Distortion meter project

[retailer]

I've recently been working on a number of valve amps and was interested in some comparisons and as one thing leads to another I decided to make a distortion meter, not only to do some testing on the current amps I have in the workshop, but I do have a bucket list of amps I would like to scratch build so a distortion meter would come in handy. I know this can be done with suitable software and a PC sound card but I don't want to have to lug gear inside to the desk where the PC sits so I chose a magazine project from the May/June 1993 Electronics Australia (no longer in publication), the authors of the project claim it is able to measure distortion down to 0.01% which I figured was more than adequate for valve amps. The meter is a spot frequency type, the major parts are a low distortion oscillator with 3 fixed frequencies of 100Hz, 1KHz and 10KHz, a corresponding notch filter, and a millivolt meter, the scale of this meter was the subject of a previous forum post. Past issues of Electronics Australia are copyright but I believe fair use allows a small portion of the build project to be posted so I've attached the circuit, I guess the moderators will delete if this is not allowed, the circuit does have a one error a 680 ohm resistor was omitted from IC2 output, pin 6.

PCB's for the project (3 of) are no longer available so no option but to make my own, this was a bit of a process - first step was to draw a blank PC board correct size and 2mm thick in Fusion 360, then I scanned the PCB tracks from the magazine article, imported then into Inkscape, scaled them to full real world size and then using the Inkscape X,Y coordinates of the component pads, drew the component pads in the corresponding X,Y location in Fusion360, next I joined the relevant pads with tracks, copying as close as I could the path taken by the tracks shown in the magazine - making the tracks and pads .07mm high, this is slightly more than the thickness the copper on a PC board.
The track holes were then drawn in followed by the ground holes (for components that are grounded). It was then onto the mill to cut the spaces between tracks, this isolates them each other. Milled with 30deg D bit cutters (Ebay) the smaller of the 3 boards takes around 4 mins to mill, there is a small burr left behind similar to that left by an ordinary drill bit, so I run a fine file over the board and then do a second run in the mill. Using the same cutter, a second cnc file drills the component holes around 0.25mm deep as I don't have a decent PCB drill bit I can use in the mill and finally onto the drill press to finish drill all the holes, I only have double PCB material on hand so I had to chamfer all the holes on the other side to remove copper from around them so component leads don't short out. All 3 boards now finished and ready to populate, the time taken to complete the boards is probably about 1/2 to 3/4 of the time needed to make boards by the photo resist film method, just as important, it is much less messy, my ferric chloride is getting quite old and when I recently inquired about buying some more from a local chemical supply company I was informed it is now a schedule 1 chemical (a possible precursor used in drug manufacture) and I now need to fill out forms and show cause as to why I need it.

I'll update as I progress.

[buggies]

Interesting methods! I hope ferric chloride is not yet another casualty.

The circuit was a bit too low-res to read the component IDs but both May and June issues are currently downloadable via the internet archive.

I note thay have put a low resistance (R49) between the two ground sections. I would also be inclined to make the mains earth to PSU ground (shown by the transformer) via an optional link to prevent any ground loop hum upsetting the measurements if your amps are earthed.

[Craig Sawyers]

Doesn't seem to be a problem here in the UK - you can buy Ferric Chloride easily from electronics suppliers like RS and Farnell.

But I usually just have boards made now by a Chinese outfit. Minimum order is five off, but when they are usually $1 each it is not a big deal. You need to use layout software of course so you can generate the Gerber files needed.

Craig

[Craig Sawyers]

And nothing about ferric chloride or iron anything here

https://www.police.sa.gov.au/__data/...A3-version.pdf

I suspect your chemical supplier is talking nonsense.

Craig

[Terry_VK5TM]

Altronics is still selling Ferric Chloride as well as some others for etching pcb's.

Jaycar is selling Sodium Persulphate for etching.

[retailer]

There was a reason I gave suppliers like Altronics a miss, I seem to recall they only have ferric chloride solution - I was looking for the solid, my 5Kg bag dating back to the mid 70's was all gone and the remaining solution I had was taking longer and longer to etc, so I phoned a local chemical supply company, the girl on the phone gave me the info. This prompted me into trying to mill PCB's which has been a success, while the mill can't read gerber files I found a conversion program called Flatcam, which converts gerber into gcode, this allows me to mill Kicad boards. Fusion360 has electronics PCB layout built in but the free hobby version only allows small boards, I think up to about 60squ cm - I have a good supply of double sided blank offcuts that I picked up from a salvage yard so I chose to make the boards my self, it wouldn't have mattered if I etched or milled I still would have had to redraw the boards one way or another.

[Terry_VK5TM]

Just checked the cost of Ferrric Chloride at Chemsupply - $427 for 5kg - ouch!

Another off the wall suggestion is to check artists supply stores, not sure if they only sell the premixed stuff or also the anhydrous (solid) stuff.

Looking at your pics, the milling option looks to have given you some very nice clean pcb's.

