Sumo Polaris 310

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jackinnj said:
replaced the original MOSFETs with IRF'S -- initially the distortion was quite high, 0.50% but by adjusting VR1 and VR2 both channels match at 0.01% to 0.02%.


If you are replacing the original parts types, you might try using a better chip for U2 and a better cap for C9.

A 741 is hardly a low-offset IC, and something like an LF411 might be a better option. There are other types, but that might be easier to find and cheap too.

DC servos are used to eliminate the feedback large cap on the gain. But that doesn't mean that no audio is passing through any caps, as it is going through the servo cap C9. This cap should be a high quality one, preferably a film type. As that value is a bit high for such a type and you might have some trouble finding pcb space to hold it, a better eletrolytic should do it.


Carlos
 
Carlos

Yes, I just wanted to get the amp up and running first -- the opamp's bypass caps are far removed from the opamp itself -- meaning that they are useless -- I can get a non-polar electrolytic and byass it with 100nF MKT for C9 --

anything more is gilding the lilly (but I am tempted) -- it sounds good, the distortion at low levels is 0.03% on both channels, and at high levels a little over 0.01%
 
jackinnj said:
Yes, I just wanted to get the amp up and running first -- the opamp's bypass caps are far removed from the opamp itself -- meaning that they are useless -- I can get a non-polar electrolytic and byass it with 100nF MKT for C9 --

anything more is gilding the lilly (but I am tempted) -- it sounds good, the distortion at low levels is 0.03% on both channels, and at high levels a little over 0.01%

Getting the amp running is certainly the most important thing.

I didn't refer to the opamp's bypass caps, but C9, which is in series with the chip's output. Changing the chip might bring the greatest gain, but do that using a chip socket, if it's not already provided for.

For the cap it might be better trying several types and values.

These two mods I would not consider gilding the lilly, as they are part of the feedback. If you go for it, change the chip first and then try different cap values and types.

Or write to Buongiorno, who designed the amp and might have something to say on potential improvements. That might be the wise thing to do.


Carlos
 
yes, when I dragged the '741 I must have crossed up the connection -- here it is on the bottom (the picture is "flopped")
 

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The original Andromeda was BJT, the later one (not a Bongiorno design) was FET.

The BJT design sounds better and is more reliable (former Sumo dealer and service center).

Not that it matters, but the source and drain connections on the schematic for your Polaris are drawn wrong, as well as the DC servo.
 
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