Audio generator for sine and square waves

[tonedeak99]

Hey fellow EEV'ers!

Looking to purchase an audio generator for audio/amplifier repair.  My research has lead me in two directions.  One for a vintage audio generator such as a Tek, Heathkit, Leader or HP.  Second is a modern USB audio adapter like the Focusrite or the QuantumAsylum QA4xx units and using REW to generate the waveform.  In the end I'm looking mainly for a clean waveform but low distortion and noise would be a plus.  The cheaper the better!

Any and all recommendations will be appreciated!

Thanks!

[KungFuJosh]

Here's three I've used for audio repair:

Tenma 72-505

UNI-T UTG932 30MHz

Siglent SDG2042X

They're all good, and you get what you pay for. I sold the UTG932 to somebody else that needed an audio generator, and I only use the Siglent these days.

[dobsonr741]

I would add the triangle waveform for quick detection of clipping and bandwidth degradation.

[tonedeak99]

Thank you both. I was hoping for some more budget friendly options which is why the usb audio cards were so attractive to me. At this time this is just a hobby for me.

[julianhigginson]

The Quantasylum units don't work with REW (I am pretty sure??) they are not meant to be soundcards. Yes they basically have all the same bits as soundcards, but they are not designed exactly the same, and have quite a few features that a soundcard would never have.

I read something once about an older generation unit (QA401 maybe??) had some kind of audio driver but I have not seen it and don't know if it was any good.

The reason you'd go with a QA403 and its special software over a USB soundcard and something like REW is that the Quantasylum is designed to do just what you want with testing, and is basically calibrated, etc. while your soundcard inputs will probably have an input level knob which guarantees every time you are measuring a signal you have that to take into consideration.... Also it has its own input range switching (again, all calibrated) to just handle different input levels with best possible S/N and is just better as a test instrument. But it costs a fair bit more (twice as much?) than a scarlet  2i2.

this is where I was recently where I just snapped and bought a QA403 because I was really really sick of having to go so slowly with my measurements of a device with weird noise problems, using a soundcard and REW, and constantly needing to recalibrate/adjust every time I changed things. Also grounding issues using the soundcard were a bit funny sometimes (Device being worked on also had a USB connection and connecting/disconnecting them would make USB drop out on either device)

if all you want is the waveform generator and you're not massively fussed about the exact level because you just want to crank the output and listen to see if it distorts or breaks up,  rather that feed its signal back into your system to measure things like THD+N or do accurate frequency swept measurements, then a soundcard and REW is probably fine. you could probably do OK with way lower quality than a focusrite scarlet there, too. I'd be more concerned about going with that over a cheaper unit if you want to make accurate low noise floor measurements.

if you're concerned about cost check out local facebook marketplace for either an older 2x2 focusrite scarlet (they are pretty ubiquitous and show up a lot) or maybe even some basic old test equipment. There's plenty of old analogue test oscillators out there if you look.

[Smokey]

It's a Wien Bridge or nothing!  Go big!
https://en.wikipedia.org/wiki/Wien_bridge_oscillator

Really though, is a decent arb not good enough these days?

[dobsonr741]

For budget there’s XR2207. More precisely clones. The sine distortion will be >1%, however it will be cheap and OK to start repairs. Output should be buffered, the Chinese kits won’t do that.

[Aldo22]

Any and all recommendations will be appreciated!

This is a cheap two-channel AWG.
https://www.aliexpress.us/item/3256805571148212.html
Not the newest model, but they work quite well.

[David Hess]

The Focusrite or QuantumAsylum are more suited for doing distortion and noise analysis, especially for low distortion and low noise designs.

A function generator which can produce square, triangle, and sine waves, along with an oscilloscope, are better for functional testing and design.  Distortion levels from function generator sine waves might be 0.5% at best, so not so good for distortion analysis.  Square waves are good for transient response analysis, and triangle waves are good for checking the transfer function.

[Gyro]

A bit of an alternative suggestion.

Decent performance if you don't mind some DIY. <0.05% THD, sine, square, triangle and sweep (with trigger out). Touch screen. Written in interpreted MMBasic so you can tweak it as desired. Probably easy to run on the Pi Pico version if you don't fancy breadboarding a PIC.

https://geoffg.net/SignalGenerator.html

[blackdog]

Hi,

I use three systems to measure for the lower frequencies.
An audio Precision measurement set.
Siglent SDG 1032X
Qaunt Asylum 403

If you so want to start doing measurements on audio equipment and the THD is not super important then the Siglent SDG 1032X is the first thing you could buy.
The THD is a little under 0.1% and for most Audio measurements that is sufficient.

