Rf path length considerations

[eddog]

I currently have just finished building Brians 3886 kit, I used components I sourced myself and the feedback resistor size (too long)demanded that it be placed in the board standing on end. I am curious if this extra length will be problematic. My measured DC offsets are 102mv and 75mv respectivily. Will shortening the Rf path decrease the offset? Thanks for any suggestions. I hope to connect input tomorrow and test speaker.

[tiltedhalo]

No it will not effect the offset, it could be possible to pick up RF but highly doubtfull. Offset at the level you recorded are well within normal ranges without a NFB capacitor. If it sounds good then all is well.

[eddog]

Great! Thanks for the input tiltedhalo, looking forward to completing the amp andcomparing it to my 3875 amp.

[Eva]

Output offset depends essentially on three things:

- Differences in threshold voltages of input devices between IN+ and IN-.

- Differences in bias currents from one input to another.

- Differences in the DC impedance as seen from each input lead of the the IC (the previously mentioned currents will cause DC voltage drops on these impedances).

The two first sources of imbalance tend to come quite well production matched on the IC. The third depends on resistor values and the use of DC blocking capacitors.

100mV won't produce any damage to conventional speakers, but are a sign of poor matching.