Modifying the tone of the Etherwave Standard

Carrying this over from the "shopping suggestions" thread.

We were discussing ways to make the tone of the Etherwave more "smooth" and less "electronic" or to put it another way, more like a bowed string and less like a saxophone.

FredM offered a great suggestion of changing the capacitor #23 to a higher value. We did this and got a result. Not a ideal result yet, but a step in a direction at least. So this is great.

Not really sure where to take this from here, but I felt it needed it's own thread so here we go.

Discuss. :)

ok -
I posted latest advice on the old thread here (http://www.thereminworld.com/forum.asp?cmd=p&T=4119&F=1&p=3)

There is absolutely no sophistication in this modification.. what it does is:

Increases the filtering effect seen on the input to U3A.. C23 is usually 4.7n, which (in combination with R23) removes frequencies above 20kHz, and thereby gets rid of the sum frequencies resulting from heterodyning..

Increasing the capacitance at C23 lowers the filter roll-off, and this has the effect of attenuating harmonics.. This also has the effect of reducing the waveforms over-all amplitude, so, at some value, controlled clipping performed by the brightness control does not occur, and the brightness control has no effect (the sound is at the undistorted setting - the only distortion heard is whatever harmonics are passed in the audio waveform.)

The results of further increase in C23 capacitance is being awaited.

Could one just use a speaker with a lower frequency response to roll off the high end? One of the speakers I have floating around has that effect.

[i]"Could one just use a speaker with a lower frequency response to roll off the high end? One of the speakers I have floating around has that effect."[/i]

Yes - you can have any sort of filter on the audio output.. electronic or electro-mechanical..

My reason for selecting the C23 filter is that it precedes the VCA/Distortion stage.. Reducing harmonic amplitude here has the added bonus of reducing (or eliminating) any nasty harmonics produced by the process of distorting the "natural" harmonics in the waveform.

I do not own an EW, and have only built bits of the circuit (and simulated the rest)to gain understanding. I am not at all impressed by the design - Some bits are wonderful.. The oscillators and the method of applying voltage control for tuning the reference oscillator, is brilliant.. Alas, in my opinion, this is where it ends -

The reason is cost and simplicity.. The diode mixer is the simplest mixer possible.. Some people have no problem with it, but I have always had problems with it - Perhaps I am too fussy, but I want the mixer to give true, reasonably accurate multiplication of the input waveforms and their harmonics - The diode mixer adds the first errors - and in MY opinion, this is audible.. Replace the diode mixer with an LM633 and the tone quality (in my opinion) improves greatly.. But the LM633 would add about 50% to the component cost (they cost about £8) .. and would be the most expensive component on the EW by far.. The diode mixer probably costs about 10p

Then there is the VCA and related tone controls.. There is nothing wrong with using an LM13700 as a VCA.. not the best, but certainly good enough for sub-pro audio (anyone who uses a LM13700 in a studio mixing desk should be shot!).. But, the way this VCA is configured and driven, and biased by the brightness control, is, in my opinion, simply horrible! To make a good VCA with an LM13700, great care must be taken over the bias on its input and lineareizing terminals.. These are the teminals used on the EW design to control 'waveform' and 'brightness' - Effectively adding clipping, and adjusting the aproximate width of the clipping.. The level where clipping starts is determined by the amplitude of the input waveform, so is (probably) quite frequency dependent.

The modification detailed should reduce the amplitude of the input waveform, therebye reducing the distortion produced.. it may be that one could achieve this by simply reducing R24 to say 2.2k which would affect all frequencies equally.. but increasing C23 instead targets the higher frequency components.

Experimenting with the values of both C23 and R24 was going to be my next suggestion... both are in parrallell with each other, so easy to play with at the connector.. One could even put a preset potentiometer to make experimenting easier.

Sadly, no report on the updated experiment yet, it'll have to wait till Monday as the only local source I have for the parts needed isn't open today. So more soon.

