Building my own balanced to unbalanced cables

I just pulled the trigger today and ordered a QA403. However, connecting cables will be an issue for me because I work almost exclusively on vintage tube preamps and amps with unbalanced inputs and outputs (plus speaker connections). The QA403 manual has some details about interfacing with various balanced connectors like XLR, but not much about connecting to unbalanced inputs and outputs. I would like to make my own cables to connect the QA403 to unbalanced inputs/outputs and think the configuration in the attached drawing would work. Note: the shield on the BNC jacks on the QA403 would not be connected. Please let me know if this looks good or if there are concerns or advice. Thanks.

No need to make your own unbalanced cables for testing preamps where you have RCA jacks, just use a bnc to rca adapter. Your diagram looks correct though. If you have not watched it, search for a video I did on Youtube on using the QA402/3- just search for “Using the QuantAsylum QA40x Audio Analyzer”- it may have some useful info since you are just getting started

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So I don’t need to connect anything to the center pin of the “-” BNC socket? The shield of the “+” BNC socket can be used as the ground or “-” connection therefore the shield of the shielded cable connects to the “+” BNC socket via the BNC to RCA adapter?


If you want to use the differential inputs on the QA (and benefit from the common mode cancellation), your wiring diagram is 100% correct.

@VAR 's suggestion to just use bnc-rca adaptors doesn’t do that with SE sources. If you are simply using SE input and output, RCA-BNC adaptors are fine and a single cable per channel.

Thanks to VAR and restorer-john. You made my life a lot easier!

Also, VAR-- I enjoyed your video on YouTube. Thanks for taking the time to make it.

@ScottRichardson- Glad you liked the video- I am debating on doing another on right now. Maybe I was not clear in that I was assuming you would be doing mainly Unbalanced (SE) testing. I may not have been clear about this as @restorer-john pointed out.

Yes, I will be doing exclusively unbalanced testing except for the speaker outputs of amps and if I test output transformers, crossovers, etc. However, all preamp connections and amp input connections will be unbalanced.

Hi @ScottRichardson. At this link
you can find very useful information to be able to reduce the noise induced in the QA and DUT connection. I hope it can be useful to you as you start to utlilize the QA403. I, at the time, applying the described rules, found benefit in measurements (reduction of induced ambient electrical noise).

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Claudio- Excellent! Clear and well-written. That will be extremely helpful in setting everything up. Thanks!

I went round and round with this.
A lot of people think the QA ground should be shorted to the DUT (device under test) shield etc. They are wrong.
The lowest noise, and most signal rejection I got was running the QA - center BNC to the shield of the DUT plug and the QA center + to the DUT + pin. The QA “ground” is just a shield over the cable but does not connect or short to anything on the device side.

I had an Ebay vendor make me the cable for 1/4" that works perfectly. I’m sure they can do it in RCA if you wish.

You might ask him to do a similar thing with XLR cables- with both male and female connectors…

I’ve bought quite a bit from this Ebay dealer. They have some great parts cheap especially their knock off of the Pomona BNC to 1/4" adapter.

I have multiple times requested a slight modification of an existing cable. They sometimes ask for a drawing to be clear but every time they did it with no problems. So if you want it with XLR or RCA or whatever just ask.

Quality is pretty good. They use really thick good RG-174 and Canare wire.
The jacks are o.k. quality, but not fantastic mostly Neutrik’s china version.

My only complaint is sometimes the strain relief is not adequate and I added extra heat shrink.

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look also here: Balanced cable for SE measurements - QuantAsylum Forum

For QA403 output → DUT input you need standart RCA cable + BNC to RCA adapter. Just leave the ‘-’ output open.

I sent a message to the cable seller on ebay about making a version with an RCA plug on the end instead of the 1/4" plug.

He already offers an RCA version: 1Pair RCA Phono to 2x BNC Male Cable 1~16FT For​ Quant Asylum QA40x audio tester | eBay

Here’s a question that I haven’t found addressed: Is the wire gauge important in connecting the dummy load to the QA403? I have some fairly thin BNC/coax cables that I can use unless something beefy is needed.

Hi @ScottRichardson. I connect my dummy load to the QA403 with BNC terminated RG316 cables. They have never given me any problems. I don’t know if that’s what you wanted to know.

I ended up making my own BNC cables using 12gauge wire into the BNC adapter (center terminal connected ownly, not the ground) that connect from my dummy load. It seems to work better for me, though I am not sure it makes a ton of difference… Kinda a pain to make them.

Cable from amplifier to dummy load (low impedance) carries all the currents, cable to QA device (high impedance) has minimal current

I have some Canare LV61S (75-ohm shielded video cable) with BNC connectors. Do you think that will be okay?