Connecting Etherwave problems

Then I will order a passive one soon. I'll write the result in this thread. Thanks again.

" I'm not actually sure what the benefit of active version is - anyone know? " - Jason

When you have a low Z output, and you want to drop the signal level, and you dont need isolation, then there is no benefit to an active interface - a simple resistive divider is usually all you need.

If you have a low level or high Z output, then an active interface can be needed for impedence matching and / or gain.

Theremins are usually low Z, line level output - so if you are feeding the signal into an instrument requiring lower signal levels (Guitar amp for example) all you need is a couple of resistors.

Out >----[Resistor R1]---------o----------> IN

                                        |

                                   Resistor

                                       R2

                                        |

    ------------------------------o------------- Ground (screen)

The ratio of resistor R1 to R2 sets the attenuation - for example R1 = 10k  R2 = 1k will reduce signal to about 1/10th.. The input impedence (Z) to the amplifier (or whatever) is seen across R2, this is usually high so makes little difference - but if, for example, it was 2k, then R2 would need ton be 2k to get aprox 10:1 gain reduction. If the input Z was 1k, R1 is all you would need.

The output sees a load of R1 + input Z .. If this loading is too great (resistance too low) the bass response could be reduced ( there is usually a series output capacitor, which, in combination with the load, forms a high-pass filter.. Usually this capacitor is large enough that the roll-off frequency is below 20Hz for expected loads - but if the manufacturer cut costs by fitting a smaller capacitor, then load can affect bass response) - If R1 is too high, one gets more noise.. A value for R1 >= 4k7 and <47k is usually a reasonable range.

The usual maximum loading expected on audio equipment is about 10k, so R1 + (R2 || Input Z)  should not be lower than this value. It should be noted that resistors produce noise - and the larger the resistor, the larger the noise.. But there is now way 'round this.. Feeding a high level signal into a low level input requires dropping the level .. If one uses resistors to do this, the noise produced by these resistors will be amplified by the high gain of the amplifier - An essential rule for low noise audio is "dont attenuate then amplify".. This is where transformers MAY give an advantage over resistors for changing signal levels and matching - but they are not without problems (and I dont want to writa a book here, so I must stop now!)

Geeks will find all this stuff in the first few pages of Douglas Self's book "Small Signal Audio Design" - it is a book I highly recomend to all designers.

By far the simplest way to do the above is to solder a couple of resistors into the plug on the lead - marking this plug as the one to go into the amplifier.

Fred.

(been looking for attenuators on-line, and am surprised at how few simple attenuators are available.. would there be a market for a lead you could plug into your EW, with a plug to go into a guitar amplifier, that took care of the levels? Perhaps having a screwdriver adjustable potentiometer in the plug to allow  trimming for differen attenuation levels - And how much would people pay for such a lead? -  )

I bought this one:

http://www.thomann.de/gb/millenium_die_dibox_passiv.htm

And it did not solve the problem. Any ideas?

Hi there,

the theremin is probably not grounded properly when not connected to the amp. I had a similar problem, when trying to use headphones with my mixer and not bothering to connect the amp.

Greetings, Randulph

Hi all,

I would probably go with randulph’s suggestion of an improper earth/ground.

I have a separate earth connection to my theremin since I often use it with my wee Vox Da5 which can run off batteries. Now on the occasions when I’ve taken the output from the headphone socket on the theremin and put it into my laptop, I get a mains hum, which disappears the moment I unplug the separate earth to the theremin.

Now this is (I think) where the relevance comes in: when I’ve been trying to connect theremin-amp-laptop all together, there have been moments when the theremin behaves so unpredictably (some high whining noises, general noise, course sound,  unpredictable pitch control and so on) that I have thought that there is something wrong with the instrument but then I check the earth connection/path to always find that there is no earth in the system whatever since there are interconnecting leads all over the place and I’ve forgotten that one wire that makes sense of the whole lot. The reconnection of the earth wire corrects the situation every time.

Roy

N.B. That said, w0ttm, Fred and Jason have much more experience with these things than I have so don't discount in the slightest what they recommend!

Some stuff from Element 14, on Theremin grounding:

The above relates to the "playing" aspects.. Below are some basics on ground wiring / cables etc.. Most important things to remember are that even tiny currents flowing through the screens on audio cables can cause horrible hum and noise - Keep the ground potential of ALL kit connected to your theremin TIGHTLY CONNECTED - But dont use the screens to achieve this!

Fred, thanks for that.

