Hi All,
I wanted to share a project I have been working on for a few months in my (limited) spare time. To allow me to make easier and better measurements of audio equipment (and a few other things as I'll mention later) I wanted to design a measurement pre-amplifier for audio band frequency. I have initially considered building something that is "readily available" like the
soundcard interface project from pmillett. However, I wanted something that is very versatile that will have a high maximum voltage range, low noise for low signal measurement, SE and BAL support for both input and output signals even for higher voltage swings at the input. Therefore I've decided to design one that will meet my specific needs.
I plan on using it for both testing audio gear for distortion and other possible issues (noise for instance), as well as being able to measure low noise densities for things like voltage regulators and a few other low noise projects I have in mind where this will come in handy.
To support all of this I went for a balanced input structure, and 6 different gain range (selected by relays) from -40dB to +60dB in 20dB steps. For the most sensitive ranges (+40dB and +60dB gain) the input referred noise is 7nV/rtHz (assuming it is driven from a low impedance source obviously). The input resistance is 100Kohm at each terminal, so 200Kohm differential.
The schematic for the main part of the circuit is shown below:

The schematic is actually fairly straight forward with a resistive voltage divider with 3 different outputs for 0dB/-20dB/-40dB settings, then an input protection circuit to keep the first stage opamp's safe. This is followed by 2 more stages with gain (0dB/+20dB in the first stage, -6dB/+14dB/+34dB in the second stage), and finally enters the output line driver which adds its own +6dB of gain.
There is also a T-RMS -> DC converter chip there to be able to measure the input signal amplitude (this isn't shown in this snip of the schematic).
The PS section shown below:

It starts with some input protection, into an isolated DC-DC module, followed by low-noise linear regulators and some filters. This is done to keep the supply noise+ripple low enough for the higher gain settings where noise coupling could be an issue.
I've used an additional smaller board to mount some of the controls to be able to place everything into a moderate size case when I'm finished.
The complete schematic can be downloaded here:
http://tolisdiy.com/wp-content/uploads/2019/08/MeasurementPreAmp_Schematic_TolisDIY-1.pdfThe assembled pre-amplifier can be seen below:

It is comprised of the main board that does all of the heavy lifting, a smaller control board for range selection and mounting of the range select LED's, and a panel mount DC voltmeter that I got from AliExpress obviously

I have done some measurements for THD, noise, and a few other parameters. I am measuring i t with a
modded EMU 0404 USB soundcard. To extent linearity of the measurement setup I use a
low distortion oscillator, and a
notch filter for 1KHz measurement. I am actually very happy with the performance, they match my expectations and calculated values quite well. I think I could potentially get more out of with a higher end soundcard or interface, but if I'm being honest, the current level of distortion is just excellent for my hobby use.
THD vs. frequency, unfortunately this is limited by the EMU 0404 USB:

Spectrum with external oscillator at 1KHz 2Vrms SE signal, through the pre-amplifier and into the soundcard, again, THD is limited by the EMU 0404 USB:

With a notch filter added before the EMU:

The notch filter transfer function can be seen here:
http://tolisdiy.com/wp-content/uploads/2019/08/fr_measure.pngOverall it seems that THD on the order of 0.0001% can be measured even with a full-scale signal. I'm saying "on the order of" and not giving an exact number because even with the notch filter, it seems like the residual distortion of the EMU is preventing me from getting an accurate reading of the distortion of the pre-amplifier.
Finally, I've measured the input noise density on the higher gain settings (terminated with 50ohm to reduce coupling and measure the noise assuming a low impedance source is connected), with 0dBFS being 4mVrms:

This translates to ~8nV/rtHz, which is very close to the calculated value of 7nV/rtHz that I've expected based on the datasheed of the input amplifier (3.3nV/rtHz typical value) and the value of resistors in the input protection and the feedback loop of the first stage.
This project isn't done yet, as I still need to design the front and rear panels for the aluminium case, to make it look nicer, easier to use, and reduce external coupling especially on the more sensitive ranges.
However, this project finally reached a point where I've been able to verify its performance meet the target I have set when I've started it, so I figured I should share it on the forum

I have tried to keep this post shorter and to the point, which is why I have omitted many measurements results, and comments about the circuit design. However, if you'd like to read in more detail, I have posted all of it with much more detail on
my blog.