Newbie Questions

Hello All,

I’m new to QuantAsylum and am expecting UPS to deliver my QA-403 later today! I’ve already been reading a lot on this site and am really looking forward to getting up to speed on the software, once I have the hardware in hand to play around with.

I’ve been accumulating gear for years and have been doing what I’d characterize as “very basic” bench testing of vintage preamps, power amps, integrated amps and receivers using a homebrew of ARTA software, a Motu M-2 USB Audio Interface, bench DVM’s, and a DSO. Mainly I do Frequency Reponse plots and spectrum analysis to get THD+N at 1kHz and at various power levels (usually 1W rms and peak “non-clipping” power).

So I’m looking to the QA-403 to enable me to do more sophisticated, rigorous and repeatable testing. But I fully expect to be on a learning curve for a while. So I’m hoping maybe you all can help me with a little advice/sanity checking as I get started? Thanks in advance!

Also, I’m interested in a reasonable degree of precision, but not going crazy over “ultra-precise” measurements. In other words, for my purposes I am not concerned with THD+N to the 5th decimal place. I’m much more concerned with repeatability and usability, and 3 decimal places is generally more than enough for the gear I measure. If I can eventually end up getting “reasonably” reliable results down to the 4th decimal from the QA and my setup (cables/loads/environment/procedures and practices/etc.) then I will be ecstatic.

Okay, so for power amp measurements I have some really nice Vishay Dale industrial non-inductive load resistors that I’ve been pretty happy with. And I’m planning to take advantage of the built-in attenuation options of the QA-403, but I also am planning on building in a -6dB attenuation network to provide for headroom and minimize the possibility of doing something dumb. Below is a schematic of my plan, I will mount it all in a metal enclosure with a heatsink. So do I have the basic idea correct?

And then can anyone give practical advice on what would be a solid choice for interconnecting the signal back to the QA-403 inputs? Advantages/disadvantages of using the QA-403 inputs single-ended with basic RG58C/U 50 Ohm BNC cables? Or is differential input really the only game in town? Getting the differential cabling done correctly seems a little intimidating based on a few of the threads I’ve seen on here, so maybe I look at tackling differential later on?

Cheers! - Steve


Welcome to the “family”. I am basically doing what you are doing- testing audio gear- mainly “vintage”. For measuring power amps I use a dummy load I made and you can watch a video on it here:

If you are going to get above about 100w/8ohms or 200w/4ohms you want to use the taps on the loads and adjust the software(dBV options) to reflect the attention (-2.5dB/ -18.1dB (8ohms) or 12dB (4ohms) for my loads). All my measurements are unbalanced. When measuring preamps (unbalanced) make sure the “-” inputs for each channel are terminated into shorts or loads. I love my QA402 and the support is great. Best piece of gear I have purchased.


THD vs Power plot
Testing Yamaha M-70
4ohm Dummy Loads
Recommended settings?

Yes, so you can see what is happening here: You have no external gain specified (otherwise you’d see the INPUT GAIN message in the upper left corner. I am using the QA403, which doesn’t display in the same manner as the prior models/

So, we have an amp gain of X, your divider gain of -6, and a reported gain of about -1 dB. So, we know that when you input -6 dBV to your amp, the amp provides 6 dB of gain, and then your attenuation provides -6 dB of gain. So, you put in -6 and you get out -6.

You need to specify your -6 dB external gain in the DBV context menu. Set the generator to -50, then crank the volume control on your amp (because we want to learn the max gain), and then read the reported gain.