I am currently collecting information for building a Gainclone amp. I have a couple of questions left over, for which I was not able to find satisfactory answers. Maybe you can help me with some of these issues.
1) LM3875 with 4 Ohm speakers: I want to use the CG to drive 4 Ohm speakers and I have read that some of you are doing this successfully. On the other hand, the datasheet describes the LM3875 as an amp for 8 ohm speakers and on page 9 of the datasheet a graph shows the output power as a function of the load impedance. According to this graph, the output power rapidly tends to zero for loads smaller than 8 ohms.
Can you help me with this apparent contradiction and is there any means to adapt the LM3875 to 4 ohm speakers? Or should I resort to the LM3886 for my 4 Ohm speakers?
2) I have noted that for inverted usually a DC-blocking input capacitor is used while this is not the case for non-inverting designs. Why is this so?
3) According to the National data sheet the local bypass condensators of the power supply should be not too large but they do not provide any numbers. In the amp designs I have found here and elsewhere on the web, usually values of 1000 to 1500uF are used. Compared to condensator values usually used in PSUs this does not seem a particularly small value. On some of the PSU designs presented, the caps used are even smaller since there seems to be a consensus that, unless you use a snubber to decrease the output impedance, a PSU with too much capacitance affects the sound negatively . Can you comment on this?
Thanks for your help.
Mick
1) LM3875 with 4 Ohm speakers: I want to use the CG to drive 4 Ohm speakers and I have read that some of you are doing this successfully. On the other hand, the datasheet describes the LM3875 as an amp for 8 ohm speakers and on page 9 of the datasheet a graph shows the output power as a function of the load impedance. According to this graph, the output power rapidly tends to zero for loads smaller than 8 ohms.
Can you help me with this apparent contradiction and is there any means to adapt the LM3875 to 4 ohm speakers? Or should I resort to the LM3886 for my 4 Ohm speakers?
2) I have noted that for inverted usually a DC-blocking input capacitor is used while this is not the case for non-inverting designs. Why is this so?
3) According to the National data sheet the local bypass condensators of the power supply should be not too large but they do not provide any numbers. In the amp designs I have found here and elsewhere on the web, usually values of 1000 to 1500uF are used. Compared to condensator values usually used in PSUs this does not seem a particularly small value. On some of the PSU designs presented, the caps used are even smaller since there seems to be a consensus that, unless you use a snubber to decrease the output impedance, a PSU with too much capacitance affects the sound negatively . Can you comment on this?
Thanks for your help.
Mick