Very interesting indeed!

The circuit you built, was it on a pcb or did you hand
wire it like I did? Just curious.

Ya, I expected some sine distortion because the XR-2206 uses pseudo sine
generation as well. Apart from testing, it's not something many of us would hear I suspect.

The ear itself does not perceive a perfect sine given the way it is constructed. With
a bone leaning against the ear drum, there is always some distortion as the motion
moves more in one direction than the other.  I'm not sure it is measurable, but
it is interesting to speculate on it.

For those who might be in the process of, or contemplating building an analog synth you may find the following results interesting from a performance perspective:

http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=basic_vco_vcf_tests

This link appears on the general notes that I have made about my synth project here:

http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=analog_synth

It's always reassuring to test what you've built to see if it is doing what it is truly what it is supposed to be doing (sometimes things seem deceptively successful when they're not).

My main disappointment in the results is only the small amount of distortion added by the VCA module. However, it is not serious enough to require a fix. It's only a problem for the perfectionist. But given how much effort goes into producing harmonics in a synth, this is not a practical problem as I see it.

The most interesting result for me, was seeing the effect of the Skew control on the Thomas Henry XR-VCO module. The wave form is shown on the middle of this page here:

http://www.birthofasynth.com/Thomas_Henry/Pages/XR-VCO.html

With the Skew control fully on, the center scope trace becomes flat.  With the triangle wave, the odd and even harmonics present, it becomes quite a harmonically rich signal to work with. The best part is the Skew control gives you control about how much of that richness you want.

LTspice simulations of the Steiner VCF indicated that the LP filter was 12 dB/octave, while the band pass and high pass filters were only 6 dB/octave. This VCF is great for simplicity and low parts count, but the BP and HP filters could be better. Yet, so far, they seem adequate for now. As a first VCF module, this is perhaps one of the best for simplicity.

My project won't win any beauty contest, but that aspect is unimportant to me. I look forward to the composition opportunities (and challenges) that it provides down the road.

I suppose you're right.  I've put a yellow sticky note on my desktop, as a reminder.

I'm currently in the middle of building my VCF module. I've been real anxious about finishing this last of the most essential parts of my analog synth.

As you're probably discovering for yourself, opamps are fairly easy to engineer useful audio circuits with. To create a buffer, you just wire it up with no resistors required (for split supply). Need some gain? Add a  few selected resistors to an opamp circuit, without any mind bending math to conquer.

To take things to the next level, I heartily recommend used books for topics such as opamps. New engineering books can cost a fortune, but I buy them used (at hobby prices) for under $12 + shipping.

A site I have had great experiences with is abebooks.com. They are a clearing house for 1000s of small used book stores across the continent (actually, they include the UK and India as well). The great thing is that they can usually find you the best used book price on the planet (be sure to use their sort by price+shipping link).

Even popular books like "The Art of Electronics" can be hard to find cheap, but I got mine from India for cheap. The paper quality of the India copies are often not as good, but I didn't really care about that (some on the internet call them the 'toilet paper' editions ).

For opamps, I heartily recommend any of these books:

• Op-Amps and Linear Integrated Circuits - Ramakant A. Gayakwad
• Op Amps for Everyone - Bruce Carter and Ron Mancini
• IC Op-amp Cookbook - Walter G. Junk (oldie but goodie)

There are probably a number of other ones out there, but these are the ones that I am familiar with.

Another source of inspiration comes from the library. If I borrow a book and like it a lot, I'll order a used copy for myself.  I've ended up with a few good books that way.

I've just added a Ceramic capacitor code calculator here:

http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=cap_codes&#code_calculator

While I can read the cap codes ok, I like to be doubly certain before I solder the part into place. There is nothing like a confirmation during a build.

[Opamp Choice]

Just breadboarded a simple 40106 oscillator and added an RC/CR. Worked a treat! So, what do I do next?

The funny thing about RC filters is they also depend upon input and output circuits that they attach to.  The source feeding into the filter should have a relatively low impedance so as not to affect the chosen filter response (which likely applies to your application).

