Simple Audio Sine Wave Generator

[Yorkie]

Haven’t been doing much vintage radio this last few months as I've been busy trying (and mostly failing) to 'de-clutter', so haven’t had anything worth writing about. Really, I should be trying to get on with finishing restoring an Ekco AC77, which has stalled because apart from limited time, I find it difficult to get motivated in winter months even though I have a cosy, heated, and well-insulated workshop.

However, I fancied a little winter project and spotted a basic audio sine wave generator that appealed to me - not that I needed one, but I had most of the bits to hand. It covers two frequency ranges 155Hz – 2,200 Hz & 150Hzx 22 KHz, and two voltage ranges .011V - .24V & .1V – 2.4V. I calibrated the voltage range in RMS rather than peak to peak volts. Had to alter the value of R1 to get it to oscillate as the 28V/40mA stabiliser lamp I used was only 62R (cold) as opposed to the specified one which was 105R. Also, to get the voltage range to be 1/10th of the full voltage I had to use a 470k for R7 as opposed to the specified 47k.

Only a basic thing, but the sine wave looks respectable enough.I found the design here:

http://www.bobsdata.com/audio_sine_wave_...index.html

I fitted the PCB on spacers behind the front panel rather than horizontally, so I could make the box shallower. I drew a front panel when I’d calibrated it and printed it on blue card, encapsulated then covered with a panel of 2mm acrylic.

Yet another 'little wooden box' project - hope I'm not borderline OCD!

Probably of limited interest, but I’ve attached a few pics anyway, including one of the component overlay which I had to work out and draw because though the PCB artwork is on the website link, there was no overlay to show component placement. The only bits I had to buy were the little lamp and the 50k dual liner pot (R5). Both ebay purchases. These rest is simply a rearrangement of bits from my 'spares box'(es) apart from the box itself, which is from an offcut from the end of a mahogany plank.

Hope it’s of interest.

[AlanBeckett]

Yet another 'little wooden box' project - hope I'm not borderline OCD!

If you report making one around 6' 6" long, 3' wide and 2' deep we'll know you're in trouble

That looks very neat and that's how I'd have mounted the PCB. It keeps the Connections nice and short.

Alan

[jimmc101]

Beautifully made David, there's something about wooden cases.

C1, C3 (22nF) and C2, C4 (220nF) look suspiciously like multilayer ceramic capacitors.
To achieve these values they will use a 'Hi K' dielectric (X7R or even Y5V) which has poor stability against temperature, time and applied voltage. See p 10-12 of http://www.johansondielectrics.com/image...007-12.pdf

The first two give rise to frequency drift and the third distortion.

Replacing them with film capacitors, which are much more stable and do not age, would be worthwhile.

Jim

[Yorkie]

Thanks for reading the thread Jim, and for your kind comments and advice re caps. Good point - as I said earlier, I just used what I had to hand!

Interesting and informative paper too at that link, which made an interesting read.

I marvel at the manufacturing techniques used in producing caps and other components (not to mention the dark arts of SMD and CPU production!).

I'm quite a fan of the 'How It's Made' series on 'Quest'.

[Nick]

Nice box, David - I'll have to up my game again and start doing finger joints instead of dovetails Mahogany too! I'm only using lowly oak!

Regarding the circuit - the use of a MES lamp to stabilise the gain of the loop is a good (though standard) trick - the use of this form of AGC goes back a long (long!) way.

Although not stated in the article David cites, the circuit is a form of a Wein Bridge originally described by William R. Hewlett in his 1939 masters thesis - and, yes, its that "Hewlett" (of him and Packard fame). HP started their life making good lab audio oscillators based on this circuit (the HP 200A - also see http://www.hp.com/hpinfo/abouthp/histnfa...index.html).

   

I would refer everyone to Jim William's exemplar series of Linear Technology application notes, specifically AN43 (1990), Page 29, where the exact circuit David is using is described in fig. 39 - see http://cds.linear.com/docs/en/applicatio.../an43f.pdf - Jim then develops the circuit into an oscillator that by fig. 48 has 0.0003% THD (3 ppm).

