Frederico,
If you have truly left, I feel badly about that. You asked for some realistic advice and asked legitimate questions. I hope you get some somewhere.
In case you still can't help yourself from sneaking a peek here, I'll explain my strategy for test gear.
I wanted to be able to measure audio gear I built. That implies measuring components, too. In addition, this is not a full time activity, so I wanted to purchase gear that didn't take up a lot of space or cost a ton. 99.96% of the time my test equipment sits on a shelf in the closet not taking up space on my work table.
Here's my list of what I bought, my reasoning, and what I found:
DMM - I've owned a Fluke 87 mark something or other for more than 30 years. It still works great.
Oscilloscope & more - I purchased a
Diligent Analog Discovery 2 a few years back. For as often as I use a scope, it works great. The software lets me do a lot. I can perform
square wave testing on an amplifier circuit. It allows me to make
amplitude response measurements far beyond the audio band. It also provides a
spectrum analyzer good up to maybe 10 MHz or so. I can
measure passive components at various frequencies. I've built some accessory pieces that allow me to
measure the closed loop gain and phase margin for amplifiers. Easily
programmed, too.
If I was using a scope as a central tool for repairs, I'd probably purchase one of the very new Rigol scopes. That is a faster tool to use because it takes no set-up - just turn it on, set a couple knobs, and probe. Less flexible than the Analog Discovery 2, but wider bandwidth. So far, I haven't found a justification for one of those, although I've tried.
🙂
Regardless of the scope you choose, it's good to have both a
differential probe system and a
current probe. Sadly, neither of the ones I bought and just linked to are available any longer. (If anybody knows of some reasonable alternatives, I'd be very interested.) It's amazing to see what currents are flowing where when you have a current probe to look at that. It makes you realize some of the imperfections that you don't easily observe in SPICE simulations unless you really make the effort to create realistic simulation circuits. Basic stuff that is basically ignored.
Basic LCR meter -
DE-5000 I bought this before I bought the Analog Discovery 2 and a lot of the measurement capabilities overlap. But, the DE-5000 allows for really fast measurements.
Semiconductor tester - For sorting semiconductors, this is really useful:
DCA75. You can do a lot of the same with the Analog Discovery, but the Peak device is really simple and fast. You can sort JFETs for Idss in seconds.
Distortion testing - When they were first available, I bought a
QuantAsylum QA401. The newer QA40x devices have much better distortion measuring capabilities because of the better DAC and ADC parts available now. But, I haven't tried those. (More on that in a bit.) The QA401 is great for lots of measurements, like noise, frequency response, and so on. It just is limited for distortion testing. I bought a
QA480 to supplement the basic QA401 and it works great and does get me to -150 dBC harmonic measurement of a 1 KHz tone.
During the great component shortage, QuantAsylum was forced to redesign their products and were held back in production for lots of months. During that time, a company called E1DA arrived with some new products:
Cosmos Audio Test Gear. You pretty much know about those. With a
Victor Oscillator as a test source, I can now reliably make simple harmonic distortion measurements at much better than -150 dBC when using the
APU and the rest. That level of test resolution really pretty much allows you to get a good understanding of the components you use and the circuits you build. Without the APU in line, the distortion limits of my
ADCiso and
Scaler combination are close to -140 dBC using the Victor oscillator as the source. That's still really great and is 30 or more dB better than I could get with the unaugmented QA401.
If you want to go beyond testing a single tone as from the Victor oscillator, like for various multitone IMD tests, you need a DAC as a source. This article caught my eye:
PMA Test System. So, I purchased a
Topping D10s. It performed exactly as described in the article. Based on a different review, I then purchased a
Topping E50 because it has both unbalanced and balanced outputs and promised better performance. Indeed, the noise performance of the E50 is about 10 dB better than with the D10s. The distortion performance is variable - it takes some adjustments of the software to make it approach what the reviews describe. (I think this shows some of the ups and downs of distortion compensation.) Both of these DACs will do the job. The E50 I have gets to better than -135 dBc for each harmonic and the D10s gets to about -125 dBc. But, if E1DA releases the
Cosmos DAC, I'll probably purchase one.
I tend to use two pieces of software for distortion testing.
REW is the prime one and works across Windows, macOS, and Linux. It really does everything and works very well indeed. I also use
Multitone.
With this test set-up, I can make measurements almost as good as what can be done with an Audio Precision test set. In some ways, even better. Yeah, it took a few hours to learn some of the subtleties, like which drivers to use and the various settings, but that's also true when you first start using an AP system. I think something similar to my system would do what you want. I also suspect that the QA403/404 would as well. I just can't speak to that because I haven't tried it, so I don't know what the new software capabilities are. But, QuantAsylum makes very well engineered and thoroughly thought out products. Having everything in one box with test software specifically designed for its use is a great advantage in some ways.
Beyond - I also have a few other pieces that are mostly used for RF testing, but also are helpful with audio projects, too. Just not that often. Here's a list:
tinySA
NanoVNA
Signal Generator
VFO/Signal Generator
OK, that's a lot. Of course, you don't need all that stuff. In fact,
I don't need all that stuff. At least a third of that is redundant and essentially has been replaced with newer, better gear, as I described. (I just need to figure out what to do with the surplus.) But, it all still fits on a single shelf in the closet.
Hope that helps you or somebody. I'm no expert, but I pretty much know what has worked for me and what hasn't.
Oh - one other thing. Experimenting with this kind of thing is what gets you to be one of the best engineers on the planet.
Usual disclaimer: I have no, zero, zip connection with any of the companies mentioned above. None. I have purchased whatever I have from them at normal retail prices like everybody else. I think I may have saved a few bucks once or twice when a piece was on sale for some reason. The people at those companies wouldn't know me if I knocked on their door and was wearing a name badge.