Another challenge is the input impedance of the stage following the filter. It should feed into a high input impedance for best results. Feeding into a circuit with lower input impedance will affect the response.  Exactly how and what ranges to watch out for is something I want to investigate. I hope to run some LTspice simulations on that purely out of curiosity.

I only mention these things because what seems simple doesn't always work as expected. So for best results, low impedance feeds into the RC filter and then the filter feeds into a high impedance following stage.

I heard that 'proper' filters incorporate op-amps (is an LM386 ok?). Is this so the quieter tones are louder? The next step would be great to see!!!!
Many thanks again!!!!

The RC filter is a passive filter. You can't beat it for simplicity. But it does suffer from what is known as "insertion loss". The signal coming out will be reduced in amplitude.

With an opamp (or transistor) circuit, you make what is known as an "active filter". It is "active" because it includes active components (that amplify).

So what is best depends upon your needs (as in most things in life).

If your RC filter is going into another amplifying stage, you really don't need to worry about insertion loss (you simply make it up the lost gain in the following stage(s)). If you want your filter to amplify a band of frequencies above the current level, then an active filter will be required.

Here's another deciding factor:

One RC filter drops power at a rate of 3 dB per octave beyond the cutoff frequency. With signals, you are usually only concerned with voltage, which drops at 6 dB / octave. [Apologies for going into decibels here... -3 dB means a drop of 50%]

If you wanted a sharper cutoff than that, you need to run multiple RC filters in series. The problem is that this doesn't work well unless you go to an active filter design. The reason is that the output impedance of a passive RC filter is relatively high. Feeding into the next RC filter means that both filters now become corrupted in filter response because of this (both filters will interact and change the frequency response).

A simple fix is to run one RC filter into a buffer (opamp), which then feeds another RC filter. The buffer provides the high input impedance that the first filter needs, while providing a very low output impedance to feed the next filter with. So now we have gone full circle in this post.

Of course, you can also have opamp circuits that are active filters in themselves, rather than just a buffer, but that gets more difficult to describe in a brief discussion.

Opamp Choice

The LM386 is really meant to be an output amplifier, driving a speaker or similar type of load.  If the output in your application is simply an audio output signal to be plugged into a stereo (for example), I would not use that.

Use a voltage amplifier opamp like the TL071, TL072, or TL074 for single, dual or quad opamps, depending upon your needs.  These will be easier to work with than the LM386 (fewer connections) and provide better quality results.

Sometimes I have seen LM386's used in pedal circuits. This is normally done when it is doing some extreme "driving" into another distortion type of stage. I still suspect that this application of the LM386 is less than optimal, but I've not checked it out in detail.  

I've overly simplified the opamp selection here, since there are lots of other things to consider. It requires a book to treat the subject properly. But the TL07x opamp is a great jellybean part for low cost - high performance. If you need lowest noise, lowest distortion, the NE5532 or NE5534 is suitable for most applications.

Update:

I added some notes about effects of inter-stage impedance effects on filters here:

http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=rc_filters_coupling

I've been busy adding a few locally hosted online electronics calculators here lately. The one I want to draw your attention to is the online RC filter calculator here:

http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=rc_simple_filter

I've attached a picture of the results.  It took me while to figure out how to create a bode plot for the low and high pass filters, but it is working now. So the tool now also plots graphs of each filter, once the frequency is known. You can calculate R, C or F given any two known values.

The other calculators are listed in the menu here:

http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=online_tools

One I find particularly useful is the standard resistor calculator. Unlike the others, with this one you can choose the closest standard value, closest higher, or closest lower standard value. Sometimes you need to choose in a particular direction.

I find the best device for holding the pcb is a iron bench vise of some sort.  Mine is attached to a bench in my work shed in the backyard. For picnic table removals, I have used clamps to the picnic table but this is not nearly as convenient.

The main trick is to get the pcb clamped so that the edge is facing you, so that each hand has easy access to the top and bottom of the pcb. Being right handed, I like to heat the bottom side with my right hand and remove components with a tool in my left.

