Connecting audio output to oscilloscope

[Cykar]

I'm a novice when it comes to oscilloscopes.
I recently got one, an old cro, which I intend to use for chasing signals, as well as trying some oscilloscope music - basically using synthesised audio to draw patterns on the oscilloscope using X-Y mode

(Jerobeam Fenderson is a good example: https://www.youtube.com/channel/UCECl4aNz5hvuRzW5fgCOHKQ)

So, initialy, I just connected up the wires from a 3.5mm jack plug with my scope probes - no problem, works just fine. But I'm after a neater more permanent cabled solution. Thing is, the more I look into adaptors and cables etc. etc. the more I seem to get confused (50ohm/75ohm stuff).

For example, I have a stereo 3.5mm jack plug to two phone/RCA male cable. I can get a couple of BNC/RCA adapters:
https://www.ebay.co.uk/itm/2-x-BNC-Male-to-RCA-Phono-Female-Connector-Adapter-Joiner-CCTV-Cable/371976030333?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20131017132637%26meid%3D3bed76ce9c014fe38765cc3040d0169a%26pid%3D100033%26rk%3D3%26rkt%3D8%26sd%3D192597486944%26itm%3D371976030333&_trksid=p2045573.c100033.m2042
that I could plug right on the end and then straight into the scope. The link is meant to be a 50ohm connectors, but most are 75ohm. Would this work? Is it advisable? Would it matter which impedance I chose?
Or else, should I be getting bnc to RCA cable then connecting them to an adapter to get it into the 3.5mm jack - but isn't that just adding a whole load of unnecessary extra cable?

I've read a lot about cables and impedances over the last 24hrs, so am I right in thinking that the impedance, 50 or 75ohm, really would make any difference when it comes to audio frequencies, it's just that if I use cables with adaptors I have to make sure the cables impedance and adaptors impedance match?

I hope someone can help clarify my confusion - thankyou.

[Yansi]

Connect it right up with the RCA adapter and don't even think about impedances.

The scope input is something like very high impedance for audio (1Mohm), and then on the other side you drive it with something like tens or hundreds of ohms source impedance.  But it won't matter much here, will work just fine.

But please be advised, the scope inputs are more then likely mains earth grounded.

Y.

[magic]

50Ω/75Ω is characteristic impedance, basically an impedance between the center conductor and ground that you see when you change input voltage into the cable faster than it even reaches the other end, which results in significant current into the cable as its internal capacitance is charged.
Analog audio signals are slow enough that it doesn't matter. Neither audio cables nor connectors are designed for any characteristic impedance, their characteristic impedance is random, anything will work.
Impedance specs of cheap adapters may be bogus, another reason not to care.

[Cykar]

magic & Yansi - thanks both.

I think I was over-thinking the whole thing. I'll just plug in and play - thanks again.

[David Hess]

At audio frequencies the transmission line impedance becomes irrelevant unless the line is thousands of feet long like a plain old telephone system's cable.

A 3.5mm to dual RCA to dual BNC adapter chain will work fine.  The thing to be careful of is that the oscilloscope ground connection through its power cord is the same as the ground connection of the audio source.  If they are not the same, there will be a ground loop which will cause 60Hz hum.

[Yansi]

It will cause 50Hz hum (because of UK)

[Electro Detective]

If you need to be absolutely sure of no BANGS or hummm...

two decent quality audio transformers that isolate/float both the signal and signal return/earth/ground

so there is no relationship to the oscilloscope's earth/ground

That said, I've been doing the RCA to BNC adapter thing for ages with no drama,
but will use a pair of audio transformers (2 or 3 way balanced config) in unknown or suspect scenarios 

It's cheap insurance you pay for once,
..and never loan out to anyone! 

[David Hess]

Unfortunately decent quality audio transformers are both rare and expensive.  If it is an issue, I usually make do with an instrumentation amplifier suitable for audio work or I transmit the signal as a current instead of a voltage.