The wonderful world of earthing made easy! And I'm not being facetious by saying that.

Roy

Thanks a lot Fred, Randulph and Roy. I will take some help from a friend with better soldering skills and understanding of electronics. Your guide made it easier to understand, Fred. Is there a finished solution to buy, this cannot only be a theremin problem?

" Fred. Is there a finished solution to buy, this cannot only be a theremin problem? " - toyelele

Its not only a theremin problem - it covers every situation where different pieces of kit are connected together and screened cables are used to shield signals  - Everything from microphones to hi-fi's to studio wiring to lab equipment..

But the huge diversity of situations these kind of problems can occur in is probably what makes a simple "off the shelf" solution difficult. Back in the '60s when people were putting their classy Hi-Fi seperate systems together (and yes - for about a decade people really did strive for Hi-Fi and did actually listen to real music.. ) the stuff I have posted would be deemed elementary.. But these days even fitting a simple adaptor, and the possible problems these could create, is beyond the understanding of most -

There were patching connectors available once, which gave one access to the screen connections and allowed one to break/make these - but I havent seen one of those in decades - Also, in them good ol days (LOL) you simply wouldnt buy a piece of kit (turntable, amplifier , whatever) which didnt have an accessible grounding terminal on it.. Equipment was designed with ground potentials and grounding in general firmly in the forefront of the thinking.. These days you need to look damn hard to find equipment with this vital connection - It astounds me that something like the theremin, which depends entirely on ground coupling, dont have a easy accessible screw terminal on its back plate for easily connecting ground...

Most of the time, having screened cable with the screen connected at both ends, is fine - Often the equipment is adequately coupled via mains earthing - but adding a seperate cable between the ground points is always better, and in a studio situation grounding all equipment at one master ground point is (or certainly was) best practice..

One usually only needs to cut the screen at one end rarely - and this is often due to poor design of the equipment sourcing or sinking the ground - for example, if a mixing desk has screened internal wiring but its grounding is not managed well.

Supplying an adaptor which broke the screen is probably a dangerous idea for any manufacturer - People whose equipment was not sharing a common ground through any mechanism other than the screens, and who had noise as a result, would not fix the problem by disconnecting the screens - this would make the problem far worse! - They would need to have a seperate ground path connecting the equipment.. I think that there would be loads of returns on such adaptors - simply not a viable product! - Those who understood how it could be used would be those capable of simply un-soldering the screen from their jack plug!

Fred.

In my diagram showing the power adaptor, I showed this adaptor with a connection to mains ground.. This is the way it SHOULD be..

BUT - Many (most?) power adaptors (wall-warts and the like) do not connect to the mains ground - they may even have a dummy plastic earth pin.. It is often only the power supplies specifically supplied with a theremin which have a correctly implemented ground pin.

Also - Even if your theremin is grounded correctly, if this is connected to any equipment which is not correctly grounded, you can / will have problems.

The best way I have found to eliminate ground-loop problems is this:

Take a mains plug, connect a heavy wire to the ground terminal, INSULATE BOTH OTHER TERMINALS - as in, cover them with hot melt or epoxy adhesive so that there is never any possibility that they will accidentally come into contact with anything.. THEN put the back on the plug (You really dont want the earth lead to come adrift and accidentally make contact with the live pin! - And you should never trust the wiring of mains sockets - L and N can be swapped over!)

Take the ground wire to a "chocolate block" screw connector strip - wire one side of this strip so that all blocks are connected to ground..

You now have a row (usually 12) of screw connectable grounding points - wire the ground of each piece of equipment directly to one of these ground connection points. If your equipment does not have ground connection points (and most equipment these days doesnt have) you need to find easily accessible points to add your own.. The nuts holding sockets in place is often a good point - Take a metal cased jack plug, plug it into the equipment, and test with an ohm-meter to find a point which is connected to the jack-plug's case, and connect a wire to this point, and fit a socket (in-line Banana type is best) .. You can now make a set of ground leads with banana plugs, going to your grounding chocolate block.. This allows one to easily connect / disconnect things without difficulty.

Its quite a lot of work and hassle - but you only usually need to do it once - And once you have done it you will wonder how you ever put up with the background noise levels you had before.

If you move equipment to other places, a seperate block to use with equipment you move (rather than dissconecting your main ground distribution every time) is a good idea..

Its also worth having at least one set of leads with the screen broken at one end - Mark this lead clearly, and use it to test if there is improvement.

Fred.