AC Couple Input Value?

[rgrayton]

If I want to AC couple my input, which, I am told I should do, is this calculation correct? At each input, I have a 22.1K to ground, so I believe with a 1uF cap, this would give me a 7Hz HP,
1/(2*PI*22.1K*1E-6). Is this right and is this a low enough break? Thanks for any inputs. -bg

[pacificblue]

Your calculation is correct. You should choose the frequency to be at least five times below the lowest frequency you can hear. The reference designs use 1 µF with a 47 k Rin, which gives 3,4 Hz and should even be sufficient for the 16 Hz the big church organ can do. With 22,1 k that would be 2,2 µF or bigger.

Take into account that AC coupled means you also need a capacitor in the feedback loop leg to ground. The frequencies should be matched or both frequencies should be sufficiently low, if unmatched.

[rgrayton]

Take into account that AC coupled means you also need a capacitor in the feedback loop leg to ground.

PBlue, thanks again for you inputs.

Since the fb is same at 22.1K, I should also put same uf in parallel with the 680 to ground in the schematic in this link? -bg
schematic

[pacificblue]

The same formula applies. 1/(2*PI*680*3,4) results in a 68 µF capactor for the 680 Ohm leg. And it goes in series with the resistor, not parallel. Like in the schematic on page 21 in the datasheet.

[AndrewT]

for Zin=22k1 or 22k I would recommend that a 3u9F or 4uF blocking cap be used at the input. This gives a High Pass input filter of F-1dB~3.5Hz & F-3dB~1.9Hz.
This requires the NFB blocking cap to be >= 1.414 * 22k1 / 680r * 3u9F >= 179uF. Use 220uF in series with the 680r.
With this value of NFB cap you can go upto 4u7F for an input blocking cap.

But to get the full benefit of this low frequency ability the series combination of the DC blocking cap in the source output and the DC blocking cap in the amp input must be ~4u7F.

If both pieces of equipment have a DC blocking cap and one of them is too small then arrange to short out the smaller cap. And ensure the one you keep is the highest quality that you think gives you good value.

[rgrayton]

But to get the full benefit of this low frequency ability the series combination of the DC blocking cap in the source output and the DC blocking cap in the amp input must be ~4u7F. If both pieces of equipment have a DC blocking cap and one of them is too small then arrange to short out the smaller cap. And ensure the one you keep is the highest quality that you think gives you good value.

Not sure what this means. I may be connecting this to any number of commercial equipment and would have not knowledge of any of this. Am I all wet here? I thought the reason for doing this would be to filter any DC caused by different ground potential between equipment. thx. -bg

[rgrayton]

his requires the NFB blocking cap to be >= 1.414 * 22k1 / 680r * 3u9F >= 179uF. Use 220uF in series with the 680r.

Could I bypass an electrolytic with a 1uF poly?

[pacificblue]

Originally posted by rgrayton
I thought the reason for doing this would be to filter any DC caused by different ground potential between equipment.

Yes, it is to filter DC. Usually those filters are present in all components. The trouble with them is that they add up to each other. And not all manufacturers have the same idea about the ideal roll-off frequency. So sometimes the filters are designed in a way that they can degrade the sound more than necessary.

Once you have the soldering iron in your hands, you might just as well disable redundant filters or adapt them to get the best out of them. Of course you have to find out, if your taste is the same as Andrew's. And to find that out, you will have to try it out.

Originally posted by rgrayton

Could I bypass an electrolytic with a 1uF poly?

Yes. It is even recommendable with the poly having around 1 % of the electrlytic's value as a rule of thumb.

[rgrayton]

Thanks PBlue. I still don't get what the FB cap is all about or how it relates, but that is just my ignorance with this stuff. -bg

[mjf]

I may be connecting this to any number of commercial equipment and would have not knowledge of any of this.
hello.
perhaps you can build in two inputs - one with a cap ,.and the other without a coupling cap(used for preamps that got one).......
greetings............