Which audio signal generator? - UK Vintage Radio Repair and Restoration Discussion Forum

vinrads · 12th Sep 2020, 9:28 am

I have an old Marconi audio sig gen boat anchor vintage, with sine wave only, I was thinking do I need one with square wave plus sine wave out, when checking amplifier for distortion?

Mick.

chriswood1900 · 12th Sep 2020, 10:55 am

Mick
There are plenty of small good used AF generators around, I would tend to go for a transistor unit from the likes of Farnell, Leader for which the manual is readily available and therefore easy to fix.
You need to consider what frequency range 10Hz to 100Khz is usually good enough and and if used for amplifiers sign and square wave and a distortion of less than 0.1%.
If it higher end amps then you will be looking at better units capable of at least 0.005% distortion and then you are into the territory of Radford, HP or one I like the Blackstar LDO 100. Avoid function generators unless you can afford a high end one as the distortion numbers are usually horrible. Finally you could consider using software on a PC but I prefer not to have the additional clutter on my bench.
I suspect quite a few people on the forum may have spare units so a request in the wanted section may bear fruit.
Good luck

Radio Wrangler · 12th Sep 2020, 11:08 am

Audio generators fall into two main camps

The Wien bridge type is a low distortion sinewave oscillator. Many of which have a squarewave output achieved by using an analogue comparator to square-up the sine. These are the best choice for distortion measurements.

The Function generator type uses an integrator with switched currents to make a triangle waveform. A square wave output is available from the current switching operation. There isn't a sinewave in the basic oscillator, so a diode-based shaping network is used to turn the triangle wave into an approximate sinewave shape. The sine output from these things is not particularly low distortion. Most are pretty awful, many times worse than the audio circuitry you may be trying to measure.

More recently, a third flavour has emerged. There are digital signal sources that essentially replay chosen waveforms at a programmable speed. In the quest for low distortion and clean signals you run into issues of whether the waveform at the resolution it has fits into the period of the frequency you've chosen. They can be quite good, but there are hidden trip-ups, you won't always get the headline performance. What they are brilliant at is making arbitrary waveforms. If you can imagine it and make a spreadsheet of it sampled, it will make it for you.

Also there is software for operating the sound card in your computer. If done well, this can also have low distortion. The problem remains of it being connected and powered inside a very dirty environment (the computer) so how do you extract the wanted signal without the crap? There are special quality external sound cards to do this, made for music production.

David

ajgriff · 12th Sep 2020, 11:09 am

Square waves are must for audio work in my view.

Alan

barrymagrec · 12th Sep 2020, 11:15 am

For general audio work I would look at a Levell TG150D or TG200D.

The TG150 wil suffice for most purposes, the TG200 has a higher output and wider frequency range.

The D suffix indicates square wave output, if you also have an M suffix that indicates an output meter.

I wouldn`t touch a function generator for anything other than the most basic audio work.

vinrads · 12th Sep 2020, 10:09 pm

Here is a couple I found for sale ,A Farnell LF1 10hz to 1mhz and a MFG 3002 ? to 2mhz any good ? Mick.

Radio Wrangler · 12th Sep 2020, 11:02 pm

Those are examples of both sorts.

The Farnell LF1 is a reasonable Wien Bridge design, although not a very low distortion one, it's fine for audio work in general. Does squarewaves as well.

The MFG 3002 I found on Amazon. The FG in the name gives it away, it's a function generator so its sine output is approximated. Best avoid this type of signal generator, as Barry suggests. They are popular in general electronics labs, but do not suit decent audio or RF work. They can be useful at times but combine great versatility with limited performance in terms of cleanliness and the range of levels they can make.

David

vinrads · 13th Sep 2020, 8:57 am

Thank's for that David , so It's the LF1 up to now , Mick.

Diabolical Artificer · 14th Sep 2020, 5:58 am

You can make a very simple sine to square wave convertor using a scmidt trigger chip, see ESP for details, which is what I use, bnc onto a small board, PP3, bnc out. Problem is most decent wein bridge sinewave genny's don't have sq wv out. It's also possible to knock up a simple Wein bridge osc at spot frequencies for THD testing, something like 100hz, 440/1000hz 10khz, 20khz, though 1k will do, using an opamp and 28v lamp and few R's & C's.

There's also a brilliant bit of free software for PC called Soundcard Scope, it has a sig gen (low THD), THD meter (as good as a HP audio analyser), scope & FFT, goes up to 20khz.

