Interfacing connection / selection box diy?

Has anyone made an interface pcb for the QA401, like an AP box?
XLR sockets.
Switches for different loading, 10K, 600ohm, 50 ohm etc…
Single ended or differential stages

I’ll possibly embark on something myself but if someone’s already made one i’m happy to pay for it and save the time!

While it may seem simple it turns out to potentially be quite a project. There are a number of considerations-
Will in include an RIAA or an attenuator to low level for testing high gain circuits?
Are load resistors internal or external? What loads?
Balanced or single ended interfaces? Multiple in and out for test sources and analyzers? Switchable?
Parallel monitor connections for a scope or level meter?
Route audio through the DUT?
Speakers as loads as well?
I have a ST1200 test panel that does all this and more. its a good example of feature creep but its also proven pretty useful. They were created for stereo service at Pacific Stereo stores long ago. I have the full docs for it if you want to see what they did. The box is bigger that the ST1700 analyzer. Its also a huge PITA to wire up or disconnect for service.

I have been thinking about this for some time as well. I believe something could be put together for this that would be really useful, especially if it supported tractor integration. It would be more expensive that it seems it should because it would need good relays and possibly some active circuitry.
The other serious issue are all the different connections you need to real with:
Double Banana
1/4" TRS
1/4" mono
3.5MM TRS/stereo

And then you would want digital SPDIF in and out. And possibly AES-EBU as well.

All these considerations are why there are so few that have been made. The ST1200 really is it, AP has some switch boxes and you can get industrial switchers from HP/Keysight and Keithley for serious $$. Nothing really audio oriented.

You need to think of your requirements otherwise as 1audio states, you’ll get feature creep i.e. massive complexity. I build valve amps so my requirements would be:

XLR and RCA inputs - for convenience
600V DC blocking ability - for measuring noise on HT supplies and protection from valve HT
46dB of attenuation (excluding the existing 20dB) - for measuring valve amp levels max probably 300VAC
20-40dBs of gain - for measuring noise…though I am still unsure about whether this is necessary as the QA401 seems to do fairly well with low level signals.
6-10dB gain for QA401 outputs - I often need more than the ~1.8V the QA401 can provide

A pair of (battery powered?) op amps per channel configured as either buffers or to provide a fixed amount of gain would be the heart of the interface. Another pair per channel would provide the output gain requirement. DC blocking on caps on input and output would provide DC protection, Zener overvoltage protection would take care of unwanted AC voltages while a switched attenuator would provide suitable AC ranging or gain if incorporated. Multiple types of input sockets could just be paralleled. A switch to short the - inputs of the QA401 would allow for single-ended/differential operation. A meter/scope output should be paralleled with the input. And switched line level load resistors would require just that: a switch and resistors! This device should be mildly complex to fiddly to design and build…

yes it could get very complex but my area of interest is fairly narrow. (for the time being!)
I’m sound engineer / service tech / amateur designer.
So i’m mostly interested in distortion and freq analysis of balanced, line level compressors / eq’s etc.
I’d just like a plug in and play box for quick tests where i don’t have to mess around with a box full of special cables etc!

XLR i/o with a switch for unbalanced connections. (plus double banana for probes)
Maybe a 3 position H-pad (600ohm) on the input so i can drive output stages into distortion and see whats going on. (-30 -40 -60dB with trim for differential signals)
Load switches, 600ohm / 10K
termination switches.

I’ll put together a schematic and then everyone can pick it to pieces!

Thanks handsome, wrote the previuos before i read your post…

600V DC blocking ability - for measuring noise on HT supplies and protection from valve HT

I think i’ll just use my DSO for PSU noise measurements but it would be nice to have a saftey net built in!

I haven’t actually used my QA401 in anger yet!
You’re right tho i may need some additional drive capability as most of my work will be with 600ohm loads…

Ok it’s getting complex now!

Not terribly complex…
Last time I saw a 600ohm load the word “wireless” just referred to something used to receive radio stations on!

You’re right but my in-head designed box with a pair of XLR’s and a couple of toggle switches may require a little more thought!

Lot’s of the desirable studio kit goes back to designs from 60s radio studios. Gently overloading these pieces often gives the ‘magic’ sound engineers want. That’s what i’d like to analyse so i can understand what’s going on and come up with my own designs with modern parts.

Just found this…
It’s for the QA400 but basically what i’m after! I can use it as a basis for my design anyway.
I could test two balanced channel with this, which would be useful from a design/mod point of view.

Remember QA400 does not have differential inputs you might want to retain our QA401s diff inputs - I would have to defer to the more knowledgeable souls on this forum as to which is would be better; THAT’s diff to SE or QA401’s diff input.

Yes, thanks. I’m aware of the changes, quite a few bits i wouldn’t need.
By the looks of things there’s a bunch of people over on the diyaudio forums who’ve already gone down this rabbit hole so i’m going to wade through the posts there. I will report back my findings captain! ta

Perhaps Matt and company will be updating the QA401, not just for the sake of progress but because of the AKM disaster. ESS, as one example, claims to be offering better ADC parts at a competitive price. How well they actually work is TBD. If they meet the marketing release, a new QA40x would come that much closer to an Audio Precision system for far less money. (Not to 1 MHz, though…) That would all involve real work on the part of QA, obviously.

Maybe QA would consider retaining the BNC inputs and outputs for the unbalanced connections, but go to XLR for balanced/differential I/O in any new product. Perhaps another step or two of attenuation, too. With these two advances, your other desires could be easily managed with passive external boxes as you need them. Of course, that doesn’t help an existing QA401 owner…

My autoranger might be an option. The latest version has selectable nominal levels from 0.5V to 4V, bal + se input and output, full auto and manual ranging. Measurements with the recommended OPA1656 have THD levels below -140dBr. Zin is 10k or 20k. Isolated from mains for absolutely no hum spurs in spectrum.
See attached example 3rd party measurement.

I somehow neglected to mention the AR approach. My bad.

Is there a suitable adaptor board you can recommend for using the OPA1656 instead of the OPA2134 at U4?

A downside to the AutoRanger is that it’s a somewhat larger box than the QA products. So, you either need to appropriately adjust the feet placement on the AR, or make sure that it goes at the bottom of the test equipment stack on the test bench. :8^)

Hit “Reply” too soon.

Is the Brown Dog adaptor a good choice? I see that they’ll sell you one with the OPA1656 already installed. If you add in the shipping for a single opamp, that’s not a bad deal.

Jan’s box looks great. Have a day off tomorrow to explore options. Not bothered about box sizes, i’m going to re-house everything with a PSU/battery.

I’ve just had some SOIC to PDIP adapter pcb’s made up. Pm me your address and i’ll post one if you need. Cost pennies and had to order tons…

When I saw the Brown Dog units that are ready to plug-in, I ordered one. Besides, I’m sure that postage for sending anything to me would cost 0.1 Audio Precision units these days…

Thanks for the offer, though!

Just wanted to mention for anyone reading this thread that the circuit above is by a user over on the Groupdiy forum, dfuruta.
There’s some info about it there.

FYI, I’ve ordered on of Jan’s Autoranger 2 boxes. Will design the output side over christmas.