My new QA401 has a problem

I bought this new about 2 weeks ago and have been using it to get familiar with it’s functions. So far I really like what I see here. But now I have a problem. My left output no longer puts out a differential signal, It works OK in a single ended mode but the out - does not put out a signal. The right output works as it should. On the left output with a 0 dBV signal I get 1 Vrms at the out + but nothing at the out - . The right outputs works correct as I get 1 Vrms at the out + and 2 Vrms measured differentially across the + and - outputs.

I have not put in any high voltage into the inputs as I have been using it to measure THD of my 3.5 watt tube amplifier that I am building.

Do I need to send this back into the company for repair? Could this be a software issue?

Hi @dbotich, my first guess is the L- output resistor has opened up. Could you please try the following:

  1. Close the app and unplug the QA401
  2. Restart the app and pick File->Reset Startup Defaults
  3. Plug in the QA401 and wait for it to confgure

The above two steps will put everything back to a known default state to ensure it’s not a settings issue. And then:

  1. Short L- Input

  2. Connect L+ Output to L+ Input

  3. Enable GEN1 at the default -10 dBV level, press the RUN button to start running

  4. After a few cycles, press STOP and do a Edit->Capture Bitmap @ 800 x 480 and paste it here

  5. Now connect L- Output to L+ input, run a few cycles, grab a capture and paste it here.


1 Like

I followed your instructions and here are the results
Here is step 7 L+ out to L+ in

Here is step 8 L- out to L- in


Thanks @dbotich , yes, it appears the output resistor has opened. This is a normal, protective action if the resistor exceeds 1/10th of a watt for some period of time. The precise part used between the opamp output and the BNC is usually a RT0603BRD0747RL, which is a 470 ohm 0603 0.1% 25 PPM thin film resistor. The resistor won’t generally open, even if the output is shorted at max amplitude into a dead short. But it can open if the outputs are momentarily exposed to a larger output (such as an amp).

I could send you a few resistors on cut tape if you’d like to keep using it and replace yourself when time permits, OR I could send you a return label and you could ship in back and we’d do a same-day repair (no charge), verify it is meeting spec and return via USPS

Please just let me know. Thanks, Matt


Is the resistor a standard type or surface mount? If it is a
standard type that goes into a PC board then I can just replace
it. Can you send me a pic of it in circuit? If it requires a
delicate operation (surface mount) then I will send it in to you.
I am sure that I did not use it incorrectly as I have been using
test equipment, both analog and digital, for years now. I have
been mostly using it to send a SE signal up to 6dBV into my
prototype SE tube amp. I did try to use a differential output
signal through a transformer to convert it to single ended. That
seemed to work OK until the L- output failed. I never shorted the
output nor did I ever input any V into the output.

My amp has an RCA SE input. Can I send a differential signal from
QA401 using the L+ into the + side of the amp input and the L-
going to the ground side of my amp? I need to be able to use the
maximum output (4V output +6dBV) of the QA401 to drive my amp to
soft clipping (5% THD and about 4 watts output into 8 ohm load or
5.6Vrms) for testing. What is the best way to convert to the
output to single ended if I want the full differential V swing?

On another note: Why does the generator run intermittently while
running tests? Can it run continuously or is this to protect the


Hi @dbotich, to generate a continuous tone, you can use the Settings->Generate Fixed Tone and enter the frequency and level to generate a continuous tone. This is useful if you are checking levels with a DVM to ensure you have all the gains/offsets set up correctly in the QA401. The QA401 doesn’t make measurements in this mode.

I’ll send you a label so we can check it out. Maybe driving into the transformer was a bit much depending on the impedance. That would be 4Vrms = 11.3Vpp across 100 ohms, so the peak power on the tips would be 1.3W or about 13X more than the part rating, though it would be knocked down by transformer Z and opamp output current limit. If you measure the or know the transformer Z at 1 kHz we can work out the max permissible drive level.