[retailer]

Now that I have the milling sorted out I doubt I'll ever etch another board, not that I make that many - over the years I've tried all the methods - etch pens, nail polish, toner transfer, blue film negative photo-resist - the milling method so far is the quickest and easiest and if I bother to invest in a set of proper cnc pcb drill bits, it will be even easier. If I ever want a really pro looking job with silk screening and solder masks etc. I'll send it out to one of the Chinese makers.
Yes $427 is a good amount, not sure what my original 5Kg bag of anhydrous cost but it did come from Ace Chemical Co. which was just off Torrens Road back then, purchased from a young school leaver who went on to start Chem Supply.

[DrStrangelove]

Love the lightbulb stabilisation in the oscillator.

Shades of the HP200A of immortal memory.

I built a piece of student demo kit using a similar arrangement.

It worked quite well once I'd found a suitable lightbulb.

On the FeCl, I think I've got more than enough in the shed to see me out, especially at the current use rate.

[chriswood1900]

It may be of interest there was a very similar project in Nuts and Volts Magazine available here:-
https://www.nutsvolts.com/magazine/a...rtion-analyzer
with full build details.

[Wellertcp]

And there's Bob Cordell's THD meter;

http://www.cordellaudio.com/instrume...analyzer.shtml

[Jez1234]

I can highly recommend the Practical Wireless "Durley" distortion analyser. It also has a setting for use as a sensitive AC Voltmeter. It was designed by Ted Rule.

I built one many many years ago and it is still my main instrument for THD measurement in (huge!) preference to my Marconi Instruments and Dymar meters which are not much use below around 0.1%. The Durley goes down to around 0.002% usably. It covers the full audio range. You need a separate low distortion oscillator and one with the old light bulb trick will not usually be good enough to measure down to these levels.

[DrStrangelove]

Thanks for mentioning the PW Dursley.

Looks interesting.

[fetteler]

I am extremely fortunate to have one of these for measuring distortion and most of the other things needed to evaluate Audio.

It only does spot frequencies though...

https://collection.sciencemuseumgrou...udio-equipment

Steve.

[Smithdoor]

Any time I look for distortion I use a Oscilloscope and AF generator. You see right on the Oscilloscope.
It does not give number but helps find the problem.

Does a distortion meter aid in repair?

Dave

[Jez1234]

Originally Posted by Smithdoor

Any time I look for distortion I use a Oscilloscope and AF generator. You see right on the Oscilloscope.
It does not give number but helps find the problem.

Does a distortion meter aid in repair?

Dave

You will not see harmonic distortion below around 3-4% using just a sig generator and oscilloscope!

A distortion analyser with a socket for "Residual Out" (or however termed) feeding your oscilloscope is what you need You can than see distortion down below 0.01% and see eg the optimum point to set bias in a class A/B amplifier... amongst many other things. It allows you to see the shape and form of the distortion and not just the averaged level on the meter and this is very useful indeed when designing hi fi audio amps etc. The PW Durley I mentioned has this facility.

It also has switchable bandwidths (full or something like 130Hz - 20KHz IIRC), earth lift, switchable input resistance (to show input into eg 50R, 600R 1K 100K etc on the AC Voltmeter. Useful) and even LED's on certain controls which light up to show which should be used in conjunction when a particular function is selected.

[retailer]

I found a number distortion meter projects to consider when searching for something suitable - ETI 1985, Audio Magazine 1981 (Bob Cordell), Nuts & Volts - I wasn't aware of the PW 1983 project, I did get a copy - it's an interesting read - I settled on the Electronics Australia Project because it was fully stand alone, those components that I didn't already have were readily available, it allows for the use of an external ultra low distortion oscillator should I ever get one, and the use of a small 12V globe in the oscillator meant I wouldn't have to dismantle my homemade audio oscillator just to salvage the thermistor - there are probably better choices but if my build meets the project author's specifications I'll be happy.

After struggling with Fusion360 for over 2 days I came up with the final front panel layout model - I now just need to generate the Gcode for the final engraving which will be a challenge as I don't have enough Y axis travel to engrave the whole panel at one setting so I'll have to do it in sections - the mV meter and oscillator will be done together, then I'll have to reposition the panel on the mill table to do the lower half, an extra issue will the actual 2mm thick aluminium sheet I plan to use.
Some years back I picked a small stack of old road signs they were cheap, a good size - most are around 900mmx600mm and only $5ea, but all have some damage mostly around the fixing holes, so they may look reasonably flat but this may not be good enough. For good engraving the aluminium sheets needs to be perfectly flat, I've set the depth of the engraving at 0.5mm, but I can vary it depending on how I set the Z axis origin.
Once the sheet is fixed down to the mill table I'll have to probe the surface of the panel with dial indicator to find if and out how badly warped it is and decide how to deal with it from there.

[Smithdoor]

It is good to know.

Thank you
Dave

Originally Posted by Jez1234

You will not see harmonic distortion below around 3-4% using just a sig generator and oscilloscope!

