Oscilloscope for basic audio testing

[ferrisio]

Hello, I'm thinking of adding a cheap basic used 'eBay special' oscilloscope to my toolbox for basic testing - nice to actually 'see' clipping when setting up systems/troubleshooting etc. I also play with valve amps, so will come in handy for biasing/setting up etc. I'm also hoping it will spur me on to learn more theory by being able to investigate real-life circuits to see whats going on, practical examples of the theory if you will.

 

Are there any 'must-have' features that I should look for? I'm guessing that a fairly basic model will do what I need. I did search before posting this so apologies if I've missed a thread somewhere.

[ChrisD]

Features all depend on how much you want to spend, and what sort of signals you want to look at. If you want a scope just for audio, you won't need much above 2KHz, but radio, and video, you'll need 20MHz or so, if not more. Other things, like triggering and storage is useful, along with a signal generator, so you can compare the input signal with the signal you are sending in. Also, go dual channel if you can, makes it easier to compare signals (i.e. on a balanced XLR, the in phase, and out of phase halves can easily be overlayed, or summed to show interference, using a dual channel 'scope).

 

Hope that is of some use to you, but just be wary of resolution and quantization with cheap digital scopes.

 

Cheers

 

Chris

[Bobbsy]

ChrisD sums it up well (except I think he meant to say going up to 20 kilohertz for sound, not 2!)

 

You'll have to decide for yourself whether you have any need to go up to video frequencies--it can be very useful but adds a fair bit to your price. Of the other features mentioned, ones I find very useful are the dual channel operation, storage...and having the right connections on the input without resorting to adaptors.

 

I'd also second the warning about the quantisation on cheap digital scopes...frankly, some of the ones I've seen have been so bad as to be useless so try before you buy if you go down that route.

 

Bob

[ChrisD]

Sorry, yes, Bobbsy is right. 2KHz would be a little futile!

[Lamplighter]

I'd second Ebay as the way to go, My trusty 30 year old, very expensive, Tectronics finally died recently, I picked up a fairly new Hameg dual beam for less than £100. Unless you want lab standard measurements, I would suggest an analogue scope. The cheapest units are suitable to look for clipping but you really need 10Mhz bandwidth if you want to see harmonic distortion, crossover artifacts and instability. You would also need a suitable low distortion tone source. It is useful to have a mu-metal tube screen if you are using it in the proximity of any mains powered equipment. These are not found on the cheapest instruments. You should be able to pick up a single beam Hameg for around £40.

Brian

[dbuckley]

I'd also second the warning about the quantisation on cheap digital scopes...frankly...

 

I think thats a little unfair; I've got a "somewhat older" Lecroy scope, and its the sort of instrument that was easily the wrong side of $10K when it was state of the art, and it's got all of 256 bits of resolution. You just have to be careful about setting up the thing to get decent measurements. On the other hand, the advantages a digital scope brings in terms of analysis whacks my old analogs into the "not used very much these days" category.

[ferrisio]

Fantastic stuff, with special thanks to ChrisD! All very common-sense stuff, which is usually the sign of good advice. Quite happy to stick in the audio range for now, keeping the price down and leaving me an upgrade path when my skills have increased to warrant the greater range/bandwidth and feature set. Now just hoping lots of other lurking Blue Roomers aren't all heading over to eBay to compete with me! Thanks again.

[Gareth Connor]

Hi Ferrisio,

 

My first post... although I have been a "lurker" for many months!

 

Just to drop in my tuppence worth.....

 

2-beam is very useful for overlaying left & right signals for basic stereo level-check differences, or for direct comparison of input & output signals, or if you have/acquire a distortion analyser, for looking at the signal and the distortion residual. With the usefulness of 2-beams, I would have nothing less.

 

Regarding bandwidth, I would go as high as your budget will allow. You can't hear much above 20kHz, but you sure can see it if you have a good 'scope. The high bandwidth is great for investigating that really annoying scratchy-pot DC that is actually an RF signal being generated by an op-amp that's going unstable due to lack of power rail bypassing due to dried out caps (real life example from a mixing console refurbishment).

 

Phase checking is also far easier with a 2-beam 'scope, especially with an external trigger input which is used to determine a reference.

 

A built-in square-wave generator for alignment of test probes is useful..... a square-wave and a well-trimmed probe will tell you a world of information about the performance of an audio circuit.

 

Delayed timebase - useful but not essential in audio work.... Can't remember when I last used it.

Storage - useful but not essential in audio..... Would possibly use it if I had it, but no examples of requirements come to mind.

 

E-bay is a good source, as are some of the military surplus establishments.

 

Gareth.

[cedd]

Ask your local colleges if they're clearing any out. That'where I got mine from (Hameg 2Mhz) as well as a couple of sig gen's and a bench PSU.

 

Factor in a decent selection of test leads, probes and some audio-based test leads as well (Jack/XLR to BNC). If you have a breakout lead from an XLR to 4mm "banana" plugs, you can look at a balanced waveform which is well worth it.

 

Anybody know of anywhere that sells audio test leads? I made my own but it might be of use to somebody.