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Old 15th May 2005, 08:21 AM   #11
taj is offline taj
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To quote my earlier post...

>> I don't plan to spend $200 a pop for Blackgates for basically a garage sale amp.

Thanks for all the advice, everyone, keep it coming. I'm learning a lot.

..TAJ
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Old 16th May 2005, 03:11 PM   #12
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Default enlargement?

Greg, Could you explain this? I'm just learning electronics, and I don't know what this means (yet).

>>>The topology of the amp gives the input/VAS stage fairly poor power supply rejection, so enalarging C11 could provide some benfit.


And 'enlarging' means a higher uF value component or high voltage rating?


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TAJ
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Old 1st June 2005, 06:49 AM   #13
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Quote:
Originally posted by Greg B.
It is a fairly early transistor design that even with the best parts would not be a great amplifier. But it is a good learning tool. The topology of the amp gives the input/VAS stage fairly poor power supply rejection, so enalarging C11 could provide some benfit. Even better would be to have a C11 for each channel and double the value of R21 to each to obtain the same voltage drop. This would isolate the two input/VAS stages from each other.

I also find that the carbon composition resistors used throughout drift pretty badly with age. This can upset the DC bias point and cause channel gain imbalances. Check the actual values R4 and R9 if the two channels do not have the same gain. I have seen this on mine.
Greg
A newbie question for the gurus here...

I finally fixed some major power supply issues with my Dynaco Stereo 80 so it runs now. One channel works fine, the other is very ill. Very low volume with lots of hums and noise.

I measured the voltage at all the test points (see link below) and the bad channel is reading about double the voltage of the good channel on most rail points. Test points #3 & 4 (on PCB-19) should both be the same (36v), but #3 is about 70 volts. Points #5 & 6 should both be 37.5 volts, but #5 is about 70 volts. I can't figure out why though. The power supply is offering up the same supply connections through resistors 16 and 17 to each channel. Could C6 or D2/D3 be causing this problem?

Schematics: http://home.indy.net/~gregdunn/dynac...T80/index.html

..TAJ
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