PC sound card based distortion analyzers.

mondialfan

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#21
I've used some sound cards along with REW in the past and it works well for distortion measurements. As you've probably already seen, all you need is a simple voltage divider to limit the voltage fed to the soundcard in order to test. You can go quite simple in the dividers design. Early on I built an RCA cord with the resistors cobbled into the cord. However if you plan to use it a lot the best thing to do is to build one of the manual attenuators that has protection diodes built into the circuit to keep from blowing your sound cards input. One over voltage event and your sound card is toast. I attached a document I found that has the build up of a simple attenuator.

You could also look at an older Quantasylum QA400. They can be had for around $100 - $150. You'll still need an attenuator for the QA400 as it is essentially a purpose built sound card but it has specialized software for measuring audio gear. The only downside to the QA400 is that the older software for it will only work on Win7 and older computers. I still own a QA400 and it works well if you have an older version of Windows to run it. If you want to drop about $500 there is a newer QA402 that will be coming later this spring. The QA402 will have much improved specs, a fully autoranging input using relays (no attenuator needed) and Win10 compatible software. The measurement capabilities of a QA402 should exceed just about anything most DIYers will measure.
 

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George S.

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#22
Yeah, been looking at the Quantasylum units. Very nice! Just ordered linear taper pots and the diodes for a sound card interface. That schematic you provided looks very similar in concrpt. Currently rebuilding a HP 1742A scope with the optional multimeter on top and got that cheap Koolatron 60 MHz function and signal generator off Amazon. Once I get the HP scope aligned to my satisfaction, then it's sound card time. Thanks for your input. A used Quantasylum unit may be something I get in the future once I see what the sound card will do. Thanks!
 
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mondialfan

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#23
There are a few caveats to the older QA's that I forgot to mention in my last post. The QA400 in addition to the fact its not supported by Win10, and the fact it has no attenuator built in also has a rather limited oscillator output level and will not drive some amps to clipping. For just analysis purposes it is a good tool but if you'll be specing out your amps after rebuilds then you'll want something else. The later QA401 has a built in -20dB automatic attenuator but that will not handle a large amp like a PL700 (stock or WO'd) and the single range attenuator is not optimal for the ADC which likes to be fed by a voltage kept in a tighter range to minimize noise. The QA401 also has a somewhat limited oscillator output, its higher than the QA400 was but once again it may have trouble driving some amps to clipping.

The QA402 that is coming later this spring looks to be taking care of all of these caveats from the earlier models. The auto-ranging attenuator has several relays to keep the input level in the sweet spot for THD on the ADC that they're using in order to minimize noise with a max of -36dB of attenuation for large amps and it will have way more output voltage from the oscillator to drive any amp to clipping. If the QA402 lives up to the preproduction promises I may be replacing my AP Sys1 + QA400 with one of them. Anyway I just wanted to be clear on the downsides to these, you'll also experience some of these problems with most soundcards, especially in regards to the output voltage to drive an amp with.

BTW I've found that the test gear rabbit hole is as deep or deeper than the audio rabbit hole so be forewarned....
 

George S.

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#24
Have a old monster Dell XPS 720 set up with REW , a Sound Blaster X-Fi sound card, fresh install of 10. Going to learn how this works on old equipment before I set up a better system.
The HP1742A scope has been rebuilt and required minimal alignment. Had some overheated molex connecters in the power supply, dirty switches, dried out contact grease on the time base cards. Flood gun works perfect and easy to get a nice crisp trace after fixing the power supply and aligning the beam focus and intensity. I have about $150 into the scope and a like parts unit that also sort of worked but has similar power supply issues, easily fixed. I just offered the sellers on EBay a really low-ball offer and they didn't even counter offer. PXL_20210213_014327720.jpg PXL_20210213_152051644.jpg
The Koolertron DDS 60MHz unit was a Amazon purchase, and I really like it, so easy to use. And being I'm a hobbyist, it meets my needs perfectly.
So I'm on my way, just have to learn how to use this stuff and correctly interpret the results. Never stop learning.
 
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BlueCrab

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#27
I have a similar generator - they must have re-badged them with at least 5 or 6 different names. But I agree - they work well, are easy to use, and cheap. I wrote some software (mine came with some s/w) to make it easy to set up as a wobulator to use for aligning the IF in AM/FM radios (this can be done manually without too much trouble also).

I'm interested in how you get on with your setup.
 

George S.

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#28
I have a similar generator - they must have re-badged them with at least 5 or 6 different names. But I agree - they work well, are easy to use, and cheap. I wrote some software (mine came with some s/w) to make it easy to set up as a wobulator to use for aligning the IF in AM/FM radios (this can be done manually without too much trouble also).

I'm interested in how you get on with your setup.
Yeah I like how you can make a custom wave form and download it to the unit from the computer. I have a pile of 10+11 meter mobile radios to align, 2 nice DC power supplies that need checked for ripple and bad caps, etc . Have a big backlog of items to fix. I have a pretty well outfitted home garage for mechanical repairs, just been missing the tools for diagnosing and fixing electronics .
 

George S.

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#30
Yup, that's it. Amazingly easy to use. Sold under many different names. Cheaper versions are available, some of the Amazon reviewers talk about the other versions. For a hobbyist, much better than buying a 20 year old used unit that may well do less, be out of alignment, or need repair.
I got the HP scope because it was a CRT, not digital, very easy to fix with lots of info on the web. Dirt cheap so I could afford 2, keeping 1 for parts. Besides, I saw one in a Popular Electronics magazine article probably around 1980. I thought that digital multimeter mounted on top was just so cool. Well I finally got one. Turns out that multimeter works O.K., but it's pretty slow, and probably not as accurate as a modern Fluke. But for that time period, wow, very cool.
 

George S.

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#31
So, REW is very easy to use, and has many features including a Real Time Analyzer. I didn't need to build a interface as the built in tone generators output can be be adjusted, so that the preamps output I was measuring, would be a line level output, back into the sound card.
If you've used sound cards before, you probably know how buggy the software can be. Just installing the driver worked best. Loopback testing of the sound cards showed the Asus Xonar U7 Mk 2 usb card had a much lower noise floor then the ancient Creative X-Fi, however the X-Fi has a flatter response.
So I would in no way say the distortion #s and graphs REW generated are dead on accurate, it's probably very good for comparing before and after changes one makes like swapping opamps in a preamp. It is also very good at measuring different sound cards using loop back.
Here is a link to the first of four very short videos that got me started on how to test with REW.
 
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