Audio Signal Generator Advise

[Bertu]

Greetings,

my son and I recently ran into the obvious issue that the integrated function generator in our Hantek scope is unbalanced, and even worse, seems to share ground with the scope probe, so it seems pretty useless to test equipment with balanced inputs and outputs.

I am looking into options to buy a separate signal or function generator. Would something really basic like this do the job?
https://www.ebay.com/itm/373938462199?hash=item5710781df7:g:2~sAAOSww-NiD-sZI am thinking anything operating off a wallwart or battery would have a floating ground and could be used to drive a balanced input?

This unit seems to be analog, but still unbalanced.
https://www.ebay.com/itm/175252526782?hash=item28cddd56be:g:mroAAOSwx59iJXPf
Or skip the budget units and go for a 'real' arbitrary function generator like a FeelTech fy6800 ?

Or would it make sense to get (or build) something like the ATI matchmaker?

Thanks for your help, any advice is appreciated.

 

[JohnRoberts]

It depends on how serious you are, back in the 70s when I was active on my test bench I built a few DIY interface fixtures to simplify testing.

Now you can buy off the shelf balancing ICs.

There are lots of old used signal generators floating around (I even designed one LOFTECH TS-1). My TS-1 had an unbalanced output. Back in the 70s I used Heathkit sine wave generators.

A external audio transformer might provide balance but not a cheap solution.

JR

 

[mad.ax]

If you go the old'n'used route, there are real gems to be found... For instance this Ferisol C902M that comes from the French Navy. All valves, high output, integrated voltmeter, switchable impedance from 1,75Ω to 5KΩ, and balanced or unbalanced output!

 

[Bertu]

Thanks John.

We are finishing up our second project. The first was the opto-compressor from DiyRe, the second is our own 500-style mic preamp with EQ. That's when we ran into the issue of testing balanced inputs and outputs. We are planning to build a few more pieces or gear - summing mixer, active DI, saturation circuits ... so this signal generator would be used for a a couple more serious projects. But, we are not talking about measuring distortion, mostly scope work to check for oscillation, EQ behavior etc.

 

[mad.ax]

Now for something much easier to find, less expensive (actually free), I've tried this Android app when I was at a friend's place who had a scope but no sig-gen, and I found it quite useful.
Function Generator – Applications sur Google Play

It uses the headphone output of an Android phone or tablet. Audio bandwidth obviously but 16bits so decent signal. Phone or tablet means output is floating. I haven't tried it but since you can tune the phase independently for each channel, I guess you could make a mini jack>XLR adaptor and assign ch1 to pin 2 and ch2 to pin 3. With the same signal on each channel, but one 180° off, you should get a balanced out.
For zero bucks, it's worth a try imo...

 

[abbey road d enfer]

Beware that most analog function generators have rather high distortion; 1% is typical.
Connecting an unbalanced generator to a balanced input somewhat works, but does not allow full performance assessment.

 

[JohnRoberts]

Beware that most analog function generators have rather high distortion; 1% is typical.
Connecting an unbalanced generator to a balanced input somewhat works, but does not allow full performance assessment.

1% distortion is usable for general troubleshooting and frequency response measurements. 1% distortion is actually audible (just barely) in critical listening tests. My only test equipment design delivered something like 0.3% THD. I could hear that in close A/B listening trials between it and lower distorion, but decided to accept it as a fair compromise for faster settling time (no free lunch in design).

Nowadays with digital everywhere we can conjure up arbitrarily pristine signal source. I decided not to revisit my old TS-1 because these days you can just about cover it functionally with a smart phone ap.

JR

 

[Bertu]

That app looks pretty useful. I'll built myself a little adapter and give it a try. Thanks!

 

[industrialarts]

Now for something much easier to find, less expensive (actually free), I've tried this Android app when I was at a friend's place who had a scope but no sig-gen, and I found it quite useful.
Function Generator – Applications sur Google Play

It uses the headphone output of an Android phone or tablet. Audio bandwidth obviously but 16bits so decent signal. Phone or tablet means output is floating. I haven't tried it but since you can tune the phase independently for each channel, I guess you could make a mini jack>XLR adaptor and assign ch1 to pin 2 and ch2 to pin 3. With the same signal on each channel, but one 180° off, you should get a balanced out.
For zero bucks, it's worth a try imo...
View attachment 95959

I have been using this program for years. It's really useful, tons of functions. I recently built a box to balance and transformer isolate the outputs. It may not be lab quality but it's served me well.

One thing to know about a tablet - it switches the speakers off and re-directs signal to the headphone jack when it detects a drop in voltage when the headphones are plugged in. So I had to add a resistor to ground on one channel to trigger the HP jack (only one channel triggers the HPs).

The box I built has DRV 135 line driver ICs to drive the xfrmrs. One side of my box is switchable -10, +4 and mic balanced out, the other side (other channel) is basically guitar level out to test amps. I don't remember if I tried to drive a transformer directly from the HP out but I think I built the box because it didn't work well.