With regard to the speaker, I've tried lots of different Amp/Preamp combinations as well as lots of different speakers, EQs, and other things. Unfortunately, none of it achieves the desired result. I'm not a huge fan of analogies but I think it's the best way to sum up this particular issue. Let's say you are a classical guitar player and you own a steel-string dreadnaught guitar. No matter how you mic it, preamp it, layer effects on it, nor what speakers or amp you use, that dreadnaught will never sound *exactly* like a nylon-string classical guitar. This is similar to the trouble I'm having. I love the playability of my Etherwave, but I loath the tone it has. I tend to describe it as "digital" or "electronic" because this is the best way I can put it, not sure if it properly conveys my meaning though.

It would be a good idea to get the following parts:

capacitors: at least one of every value from 47nF to 1uF .. these must not be electrolytics, need to be rated >16V, but can be any type or tolerance.. cheap resin ceramic capacitors are fine - get ones with 0.2" pitch and reasonable lead length if possible, you want to be able to fit them at the connector without needing to solder extra wire to them.

one 220 ohm resistor - small cheap leaded part.
one 10k ohm preset potentiometer, any small part will do.

The idea will be to connect the 220 ohm resistor and the preset in series, and wire these in parrallel with the capacitor - this will enable you to adjust the signal level and play with different values of capacitor.

Start by increasing the capacitance until you get to the point where the loudness of the high notes (compared to the low notes) is too low for your taste..

Then halve this capacitance, and fit the resistor / preset.. adjust the preset to get the balance you want.

if sound is too buzzy - add more capacitance, and trim the preset.

With luck, you will find the sound you want.. You will only have that sound available, as the tone controlls are unlikely to have much, or any effect.. You could add a switch to disconnect the extra capacitors and give the original sound back, if you wanted.. probably best to use a screened lead to this switch

Ok, this has been a fun experiment, though it has turned out at all like predicted.

First, I'll recap the original result. We connected a 0.047uF capacitor in parallel to the C23 on the Etherwave. The effect it had was not noticeable at all with the "waveform" knob turned all the way counterclockwise, but noticeable with the "waveform" knob turned the other direction by the fact that the tone no longer changed.

So next. We removed the parallel 0.047uF capacitor and added a connector to the Aux port on the board. The socket we added was a 16pin IC socket that we cut in half, allowing us to connect all manner of different capacitor values without further soldering/desoldering (my tech savvy friends idea, seemed a good one to me).

So, we then fitted the resistor and capacitor together in parallel and then in series with the potentiometer (220 ohm and 0.047uF). This seemed to do nothing different than the 0.047uF capacitor alone at all. As far as my ears could tell exactly the same effect, no change in loudness at either end of the range.

So, here is where we deviated.

We began to experiment with just different capacitors plugged into the socket. We went from 0.001uF to 1.0uF. The effect of values below 0.047uF was that the "waveform" knob still changed the tone when turned clockwise, however it's effect diminished until with the 0.047 capacitor it stopped effecting the sound. Still no change in the loudness at any point in the range. We got all the way up to 1.0uF and still, the Etherwave sounds exactly like it did without the mod when the "waveform" was turn fully counterclockwise, just that now turning it clockwise does nothing at all.

One thing did change though, with the 1.0uF capacitor in place, the "brightness" (or as I've always called it, "distortion") no longer changes the tone, but now acts as a volume knob. All the way ccw and the Etherwave is full volume, all the way cw and it's muted. A very nice side effect.

But through all this, the tone is still the same as it ever was. You can hear this best in the lower part of the range. Low notes kind of go "dur dur dur dur dur" like they have a forced staccato instead of "duuuuuuuuuuuuuuuuur".

So, seems that what we've achieved is a quick dirty hack to turn the "brightness" control into a volume control. Useful, I'm going to leave it in place, but the tone of the Etherwave hasn't changed at all.

How strange! -

In particular:

[i]"So, we then fitted the resistor and capacitor together in parallel and then in series with the potentiometer (220 ohm and 0.047uF). This seemed to do nothing different than the 0.047uF capacitor alone at all. As far as my ears could tell exactly the same effect, no change in loudness at either end of the range."[/i]

Usually, with the default values (R23=10k, R24=4.7k) these would form a potential divider (attenuator) reducing the signal to 1/3rd (0.319)of the value seen across D4 (this is calculated on DC.. The capacitor acts as frequency dependent resistance for AC signals). With R24 at 220 ohms, the attenuation would be (1/(10k+220))*220 = 0.0215 ... Even allowing for the exponential nature of loudness, one should hardly have heard any signal at all! (cant be bothered to calculate the db's right now)

I suspect some physical problem (220k resistor instead of 220 ohm resistor, faulty / miswired preset, wrong connections - something like that..)


