!st post, a little history where I am coming from: With revealing my age, I still use the Heathkit IM and Harmonic distortion analyzers to test my audio projects. Decided to get Multi-Instrument software. While it is nice software, I find it cumbersome to use the sound card interface. Seems like the QA402 is a good answer for me.
So, the question is can the QA402 do IM measurements or is this something I no longer need to worry about?
Hi @JWB474, yes, I think IM measurements are still very relevant, especially ITU (19 + 20 kHz) tones on a class D amp. There’s an old link here on manually making the measurements on the QA400. The QA401 added a plugin that sweeps the IMD performance over a range of analyzer output levels, and graphs the level at 1 kHz.
The QA402 will eventually pull that too, probably with some enhancements.
Great, Thanks for the response. Will the QA402 be able to be set up to run the old SMPTE standard of 60Hz and 7Khz in the 4:1 ratio?
Hi @JWB474, yes the QA402 has two sine generators and you can set the frequency and amplitude on each.
I became more interested in IMD distortion as I saw it spec’d in a lot of the “vintage” gear that I test. The automated tests in the QA40x do not appear to be testing the % of IM that I see in CEA-490-A para. 3.18 (to be honest, I am not sure if this is the latest spec which may not have been the one used in a lot of vintage gear), but will show here:
I decided to try doing a manual test using a span of 10Hz-50kHz on an amp I had which lists a maximum IMD of 0.005% for any combination of frequencies from 20hz-20khz at up to twice its maximum peak power level. I set up for the 19 &20khz tones at -13dBV and got the plot below:
I set markers at all the pertinent frequency and enter the dBV’s into an Excel (after converting dBV to RMS volts) and came up with an IMD of 0.00003151 %. This is a couple of orders of magnitude better than the worst case spec, but may be reasonable not being anywhere near the amp’s maximum output power. My question is whether or not this could be an automated test at some point in time ? I suppose one could dump the data as a CSV and import that into an Excel spreadsheet that just looked at the data for those five frequency points (after converting to RMS volts).Thoughts ?
In Kenwood old gear it is specified the IMD – SMPTE instead of the ITU-R you showed. I think the SMPTE is more actual.
The amp is a McIntosh from around 2000, and does not refer to SMPTE or ITU-R in its IMD call out. But it is a %.
Not knowing how Mcintosh measured the IMD, it’s a useless information. On AP site there is an explanation of various IMD standards. Clio uses the SMPTE.
Not sure who Clio is. I looked and an older IHF IMD spec (202) and it looks the same as the one that I quoted, without the equation:
I think this is what is being referenced as a % in their data sheet…?
Hi Scott,
in the '80 at the beginning of “computing aided audio measurement”, there were only 3 brands in the field (as far as I remember): Audiomatica with Clio, Audio Precision, MLSSA. The first two are still on the market. Reading their manuals, is very useful when entering the audio measuring world. In our days there are more options, and Quantasylum takes an important role in this area.
Now, from AP about IMD types:
So, to make a comparison between Mac (or any other brand) specifications and what you measure, you need to know HOW they measured, using which standard.
Thanks for the info. I will reach out to McIntosh and ask them and “report back” 