Guitar Transmitter Report (+ PCB)

[RaceDriver205]

Just built the Beetle MKIII guitar transmitter.
Used this pcb: http://www.tothemax.web1000.com/mkIII.html and it works (confirmed).
Ready for PnP transfer, but PCB should be printed shrunk to 14x35mm.
Other required info: http://www.talkingelectronics.com/FreeProjects/FM-Bugs/BeetleMkIII/BeetleMkIII.html

You have to adjust the input potentiometer to prevent it distorting (play loud 6-string cords at max guitar vol. to test it).
Finding out where to tune the radio to takes a bit of fiddling (has to be tuned very precisely), so dont assume its not working.

[Jaicen_solo]

I built one of these (A transmitter that is) with a standalone receiver but I found there were a number of issues I never got around.
Firstly, in it's stock form, the transmission introduces a good degree of hiss which is very noticable, even at moderate gain levels.
The only way around this is to provide pre-emphasis of the high frequencies in the guitar signal prior to transmission then de-emphasis on the receiver but I never got anything satisfactory.
The second problem you've noted is one of dynamics. FM transmitters, particularly simple ones like this are just not capable of handling the dynamic range produced by a guitar and will tend to produce intermodulated distortion on heavy attacks.
I gave up on my transmitter due to these reasons, but lately i've been thinking it would make a pretty good in-ear monitoring system paired with a 386 amp. I definitely wouldn't use it to play live, but maybe you have better results??

[AzzR]

Ive been wanting to build a wireless guitar transmitter for a while.

What this one like in terms of putting out a clean signal

Dream

[powerplayj]

I just built this FM microphone and thought about adapting it for guitar with a 1/4 input.  Then I thought, why not try to make an entire wireless system (UHF/VHF) with a transmitter and receiver like those that are commercially available.  However, I'm assuming there are some caviats or else it would be a more popular project.

[Jaicen_solo]

Well if you can implement a UHF/VHF crystal oscillator, you should be good to go.
I think the major obstacle to a DIY design is that you can pick up a true diversity VHF set on ebay for a little more than it would cost to DIY something which may not work as well, but by all means give it a go.

[RaceDriver205]

I would agree that this is not a 'professional high quality' device. I got very little hiss at close range (when i was testing), but I have yet to test it at larger ranges.

However it is very small and simple, and providing you adjust the volume input to it such that it does not distort, the clean signal seems good enough. I built this device for its novelty, and transmitters tend to get real complicated real quick when you look to improve performance.

If you are looking for high-fidelity I think you'll have to buy the 'real deal'.

[Jaicen_solo]

Well if you were using a commercial reciever, that would likely have de-emphasis built in. I found this to rob my signal of any sparkle which is why I used standalone reciever arrangement.

[RaceDriver205]

Interesting. How did you make this stand-alone receiver?

[Jaicen_solo]

I didn't build the reciever, I just recycled one from a dead karaeoke mic. It was stuffed with smt ic's so I'm not sure how feasible it would be to DIY. Besides it came with a (spare) transmitter, and lo-fi mic for about £30, and at that price it's not really worth the hassle of etching pcb's in my view. Sorry for the confusion.

[RaceDriver205]

Just tested it again, this time more thoroughly. Had the transmitter 5 metres away with no antenna.
There was no perceivable buzz/hum, and the signal was very clean (zero distortion) but:

• The volume was lower than normal FM stations (But I have found this with all small transmitters)
• There was a treble cut

I think this deemphasis could be countered by a 'reemphasis' or just simple treble boosting, and another fixed MOSFET booster to correct the volume.

[TELEFUNKON]

"normal" FM stations compress heavily and don`t need to be afraid of the FCC.

[Mark Hammer]

Here's a wireless project scanned in from ETI that I've had posted for a while.  This one comes with, believe it or not, a built-in Boss SD-1.  Seriously.  Haven't built it but my understanding is that it works but is not the highest of fidelities.

http://ampage.org/hammer/files/Wireless.PDF

[Mihkel]

I just built it and after making a new coil it works nicely. I can't say anything more about it right now, 'cause im playing it through an old radio, and it probably will sound different through a guitar amp. I do notice the overall volume and treble loss though (an excuse to build the new GGG EQ i guess  ). RaceDriver205, what did you use for the receiver?

Thanks for sharing the project

[JimRayden]

Any chance of increasing treble response within the emitter circuit?

----------
Jimbo

[Mihkel]

I was thinking about trying to make the 100nf cap at the input smaller, but that just seems far too obvious.

[JimRayden]

It'll cut off the bass and propably leave you with a puny midrange honk.

-----------
Jimbo

[RaceDriver205]

I think when I come to install it in my FX system, I will either use treble boost or the 'proper' FM emphasis circuit that they use - at the receiver.
The problem is not that the transmitter looses treble, but that commerical radio stations and FM receivers use 'emphasis'.

Emphasis is where the high-frequency component of the audio is boosted in amplitude prior to transmission, in order to get better signal to noise ratio. Because the MkIII does not use this pre-emphasis, but a standard radio set does, this results in a loss of treble - the radio 'deemphasises' the plain audio signal.
To get a large improvement, you could add a proper pre-emphasis circuit between the guitar input and the transmitter.

Also, I have realised that the volume drop is not going to be an issue. The idea of the transmitter is to get the guitar signal to the amp, and a guitar signal is very very quiet. So really, I need to actually cut the volume at the receiving end, to get the correct input for my amp/effects. I will probably use a cheap/simple mini radio and connect it straight into my multi-FX box, so hopefully the transmitter will keep its frequency constant.

[George Giblet]

You need a buffer to stop that ckt loading the pickup.  Then you need to add an RC network across the emiter resistor to pre-emphasis.
As someone already mentioned the quality of that circuit isn't that great.  The frequency stability is a little rough as well.

The circuit Mark gave is much higher quality (thanks Mark).

[Mihkel]

You need a buffer to stop that ckt loading the pickup. 

Would running this buffer on 3V work?

Then you need to add an RC network across the emiter resistor to pre-emphasis.

Can you give any examples of ciruits that would work?

[George Giblet]

> Would running this buffer on 3V work?

It would have to be a single transistor buffer.  A divider placed at the input could be used to reduce overload.

When you add the buffer I  suggest putting at least 1k in series with the 100n input cap to the oscillator stage.

>Can you give any examples of ciruits that would work?

No examples but I know it can be done.

The simplest is to add an resistor in series with a cap across the 470ohm resistor.   The circuit may take some adjusting to get to work.

Try replace 470 ohm resistor with 1k, add 68n series with 100ohm.  For non-US use 47n.

Try replace 470 ohm resistor with 2k2,  add 33n  series with 220ohm. For non-US use 22n.

Play around with the 10pF cap from C to E.

Another way to do it is,

http://www.techlib.com/Karen.htm#FM%20Transmitter

A less efficient method in terms of parts (which won't work on 3V) is,

http://sound.westhost.com/project54.htm


On these simple circuits you will find the frequency drifts as the battery dies - a real pain in the butt.