Audio Test Set - UK Vintage Radio Repair and Restoration Discussion Forum

Radio Tech · 25th Aug 2016, 6:57 pm

I have a circuit for an audio test set and built this unit, the set only had one range, I decided to extend the ranges by adding a rotary switch, I hope this is useful.

Ken

Diabolical Artificer · 26th Aug 2016, 3:35 pm

How is it Ken? Is it stable and did it oscillate first time? I've been looking for and at several sig gen circuits but one that has very low distortion, 0.005% or better and good stability.

I've found some very simple oscillator designs on the net, ones that use a single transistor for instance that use a transistors negative base current.

I'll knock your circuit up for the craic and see how it performs.

Andy.

Radio Wrangler · 26th Aug 2016, 5:07 pm

That one uses a 555 timer chip, Andy. It gives a rectangular wave output, not a sine.

If your goal is 0.005% distortion, then you're into Wien bridge territory with a very carefully designed FET type amplitude controller. Have a look for a Radford low distortion oscillator. There is also a matching distortion meter. Otherwise, there.s always an HP 8903 from Stewart's... Or one of audio precision's soundcard instruments.

David

boxdoctor · 26th Aug 2016, 6:51 pm

The good old ICL 8038 chip generates sine, square, and triangle outputs. Peripheral circuit is fairly simple, and the addition of a bit of extra circuitry will give you sweep generating facilities as well. Data sheet here: http://www.alldatasheet.com/view.jsp...L8038&sField=4
They are still about, and are comparitively inexpensive..Tony

Radio Wrangler · 26th Aug 2016, 7:56 pm

The 8038 is a function generator on a chip, it makes square and triangle waves in its oscillator and then shapes the triangles into an approximation of a sine using a nonlinear diode network. It's an awful lot better than a 555 if you want a sinewave, and sweeps can be useful, but it still won't get you anywhere near 0.005% harmonic distortion. That sort of purity is still a serious achievement.

David

mhennessy · 26th Aug 2016, 10:40 pm

0.005% is -86dB. It's possible with a Wien bridge, but not trivial to achieve. The amplitude-control "sidechain" will be tricky to get right - especially if you want to vary the frequency - and attention to detail will be needed everywhere.

One of the best Wien bridge oscillators out there is by Cyril Bateman. It was built as a low-distortion source for his capacitor distortion investigations - a series of articles well-worth checking out if you haven't done so already. You can download them here:

https://linearaudio.nl/cyril-bateman...sound-articles

The first article covers the oscillator. It gets to -128dB, which is about 0.00005%!

Oscillators based around variable-state filters usually give better results than a Wien bridge, but they are similarly difficult. IIRC, that's what the HP8903A uses.

There's some good stuff about SVFs in this book: https://www.amazon.co.uk/Linear-Elec.../dp/0750608684 - no idea if it's available as a PDF anywhere...

But there's a lot to be said for a good soundcard...

To expand a bit about "function generators", these are very different to low-distortion sine-wave oscillators based on frequency-selective feedback. A function generator starts by making triangle and square waves. Then, the triangle wave is "bent" into an approximation of a sine wave using many diodes or transistors (the non-linear relationship between current and voltage being the "MO" there). At best, distortion might be 0.5%, give or take (I think my HP3314A might be a tiny bit better, but still worse than 0.1% IIRC).

Function generators work by having a pair of current generators that charge a capacitor in one direction, then flip around to charge in the opposite. You could make one with 2 op-amps - one being an integrator and the other a simple Schmitt trigger (literally, 2 op-amps, 1 capacitor and 3 resistors). The current source method is preferred because it's easier to play games like altering the ratio of the two currents to give sawtooths rather than triangles (or pulses rather than 50:50 square waves). Many will also have various modes like triggered and gated, and a frequency modulation input. Also, a function generator doesn't have "amplitude bounce" as you alter the frequency - sine wave oscillators will to some extent. Those using bead thermistors to stabilise the amplitude usually have lots. Some Farnell oscillators used a heavily damped slow motion drive to disguise it!

In short, function generators are incredibly flexible and useful to have around, but they don't do low distortion. In practice, you need both types of generator in a workshop.

Forget about the ICL8038 - it's a PITA to use, and gives poor results. And it's obsolete. I assume the ones available today are clones or NOS.

Oh - if a low-distortion oscillator offers a square wave option, then it's a Schmitt trigger fed from the sine wave. The absence of a triangle wave option is the first clue that you might be dealing with a low distortion product. But check the specs, naturally...