But more important in my eyes is the flawless pulse response of the Siglent SDG 1032X, there are no abbarations if you are terminated with 50 Ohm and you do not go higher than 1Vpp,
the square wave is then perfect.

This is what the square wave pulse looks like, no abberations!


This is a measurement also done with the Siglent SDG 10132X but measured with the Quant Asylum 403.
What was already pointed out by another forum user, this measurement instrument is better suited for distortion measurements and noise behavior.
The generator of the Quant Asylum also has a nice low distortion.

This picture shows the distortion of the SDG 1032X along with some random other signals always present with DDS generators like this Sigelent SDG 1032X.
This is a 1KHz Sine generator Loaded with 50 Ohm.


Now which is best for the topic starter, actually a Siglent SDG 1032X and a Quant Asylum :-)

It is better not to go any cheaper in terms of function generator than the Siglent SDG 1032X which I have had about four years of experience with now.
It is together on the measurement shelf with the SDG 2042X and also a Rigol DG4162.
But usually the first one that comes on to do measurements is the SDG 10132X, because I often need a clean pulse signal and if I stay below 1Vtt at 50 Ohm load I don't have to worry about my signal.
Above this signal level a small riple is visible just like with other function generators.

Also remember that with a function generator you can do much more than pushing a 1KHZ signal into and amplifier?

You can also tune radios with it, I test power transistors with it, the generator drives the base and with an external power supply and collector resistor I then do some basic measurements.
The function generator can also be used as a DC source, keep in mind that the output resistance is then 50 or 25 Ohms when closed.

There are also so many possibilities with a modern function generator, don't forget to take that into consideration.

Kind regards,
Bram

[tonedeak99]

Wow so many great recommendations.  Thank you all.  So from what I gather, the QuantAsylum and Focusrites are not the best for signal generation needs in audio repair.  I was just trying to keep the costs down and save desk space by using a small device connected to my computer.  Also, if I decide to quit the hobby I thought these would have a high resale value.

I understand the Siglent recommendations but that's way out of budget for me.  I want to learn about basic audio circuits and repairs.  I'm currently using my phone with a tone generation app, but the waveforms, especially the square are terrible.  See pic.

I really like the DIY route proposed by @Gyro and @Smokey.  Thank you I'll seriously consider them.  Also the Feeltech recommendation by @Aldo22 is a good deal but I have read they have grounding issues and don't produce the cleanest waveforms.  This is why I was leaning toward a vintage HP, Tenma, Heathkit, Leader, Tek, etc, unit that even though quite large but they are cheap and able to produce clean waveforms. Many with DC offset and/or frequency counter functionality built in. How important and useful is it to have DC offset and counter functionality?  FYI, my oscilloscope is an old analog that doesn't have any measurement or counter functions.

I would definitely like to keep the generator cost down to <$50.

[KungFuJosh]

I haven't tried this one, but it seems to be the replacement for the Tenma I previously had, and fits your budget: https://www.newark.com/tenma/72-490/compact-audio-generator/dp/09P2110

[Aldo22]

Also the Feeltech recommendation by @Aldo22 is a good deal but I have read they have grounding issues and don't produce the cleanest waveforms. 

Grounding issues are easily fixed.
I didn't even buy a new connector, just pulled in a 3-core cable and connected PE to the BNCs (photo).

No, it definitely looks better than on your oscilloscope photo. (Attached screenshot 20kHz Square/Sine FY3224s) 

[Martin72]

Hi,

I would definitely like to keep the generator cost down to <$50.

DDS Signalgenerator 500khz

[David Hess]

I have a couple of old style analog function generators, and a Tektronix FG502 function generator which operates up to 11 MHz.  The lowest distortion analog function generators that I am aware of are the Tektronix FG501A and FG507 which were intended for audio applications, but of course they are not that good at 0.25%.  A modern cheap DDS *should* be better.  Function generators are really the wrong instrument for audio distortion testing.

Old dedicated analog sine sources achieved 0.0008%, which may not be quite as good as the best audio instruments like the QuantAsylum, however it is relatively straightforward to build an analog source which is much better, and it is the only way to verify the performance of 20+ bit audio ADCs.  These are better than even anything from Standford Research Systems or Audio Precision.

The QuantAsylum QA403 probably has the best performance for cost if you are interested in noise and distortion measurements, but it would not be my first choice for design because it does not support network analysis even though it should.  Something from Cleverscope would be more suitable for design work, but maybe someone else here knows of a less expensive option for network analysis.