Yes- I understood that the moist rag was applied during the removal. For special ICs, this would be very useful. But in my opinion it will be too much of a hassle for most parts. Perhaps a lower hassle way to do this is to spray a light atomized spray of water on the top side prior to component removal.

That is a good idea for special chips being extracted. Keeping them moist is good. I wouldn't cool them after removal though. I'd rather they cool slowly so as not to crack or stress anything as it shrinks with temperature.

I personally find that the process is already complicated by needing three hands and trying not to burn yourself (when using a torch). Also, if you're doing this outside at the picnic table to avoid fumes (as I often do), you have the added complication of not losing the chips as they fall out. 

I know there are a few op amps loose in my backyard.

After I posted here, I ended up finding several youtube videos demonstrating their salvaging skills and techniques. I did find at times, by accident, that many components will just fall out when using the propane torch (and sometimes with the heat gun).

But I think the general success of that really depends upon the way the board was assembled. The boards that I had salvaged, had many components with the leads bent over. This pretty much forced a bit of a pull to get them out. The caps proved enough of a hassle, that I just left them and other small stuff like transistors there (they weren't worth the electricity or the propane to get them out!)

One recommendation I saw somewhere was to use a wood chisel to pry component leads straight before heating them for removal. I think this is a very good idea, which I intend to try out next time. You lay the chisel on it's backside, and bend the lead by moving the chisel handle towards the pcb (thus moving the chisel end up and away from the pcb - bending the lead straight). The chisel end gives you that tiny lever you need to force the issue.

Using some of the salvage from last week, I've started building the noise module. See attached image. I used one of the original front aluminum panels (cut to my module size of 9 inches), which already had holes for the potentiometers and the power push on/off switch (with LED).  There are some square holes left that I need to cover or fill later. Don't worry if the current print on the module face is upside down (this will be covered/painted over).

What I have soldered up today includes the power on/off, and the white noise generation circuit. I didn't need the push on/off power switch, but I found a way to include it because of the cool little LED that is wired to shine through the white button.

The main thing that I wanted to report here today, is that I discovered that the circuit works much better when you wire it correctly!  I must have had the 4.7k and the 47k resistors reversed in the breadboard incarnation. When I soldered it up correctly today, I was able to get about a 2.5 volt pp output from the transistor pair (one being the zener).  I've attached the scope trace, where each graticule is 2 volts each. So I'll be reducing the white noise gain opamp accordingly in the final build.

The other thing that I thought I'd mention is a realization I had about the pink noise filter opamp stage. The feedback circuit that I had previously adopted- rather than have multiple RC pairs, actually accomplishes the same thing by cascading the capacitors. While this looks cool on paper, this could be a nuisance in the build.

Capacitors in a series, as everyone knows, results in a reduced total capacitance (like resistors in parallel). This is ok except, what happens if you can't [easily] get the precise value of the capacitor you want?  If you sub something else, this changes the values of all of the downstream caps (and/or R's).

So while the original design works ok, I plan to rework it into a more traditional form of independent RC pairs. That way, changing one R or C does not upset the remaining pairs.

There is always lots of fun, when it comes to building something!

[You must do this outside, since it is not only very smelly, but the fumes are very bad for your health.]

I have occasionally scrounged parts from discarded circuit boards. I recently was given about four audio mixer boards from a large TV production console, through a friend. This was not only a great source of pots and switches, but also had a bonanza of opamps and other associated chips.

I've traditionally extracted parts like this using a propane torch, applied to the solder side while the PCB is held in a vice on the picnic table. You must do this outside, since it is not only very smelly, but the fumes are very bad for your health.  It probably is a good idea to wear a mask. Be prepared to be stopped by your spouse-- it will attach to your clothes, and you'll need a shower afterwards also.

The propane torch approach works very well at getting parts out, since they'll often just drop out once the solder is melted. However, many parts are hooked in and will require some pulling. Chips usually pull out well, and I have a special little IC puller that hooks under each end of the chip. All of this works very well, except for the smell(!)  It is of course, destructive to the PCB.

To avoid all the spousal hassle associated with the smelly process above, I tried out a different process with the remaining pair of boards (I got through the first two with the propane torch).