Another class of oscillator involves digital means to generate waveforms. Lots of things there - another day

26th Aug 2016, 3:35 pm	#2
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,658	Re: Audio Test Set How is it Ken? Is it stable and did it oscillate first time? I've been looking for and at several sig gen circuits but one that has very low distortion, 0.005% or better and good stability. I've found some very simple oscillator designs on the net, ones that use a single transistor for instance that use a transistors negative base current. I'll knock your circuit up for the craic and see how it performs. Andy. __________________ Curiosity hasn't killed this cat...so far.

26th Aug 2016, 5:07 pm	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,869	Re: Audio Test Set That one uses a 555 timer chip, Andy. It gives a rectangular wave output, not a sine. If your goal is 0.005% distortion, then you're into Wien bridge territory with a very carefully designed FET type amplitude controller. Have a look for a Radford low distortion oscillator. There is also a matching distortion meter. Otherwise, there.s always an HP 8903 from Stewart's... Or one of audio precision's soundcard instruments. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

26th Aug 2016, 6:51 pm	#4
boxdoctor Rest in Peace Join Date: Sep 2010 Location: Ripley, Derbyshire, UK. Posts: 785	Re: Audio Test Set The good old ICL 8038 chip generates sine, square, and triangle outputs. Peripheral circuit is fairly simple, and the addition of a bit of extra circuitry will give you sweep generating facilities as well. Data sheet here: http://www.alldatasheet.com/view.jsp...L8038&sField=4 They are still about, and are comparitively inexpensive..Tony

26th Aug 2016, 7:56 pm	#5
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,869	Re: Audio Test Set The 8038 is a function generator on a chip, it makes square and triangle waves in its oscillator and then shapes the triangles into an approximation of a sine using a nonlinear diode network. It's an awful lot better than a 555 if you want a sinewave, and sweeps can be useful, but it still won't get you anywhere near 0.005% harmonic distortion. That sort of purity is still a serious achievement. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done Last edited by Radio Wrangler; 26th Aug 2016 at 8:02 pm.

26th Aug 2016, 10:40 pm	#6
mhennessy Dekatron Join Date: Jul 2007 Location: Evesham, Worcestershire, UK. Posts: 4,244	Re: Audio Test Set 0.005% is -86dB. It's possible with a Wien bridge, but not trivial to achieve. The amplitude-control "sidechain" will be tricky to get right - especially if you want to vary the frequency - and attention to detail will be needed everywhere. One of the best Wien bridge oscillators out there is by Cyril Bateman. It was built as a low-distortion source for his capacitor distortion investigations - a series of articles well-worth checking out if you haven't done so already. You can download them here: https://linearaudio.nl/cyril-bateman...sound-articles The first article covers the oscillator. It gets to -128dB, which is about 0.00005%! Oscillators based around variable-state filters usually give better results than a Wien bridge, but they are similarly difficult. IIRC, that's what the HP8903A uses. There's some good stuff about SVFs in this book: https://www.amazon.co.uk/Linear-Elec.../dp/0750608684 - no idea if it's available as a PDF anywhere... But there's a lot to be said for a good soundcard... To expand a bit about "function generators", these are very different to low-distortion sine-wave oscillators based on frequency-selective feedback. A function generator starts by making triangle and square waves. Then, the triangle wave is "bent" into an approximation of a sine wave using many diodes or transistors (the non-linear relationship between current and voltage being the "MO" there). At best, distortion might be 0.5%, give or take (I think my HP3314A might be a tiny bit better, but still worse than 0.1% IIRC). Function generators work by having a pair of current generators that charge a capacitor in one direction, then flip around to charge in the opposite. You could make one with 2 op-amps - one being an integrator and the other a simple Schmitt trigger (literally, 2 op-amps, 1 capacitor and 3 resistors). The current source method is preferred because it's easier to play games like altering the ratio of the two currents to give sawtooths rather than triangles (or pulses rather than 50:50 square waves). Many will also have various modes like triggered and gated, and a frequency modulation input. Also, a function generator doesn't have "amplitude bounce" as you alter the frequency - sine wave oscillators will to some extent. Those using bead thermistors to stabilise the amplitude usually have lots. Some Farnell oscillators used a heavily damped slow motion drive to disguise it! In short, function generators are incredibly flexible and useful to have around, but they don't do low distortion. In practice, you need both types of generator in a workshop. Forget about the ICL8038 - it's a PITA to use, and gives poor results. And it's obsolete. I assume the ones available today are clones or NOS. Oh - if a low-distortion oscillator offers a square wave option, then it's a Schmitt trigger fed from the sine wave. The absence of a triangle wave option is the first clue that you might be dealing with a low distortion product. But check the specs, naturally... Another class of oscillator involves digital means to generate waveforms. Lots of things there - another day