[jomaroliv]

Hello,  regarding the audio transformers will this one be adequate (cheap)

https://www.conrad.ch/fr/p/neutrik-nte10-3-impedance-50000-tension-primaire-1-2-v-contenu-1-pc-s-515964.html

Or this most expensive https://www.conrad.ch/fr/p/neutrik-ntm4-impedance-10000-contenu-1-pc-s-516000.html

Thanks

[WatchfulEye]

Issues of grounding (loops and ground potential mismatch) can be avoided if you use a signal source powered by an internal battery and with no other electrical connections. (no other accessories, charger, etc. connected)

Smart phones and portable media players with an analogue headphone jack will work OK with just a 3.5mm jack to BNC cable or suitable series of adaptors.

Connecting anything mains powered or connected to other equipment needs care to avoid 50Hz hum contamination or equipment damage.

[Terry Bites]

Mike it up.

[GumpyGus]

Just hook them up directly.  For audio up to maybe 50KHz  any kind of wiring and connections will do.

Potential problemss:

(1) Ground loops.  Scopes almost always have one side of the input grounded to the case, and if its plugged into an AC outlet with a ground pin, that side of the input will share any ground hum present on the ground pin.   If your signal source is also grounded, there could be a few volts of hum which the low side wire may drop a few millivolts, adding a little hum to the input signal.  Solutions:  Plug scope and signal source into the same outlet.  Or if your scope has an A-B mode, plug your signal source across both A and B inputs.  With say a 1K resistor from A to scope ground to minimize the comm-mode junk.

(2)   You can also reduce hum by using an isolation transformer on one or both signal lines.  Problem is transformers have phase-shift so any A-B or X-Y signals will be less than straight lines and more like ellipses unless the transformers are identical.

(3)  You can also reduce hum by plugging the scope into an isolation transformer.  But watch out, don't put copious voltage on the scope case!   Don't ask me how I learned this.

[Ian.M]

(3)  You can also reduce hum by plugging the scope into an isolation transformer.  But watch out, don't put copious voltage on the scope case!   Don't ask me how I learned this.

*PLEASE* don't 'float your scope', (unless its officially designed and intended for floating operation).  Floating scopes have killed good engineers who *thought* they knew what they were doing!

Another alternative is a differential amplifier.   One dual OPAMP and eight 10K 0.1% resistors can be used to build a two channel unit gain differential amplifier, and then you simply take the incoming signal ground to the negative inputs of the differential amplifier, and the output signals will be re-referenced to the scope ground.  If you've only got a single (floating) supply, you may want to buy ten resistors and a quad OPAMP, and use one of the spare OPAMPs + 2 resistors as a rail-splitter for the internal (and output) ground.  If you've got  a 5 digit or better bench ohmmeter or DMM, you can select resistor pairs for as close to 1:1 ratio as you can get for better accuracy and CMRR.  The resistors connected to a single OPAMP input are a pair that must match, but the pair connected to the other input can differ in value a bit as long as their ratio is matched.  Use the worst pair for the rail splitter as supply rail balance is non-critical.

[Wallace Gasiewicz]

I have used the direct connection method. It is OK. Nice when you are aligning a receiver SSB.
I usually I just use a probe on the speaker leads with the speakers hooked up, like you did. This works fine also. You can install a jack, RCA or any other phono jack in parallel to the speaker and use the high impedance  of the scope also, if you want it to be a semi permanent installation. It might be fun to show it to the kids....Play scary music for Halloween and show it on the scope!!!!
You can use any coax or even speaker wire to the scope BNC, Whatever....
This will not upset the speaker impedance much at all. And I doubt that you could exceed the input volts of your scope.
But you have to listen to the speaker this way, I don't know if you want to listen.

PS: RCA jacks are 50 ohm.

[themadhippy]

PS: RCA jacks are 50 ohm.

apart from the 75 ohm ones

[Jeff eelcr]

While this is true for home or portable equipment be very careful with
Automotive equipment not all is common ground and hook up of two
channels without isolation can be very damaging to the equipment you
connect.
A or several 5000uf 50V capacitor can be used to isolate the - audio from
each of the scope BNC grounds.
Jeff

[Jeff eelcr]

Just a note on many of the 3.5mm adaptors.
They are often wired wrong (Left is tip).
Jeff