All the QA401 output and inputs share the same ground, which is the BNC outer conductor. You want that to be tied to your amp ground. So, if your amp is SE input, then you cannot drive as you are proposing.

If you need a lot more drive, your best bet is to build a simple +10 dBV amp. The QA470 has two general purpose low-noise amps (based on OPA1612). One amp is +10 dB, the other is +30. Both amps can output up to 8Vrms.

OK I will send it back to you.

I really appreciate you helping me with and your expert information is great.

So this brings up the question, can the 401 be damaged by it’s own output? By impedance mismatch of DUT? Is the SE output less likely to be damaged vs using the differential output?

I really want to understand how not to damage this by my setups. I have been using ARTA with an ASIO IO and Picoscope and Siglent O scope for a few years. I really like what I see so far (other then this issue) with the QA401 and I think it will meet my audio amp testing needs once I get this figured out.

I understand that for me to get the full 4 Vrms output of the 401 I can;

  1. Use the SE output with an amplifier with at least 6 dB gain

  2. Use a differential to SE converter/amplifier such as attached PDF from Analog Devices.

Will a balanced line to SE converter work with a differential output? Such as this one on eBay attached image below


(Attachment raq-issue-145.pdf is missing)

Hi @dbotich, happy to help!

If the QA401 + or - output is shorted momentarily at max voltage, or for extended periods at less than maximum voltage, it’s not generally a problem. But if the + output is shorted directly into the - output, then that can open the output resistor at higher voltage levels. This is by design: If excessive currents are flowing you want the resistor to open rather than the opamp to get so hot that it causes a permanent and silent shift in performance.

The ebay circuit would probably work, but you’d want to make sure its distortion and noise is less than the amp you are measuring.

OK so maybe my transformer across the out + and out - was acting like a short which caused the output resistor to open. I won’t do that again. If I want to test a transformer is it OK to use it in SE output mode without causing any problems? Say I want to look at freq response and THD of a transformer and I load the secondary with the appropriate impedance load. I have a GenRad watt meter that has multiply reactance loads from around 1 ohm to 32K ohms that I use for loading the DUT.

On another note I would like to be able to measure noise and ripple of my high DC voltage B+ power supply (350VDC ). I know not to put that into the 401 as it will fry it. I have used a DC blocking capacitor and resistor and measured the AC across the resistor in the past to do this. That usually measures in the mv range which I can convert to dB. What is your recommendation to measure high V DC power supplies?

Yes, you can use the QA401 to drive single-ended or differentially into a transformer, you just need to limit the levels depending on your load impedance. This is true on all test equipment.

Remember the output opamp max output is around 65 mA (round number) and you can find that on the TI data sheet for the OPA1612. TI doesn’t go into great detail on the specs versus the output current, but a reasonable assumption is that the opamp isn’t meeting normal specs when you are asking it to drive that hard. You can get a bit of an idea on the degradation from the data sheet. The plot below is from the OPA1612 data sheet. Look how much the performance of the opamp degrades between a 2K load and 600 ohm load. These are for 3Vrms signals–about 5 mA of drive current. It’s pretty substantial, and the OPA1612 is a stellar modern opamp.


But let’s say we want to limit the opamp currents to +/- 20 mA peak to ensure it’s performance will be solid. And let’s say we want to drive differentially into a 10 ohm load (a near short) across the + and - outputs. And we know the QA401 output impedance is 50 ohms on the + and 50 ohms on the minus. That means a total load of 110 ohms (50 + 50 + 10). If we want to limit peak output to 20 mA, then that means the max voltage (the tip of the sine) will be 0.02 A * 110 = 2.2V peak. Now, since this is differential, that means the high side is driving at 1.1V, and the low side is driving at -1.1V.

And these are peaks, so the RMS is 1.1/1.41 = 780 mV = -2 dBV.

