Hello,
I wonder if someone could suggest a good source of information, either in print form or online is fine, for understanding a particular negative feedback circuit as it applies to shaping an EQ response. In other words I would like to learn how it functions (I have a good idea of the basics) and what the mathematics are for determining component values and how to change them to achieve different requirements. This is the circuit I'm interested in:
I wonder if someone could suggest a good source of information, either in print form or online is fine, for understanding a particular negative feedback circuit as it applies to shaping an EQ response. In other words I would like to learn how it functions (I have a good idea of the basics) and what the mathematics are for determining component values and how to change them to achieve different requirements. This is the circuit I'm interested in:
Just Googling "active filter design" should turn up a lot of resources.
A couple of examples:
https://www.tinaja.com/ebooks/afcb.pdf
https://gctjaipur.wordpress.com/wp-content/uploads/2015/08/continuous-time-active-filter-design.pdf
Also, often it is assumed that the reader is familiar with the Laplace Transform. Some possible resources, if needed (basically, it is used to reduce problems involving differential equations into algebra problems):
https://www.sjsu.edu/me/docs/hsu-Chapter 6 Laplace transform.pdf
https://www.analog.com/media/en/technical-documentation/dsp-book/dsp_book_Ch32.pdf
A couple of examples:
https://www.tinaja.com/ebooks/afcb.pdf
https://gctjaipur.wordpress.com/wp-content/uploads/2015/08/continuous-time-active-filter-design.pdf
Also, often it is assumed that the reader is familiar with the Laplace Transform. Some possible resources, if needed (basically, it is used to reduce problems involving differential equations into algebra problems):
https://www.sjsu.edu/me/docs/hsu-Chapter 6 Laplace transform.pdf
https://www.analog.com/media/en/technical-documentation/dsp-book/dsp_book_Ch32.pdf
Last edited:
It's actually pretty simple ... in theory. As you've undoubtedly found if you read Lipshitz' paper, the math gets hairy in a hurry. But in theory the feedback network is "just" a voltage divider:
Pardon the mirror image for Z1. So now you can derive the divider ratio as:
Vout/Vin = Z2/(Z1+Z2)
Trouble arises because Z1 depends on frequency and that makes the math a bit complicated. It should be no trouble to play with the divider in a simulator and get some understanding that way, though. Keep in mind that the frequency response of the amplifier overall will be approx. 1/ß, so (Z1+Z2)/Z2, assuming you have sufficient forward gain.
If you're new to circuit simulation, I suggest learning QSpice. I'm thinking LTspice has become abandonware now that the author is with Qorvo.
Tom
Pardon the mirror image for Z1. So now you can derive the divider ratio as:
Vout/Vin = Z2/(Z1+Z2)
Trouble arises because Z1 depends on frequency and that makes the math a bit complicated. It should be no trouble to play with the divider in a simulator and get some understanding that way, though. Keep in mind that the frequency response of the amplifier overall will be approx. 1/ß, so (Z1+Z2)/Z2, assuming you have sufficient forward gain.
If you're new to circuit simulation, I suggest learning QSpice. I'm thinking LTspice has become abandonware now that the author is with Qorvo.
Tom
Thank you for these great suggestions. I've considered LTspice and QSpice for a long time and understand its potential. I have a Mac laptop and haven't found a clear answer to potential compatibility issues. I've read the Lipshitz paper a few times and must admit I get a little further with it each time. I also remember the article in Audio Amateur that used it to create tighter tolerance RIAA feedback values. Also thank you for those resource links, I will carefully absorb them.
The overall reason for this thread is that I have built a tape head preamp based on the Marantz 7 circuit that was inspired by a YouTube contributor who described how to convert the RIAA circuit into an IEC and NAB playback EQ. I built the unit with the ability to adjust the gain, input loading and low and high frequency parts of the curve. I have a calibration tape and can measure the playback frequency response and make adjustments. And actually the results are quite good compared to other tape preamps I have.
I sort of feel like I'm a fairly skilled builder and that a real engineer hands me a set of schematics and says, "here, build this". I'd like to know a bit more of what it is I'm building and the theory behind the results I'm getting. One example is when I change the pot for gain (the 510 ohm resistor) it effects not only the output level but it changes the way the bass response was matching the NAB curve. From Tom's post I believe I understand now that this resistor is part of a voltage divider. Thank you for that.
The overall reason for this thread is that I have built a tape head preamp based on the Marantz 7 circuit that was inspired by a YouTube contributor who described how to convert the RIAA circuit into an IEC and NAB playback EQ. I built the unit with the ability to adjust the gain, input loading and low and high frequency parts of the curve. I have a calibration tape and can measure the playback frequency response and make adjustments. And actually the results are quite good compared to other tape preamps I have.
I sort of feel like I'm a fairly skilled builder and that a real engineer hands me a set of schematics and says, "here, build this". I'd like to know a bit more of what it is I'm building and the theory behind the results I'm getting. One example is when I change the pot for gain (the 510 ohm resistor) it effects not only the output level but it changes the way the bass response was matching the NAB curve. From Tom's post I believe I understand now that this resistor is part of a voltage divider. Thank you for that.