QA403 Input Clipping way below level

Having an issue with my new QA403, I have a signal coming in @ 1.37 dBv and it clips the 6,12, and 18 dbv setting, when I change the attenuator, it seems like the ranges are not changing. 0 input works normal upto around 0.6 dBv, then it clips. Also the range 24-42 is fine as well and switches. I have verified the incoming voltage with my HP 34401A.


Hi @Fireanimal,

Can you try the procedure at the link below and check the calibration in working as expected in loopback? When you say it “clips” does that mean the distortion is very high, or that the attenuator clicks on momentarily over and over?

Ok I just checked, and loopback is normal, so not an issue with the QA403, seems to be my amplifier load bank causing it. Not sure why, its not clicking its visible distortion but only on 6,12,18 not the others? Dont see any thing on the scope either.

If you have a large-ish DC present, then when that is applied it will overload the ADC, which will activate the attenuator, which will attenuate the DC, and then when the DC settles the attenuator will de-active, which will cause the DC to be applied again, which causes overload.

See if you can settle the system at +24 dBV input, and then once settled and measuring as expected, then switch to +18 dBV, and see if that settles, and then try +12.

Some class D amps idle the output at half the rail voltage, which can result in some surprising DC levels. What type of amp is this? Looking at your schematic, your assumption is the amp is single-ended, is that correct?

Its a single ended class AB amp I am building. No DC offset issues, less then 1 mV. Strange thing, I moved the sense resistors to the end of the resistor string instead of the middle, and I was able to measure without issue. But then I hooked up a differential class D amp, and ran into the same issue I was having before, same type of clipping at the same power levels, but changing to +24 dBv fixed the problem again? Not sure what the root cause of this is. Thanks

There are two DC offset issues: The difference between measurement ground (BNC shells) and the difference between signal (eg common mode, or L+ versus L-).

To verify it’s a DC offset issue, connect the amp with the input of the amp shorted (eg no signal output). If you hear the relay click, it’s because there’s DC present.

The other thing to keep in mind is that if you apply DC to the input (for example -40V), then that voltage will be “held” on the input cap, and when you short the input, you’ll impress a large swing on the other side of the cap. You’ll see this if you apply a large DC (<50V absolute) to the input and then remove it, and then short the input and you’ll hear the atten relay click.

Your amp ground is connected to a BNC shell on the QA403, is that correct?

I have 2 BNC connectors, one is + and one is -. Only center pins, nothing connected to the shell.

OK, thanks. Is the ground at the input of the amp connected to the BNC shell on the QA40x?

Yes I am using a custom RCA to XLR cable I made, with RCA to BNC adapters. XLR Hot to L+ XLR Cold to L - and XLR Pin1 to both RCA/BNC Shells, this is to the input of the DUT. On my load bank, I only use Speaker + to L+ and Speaker - to L-. BNC shells are all floating. Thanks

Hi @Fireanimal, if the BNC shells on the QA403 are floating, then you haven’t established a common ground and so even if you have 0V offset in your amp, relative to the QA403 (which is floating) it could look like a very large offset.

And just to clarify, all the BNC shells on the QA403 are tied together.

Is there a connection between a QA403 BNC shell and your amp ground? And the amp ground is likely earth ground from the wall, is that right?

The output BNC’s of the QA403 are tied to ground on the input of the Amp. The amp is grounded to earth, but if I try to ground the input BNC’s to the amp ground point I get excess noise.

but if I try to ground the input BNC’s to the amp ground point I get excess noise.

The input and output BNCs shells are all tied together, so I’m puzzled why this would matter.

What DC voltage do you measure from the BNC shell to the amp ground and also to the amp output?

And with your amp and QA403 both plugged in, what is the voltage from the USB BNC shell on the QA403 to your amp ground?

Finally, what power supply are you using for amp? It does have a 3rd prong for earth ground, is that right?