OK, so we know if we drive the 10 ohms load differentially the current the output opamp is dealing with with will be in the +/- 20 mA peak = 14.1mA RMS. The output 50 ohm resistors will be seeing 0.02^2 * 100 = 20 mW, well below the 100 mW rating.

So, if you set the QA401 to drive at -2 dBV, and then connect a 10 ohm resistor across the + and - outputs, the QA401 will happily drive it all day. In short, you need to do a bit of math to make sure you aren’t asking the OPA1612 to drive too hard. Because if you are, you don’t have any way to know if the measurements you are seeing is due to the DUT or due to the output opamp being asked to work too hard.

For measuring high voltage supplies, the reason just a cap won’t work is as follows. Let’s say you insert a 1uF cap in series with the QA401 input, and you pick a cap rated for 500V. With the 100K input impedance of the QA401, you get a high-pass corner of 1.5 Hz. Your circuit looks as follows, where C1 is your added cap and R1 is the QA401 input impedance. The VF1 measurement is the voltage into the QA401.


And then you connect to the 300V power supply at t = 1 second. Below in the transient simulation you see the output presented to the QA401 nearly approaches 300V for 100 mS. And that 300V for 100 mS will kill the QA401. And you’ll get a similar response when you remove the high voltage supply.

What you really need to measure high voltage supplies is a circuit as follows. C1 is your DC blocking cap. The 1K R2 and R3 are current limiting caps. And Z1 is a very, very fast TVS device that will clamp at 3-4 volts (ignore the part number on the schematic).

The thermal requirements on R2 are pretty tough. R2 will need to cope with momentary currents of 350/1k = 350 mA, which means momentary power of 122 watts. So these are big resistors. You can use higher value resistors to help with power, but they will add noise. So, you’d need to consider your supply noise and potentially bump up the 1K to help with dissipation. The output, as you can see below, is clamped at +6V when connected to HV supply, and minus ~1V when removed form HV supply OR when supply is connected backwards on accident.

Again, the above is just a concept and a lot of thought would be needed into part selection to ensure it was safe. I’m not saying the above is safe–it’s just to show that a simple cap won’t accomplish the job.

Would this TVS work in this application:

It’s hard to find any lower voltage clamping TVSs…

Hi @Ilya, yes, I think this would work. But before you connect to the QA401, verify it’s clamping as expected with a scope when you connect to a HV supply. TVS devices are insanely fast. But suboptimal layout + inductance can sometimes mean it’s not clamping as you’d normally expect.

@matt thanks for your comments. I’m planning to build a DC iso box that will be useful for other low voltage devices besides QA401. I’ll post my build and measurements separately when the device is ready.
I’ve run a quick sim of the circuit and changed a power resistor to 2.2k (calculates to 50W instead of 1k/100W), and a cap can be smaller as well - 0.47uF shows -3dB point at 3.5Hz which is fine with me.

However, my sim shows that the top rolls off at ~20kH (-3dB point). Is this because of the TVS junction capacitance (can it be that large though)? Or is it just the sim goofing up?

The part you referenced doesn’t have a typical capacitance that I can see. But you can get a flavor of how the capacitance might change on a part such the SMAJxx. There you can see the capacitance is probably 1-2 nF. With the 2.2K that puts you north of 20K. So, perhaps the part you referenced is higher capacitance (lower voltage) and the 20k corner is legit.

Do you know roughly what the output impedance of a tube HV power supply might be for a power amp?


Looks like that TVS has a high junction capacitance indeed. I’ve searched for other parts that I can get locally (that means fast). Here’s a promising one (just 80pF!):

This should leave plenty of room at the top end.

The second question is a tough one. It’s a SMPS booster for tube preamp. As far as I can find information about output impedance of boost converters, it’s an output cap in parallel with the load. Output cap is 10uF, a load is mostly resistive for these purposes, drawing 20mA (15k @ 300V). So it boils down to the ESR of the cap which is rather low, no more than 1-2R worst case. But if you count in 100n ceramic in parallel, this will be order of magnitude less.