Oscilloscope/Audio Analyzer recommendations?

Hi all. Was wondering if anyone had any suggestions for an oscilloscope, keeping in mind that what it will be used for predominately is valve amp building. I'm not too up to date on current models, and didn't know if there might be something out there that could serve as an Oscope to measure waveforms, voltages, etc but also be tailored towards audio use (spectrum analyzer, thd, etc?)

I've been using an Elenco 30Mhz dual trace, but figured there are probably lots of options these days for audio analyzers. Just need to be able to measure THD from stage to stage and at output, and regular scope functions.

Thanks much for any input,

LC
 
Last edited:

tomchr

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2009-02-11 12:58 am
Calgary
www.neurochrome.com
Some modern scopes offer built-in FFT which will allow you to peek at the THD. Most inexpensive scopes use 8-10 bit ADCs so don't expect to be able to resolve THD better than 0.1 %. That's borderline useful even for tube work. You'll also need to add a low-THD source. A regular function generator will only get you to about 1 % THD as they commonly use sine shaping rather than an actual sine generator.

If the scope you have is working and you like it, I suggest sticking with it. You can add an audio analyzer. I found my HP 8903A to be incredibly useful for tube work. It's good enough for most solid state work as well. Mine got down to 0.002 % THD+N (1 kHz). The spec is 0.01 %, if I recall correctly. There's an "A" version and a "B" version. The difference is subtile. You can read my synopsis here.

A computer sound card will get you very far. I'd add Pete Millett's interface both for the gain selection and the input protection. You'll need some software. True RTA is one that you see quite a few people here use. It's the best I've found as well and it's very reasonably priced. That's not to say there aren't other options, but I haven't searched extensively.

For sound cards, I recommend anything in the Focusrite lineup. In particular the Saffire 6 USB or any of the Scarlett ones would be of interest.

Now a sound card + interface + software will likely set you back more than the cost of a dedicated analyzer. The Quantasylum QA401 looks like a very capable unit. I have no personal experience with it, but it is spec'ed well.

Tom
 
Thanks Tom, actually

my scope is not functioning, not correctly at least. It was an e-bay find for 75$ and served me well, but after a fall from the test bench the vertical hold has gone awry....so every time you want to measure above 3 div or so you need to switch scales, and the calibration is off a bit as well.

i know I could fix this, but I have a friend just getting into the hobby that I'm going to give it to, let him learn a little.

I use a Mac so the software thing is limited i must say....That, and I really like analog equipment (well ok some digital mixed in is OK, but dedicated equipment).

I am surprised that the scopes average around .1%. You're right, that is not good enough when measuring stages that should only have .05, etc...

I'll take a look at the 8903 you mentioned, and keep looking about scopes. I would think that at least there must be one with a good generator, though that would be covered by the 8903 anyhow huh....

Tnx again,

Loren
 
Just one more question on this, Input Probe?

Hi Tom. I did my homework and was pleased with the Scarlett 2i2, and I scrounged up a PC with the correct software. I know I still need an interface to go between the two, but in the meantime I want to start learning. One thing i have not been able to find info on, is how to make a probe. I am assuming it is just a simple resistor divider/cap type scenario, but not sure. Lets say I want to measure a 5687 DC to a 5687, by the end there I'm going to be seeing upwards of 150v...obviously too much for the Focusrite...so Do I just load the circuit with the true load it would see, and then make a probe with a divider?

Thanks much again for the info, I REALLY appreciate it.

Loren