So to reprise, old valve genny for manual sweep, spot genny for THD, bolt on sq wv convertor, PC for FFT, this gives you a very flexible audio test rig. Failing that Sugden did a test meter and low THD sine all in one little box, well worth getting one if one comes up.

Andy.

McMurdo · 14th Sep 2020, 8:19 pm

This one's like mine, available as Gould or Advance or possibly Farnell

https://www.sglabs.it/en/product.php?s=gould-j3b&id=327

Low distortion, takes a few seconds to warm up and start oscillating (its not valve!).

Sine and square with balanced, unbalanced and TTL outputs, calibrated attenuators. Sufficient power output to drive speaker coil for reconing purposes.

Great for audio repairs and available quite cheaply (some on ebay now).

vinrads · 14th Sep 2020, 10:23 pm

Thank's for that Kevin , I have taken the plunge and purchased the Farnell LF1 , Mick.

Julesomega · 15th Sep 2020, 12:01 am

You're lucky, Mick, the full service manual has just appeared on this forum thanks to Bryan M. I'd been looking out for this for ages, it hadn't previously made it onto the 'web. I'd wanted to say a thanks to Bryan but the thread was abruptly closed so I'll take the opportunity now

Boulevardier · 15th Sep 2020, 1:35 am

Quote:

Originally Posted by Julesomega

You're lucky, Mick, the full service manual has just appeared on this forum thanks to Bryan M. I'd been looking out for this for ages, it hadn't previously made it onto the 'web. I'd wanted to say a thanks to Bryan but the thread was abruptly closed so I'll take the opportunity now

Sorry if I'm being dense, but I can't see Bryan M's posting with the service manual. Can you point me to it?

Mike

chriswood1900 · 15th Sep 2020, 7:03 am

https://www.vintage-radio.net/forum/...hlight=Farnell

AC/HL · 15th Sep 2020, 12:29 pm

That was the same thread. As a wanted it would have been lost, so I moved it to the test gear section for posterity. It is searchable.

12th Sep 2020, 9:28 am	#1
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,755	Which audio signal generator? I have an old Marconi audio sig gen boat anchor vintage, with sine wave only, I was thinking do I need one with square wave plus sine wave out, when checking amplifier for distortion? Mick.

12th Sep 2020, 10:55 am	#2
chriswood1900 Octode Join Date: Jan 2007 Location: Dorridge, West Midlands, UK. Posts: 1,479	Re: Which audio signal generator. Mick There are plenty of small good used AF generators around, I would tend to go for a transistor unit from the likes of Farnell, Leader for which the manual is readily available and therefore easy to fix. You need to consider what frequency range 10Hz to 100Khz is usually good enough and and if used for amplifiers sign and square wave and a distortion of less than 0.1%. If it higher end amps then you will be looking at better units capable of at least 0.005% distortion and then you are into the territory of Radford, HP or one I like the Blackstar LDO 100. Avoid function generators unless you can afford a high end one as the distortion numbers are usually horrible. Finally you could consider using software on a PC but I prefer not to have the additional clutter on my bench. I suspect quite a few people on the forum may have spare units so a request in the wanted section may bear fruit. Good luck __________________ Chris Wood BVWS Member

12th Sep 2020, 11:08 am	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,864	Re: Which audio signal generator. Audio generators fall into two main camps The Wien bridge type is a low distortion sinewave oscillator. Many of which have a squarewave output achieved by using an analogue comparator to square-up the sine. These are the best choice for distortion measurements. The Function generator type uses an integrator with switched currents to make a triangle waveform. A square wave output is available from the current switching operation. There isn't a sinewave in the basic oscillator, so a diode-based shaping network is used to turn the triangle wave into an approximate sinewave shape. The sine output from these things is not particularly low distortion. Most are pretty awful, many times worse than the audio circuitry you may be trying to measure. More recently, a third flavour has emerged. There are digital signal sources that essentially replay chosen waveforms at a programmable speed. In the quest for low distortion and clean signals you run into issues of whether the waveform at the resolution it has fits into the period of the frequency you've chosen. They can be quite good, but there are hidden trip-ups, you won't always get the headline performance. What they are brilliant at is making arbitrary waveforms. If you can imagine it and make a spreadsheet of it sampled, it will make it for you. Also there is software for operating the sound card in your computer. If done well, this can also have low distortion. The problem remains of it being connected and powered inside a very dirty environment (the computer) so how do you extract the wanted signal without the crap? There are special quality external sound cards to do this, made for music production. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

