Audio Measurement Pre-Amplifier – Part 5 – Updated Ver1.1

Over the past year and a half since posting the series of pages about the measurement pre-amplifier I’ve designed and built, I’ve received emails from multiple people who were interested in building the pre-amp. I’ve happily shared with them the remaining boards I’ve had from that first batch I ordered at the time. These few boards were all given out, and I’ve therefore ordered a few extra boards to be able to keep offering these boards to people who would like to build such an instrument for themselves. Unlike the first batch, this time I’ve printed boards that fixed the issues I’ve reported when building my own unit and were fixed by a “bodge” over the original board. These modifications to the board design (and the writing of this post) were all done well over a year a go when I’ve originally assembled my unit, but I didn’t see a need to post them until this point in time.

This post is meant to share the updated schematic, as well as to offer additional information that can be of help to people who would like to assemble such an instrument.  I didn’t make any functional changes to the pre-amp at this revision, therefore I will offer no additional measurements in this post.

Lets start with the change log of what was changed in v1.1 of the PCB:
1 – TRMS->DC converter uses low-Z input for extra BW. R50 changed to match differential load
2 – Protection diodes revised with the exception of the low capacitance nodes
3 – Added TP’s for VCCP15 and VCCM15
4 – Implemented optional CM control for OPA1632 via R62/R78/R79/C41
5 – Replace Diode (1N914) footprint to 0805 for easier soldering
6 – Correct footprint for DC-DC
7 – Power LED polarity fixed
8 – Remove DC-DC shielded case option

The changes are added as text note onto the schematic of v1.1 which is attached at the bottom of this post in PDF format.

An example Batch-Of-Materials (BOM) is attached as an excel file at the bottom of this post as well. I’ve included some notes for most of the parts. As always, you can find alternatives for most parts if there is a supply shortage or to save cost. Feel free to contact me if you think there is a mistake or something that is missing from the BOM.
One important note about the BOM has to do with the input switch (Loop-Back option). Please note that while this switch has a fairly high withstand voltage, it isn’t rated for switching high DC voltages. Therefore, if you are connecting the pre-amplifier to a source of high DC voltage, its recommended to avoid switching to the internal loop-back mode during this time, or else it might shorten the life of the components.

General assembly instructions are mostly what you expect to see in all DIY projects, with a few additions:
1 – Start with the smaller devices, so that you have easy access and aren’t blocked by larger devices that will get in the way of your soldering iron.
2 – Make sure you clean flux residue when you are done. Depending on the flux you use (explicitly or as the core of your solder wire), it might have undesired side effects. Therefore, unless you know you use a flux that is safe to leave on the board, I recommend cleaning it properly with either IPA or other PCB cleaning compound.  In fact, I recommend cleaning it either way, as you might be using high DC on the input, or low level signals for low noise measurements, so leaving a significant amount of flux on the board might not be good practice.
3 – The main board offers 2 optional solder jumpers to connect to the exposed metal lines at the edges. Choose which of them you want to use as the case ground, and solder the appropriate jumper (J16/J17) accordingly. Default choice is J17 for the signal ground.
4 – To make sure the case has proper connection to the PCB exposed metal stripes, its advised to scrape off some of the anodized covering of the case in the internal slot where the PCB will slide. You can even add a bit of solder on the PCB edge that makes contact with the case to make sure there is some pressure for a more robust contact. You can verify there is a good contact by measuring continuity with your DMM from the case to the PCB ground.
5 – As with any DIY electronics project. Once finished with assembly and the flux has been cleaned, verifying DC rail voltages is always good practice before moving on to using/testing the instrument.


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