[i]"We went from 0.001uF to 1.0uF"[/i]

Presumably the original 4700pF (0.0047uF) capacitor was still on the board..


[i] The effect of values below 0.047uF was that the "waveform" knob still changed the tone when turned clockwise, however it's effect diminished until with the 0.047 capacitor it stopped effecting the sound. Still no change in the loudness at any point in the range.[/i]

Ok,


[i]" We got all the way up to 1.0uF and still, the Etherwave sounds exactly like it did without the mod when the "waveform" was turn fully counterclockwise, just that now turning it clockwise does nothing at all. "[/i]

This does not make sense - you should hear SOME change in the tonal quality as you increase the capacitance from say 100nF to 470nf, or certainly from 100nF to 1uF.

The ONLY thing I can think of here is that, at about 100nF, you have reduced all audible harmonics to below hearing - and that you have a pure sine wave at this point .. but that this sine wave is being distorted by some subsequent part.

If you have access to an oscilloscope, check the waveform across the capacitor (GND - Audio on the connector).. Alternatively, connect an amplifier to this point and see if the sound you get here is cleaner than the sound you are getting from the EW output... You will not be able to control the volume, but it would be a valuable clue.

[i]"One thing did change though, with the 1.0uF capacitor in place, the "brightness" (or as I've always called it, "distortion") no longer changes the tone, but now acts as a volume knob. All the way ccw and the Etherwave is full volume, all the way cw and it's muted. A very nice side effect."[/i]

I have no explanation at this time.. To be honest, I expected this to be a quick "Yeah, it sounds great" or "Nah, it only starts to sound ok when there isnt any volume" or "it changes the sound, but its like shoving cotton wool into the speaker" (I actually expected the latter response!) - After all, its just changing a capacitor in a simple RC filter... I should have known better! I should have learned by now that even a simple RC filter becomes a quagmire with bloody Theremins! DAMN THEM - They are stealing my life!! ;-)

[i]"But through all this, the tone is still the same as it ever was. You can hear this best in the lower part of the range. Low notes kind of go "dur dur dur dur dur" like they have a forced staccato instead of "duuuuuuuuuuuuuuuuur". [/i] -

I think this is possibly a big clue.. but, as I have said (I think) - I do not own an EW.. This 'sounds' to me that the reference and variable oscillators are drifting in and out of lock.. and that if this is a regular "Durr Durr" it could indicate a problem such as power pulsing or beating with something else... Thierry! We need an EW expert!

[i]"So, seems that what we've achieved is a quick dirty hack to turn the "brightness" control into a volume

I would also suggest (remind) you that there is the option of packing the EW up, and sending it to Thierry..

I strongly suspect there are things which will not be fully resolved, even if the modification gets rid of some of the problems - I think you may well find that you have got used to, and tolerated things which are needlessly making your Theremin expieriences more stressful than they should be!

Seems that I may in fact be able to get access to an oscilloscope, but it will be a few days. So, we'll carry on then.

I'm beginning to suspect that despite my best efforts I'm simply not explaining what about the EW's sound I dislike in ways that folks on this forum understand. This sound that I am describing is what every single EW (even the EW Pro) sounds like. I tried to find a sound clip on the moog site but it seems they prefer their theremins to be novelty synth controllers these days so the only videos they have up are to that effect. This video though demonstrates it rather well:

http://www.youtube.com/watch?v=4nM9UpBBPhI

At about 4:00 into this video the gentleman doing the presentation is talking about "tuning" the theremin and he holds a few of the low notes for a long time. In this you can hear the "dur dur dur dur" that I am trying to describe. This "stuttering" is in fact present in every sound the EW makes, but in the high ranges it's so fast you don't hear the drop in volume. I think this makes the EW sound like a toy instrument, and causes playing pieces that have low notes in them to just sound bad.

This is why I originally asked the community at large about different "higher grade" theremins, I feel like the EW is a beginners instrument. But despite my best efforts it seems "get a degree in electrical engineering" and "play a moog" are my only options, as no, no one produces better theremins that a musician can buy.