In the 2nd process, I used a heat gun on high. This requires considerably more patience, since it takes longer to heat up a chip (or a pot) so that all the solder melts. In fact, you may need to do a little gentle rocking etc. to help it come out.
This heat gun process however, does heat the chips considerably more, since the heat is applied longer until the solder begins to melt. Again, you should do this outside, or at least in a well ventilated place like a garage.

I got a lot of good parts extracted this way. What I didn't know until this morning, was whether or not the extra hot chip extractions were successful or not. I managed to test several TL072, NE5523, NE5534 and a pair of LF412 opamps and happy to report that they all seemed to test ok.

To test them, I bread boarded them in unity buffer configuration, for each opamp. I supplied a signal into the non-inverting input and scoped the output to see if it looked like a good sine wave (that was the input).  Someday, I want to design a generalized opamp tester along these lines and add some other fancy tests so that they can be tested without a scope.

Anyway, the point of this post is simply to say that the heat gun extraction of opamps appears to be a successful approach. The chips do get hot, but they seem to survive it ok.  The trick is simply to get in there, get them out, and let them cool off as best as you can manage it.

I also found that by extracting chips in neighbouring groups is helpful, because they already have the starting heat they need (it saves time, and reduces the stress on the chips being extracted).

I've attached a picture of the dining room table as I was sorting out the parts that were extracted (while the wifey was away).

[Update:]

Actually my pink filter is more based upon the TLC2272 circuit here http://www.techlib.com/electronics/pinknoise.htm. The evidence is in the way the capacitors and resistors are arranged in the feedback loop.

My modifications included changing the gain, so that his 20k feedback resistor became 220k, his 470 ohm resistor became a 330 ohm and I added RC pair C7 and R12 to further correct the response near 20kHz. The TLC2272 circuit was designed to flatten out after 4kHz, which was not good enough for my intents.

The MC4558 circuit does have a good pink response, with fewer FB components. But I didn't like the white response (and I intend on bringing both out). The white tapers off rapidly as you go below 60 Hz, which I felt was excessive. The white also peaks above 22kHz but this can probably be ignored (though it could introduce distortion somewhere down the pipeline if it was too high in amplitude).

The MC4558 pink response is dependent on the white response of the prior stage, so if you were to fix that, then you'd have to revisit the pink filter design.  

I also didn't like the opamp used. You could sub out the MC4558 but then then you'd have to rework things somewhat. The MC4558 has a minimum slew rate of 1.5V/us, which seems low - I need to check this, but it is probably barely adequate.

The big objection was the noise rating, which is 12nV/sqrt(Hz) for the MC4558. I have read that you should use an opamp with less than about 5nV/sqrt(Hz) so that you're amplifying your noise source rather than the opamp's own noise.

So I chose to use the NE5532 opamp which has a typical noise rating of 5nV/sqrt(Hz) at 1kHz. It also has a slew rate of 9V/usec, which may or may not be significant but is considerably higher.  The other factor for choosing the NE5532, is that I acquired several of these, pulling them from discarded TV station audio console cards, that I got from a friend.

When I first saw circuits and the advice for using low noise opamps in the pink filters, I thought this was nuts (high quality noise?)  But there seems to be sound logic to it. I'm just not sure I would be able to tell the difference.

Every module build has been a great learning experience!

Update: I went back and calculated that SR=1.5V/usecs is good up until about an swing of 9.5 volts @ 20kHz. Given that noise waveforms may need a minimum of a 2nd harmonic (for faithful reproduction), this is good to about 4.25 swing volts. So this would seem to barely cover the application ok, if the signal output level was at or below 2.125 volts amplitude (4.25 volts swing).

On the other hand, a SR=9V/usec, is good for 71.6V @ 20kHz, or about 8.95V swing at the 4th harmonic of 20kHz.

Clearly a cleaner top end, though few of us would probably hear the difference.

I had no idea that generating pink noise was so tricky.  I've simulated several pink noise implementations and performed some AC analysis on each here, which folks might find interesting reading:

http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=noise_generator

I think I've also arrived at a design that I plan to build now.