12th Sep 2020, 11:09 am	#4
ajgriff Nonode Join Date: May 2004 Location: Halifax, West Yorkshire, UK. Posts: 2,587	Re: Which audio signal generator. Square waves are must for audio work in my view. Alan

12th Sep 2020, 11:15 am	#5
barrymagrec Octode Join Date: Dec 2017 Location: Morden, Surrey, UK. Posts: 1,557	Re: Which audio signal generator. For general audio work I would look at a Levell TG150D or TG200D. The TG150 wil suffice for most purposes, the TG200 has a higher output and wider frequency range. The D suffix indicates square wave output, if you also have an M suffix that indicates an output meter. I wouldn`t touch a function generator for anything other than the most basic audio work.

12th Sep 2020, 10:09 pm	#6
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,755	Re: Which audio signal generator? Here is a couple I found for sale ,A Farnell LF1 10hz to 1mhz and a MFG 3002 ? to 2mhz any good ? Mick.

12th Sep 2020, 11:02 pm	#7
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,864	Re: Which audio signal generator? Those are examples of both sorts. The Farnell LF1 is a reasonable Wien Bridge design, although not a very low distortion one, it's fine for audio work in general. Does squarewaves as well. The MFG 3002 I found on Amazon. The FG in the name gives it away, it's a function generator so its sine output is approximated. Best avoid this type of signal generator, as Barry suggests. They are popular in general electronics labs, but do not suit decent audio or RF work. They can be useful at times but combine great versatility with limited performance in terms of cleanliness and the range of levels they can make. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

13th Sep 2020, 8:57 am	#8
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,755	Re: Which audio signal generator? Thank's for that David , so It's the LF1 up to now , Mick.

14th Sep 2020, 5:58 am	#9
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,657	Re: Which audio signal generator? You can make a very simple sine to square wave convertor using a scmidt trigger chip, see ESP for details, which is what I use, bnc onto a small board, PP3, bnc out. Problem is most decent wein bridge sinewave genny's don't have sq wv out. It's also possible to knock up a simple Wein bridge osc at spot frequencies for THD testing, something like 100hz, 440/1000hz 10khz, 20khz, though 1k will do, using an opamp and 28v lamp and few R's & C's. There's also a brilliant bit of free software for PC called Soundcard Scope, it has a sig gen (low THD), THD meter (as good as a HP audio analyser), scope & FFT, goes up to 20khz. So to reprise, old valve genny for manual sweep, spot genny for THD, bolt on sq wv convertor, PC for FFT, this gives you a very flexible audio test rig. Failing that Sugden did a test meter and low THD sine all in one little box, well worth getting one if one comes up. Andy. __________________ Curiosity hasn't killed this cat...so far.

14th Sep 2020, 8:19 pm	#10
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 5,270	Re: Which audio signal generator? This one's like mine, available as Gould or Advance or possibly Farnell https://www.sglabs.it/en/product.php?s=gould-j3b&id=327 Low distortion, takes a few seconds to warm up and start oscillating (its not valve!). Sine and square with balanced, unbalanced and TTL outputs, calibrated attenuators. Sufficient power output to drive speaker coil for reconing purposes. Great for audio repairs and available quite cheaply (some on ebay now). __________________ Kevin

14th Sep 2020, 10:23 pm	#11
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,755	Re: Which audio signal generator? Thank's for that Kevin , I have taken the plunge and purchased the Farnell LF1 , Mick.

15th Sep 2020, 12:01 am	#12
Julesomega Nonode Join Date: Dec 2012 Location: Stockport, Greater Manchester, UK. Posts: 2,102	Re: Which audio signal generator? You're lucky, Mick, the full service manual has just appeared on this forum thanks to Bryan M. I'd been looking out for this for ages, it hadn't previously made it onto the 'web. I'd wanted to say a thanks to Bryan but the thread was abruptly closed so I'll take the opportunity now __________________ - Julian It's good here

15th Sep 2020, 7:03 am	#14
chriswood1900 Octode Join Date: Jan 2007 Location: Dorridge, West Midlands, UK. Posts: 1,479	Re: Which audio signal generator? https://www.vintage-radio.net/forum/...hlight=Farnell __________________ Chris Wood BVWS Member

15th Sep 2020, 12:29 pm	#15
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: Which audio signal generator? That was the same thread. As a wanted it would have been lost, so I moved it to the test gear section for posterity. It is searchable.