parts and my input stage still worked fine. As you can see from the above I switched to BC550C/BC560C pairs as I can get a perfect match with these at a higher hfe. Amplitude, I haven't finished my output stage yet. Board is mostly populated and front end is tested but I need to get some metalwork done now. Also been a bit distracted with building prototypes of my mirror vas superamp. Given me a little time to look over the schematic of the Bryston clone power amp in detail, and I notice that the vas current is quite low at about 2mA. Ok the output stage has a high input impedance, but even so. Maybe a bit of value engineering going on there. I might try raising the vas current to 4 or 6mA to see what effect this has.
Biasing problems
Hi All,
I wonder if anyone could shed some light on my amp set up? I am only setting up the main board I have shorted the input to earth and have very little dc offset. The problem I have is with the bias, measuring across r45 I can take it to about 20mV and it then takes off to 600mV and of course gets very hot. Would I be correct in assuming it's oscillating? Any help would be much appreciated. Thanks
Hi All,
I wonder if anyone could shed some light on my amp set up? I am only setting up the main board I have shorted the input to earth and have very little dc offset. The problem I have is with the bias, measuring across r45 I can take it to about 20mV and it then takes off to 600mV and of course gets very hot. Would I be correct in assuming it's oscillating? Any help would be much appreciated. Thanks
Hi Chalky, thanks for the reply. R59 does not get warm at all. I have measured all of the 0.3ohm resistors and there is quite a difference.
R52 – 8.4mV
R53 – 3.7mV
R56 – 8.5mV
R57 – 3.5mV
R44 – 10.8mV
R45 – 15mV
R48 – 10.8mV
R49 – 15.8mV
Not sure if these look OK? It's only when I get the bias to around 18mV on R45 that it goes very high very quickly.
R52 – 8.4mV
R53 – 3.7mV
R56 – 8.5mV
R57 – 3.5mV
R44 – 10.8mV
R45 – 15mV
R48 – 10.8mV
R49 – 15.8mV
Not sure if these look OK? It's only when I get the bias to around 18mV on R45 that it goes very high very quickly.
Did you get a copy of the "debugging-instructions" with your kit? If so did you check out the board before you inserted the main output devices? Did you remember to connect up all of the ground connections? Its easy to insert diodes the wrong way round so check and double check yours. Also check all of the resistor values carefully. Most construction problems arise from misplaced components, omitted connections, or mixed up connections. Been there done all of that. Its certainly odd that the output transistors driven from the emitters of the driver transistors all seem to have low current.
I didn't get any documentation with my boards, so I used the ones posted here together with the Bryston schematic. I posted the voltages of the output transistors to show the difference between them. They seem OK up to around 18mV and when I turn the pot to increase the voltage it gets to a point and then rapidly goes up to 300mV and keeps going. Is there any particular component that is likely to have the wrong value that could be causing this?