Just got my new QA403 along with QA451B and had some questions

I’m extremely new to using audio analyzers and know that I have a lot of reading to do. I thought I’d start off with some (hopefully) simple questions to get me started, like using BNC terminators on the unused inputs and outputs, are 50 ohm or 75 ohm better for that?

First off in the manual under QA40X analyzer features it states that a difference between the QA402 and Qa403 is that the QA402 uses a 24 bit ADC and DAC where the QA403 used a 32 bit ADC and DAC.

I’m sure this has been answered before, but on the QA403 itself it states that it features 24 bit ADC and DAC

I’m curious as to which it is, 24 or 32?

Next, I plan on measuring some fairly powerful amps with this, nearing 1000w at 8 ohms and doubt I’d do much distortion measurements with high power 4 ohm. I have a pair of 1000w 8 ohm non inductive loads I plan on using as external loads for the QA451, but from what I understand that being 89.5v ac rms it is but too much even with the 12db of attenuation of the QA451, so for the higher power amplifiers I’m guessing more external attenuation, but confused as to what that would look like when using single 8 ohm load resistors.

To get me started, is there any videos out there you folks would recommend to get me started on measuring a preamp and power amp. The first power amp I want to measure is roughly 250w at 8 ohms and I want to make sure I do this correctly so I don’t damage these new pieces of equipment. I learn MUCH better from watching rather than reading, so if there was something out there that could help me in that aspect it would be greatly appreciated!!!


Oh and one other question I had: the inputs of the QA403 4.7uF 50v non polar electrolytics to protect the circuitry from high DC voltage. I’ve seen amplifiers short an output and put their rail voltages on the output right in front of me. I have some amplifiers I am wanting to measure that have rail voltages of +/- 115 Vdc. Would it be wise to put in a capacitor in series with the output as a “just in case” I have a few 75uF and 100uF and I think even 140uF polypropylene 600v. I imagine that two 75s in parallel making 150uF wouldn’t skew any measurements. Thoughts?


Hi @saabracer23, these are all good questions. A great place to start, if you’ve not been there already, is the github wiki located HERE. In the Getting Started linked off the main page (see the table of contents on the right side), you’ll see that terminators are for unused INPUTS. If you short the outputs for long periods, they excessive current can cause the output opamps to overheat. They are thermally protected, but extended operation at high temps can cause some parameters of the opamps to shift according to TI.

The 24- versus 32-bit ADC are what is advertised by the ADC makers (TI for the QA402 and ESS for the QA403). Usually, these numbers represent some of the internal signal paths. In reality, what is most interesting from a performance perspective is equivalent number of bits (ENOB). The QA402 is probably somewhere around 18-19 bits, while the QA403 is around 19-20 bits.

At the link below, there’s a discussion on how to build an attenuator. For the measurement at the link, the aim was a two ohm load. But if you took the 1 ohm resistors in the picture below, and made them 4 ohms, that would give you a total load of 8.1 ohms and an attenuation of 20*log10(0.1/8.1)=-38.2 dB. So, that would take your 89.5rms = 39.03 dBV down to 0.8 dBV, and you’d measure that on the 6 dBV full scale input.


User @VAR does a lot of amp measurements, which are very thorough. Take a look at his analysis on the McIntosh MC352 located HERE.

The easiest way to protect the QA403 against a catastrophic fault is to engage the attenuator (set full scale input to 24 dBV or more) . That will put a series 93Kohm resistor in the input, and even with 200V of DC at the input, the current will be limited to 2 mA. Depending on how long the fault exists, the input resistor might open up. But if so, it’s easy to diagnose and easy to fix. But with the attenuator engaged, a 100V DC fault can be sustained for a long time. At some point at that DC voltage, the input capacitor will probably start to fail. But, again, it’s easy to diagnose and replace.

In the end, aside from practicing normal safety procedures so that YOU don’t get hurt, you should feel comfortable leaving the attenuator on while measuring high-power amps confident that the analyzer can weather common faults. And once you are confident with your setup and levels, switch the attenuator off if needed for the noise. So, let’s assume you built the attenuator suggested above (dual 4 ohm and 0.1 sense). Put the analyzer at 24 dBV full scale input, verify the levels measured match what is expected (0.8 dBV), make a few manual measurements as needed with DVM, and then once you are sure everything is running the way you like, switch to +6 dBV.

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@saabracer23- Matt referred to a video I did on a high power (350w) amp, but it did not really go into detail as to my setups. I decided to put together a video that shows how test test a power amp and what my load looks like and how I did my taps when measuring high power (up to 800w into 4 or 8ohms as it is configured). It covers the cables and connectors that I use, as well as what powered usb adapter hub works for me. The video is scheduled to be released on Jan21st on my little Youtube channel. Hopefully it will be helpful to you and others. Outlet strip Circuit breakers and equipment fuses start to blow when you start pushing the amp towards it limits- tones are harder on them than most music from what I have seen.

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Beautiful! That’s exactly what I was hoping for. That video is going to absolutely help me, to be honest more than help. I’ll wait for the video, now to read on the QA451. I haven’t watched a video of yours yet, but will as soon as I leave here.

Thank you!

Good luck & thanks for watching

Just so I know, let’s say I want to test a lower power receiver. Like 100 W per channel at 8 ohms, which is about 28.28 vac rms.

To make sure I hook it up right, hook the output of the amp to an 8 ohm load as usual and then use a BNC to alligator clips and hook the QA403 directly across that same 8 ohm load? And being that low no need for attenuation correct.

I’m not going to be doing any big amps until I see your video, but in the mean time I have some 25w to 90w @ 8 ohm amps I’d like to check as practice in the meantime. Need to get used to using this to get it lol.


Yes, you should be able to connect directly up to the 8ohm resistor the way you mentioned and measure up to 100w/ohms- you will probably need to set the QA403’s attenuator when you are measuring 100w.

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