Comments welcomed on test fixture PCB for impedance measurements

Hi everyone,

I’m going to get this text fixture made. It’s basically the same as @matt 's from the blog posts on measuring speaker impedance and amplifier input impedance Speaker Impedance Measurements – QuantAsylum Measuring Input Impedance – QuantAsylum

I have tried to make things a little bit more versatile:

  • Dual jack socket/screw terminal block input and output (place one)
  • Multiple SMD footprint size (place one)
  • A through-hole resistor footprint for the non-cool kids

Any comments on preferred SMD size which should be included, layout or anything else?

I plan on putting this on GitHub with the BOM so anyone can order it.

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Very, very useful! Does the Input L- tie to ground?

Hi Matt,

Well there is a question for you. I have AMP- and SPK- connections which I have labelled as the COM net. I was unsure if this would reference the same ground as the QA403.

So I made the all QA403 reference AGND and all signals reference COM. Are they tied together inside the analyser?

Schematic attached

Hi @Dan, OK, I get it. I would normally terminate the L- analyzer input with a short as it’s unused. But your setup is better because it could be used to measure a push-pull amp differentially or not.

OK, thanks Matt,

I can tied those two grounds together at the input of the analyser. Then I’ll get these made. I did make a bit of a guess on which SMD sizes to use. Nothing very small to allow hand soldering.

@Dan I’d love to get this made but have never ordered a pcb board. Would it be too much to ask for you to document how you order it?

Hi @Moto. I order my PCBs from JLCPCB. I have always been fine with them and they are cheap and reliable. The order is easy to place. This link has all the necessary explanations: How do I place an order?. Obviously to place an order you need to have the gerber file of the PCB and the NCDRILL file of the holes to be drilled on the PCB available. JLCPCB accepts payments made by PayPal.

Hi both,

Yeah that’s good advice, I have also used them. Although PCBWay seems to be sponsoring all my YouTube content these days, maybe I should try :wink:

I will try and make it as easy as possible once I’ve put it on GitHub. Any improvement will be accepted!

PCB gerbers can be dowloaded from the project page on GitHub. Please don’t order this just yet until I can confirm it works!

There is a layout bug with revision 1 when using jack plugs. I linked the tip and and ring pins, this works fine for mono signals on stereo jack. However, insert a mono jack and everything is shorted to ground :face_with_peeking_eye:

Revision 2 update is in the works.

Revision 1 should work fine with terminal blocks.

Rev 1 is working provided you use mono jack sockets or coerce a TRS jack socket into a mono jack by pulling out the ring connections.

Rev 2 fixes this so either mono or stereo jack sockets can be used. I also improved the silk screen a little bit.

Thx @Dan . Ordered mine today.

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Received them already and built one except for the resistor. @matt what would you recommend using as a resistor value if I’m trying to measure the input impedance on an amp which should be about 10k?

Hi @Moto the rule of thumb works really well in the different measurements I have done, so something around 100 Ohms would work fine.

Thx @Dan . I’ll try that. I wonder. How much the measurements would suffer if I put a decade resistor box hookup there.

You could wire a decade box quite easily using the through-hole footprint. But the PCBs are pretty inexpensive, I got 5 made and put in 0.56, 1.0, 7.5, 100 Ohm resistors to cover a range of usages.

I used a 100 ohm resistor and Dan’s pcb and measured the input impedance of a unity gain buffer. It is incredibly simple. Unity gain, opa1656 noninverting, 10k ohm resistor from + to ground, no series output resistor.
Here is the measurement I’m getting. Could it be a set up problem or is the real impedance coming in around 7k with that

amount of variance?

Hi Moto,

I’m no expert, but this is almost certainly noise and not the actual impedance of your amplifier. Could you post the settings used in the software and also the physical connections and equipment used (or take a photo of the bench)?

Are you measuring the input impedance of this opamp in parallel with a 10k resistor? Why not just measure a 10k resistor to start with? Simplifying the setup might help?

I think you will get better results by improving the signal to noise ratio, which means lager input signal and no input attenuator.

If you change to a frequency response plot in the main GUI what are the magnitudes of the signals of the left and right channel inputs? Also I’ve found the oscilloscope viewer invaluable for debugging a real distortion issue in my setup recently.

tl;dr: play around with settings.


@Dan I ran it with just a 10k resistor. Results in pic. I had left 1.0 in gain when I ran the results in the pic above so ignore that. When I reran it with 0db gain it was almost the same as the 10k resistor.
When the output of the qa402 is attached to Driver on your board and the 10k ohm resistor is attached to Speaker, with no power on, the dc resistance is 6k ohms at Driver and Speaker terminals.
Pic is at 0dbv output from qa402.

Thanks for posting that, it’s still quite hard to debug this without seeing the individual signals from each channel. But this looks nothing like the impedance of a resistor! If I had to make a guess, something is wrong with the connections or board.

Is this a Rev B board? Using terminal blocks of jack plugs?