A distortion analyser with a socket for "Residual Out" (or however termed) feeding your oscilloscope is what you need You can than see distortion down below 0.01% and see eg the optimum point to set bias in a class A/B amplifier... amongst many other things. It allows you to see the shape and form of the distortion and not just the averaged level on the meter and this is very useful indeed when designing hi fi audio amps etc. The PW Durley I mentioned has this facility.

It also has switchable bandwidths (full or something like 130Hz - 20KHz IIRC), earth lift, switchable input resistance (to show input into eg 50R, 600R 1K 100K etc on the AC Voltmeter. Useful) and even LED's on certain controls which light up to show which should be used in conjunction when a particular function is selected.

[Radio Wrangler]

You can use a low frequency spectrum analyser to look at the harmonic series and compare amplitudes. This also gives clues as to what mechanism is creating the non-linearity, but when you get down to low levels of distortion, you can get limited by your analyser's dynamic range. So use your spectrum analyser on the residual output of your distortion meter. Here the fundamental has been nulled and you can use gain to bring up the things you are interested on the analyser. Scopes and spectrum analysers show things differently and thereby give different clues as to what to do to improve things.

As Jezz says, by the time distortion is noticeable on a waveform seen directly on an oscilloscope, it's too late if you're working on a decent quality amplifier.

The scope direct view is good for spotting little bursts of RF oscillation in amplifiers where there are some stability issues. For example this is where you see stability problems with replacement output transistors in Quad 303s. This is also something missed if using a distortion meter alone. The RF oscillation is outside the bandwidth of the meter, and may not even make it to the residual output of it, and it'll be outside the range of audio type spectrum analysers and soundcards.

David

[retailer]

A small update on my distortion meter project - adventures with engraving the front panel - I had planned to have all of the hardware/metalwork finished by now and be starting final assembly but encountered a series of issues with the engraving - broken cutters, engraving too deep, bad choice of engraving font etc. etc. Fusion360 has 2 ways (that I know of) to engrave, one method uses true type fonts (same as Windows) - the text is placed on a 3d drawing of the front panel and then given a negative height this gives it the appearance of being cut into the panel, once all of the text/numbers have been placed in the correct position, the cutter type is chosen, the text is selected and Gcode is generated for use on the mill. Engraving cutters are quite simple - 'D' bits taper ground to a point, I was using 30deg cutter, when engraving true type fonts Fusion tries to maintain the correct width of the font by varying the depth of cut as needed, choosing a fat or bold font will results in greater depth of cut (cutter is tapered). I used this method as I'd used it in the past and had acceptable results.

The first panel made from Al was a fail because of an error on my part in defining the tool path which resulted in some text being engraved as .5mm deep hole, this happened almost straight away, with the first few engraved numbers, I noticed and stopped the job but the damage was done I had to discard it and start again. Had I let the engrave proceed I would have been alerted to other problems, which doomed the second attempt to failure.

Searching for a second piece of suitable aluminium I came across some black acrylic 3mm sheet, just enough for a panel plus one extra in-case I stuff it up. The first piece of acrylic was a fail - my choice of font was bad - I chose a compressed bold font which resulted in engraving that went far too deep, Fusion plunged the cutter in to maintain the width of the font, as the font was compressed the letters were too close together and ran into each other, the fine pieces of acrylic between the letters just crumbled - I could read the numbers and letters but only with a bit of imagination - this is what I would have noticed if I had let the first attempt continue - I still had one other piece of acrylic left, perhaps - I should view some youtube videos on how others have engraved.

One youtube video also described the second method of engraving, in Fusion this is called trace - this uses single line text - it has no width - the text width is set by a combination of the cutter taper/angle and also the cutter's depth of cut, this is more like traditional pantograph engraving where the cutter depth remains constant, knowing the cutter angle one just sets the depth of cut to achieve the correct width text/numbers, the choices of font are much smaller but this is probably more applicable for panel engraving, I decided to try this but was faced with the prospect of going through my design and edit all of the numbers and text from true type to a single line font, rather than miss something I chose to redesign the entire front panel. With the new setup/gcode I had the depth set at 0.3mm this would allow for slight upward bowing of the acrylic panel - with the cutter being quite short, the mill chuck is very close to the panel and this obscured my view, so I stopped it after a 1/2 a minute or so to check - it looked great, but something wasn't right, the text was placed too high on the panel and didn't line up with the position of the rotary switch - I won't say what I was thinking - back at the computer in front of fusion360 I could see I my origin point was 10mm out - I'll complete all of the engraving, on top of the last error so at least I'll see if there were any more issues, for a while I entertained the thought that maybe I could fill the error with black paint or epoxy but acrylic sheet is not that costly so for now I'll use this panel and when my new piece arrives I'll redo it.

I know some are thinking surely just printing the front panel artwork on paper and gluing it on would be easier - yes I agree but then I would have missed out on all that fun, forewarned is forearmed so for the next project, if there is one, I'll know what not to do.