You could also look into an external DAC with balanced outs. Most of the current DACs are compatible with newer tablets

 

[ccaudle]

You could also use a half-decent USB audio interface and use a laptop as signal generator. Possibly not quite as convenient in the sense of just turn it on and go, but you may have the pieces already and just didn't consider using them for that purpose.

 

[Flatpicker]

I use this. Connect channel 1 to XLR pin 2, channel 2 to XLR pin 3, and the commons to XLR pin 1. Set channel 1 and channel 2 to equal frequency and amplitude, and set channel 2 phase to 180 deg (amplitudes don't have to be the same, but I always make them equal). Works like a charm.

 

[57sputnik]

I use a Focusrite 2i2 USB which provides balanced outputs to about +15 dBu. I use it for general test and measurement.

For ultra low distortion (<-140 dBc) 1 kHz tones with balanced output I built this: ULDO-Nacho-Atto Ultra Low Distortion Oscillator, Notch Filter and Attenuator Construction - Pro Audio Design Forum

About the only time I use function generator these days is to check out-of-band performance.

Got a link to the generator Flatpicker? That looks like a nice instrument.

 

[Tubetec]

I have a couple of the Feeltech generators ,
One I modified , adding a three rail linear supply to replace the original SMPS which sagged a bit under load .
I do use both outputs set 180 degree out of phase as balanced . I also use a calibrated multimeter to set up the balance between the two channels as some level differences exist .
The distortion on the Feeltech varies with frequency to some extent , certain frequencies having lower distortion
, Ive measured thd as low as 0.07% if memory serves me correctly .

 

[neil.johnson]

If you want old school look for a Gould J3B. Classic Wien oscillator, covering the audio range, 600 ohm transformer balanced output.

 

[Flatpicker]

Got a link to the generator Flatpicker? That looks like a nice instrument.

Just search for DDS Function Generator. They're all over Amazon. Here's a link to the least expensive one which is the older 15MHz version. The newer ones are 60Mhz, but obviously both are way overkill for audio.
https://www.amazon.com/XCFDP-Genera...jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==

 

[RuudNL]

What is the problem grounding one 'leg' of a balanced input during testing?

 

[abbey road d enfer]

What is the problem grounding one 'leg' of a balanced input during testing?

For transformer-balanced inputs, the frequency response is often different when one leg or the other is grounded. Having a floating output may be useful for this assessment.
Regarding xfmr-less inputs, distortion is different between balanced drive (near zero CM voltage) an unbalanced (CMV=1/2Vin). These DDS generators have a higher intrinsic distortion, though.
Noise is probably the most significant problem.

 

[RuudNL]

That's not a very good transformer then!
(Of course you can always do two measurements: just exchange the wires going to the input.)

 

[user 37518]

For transformer-balanced inputs, the frequency response is often different when one leg or the other is grounded. Having a floating output may be useful for this assessment.
Regarding xfmr-less inputs, distortion is different between balanced drive (near zero CM voltage) an unbalanced (CMV=1/2Vin). These DDS generators have a higher intrinsic distortion, though.
Noise is probably the most significant problem.

Everything you say is true, but the OP mentioned that he really only wants it to test his equipment with a scope and stuff like that, I don't think that in such case it is very relevant to use a very low distortion and noise signal generator or even a balanced one. I own a very crude GW Instek analog function generator and I have measured its THD+N; when its cold, THD is around 0.3%-0.4%, but it drops to around 0.1% after it stabilizes; I really don't see any problem with that. If a balanced output is required you can just add a unity gain inverting amplifier. The DDS generators shouldn't be very far off from those figures. Besides, if the OP is really serious about measuring THD in modern audio equipment, he really needs something which has extremely low distortion, 0.01% max THD won't even come close to what is necessary nowadays.

I also have a Tektronix SG505 signal gen. which has very low distortion (specs say < 0.0008%), but I barely use it since I got my AP SYS-1, which has even lower distortion. The main problem with using a low distortion generator is that you can only measure THD at one frequency, if you want a plot of THD+N vs frequency, you really need a computer controlled solution or lots of time to manually change the frequency and record several points in something like Excel. My advice is that the OP should buy one of the alternatives others have already posted here and, if he wants to measure distortion or more advanced stuff, he should invest in something like a Quant Asylum unit; they recently came up with a new version. The price of a very low distortion generator (like the Tek SG505) can be higher than the price of a Quant Asylum unit.

 

[DKaye]

It depends on how serious you are, back in the 70s when I was active on my test bench I built a few DIY interface fixtures to simplify testing.

Now you can buy off the shelf balancing ICs.

There are lots of old used signal generators floating around (I even designed one LOFTECH TS-1). My TS-1 had an unbalanced output. Back in the 70s I used Heathkit sine wave generators.

A external audio transformer might provide balance but not a cheap solution.

JR

Would adding a SparkFun THAT 1646 OutSmarts Breakout (SparkFun THAT 1646 OutSmarts Breakout - BOB-14003 - SparkFun Electronics)
be useful in providing a balanced output to the TS-1? Ours is at least 40 years old and still gets frequent use. Thanks, JR.