dartecchia
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- Jan 5, 2025
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Hello everyone!
I'm embarking onto the design of an hifi preamp and I'd like to share my view on different topics in a detailed way, so that it could be decently understandable, and to receive a feedback if you're interested in them. The following solutions will be nothing new, most of them are based on well established literature, I'm particularly fond of the following textbooks:
The Philosophy
Basic Design Choices
First thing first: main active device. I feel that in this case the most viable solution, with respect to cost and complexity, is an Op-Amp design: a complete and very careful discrete design could, maybe, provide a lower noise solution, but I don't feel good enough to dwelve into something like this given it would be a much more complex to design and a better result is not really assured.
So I had to choose an Op-Amp that suited the scope.
I know what you all are thinking: NE5532/34. For what it offers it's really difficult to beat: low cost (0.32€/pcs!), pretty low noise (5 nV/√Hz) but I wanted something better! And with a better drive capability, even at a higher cost.
Why a better drive capability? To drive lower value resistor and consequently have lower noise.
Let's evaluate some other Op-Amps:
Unfortunately THD+N is not specified for a 150Ω load, and the harmonic distortion are not specified for less than 100kHz. It could be interesting to perform some measurements in this range, but given the reported specs we are in for a good time.
This is not a cheap Op-Amp though, on Mouser the dual version (OPA2891) goes for 6.34€/pcs.
Still, given the specs, it's the best shot for what I want to achieve.
To maximaze dynamic range and THD specs I'll power it up with a Dual 18V Linear Power Supply (not sure yet if I'll go for reg or unreg).
I want to implement an Equalization Stage with variable cut/boost and frequency controls for LF and HF, though I'm not yet sure which topology to use.
This preamp will also have an Headphone Amp capable to drive up to 32Ω loads.
Input Switching will be done with SPDT Relays, to minimize channel crosstalk.
That seems all the requirement I've given myself. Next topic to be addressed will be the MM Phono Stage, let me recollect my thoughts about it and I'll update the topic in a short time.
Let me know if this project could be of interest to someone and if you see any major flaw or technical difficulties!
Best Regards
I'm embarking onto the design of an hifi preamp and I'd like to share my view on different topics in a detailed way, so that it could be decently understandable, and to receive a feedback if you're interested in them. The following solutions will be nothing new, most of them are based on well established literature, I'm particularly fond of the following textbooks:
- The Art of Electronics by Paul Horowitz, Winfield Hill
- Small Signal Audio Design by Douglas Self
- Microelectronic Circuits by Adel Sedra, Kenneth Smith
The Philosophy
- Cost and Complexity: I think that simple solutions are, most of the time, the best one. Still there are elegant simple solutions and bad simple design: this preamp doesn't want to be simple at all costs and the design tradeoffs will be evaluated case by case. This means that the overall price is not gonna be in the tenth of thousands, but I want to implement good components and solutions, trying to mantain BoM cost (just components) less that 500€ (I'm european).
- Low Noise and Low Distortion: one of the main characteristic of a preamp is the ability to reproduce whatever input signal in the most linear way possible, adding the lowest amount of noise. This will be the main drive of the design, unless it will provide exceptional degrees of complexity with respect to very little improvements.
- Number of inputs: I want this preamp to have the following inputs:
- MM Phono Balanced/Unbalanced
- MC Phono Balanced/Unbalanced
- Line IN Balanced/Unbalanced
- CD Balanced/Unbalanced
- USB (Integrated DAC)
Basic Design Choices
First thing first: main active device. I feel that in this case the most viable solution, with respect to cost and complexity, is an Op-Amp design: a complete and very careful discrete design could, maybe, provide a lower noise solution, but I don't feel good enough to dwelve into something like this given it would be a much more complex to design and a better result is not really assured.
So I had to choose an Op-Amp that suited the scope.
I know what you all are thinking: NE5532/34. For what it offers it's really difficult to beat: low cost (0.32€/pcs!), pretty low noise (5 nV/√Hz) but I wanted something better! And with a better drive capability, even at a higher cost.
Why a better drive capability? To drive lower value resistor and consequently have lower noise.
Let's evaluate some other Op-Amps:
- LT1028: https://www.analog.com/media/en/technical-documentation/data-sheets/1028fd.pdf
- Voltage Noise: 0.85 nV/√Hz [best of class]
- Short Circuit Current: 35mA [not great, ok for 600Ω]
- AD797: https://www.analog.com/media/en/technical-documentation/data-sheets/AD797.pdf
- Voltage Noise: 0.9 nV/√Hz [great]
- Short Circuit Current: 80mA [good, more than ok for 600Ω]
- OPAx891: https://www.ti.com/lit/ds/symlink/opa2891.pdf
- Voltage Noise: 0.95 nV/√Hz [great]
- Short Circuit Current: 200mA [great, ok for 150Ω]
Unfortunately THD+N is not specified for a 150Ω load, and the harmonic distortion are not specified for less than 100kHz. It could be interesting to perform some measurements in this range, but given the reported specs we are in for a good time.
This is not a cheap Op-Amp though, on Mouser the dual version (OPA2891) goes for 6.34€/pcs.
Still, given the specs, it's the best shot for what I want to achieve.
To maximaze dynamic range and THD specs I'll power it up with a Dual 18V Linear Power Supply (not sure yet if I'll go for reg or unreg).
I want to implement an Equalization Stage with variable cut/boost and frequency controls for LF and HF, though I'm not yet sure which topology to use.
This preamp will also have an Headphone Amp capable to drive up to 32Ω loads.
Input Switching will be done with SPDT Relays, to minimize channel crosstalk.
That seems all the requirement I've given myself. Next topic to be addressed will be the MM Phono Stage, let me recollect my thoughts about it and I'll update the topic in a short time.
Let me know if this project could be of interest to someone and if you see any major flaw or technical difficulties!
Best Regards