I built this circuit (the fig. 48 version, which is widely regarded as an absolute classic of analogue design) many years ago for distortion testing of high-end amps I was then designing and I had some loop stability problems with it. I emailed Jim on the off chance that someone in his exalted position would take pity on simple me, and to his eternal credit he was very helpful and we had an incredibly useful email exchange.

He did admit that the circuit, whilst having extremely low THD, does suffer from loop stability problems, so don't expect too much !

Anything by Jim Williams is worth a read

Here's a photo of that ultra-low THD variant under test on my bench as it was 10 years or so ago...

   

[Yorkie]

Thanks for reading the thread and for your insightful response and the interesting links Nick. Hadn't realised how far back the design went, or the use of a lamp as a stabiliser. Some time back I built a PW L/C bridge which used a little wire-ended lamp, but the bridge stopped working and I need to take a look at it. I think the IC may have failed. (I've got spare but can't recall which it is).

As to the box, the mahogany was the end if a plank used to make some new doors at the Hull Dock Offices I believe! My constructional abilities (practical skills) have always exceeded my technical knowledge, and as a consequence, right back to the days when I wore short trousers, I've built countless projects that look quite presentable but have often not worked - sometimes down to my having made an error, or other times, because I've faithfully reproduced errors or shortcomings in the design that weren't apparent to me.

In my teens, (when PW was known affectionately as 'Camm's Comic'), as my gullibility threshold increased, it dawned on me that terms such as 'An Enthusiast's Power Supply' or 'An Experimenter's Short-Wave Three Valver' meant 'you need to be enthusiastic to build this and expect it to work' or 'we experimented with this design to try to make it work but couldn't - maybe if you experiment a bit more, you might get it to work'? As my technical knowledge has increased over the years, the pace of change has accelerated apace, leaving me further down the field - PICs, SMDs etc. But I'm happy in my own skin, bobbing along and enjoying myself along the lines of my sig block!

[Nick]

As to the box, the mahogany was the end if a plank used to make some new doors at the Hull Dock Offices I believe!

My oak is the left-overs from 8 years ago when we rebuilt our house as a green & eco-friendly one.

We had about 150 sq metres of solid oak flooring laid, plus the same again of limestone (and 38 tonnes of sandstone outside!) - as usual, we slightly over-ordered as you assume maybe 5% of wastage, be we actually got very little, even with there being 3 roundel rooms. Main reason for so little wastage was that the planks were of three widths and had T&G on all 4 edges, so you could abut them between joists, not just on a joist - worth remembering !

So my oak is all from that - it has T&G plus some stability ridges on the back, but machines down to solid 20mm x 150 to 200mm planks - it's "character" oak, so has the occasional knot, but I have loads of it.

Don't have much else except a load of cedar that gets used for the bee equipment...

[Mark Hennessy]

I'd add to the recommendation to read the Linear Technology app notes. I'm lucky enough to have 3 books of them - acquired before the internet was the primary source of date. I'm guessing they are long out of print, but they are definitely worth seeking out. There's something nice about having a compilation in book form, as just flicking through the pages and landing on a random circuit diagram can trigger off a thought-process that leads you to the solution or insight that you were looking for.

Jim Williams was one of the greats, along with Bob Pease, who tragically died in a car accident on the way home from Jim Williams' funeral. Bob Widlar was another great analogue designer - well-known for being a "character". His views about digital were well-known, as shown on this famous poster: http://www.theamphour.com/a-widlar-poster-for-the-ages/ (don't click if you're offended by an image of an analogue genius "flipping the bird" )

[pwdrive]

I had to look away from that poster, it was turning me into Satan.

Lawrence.

[Mark Hennessy]

I had to look away from that poster, it was turning me into Satan.

Lawrence.

Believe me, we've had complaints for much less