youyoung21147 said:Are you sure the LM4780 has a high impedance on its two inputs ?
If so, I can go back to my low-impedance feedback network
Not sure, but most probably it does. I had a brief look at the datasheet and didn't find any figures for input impedance, but that is a strong indicator that it is high. The theoretical op amp model prescribes infinite input impedance on both inputs, so a high input impedance is the default for op amps. The exception would be if these were CFB op amps, but that is not very likely and the datasheet would most certainly say clearly that that is the case.
Oh BTW I have a few stupid questions to ask here, which I'm not sure about the answers :
- Is it true that a lower-impedance feedback network helps reducing noise and distorsion ?
- is there any signal going through Ri1 and Ci1, i.e do I need to put some audio-grade components in this path in the case I need Ci ?
- Is there a rule for setting Rin, on the non-inverting input ? Is lower better (well, not too low) or higher ?
- How can you calculate the input impedance on the inverting part ? What is the total input impedance for the whole bridged circuit ?
- is the gain prefectly equal from an op-amp to another using the same feedback network, even if they have some manufacturing differences ? (a chip is never similar to another)
- What sort of pot should I use in the volume control ? I heard that 50k increased the dynamic range, but that it gave more noise than a 10k pot. Though I see that most Gainclones use a 50k pot ?! How to choose it ? Does it depend of Rin or is it the contrary ?
Better ask this here than have stupid questions remaining in my head lol
- Is it true that a lower-impedance feedback network helps reducing noise and distorsion ?
- is there any signal going through Ri1 and Ci1, i.e do I need to put some audio-grade components in this path in the case I need Ci ?
- Is there a rule for setting Rin, on the non-inverting input ? Is lower better (well, not too low) or higher ?
- How can you calculate the input impedance on the inverting part ? What is the total input impedance for the whole bridged circuit ?
- is the gain prefectly equal from an op-amp to another using the same feedback network, even if they have some manufacturing differences ? (a chip is never similar to another)
- What sort of pot should I use in the volume control ? I heard that 50k increased the dynamic range, but that it gave more noise than a 10k pot. Though I see that most Gainclones use a 50k pot ?! How to choose it ? Does it depend of Rin or is it the contrary ?
Better ask this here than have stupid questions remaining in my head lol
So I found some answers to my question in this nice ebook.
It encourages me to try the low-impedance feedback circuit and compare it with the regular National schematic (anyway, resistors are not lost )
But I still have a little problems with the pot : can its nominal impedance (i.e 10k, 50k, 100k) be considered as the virtual input impedance ?
I think a 50k pot will make more noise, but will the volume control be smoother at low levels or has it no importance ? Most people seem to use 50k pots, but some other say 10k or 20k give better results.
So, in my case, what model would be the most suitable, assuming Rin being 22k or 47k ? (I can only get an ALPS rated at 10k or 50k )
I think it will be my last novice question here, I'm now ready to order everything. (BTW it won't be exactly a cheap gainclone
)
It encourages me to try the low-impedance feedback circuit and compare it with the regular National schematic (anyway, resistors are not lost
)But I still have a little problems with the pot : can its nominal impedance (i.e 10k, 50k, 100k) be considered as the virtual input impedance ?
I think a 50k pot will make more noise, but will the volume control be smoother at low levels or has it no importance ? Most people seem to use 50k pots, but some other say 10k or 20k give better results.
So, in my case, what model would be the most suitable, assuming Rin being 22k or 47k ? (I can only get an ALPS rated at 10k or 50k )
I think it will be my last novice question here, I'm now ready to order everything. (BTW it won't be exactly a cheap gainclone
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